The changing face of lower tropospheric sulfur oxides in the United States

The Anthropocene has been proposed as a new epoch beginning around 1950, possibly earlier, distinct from the Holocene because of the measurable effects of human activity on Earth system processes (e.g., Steffen et al., 2011; Edwards, 2016). Human effects on Earth’s atmosphere include climate forcing by long-lived contaminants, such as carbon dioxide (CO₂), and by short-lived species, such as suspended particles containing sulfate (SO₄) and carbon. Both long-lived and short-lived species derive from geochemical cycles and from anthropogenic emissions, with the latter mainly associated with fossil fuel combustion. These sources are believed to have a major impact on Earth’s atmosphere, especially the lower troposphere, and in turn not only modify the Earth’s climate, but also affect the biosphere and human environment in potentially adverse ways (e.g., Crutzen, 2012). Unlike CO₂ which has a tropospheric lifetime of decades, sulfur oxides (SO_x, including sulfur dioxide, SO₂, and sulfate, SO₄) and particulate carbon have tropospheric lifetimes of a few days. Even though the lifetime of SO_x, is short, its anthropogenic disturbance to tropospheric chemistry has extended for more than a century as a result of SO_x emissions growth. Compared to the probable effects of longer-lived species, such as CO₂, anthropogenic SO_x has so far been a relatively brief disturbance, especially when considered on geological time scales. The short lifetime of SO_x historically meant that SO_x was managed as a local to regional (<1000 km) problem; however, recent research also has included transcontinental S pollution (e.g. Irving, 1991; Ramanathan and Yeng, 2009). The principal effect of SO_x on climate change involves SO₄ particles that produce spatial gradients in radiative cooling and cloud nucleating agents across continents.

The sulfur (S) cycle is an example of an important geochemical cycle that is affected by humans. A portion of global geochemical sulfur resides in the atmosphere as gases and in airborne particles (e.g. Friend, 1973; Andreae and Jaeschke, 1992; Lens, 2009). Sulfur oxide fluxes in the S cycle have departed from a prior state mainly governed by natural emissions of S gases and particulate sulfate to an anthropogenically disturbed state over a time period of less than a few centuries. This departure is attributed to “byproducts” of increasingly diverse populations, requirements for energy and the use of fossil fuels associated with the growth of large metropolitan areas. Concentration and deposition gradients, which are most evident in the Northern Hemisphere, delineate the spatial scales of the impacts. In affected regions, SO_x contributes to S mass fluxes and modifies the S cycle (e.g., Friend, 1973; Lens, 2009). Exposure to anthropogenic SO_x adversely affects some terrestrial and aquatic ecosystems (e.g., Clair et al., 2011) and human health (e.g., Krewski et al., 2000).

To characterize changes in SO_x from actions in the developed world, a consistent, long-term record is needed for SO_x emissions, ambient concentrations and deposition. Such a record exists for surface data in the contiguous United States (US) from the early 1960s to the present. Thus, we concentrate on the US as an example, taking advantage of sustained, accessible public records over the last half century or more. Multiple measures of change in SO_x are adopted for comparison with annual emission trends. First, ambient SO₂ and SO₄ concentration data are examined as indicators of near-surface conditions in air chemistry. Both measures are assumed to be applicable to the tropospheric boundary layer. Then, SO₄ concentrations in precipitation are examined as a coarse measure of SO₄ content integrated from the tropospheric boundary layer through the middle troposphere. The survey of this large collection of data shows the effects of expanding energy production and use on their combustion byproduct, SO_x. The initial increase in S emissions with rapid industrialization and urbanization resulted in increasing environmental risk. Recognizing the value of natural and human resources, an increasingly affluent public took social action that yielded environmental improvement. Economic growth, adverse effects on populations, and subsequent improvement in public well-being closely paralleled industrialization. This sequence follows an ecological-economic characterization sometimes represented by the Kuznets Curve (e.g., Stern, 2003).

Our goals in this study are to illustrate how surface SO_x concentrations, serving as a proxy for concentrations in the lower troposphere, have changed with anthropogenic emissions, and to discuss how these changes have affected the S component of the Earth system. The changes are measured (a) empirically using ambient concentrations and wet deposition chemistry, and (b) as disturbances in natural atmospheric chemistry involving interactions with a range of gaseous and particle species. Our narrative begins with a historical context of SO_x observations from the 19^th century to the 21^st century. Then, we summarize US data from the 1990s to the present to describe nationwide changes in emissions, ambient concentrations, and deposition patterns. These data show the environmental risk perhaps reaching a maximum by the early 2000s, followed by a steady decline toward a low continental baseline. Recognizing that spatial gradients are important for SO_x characterization, we discuss regional examples of SO_x change across the US and their implications. The observations of SO_x changes yield an important picture of environmental risk reduction during the latter 20^th century and early 21^st century through direct interactions with humans and vulnerable ecosystems. This part of our study is complemented by examples of SO_x “interference” with multi-species natural air chemistry in the lower troposphere. The study is completed with a summary of the important changes in SO_x as part of the Anthropocene phenomenon.

Anthropogenic S is suspected to have been present in the troposphere before the 19^th century. The longest record of change relevant to the Anthropocene exists for locations in Europe and North America. Atmospheric S from man’s activities has been documented with industrialization in England since the 19^th century. Sulfur was identified in deposition across the US in the early 20^th century. Routine urban SO₂ and SO₄ observations have been reported separately in the US after the 1950s. By the 1990s, simultaneous monitoring of SO₂ and SO₄ exist with emissions estimates in the US.

Sulfur is emitted in the form of gases and particles from natural sources, creating a tropospheric “background”. Naturally emitted S species include hydrogen sulfide, carbonyl sulfide, methyl sulfides and sulfur dioxide. These gases are reactive in the troposphere and are oxidized to form sulfuric acid; combined with cations, they form particulate SO₄ salts, adding to ambient SO₄ from sea salt and soil dust. Combustion of fuels for energy production, especially coal and petroleum, has increased S levels both locally and globally, mainly in the Northern Hemisphere. The principal S source from combustion is SO₂, which oxidizes readily to form sulfuric acid, which in turn combines with ammonia or other cationic species to form other SO₄ compounds. The chemical reactivity of SO₂ involves important links with oxidant and organic chemistry, modifying natural processes (e.g., Seinfeld and Pandis, 1998). The SO_x species are removed from the troposphere rapidly by wet and dry deposition to the Earth’s surface, unlike longer-lived contaminants such as CO₂, methane or halocarbons. Sulfur oxides are soluble or absorbable in precipitation, enhancing their removal in a few days (e.g., Hidy, 1973).

Sulfur from air pollution often has been identified visually with smoke from fuel combustion; smoke has been recognized as a nuisance and respiratory irritant since Roman times (e.g., Brimblecombe, 1987). Public attempts to control S emissions, with smoke as a proxy, began in a serious way in the 19^th century (e.g., Mosley, 2014) in a reaction to the industrial revolution in England, Europe and the US. Britain and the US adopted a similar populist approach to smoke abatement in the latter 19^th and early 20^th centuries through community initiatives, litigation and later national legislative attention (e.g., Stradling and Thorsheim, 1999). Early attempts to reduce smoke and S in cities probably had limited influence on the growth or expansion of fossil fuel use at the time. In North America and Europe, S from combustion has been curtailed largely with burning of low sulfur fuels combined with post-combustion emission controls. This reversed the previously rising tropospheric concentrations of these species that accompanied rising emissions driven by urbanization and the industrial revolution (e.g., Smith et al., 2011). A similar pattern of growth and curtailment in the energy sector is taking place in the developing world, especially China and India. To date, curtailing of SO₂ emissions in Asia has met with mixed results, as discussed by Klimont et al. (2013).

Anthropogenic S as rainfall SO₄ and ambient SO₂ occurred at high levels in the United Kingdom by the mid-19^th century and early 20^th century. In the United States, S was reported in bulk (combined wet and dry) deposition in 1910 and as wet deposition after the 1970s (e.g., Hidy et al., 1984). Historic SO_x data are subject to large uncertainties given “primitive” sampling and analytical techniques available at the time of reporting (Text S1), including gas sampling and analysis, chemical analysis of water extracts from particle samples of differing size fractions, bulk (dry and wet) deposition sample collection in open containers, or estimation of ambient concentrations by vegetation stress (e.g., lichen samples). Assuming that the early data are reliable and comparable, SO_x found in the polluted troposphere and in surface deposition show high levels, some of which far exceed those reported after the 1970s.

Data from Britain and the US are listed in Table 1 to “scale,” in magnitude and time, the S levels reported for these two highly industrialized nations. 19^th century data in England suggest very high local levels of SO_x from industrialization and residential heating, which relied on imported and domestic coal for energy (e.g., Brimblecombe, 1987). By mid-20^th century, levels of deposition and ambient concentrations declined significantly compared with the 19^th and early 20^th centuries. Likewise, the US data showed high levels of SO₄ in precipitation, implying high ambient concentrations. By mid-to-late 20^th century, SO₄ levels had declined substantially in rainwater and ambient air despite greatly expanded post-1950s emissions of SO_x across the eastern US and in parts of the western US.

Without air quality data, assessments of human health effects from exposure to industrial air pollution were largely intuitive in the 19^th century. Recently, Hanlon (2015) found that industrial pollution substantially increased mortality in urban Britain between 1851 and 1860, and increased mortality grew as coal use intensity rose from 1851–1900. In the US, public health concern for SO_x pollution was catalyzed in 1948 with the Donora, Pennsylvania event involving the deaths of 17 people from pollution exposure during a five-day event (e.g., Schrenk et al., 1949). Direct ambient measurements were not made during this event (e.g., Williamson, 1973), but in 1949, SO₂ concentrations reached ∼500 ppbv with particle concentrations (containing SO₄) of ∼4000 µg/m³ (< 35µm diameter) in similar meteorological conditions. Contemporary maximum daily values of US urban SO₂ concentrations are <30 ppbv; daily particulate matter concentrations are generally < 30 µg/m³ for particles < 2.5µm in diameter.

Losses of SO_x by dry deposition and wet deposition from precipitation reflect emission levels and their changes. Dry deposition and precipitation scavenging are proportional to ambient concentrations and essentially dictate the lifetime of SO_x in the troposphere. The amount of dry deposition is proportional to ambient concentration; the rate of deposition is characterized by a proportionality coefficient known as the deposition velocity. Wet deposition of SO_x depends on the absorption of SO₂, its oxidation in the presence of H₂O₂ or O₃, and collection of aerosol particles containing SO₄. The phase of condensate and the processes of nucleation and growth of hydrometeors are also factors in SO_x collection. Junge (1963) was one of the first to articulate the SO_x proportionality in wet removal process taking place in clouds (rainout), and below clouds (washout). This approach was a simple way to connect ambient SO₂ and SO₄ concentrations with precipitation chemistry through the water content of clouds and an empirical scavenging coefficient. Adopting Junge’s method, the 19^th century issue of linking ambient concentrations with wet chemistry could be resolved. The SO₄ composition of rainwater provides a consistent measure of average SO₄ content in precipitation, assuming that SO₂ absorbed in precipitation water is identified as SO₄.

In addition to adversely affecting human health, SO_x modifies aquatic and terrestrial ecosystems through deposition to the Earth’s surface. In the 1930s, agriculturists became interested in the levels of natural S deposition that fertilized crops in parts of the US. This led to the early deposition observations reported in several areas (Table 1; e.g., Hidy et al., 1984). In the 1960s, Scandinavian scientists found that excessive wet deposition of acidity associated with SO₄ was affecting low alkalinity fresh waters (e.g., Oden, 1968). Studies also showed that terrestrial systems, especially Scandinavian forests with poorly buffered soils, were at risk. Later studies indicated excess acidity issues existed in parts of Europe, Canada and the US (e.g., Cowling, 1995). The acid rain phenomenon associated with SO₄ deposition became an international environmental issue by the mid-1970s. The risk of ecosystem damage in parts of the US and Canada resulted in major studies of vulnerable ecosystems between the mid-1970s and 1980s (e.g., Irving, 1991). A national plan for reductions in S deposition resulted from the amendments to the US Clean Air Act in 1990.

Strategies for returning to less anthropogenically influenced conditions have aimed to reduce the impact of SO_x deposition from the troposphere. One approach taken after the 1980s was to try to find deposition loading rates associated with significant damage (critical loads) of vulnerable ecosystems. The loadings provide a basis for reducing SO_x and other pollutant levels, including nitrogen species, to conditions that would have no “significant” effect on the exposed systems (New York State Energy Research and Development Authority - NYSERDA, 2014). In North America, most of the initial research on this concept derived from European and Canadian investigators and emerged in regulatory discussions in the 1980s (e.g., Clair et al., 2011). Later, many US ecologists embraced the idea of critical loads. Critical loads for vulnerable terrestrial and aquatic systems are estimated to be in the range of <100 to 700 eq/ha-y for eastern Canada (Environment Canada, 2012). These levels are believed to be similar to the northeastern US. The development of critical load values has been controversial—especially in relating values to ambient concentrations of SO_x. The controversy centers on quantification of the levels of sulfur deposition occurring in a pristine, ecologically relevant state having minimal anthropogenic influence. With reduction in SO_x across the US, vulnerable aquatic and terrestrial ecosystems have been subjected to less stress, as conditions move towards achieving estimated critical load levels. Ironically, concern for availability of S through deposition has emerged again for certain crops in the US and Europe with the major declines in deposition from S pollution (e.g., Agron, 2001; Camberato and Casteel, 2010). This, of course, results in added agricultural S fertilizer dispersal, affecting the “natural” S cycle in another way.

The impact of SO_x on the atmospheric component of the sulfur cycle has varied historically with changing levels of anthropogenic emissions superimposed on temporally variable natural emissions. Both anthropogenic and natural emissions have been studied extensively. Natural sources consist of marine and terrestrial emissions, including a variety of S-containing gases of biogenic origin, SO₂ and H₂S from volcanic disturbances, sea salt and soil dust (e.g., Aneya, 1990; Andreae and Jaeschke, 1992). Examples of natural tropospheric levels of gases and particles from marine or terrestrial sources are listed in Table S1. Global marine and terrestrial S emissions attributed to the natural S cycle are on the order of 3 × 10⁴ ktonnes/y (e.g. Bates et al., 1992), and about 16% of 1980s northern hemispheric S emissions. After 1850, global SO₂ emission rates from anthropogenic sources began to increase upwards from approximately 2000 ktonnes/y. Smith et al. (2011) indicate a global combustion and process related SO₂ emissions peak at 13 × 10⁴ ktonnes/y in 1975, with a subsequent decline to ∼12 × 10⁴ ktonnes/y in 2005. Prior to 1900, anthropogenic SO₂ emissions were dominated by European sources; between 1900 and 1960, North American emissions became predominant. By 1990 anthropogenic emissions from Asia began to exceed the combined Europe and US emissions. Sulfur dioxide emissions from biomass burning exceeded fossil fuel combustion and industrial processing emissions in 1850, but by 1870 the latter categories began to dominate. By 2005, SO₂ from biomass burning amounted to about 34% of the fossil fuel combustion and processing. Natural S emissions are estimated to be less than a few percent of emissions affecting the US (natural to anthropogenic ratio < 0.1; e.g., Irving, 1991).

A history of estimated worldwide anthropogenic emissions from major contributors is shown in Figure 1. The largest contributors from 1850 to the 1950s are North America and Europe. Asia and India, and other developing nations, began to increase their emissions after 1950. US anthropogenic SO₂ emissions generally are much greater than those of Canada, so that the North American trends in Figure 1 approximate those of the US. We concentrate on anthropogenic components, primarily SO₂, noting that SO₃ emissions are small relative to gaseous SO₂, <1% (e.g., Hardman et al., 1998). Estimated annual North American SO₂ emissions from 1850–2005 are indicated in Figure 1, showing an increase from ∼300 ktonnes/y in 1850 to a peak in the 1970s of ∼30 × 10³ ktonnes/y, followed by a decline, which has continued since 2000 (e.g., Environmental Protection Agency - EPA, 2014; Xing et al., 2013). In most locations, the current US SO_x emissions tend to be dominated by large non-urban power plants, many of which have used coal as fuel. Systematic emission reduction from these sources since the 1980s has come from switching to increasingly low sulfur coal, natural gas, or effluent emission control using scrubbers. Adoption of these strategies has greatly reduced the presence of anthropogenic SO_x in the troposphere, and in deposition to Earth’s surface.

The US domestic trends illustrate the emission changes occurring in response to a policy to reduce air pollution and ambient SO₂. Emission reductions were mandated by the US Clean Air Act of 1970 and the 1977–1990 amendments, which addressed public health concerns, acid deposition, and visibility impairment. Regional emission inventories show similar temporal characteristics, but the peak values of emissions and ambient concentrations vary substantially across the US with population and industrial distributions. 2010 SO₂ emission levels in the US are approximately 7.7 × 10³ ktonnes/y (Xing et al., 2013). Estimated natural emissions influencing the US, including terrestrial biospheric sources and S entering the marine borders of the US, are about 400 ktonnes/y as SO₂ (e.g., Irving, 1991). Thus, as noted above, if the natural emissions are reasonably constant, they represent a minor fraction of the total contemporary US SO₂ emissions.

Examples of spatially disaggregated US SO₂ emission distributions for 1985 and 2011 are shown in Figure S1. The geographical distribution of anthropogenic emissions follows population and industrial centers across the US with increasingly isolated sources west of the Mississippi River and in part of the Southeast. East of the Mississippi River, especially through the Ohio River Valley and Pennsylvania, a number of large SO₂ sources were built in close proximity with a west-east orientation. This geographical configuration, along with prevailing westerly winds, created nearly ideal conditions for accumulation and regional transport of SO₂ and SO₄ in ambient air moving toward the US East Coast (e.g., Hidy, 1994). In the Southeast, regional SO_x accumulation takes place in association with large spatial scale air mass stagnation over relatively isolated large sources, especially in summer, unlike conditions in the Northeast. In the West, with some exceptions such as southern California, large sources are geographically isolated by >∼100km. This allows for dilution of emitted pollution, and regional scale air mass stagnation or transport accumulation is infrequent compared with the East.

Corresponding to the SO₂ emission distributions are the geographical distributions of ambient concentrations for SO₂ and SO₄. These are shown for 1989–2001 and 2011–2013 in Figures S2 and S3. Ambient SO₂ concentration variations tend to overlay regions of different emission density, as expected. Through much of the US, estimates of 1989–1991 annual average concentration range from 2–3 µg/m³ (0.8–1 ppbv at standard temperature and pressure). A noted exception is in the midwestern and northeastern regions with concentrations ranging from 14–16 µg/m³ (5–6 ppbv). As of 2011–2013, most of the US is in a regime having annual average SO₂ concentrations of <∼2 µg/m³ (0.8 ppbv). In the Midwest and East, ambient SO₂ concentrations have declined to 6–8 µg/m³ range (2.3–3 ppbv), responding to the national reductions in SO₂ emissions. Ambient annual average SO₄ concentrations have shown a similar decline between the two periods. In 1989–1991, SO₄ concentrations are ∼< 2 µg/m³, while in 2011–2013, SO₄ concentrations west of the Mississippi River are <1–2 µg/m³; eastward from the river concentrations are in the range of 2–4 µg/m³. The region of “elevated” SO₄ concentrations east of the Mississippi River is broader than the area having high emission densities (e.g.,Mueller et al., 1980). Tropospheric oxidation of SO₂ that is nominally in a range of 0.2–4%/h generates spatial distributions of SO₄ deposition extending 400–1000 km downwind of emission sources.

The rates of reduction in annual average ambient SO₂ concentration, by region, generally show a linear, 1:1 proportionality relationship with regional rates of reduction in annual SO₂ emissions (e.g., Hidy et al., 2014). The decline in SO₄ concentrations vs. emissions or ambient SO₂ tends to be linear, but less than 1:1 (e.g. Hidy et al., 2014; Sickles and Shadwick, 2015). As discussed later, the difference between the responses of ambient SO₂ and SO₄ is identified with the atmospheric production of SO₄ during dispersal from primary SO₂ sources. This process is analogous to the dispersal of tropospheric ozone produced from the chemistry of organic vapors and nitrogen oxides (e.g., Seinfeld and Pandis, 1998; Cooper et al., 2015).

Although there have been major reductions in US ambient SO_x concentrations over the past 25 years, concentrations remain higher than (nominal) background levels without human disturbance. A continental background depends on the strength of natural sources, and will vary in time and space (Hidy and Blanchard, 2005). Estimates of baseline contributions, including a natural and anthropogenic residue, can be made from ambient concentration measurements. Typical values for baseline SO₂ concentrations in the troposphere cited by Friend (1973) are 0.1 ppbv with a range from 0–1.5 ppbv. More recent reported values are illustrated in Table S1, which show a 5–160 pptv range of concentrations. Data from remote sites worldwide reported by Carmichael et al. (2003) indicate a range from 30–160 pptv. For SO₄, Cadle (1973) reported ambient concentrations of 0.7 µg/m³ from sampling over the Northern Pacific Ocean (at altitude 0.015 km) in the 1970s, while Leck et al. (2002) reported concentrations from shipboard non-sea salt (nss) measurements of ∼<0.5 µg/m³ in the South Atlantic Ocean. Hidy (2009) summarized data from several observational campaigns over the Pacific and Atlantic regions that indicated concentrations in the ∼0.1 µg/m³ range. Observations over remote North American locations range from <0.1 to ∼0.5 µg/m³ SO₄ (e.g., Van Curen, 2003; Hidy and Blanchard, 2005; Heald et al., 2006). Observations of SO₄ concentrations are approximately 0.5 µg/m³ at remote sites along the Pacific Coast and immediately inland. From South America, values <0.1 µg/m³ have been reported (e.g., Hidy, 2009). We hypothesize that a baseline US level of SO₂ is about 30–160 pptv, and varies depending on location and season. We also suggest that the US baseline SO₄ is in the range of <0.1–0.5 µg/m³, including natural and anthropogenic components. The sulfate baseline exhibits a variability >1 µg/m³ if the contributions of sea salt and other natural S sources are included. Assuming this range of natural SO₄ occurrence is reliable, we conclude that although SO_x concentrations have declined, the current levels reached in the US remain well above expected continental baseline levels, except perhaps in remote locations mainly in the western and far-western US.

Sulfur deposition provides another measure of change that follows emission trends. Dry deposition rates have been estimated typically for relatively smooth surfaces sometimes covered with low vegetation; forested, hilly and mountainous terrain have not been characterized with the same certainty. (e.g., Sickles and Shadwick, 2015). Trends in dry deposition have been found to follow trends in ambient concentrations of SO_x, accounting for surface inhomogeneities in spatially differentiated models. Changes in dry deposition have been estimated from measurements at a number of rural locations since the 1980s (e.g. Clean Air Status and Trends Network - CASTNET, 2013; 2016). Deposition in precipitation depends on rainwater concentration and precipitation rate. SO₄ concentrations in precipitation also provide an extended, well documented history of change in precipitation SO₄ corresponding to ambient SO_x and emissions changes since the 1950s. As an example of wet deposition trends, Figure S4 shows a comparison of 1989–1991 with 2011–2013 maps of national rainwater sulfate from the National Acid Deposition Program (NADP, e.g. EPA, 2016). This continuing program of sampling and chemical assessment of rainwater has existed in the US since about 1980 (e.g., Hidy et al., 1984; NADP, 2016). A comparison in Figure S5 indicates a major reduction of rainwater SO₄ that accompanies the reduction in national SO₂ and SO₄ concentration shown in Figure S2 and S3. Figure S4a shows concentrations >∼ 3 mg/L (mgL^-1) in the eastern US with generally concentrations <∼ 1 mg/L west of the Mississippi River. One exception is the reported high concentrations in northern Utah, related to large rural sources in this area. By 2011–2013, SO₄ in rainwater (Figure S4b) declined across the US to levels < 1.5 mg/L in the East and <∼ 1 µg/m³ in the West. These levels are similar to current levels in Europe but are much higher than contemporary levels in remote areas. For example, snowpack SO₄ from the Upper Fremont Glacier in Wyoming is 0.06–0.08 mg/l between 1977 and 1985, with earlier values near zero in the snow-ice strata (Schuster et al., 2000). This glacier is the only one reporting SO₄ in snow after the 1700s in the continental US, but is most relevant to the western states. For comparison, Preunkert and Legrand (2001) report that winter preindustrial levels of SO₄ in snow in the French Alps is about 0.02 mg/L reaching 0.07 mg/L in 1980. Using the Fremont Glacier levels as a proxy for tropospheric SO₄ in the US, there evidently remains substantial ambient SO₄ to decrease to achieve a level comparable to a pre-20^th century “natural” state in winter (when biogenic production is minimal).

The short lifetime of SO_x in the troposphere favors the local and regional character of impacts as discussed above. Sulfate is often used as an index for changes in the regional distribution of SO_x in response to changes in SO₂ emissions (e.g., Hidy, 1994). The distribution of ambient SO₄ concentrations in multistate regions of the US is illustrated in Table 2, and graphically in Figure 2. These data cover the period between the late 1960s through 2010. Shown in Figure 2 are post-1990 trends in regional emissions compared with SO₄ changes from the national networks (chemical speciation network (EPA, 2016), and IMPROVE (CIRA, 2016)). The table suggests a peaking of SO₄ with SO₂ emissions across the US, followed by a decline to lower annual average concentrations after the late 1990s and beyond. The changes follow the national timetable for SO_x reductions set by the 1990 Clean Air Act amendments.

Figure 2 includes rural or nonurban annual average SO₄ data from the two national monitoring networks, IMPROVE and CSN. Hand et al. (2012b) report trends in terms of long- term (IMPROVE) sites (LT), and CSN data after 2000 compared with shorter-term IMPROVE data (ST). These ambient data are compared with six regional trends in SO₂ emissions derived from the continuous emission monitoring (CEM) of power plant effluents (e.g., EPA, 2014). As indicated in the figure there has been a regional trend downward essentially everywhere in the US coinciding with reductions in SO₂ emissions. By 2010 the ambient SO₄ across the US declined to a range of 0.5–2 µg/m³. The lowest current levels are seen in the West and Great Plains where they are believed to approach a range expected for a continental baseline, which includes an impact of transpacific transport of pollution from Asia (e.g., Hidy and Blanchard, 2005).

It’s beyond our scope to look at all of the US changes in detail by region, so we select three locations with long-term data to exemplify SO_x changes on a regional and local scale for different demographic, emissions and climatological conditions. The choices are: The Northeast, the Southeast and the West, particularly southern California. The northeastern US represents a location of high population density and an intensive industrial complex. It is widely known to be susceptible to the influence of transported SO_x from regional sources to the west and to the south. The Northeast is affected by waves of widespread air mass stagnation, especially in summer. The stagnation conditions are followed by intervals with westerly, long-range pollution transport paralleling frontal systems (e.g., Irving et al., 1991; Hidy, 1994). People living in the Northeast obviously have the potential for exposure to, and effects from, anthropogenic SO_x. There also are major terrestrial and aquatic ecosystem resources in the region that are vulnerable to SO_x exposure either in dry or wet form. The eastern parts of the region are considered to be influenced by pollution originating upwind in the Midwest and Southeast. The linear scale of the Northeast ranges from ∼400−1000 km in extent. The southeastern US is a subtropical climatic regime that has experienced increasing SO_x and other pollution after the mid-20^th century, associated with major increases in industrial activity, energy production and population growth. The Southeast experiences frequent air mass stagnation periods especially in summer that result in high levels of SO4 over ∼500 km extent. Ecological systems in parts of the Southeast also are susceptible to S deposition exposure. The urbanization of the Southeast has a potential for widespread adverse population exposure. The West contains extensive arid regions, which contrast with wetter locales along the Pacific Coast. Discrete metropolitan areas with high population densities and more industry are generally separated by hundreds of kilometers. Isolated energy production activities are scattered throughout the region. Many parts of the West are geographically remote and exhibit low SO_x levels compared with elevated SO_x concentrations occurring in more highly populated areas. However, there are large metropolitan areas, e.g., in California and other states, that have had major historic SO_x impacts associated with photochemical pollution. Emission sources in western cities are complemented inland by widespread, isolated rural coal extraction, rural electricity and petroleum energy production, and non-ferrous metal smelting. Some mountainous ecological systems in the West are vulnerable to SO₄ deposition exposure analogous to the East. Listed in Table S2 are mid-1960s data from cities in these three regions and rural locations. These are compared with values in Tables 1 and 2 to indicate their representativeness relative to aggregated data for the US.

The northeastern US historically has had the highest sustained urban SO_x concentrations in the country, especially after World War II. In addition, the rural and remote areas of the region have recorded high levels of SO_x deposition since contemporary observations began in the 1970s, particularly in upper New York State and Pennsylvania. Perhaps most relevant to our study are rural sites with a long-term record. These are exemplified by Whiteface Mountain in the Adirondack Mountains (44.366^oN; 73.903^oW). Whiteface Mountain has a record of SO_x observations that extends from the 1970s to the present (e.g., Husain et al., 2004; Schwab et al., 2016; Brandt et al., 2016; Rattigan et al., 2016). Whiteface Mountain has two sampling locations, one at the base of the mountain (600m elevation) and one at a summit observatory at 1.5 km elevation. The SO_x sampling is done at the base of the mountain.

The annual average ambient SO_x concentration trends at Whiteface are compared with regional emissions based on the National Emission Inventory (NEI) for the Northeast region in Figure 3. Emissions and ambient SO₂ track closely, as indicated in Figures 3a and 3b. Ambient SO₂ levels decline from 1.1 ppbv to 0.2 ppbv between 1989 and 2010. Prior to 1992, three-year means of SO₄ concentrations were 4 µg/m³ in 1979, 3.2 µg/m³ in 1982 and 2.4 µg/m³ later. Paralleling the emissions and ambient SO₂ decreases after 2000 are the annual average SO₄ concentration declines exemplified in Figure 3c. Sulfate decreased at a linear rate that is approximately the same as the 1986-2010 rate of ambient SO₂ reduction. After 2010, ambient concentrations of SO₂ and SO₄ tend to level at ∼1 µg/m³. The latter SO₄ concentration is essentially the same as reported at Mt. Washington, New Hampshire (1.92 km height) for 1999–2004 (Fischer et al., 2007). The pre-1990s concentrations for the rural Northeast are similar to the Whiteface conditions. For example, Bowne et al. (1991) show 1988–1990 annual median concentrations of SO₂ and SO₄ of 6.7 µg/m³ (2.5 ppbv), and 3.4 µg/m³ respectively.

Husain et al. (2004) compared Whiteface SO₄ data between 1979 and 2002 with data from another rural site, Maysville, New York, located to the WSW of Whiteface Mountain near Lake Erie. Sulfate concentrations at Maysville during this period were approximately twice those of Whiteface. Both showed 30% or more decrease with decreasing regional SO₂ emissions. At other rural sites in the Northeast, annual average SO₄ concentrations in 2001 were 2–3.7 µg/m³. For comparison, SO₄ concentrations in the New York City metropolitan area between 2000 and 2014 ranged from 4 µg/m³ to approximately 2 µg/m³, much lower than the 14–16 µg/m³ concentrations reported in 1972 by Lynn et al. (1975).

The current concentrations at Whiteface Mountain and Mt. Washington begin to approach a hypothetical regional baseline for the eastern US, but still remain at least a factor of two or more above it, based on rural western US and southern hemispheric conditions.

Measurements of wet deposition and water chemistry also have been made at Whiteface Mountain. The trend in precipitation sulfate concentrations is shown in Figure 4, and is compared with Biscuit Creek in central New York. These data suggest a decline in precipitation SO₄ since 1980, with a similar rate as that seen in the ambient SO_x concentrations. The decline is a change of 0.13 mg/L-y between 1990 and 2010, amounting to about 5%/y, which is comparable with emission reductions. In both ambient SO_x and SO₄ precipitation concentrations, there is leveling off to near zero change after 2010. Because the precipitation trend depends on both the annual precipitation quantity and the frequency of precipitation, a direct linear correspondence between precipitation SO₄ and ambient SO_x concentrations would not necessarily be expected.

The southeastern US has been strongly influenced by fossil fuel power plant SO₂ emissions, which grew steadily from the 1950s throughout the Southeast. This growth was superimposed on local emissions from ferrous metal processing (e.g., Birmingham, Alabama), paper and pulp mills, textile and forest product industry operations and agriculture. In Figure 2, trends in SO₄ compared with emissions are shown for the region. After 2000, SO₄ has declined with emissions, largely in response to reduction in S emissions from large coal-fired power plants in the region after 2004.

For air quality, the region is characterized by a regional (Southeastern Aerosol Research and Characterization –SEARCH) network of eight sites located inland and near the coast of the Gulf of Mexico, and extending west to east by ∼500 km (e.g., Hansen et al., 2003). The SEARCH data are of particular interest in that each of the stations measures ambient SO₂ and SO₄, and recently two of the sites have been instrumented to collect precipitation samples. The regional SO_x trends are documented using early observations beginning in the mid-1960s through the 1990s (e.g., Tables 1 and 2) and later with the SEARCH measurements since 2000. The decline in post-2000 SO₂ emissions in the Southeast are quantified using the EPA NEI (e.g., Blanchard et al., 2013) for Alabama, Georgia, Mississippi and northern Florida, as shown in Figure 5. The data in the figure compare the decrease in ambient SO₂ and SO₄ with regional emissions of SO₂. SO₂ concentrations follow the emissions decline in 1:1 proportionality; SO₄ concentrations also track the emissions and ambient SO₂, but with a less than 1:1 proportionality. By 2013, the regional (rural) annual SO₄ concentrations have decreased to approximately 2 µg/m³, with the SO₂ concentration decrease of ∼ 1 µg/m³. The current rural levels of SO_x in the SEARCH domain remain well above those expected to be representative of conditions in the much of the West, which are minimally influenced by local anthropogenic activity.

Over the period 2000–2012, ambient SO₂ concentration reductions of ∼ 7%/y are linear, and 1:1 proportional, with emissions reductions. SO₄ declined ∼5%/y. The different responses of ambient SO₄ and SO₂ concentrations to southeastern emission reductions could be caused by chemical and meteorological processes that result in less linear oxidation rates than have been observed at Whiteface Mountain.

Trends in precipitation SO₄ in the Southeast qualitatively follow SO₂ emission reductions, as indicated in Figure 6. Included in the rainwater concentrations is a maximum level in 2005–2007 followed by a sharp decrease as in the case of the emissions and SO_x patterns. After 2010, sulfate in precipitation has decreased modestly to 0.6 mg/L as of 2014. This concentration is similar to earlier levels reported in the mid-1950s (Table 1), and essentially the same level seen in the New York results (Figure 3). Precipitation sulfate from 2000–2012 declined ∼6%/y, similar to SO₂ emission reductions.

The western US ranges from remote locations to highly populated areas, especially in California. Figure 2 shows data from western locations on or near the Pacific Coast, inland over the Sierra Nevada Mountains to Nevada, across the Cascade Range to Idaho, the arid Southwest, and the Great Plains. In these regions the post-1990 trends follow qualitatively SO₂ emissions, declining through 2010. As noted earlier, the western SO₄ concentrations are generally lower than those occurring in the East, but are similar in recent years to rural areas of the Northeast. Data in Table 2 compare annual average SO₄ trends by region, separating the coastal and inland West. In the late 1980s, for example, annual average SO₄ levels are similar between the rural coastal sites and inland. In contrast, Miller and Flores (1992) reported that 1986–1987 SO₂ concentrations in the inland National Parks (and in Hawaii and Alaska) averaged 1.3 µg/m³, with a range of 0.8–1.6 µg/m³, compared with ∼ 4 µg/m³ at urban locations in southern California.

Because of its long-lasting concern about air pollution since the 1940s, southern California has perhaps the longest record of air pollution measurements available in the West, dating back prior to the 1950s. Sulfur dioxide measurements began systematically in the 1960s. The unusually strong influence of photochemical smog in southern California has favored rapid oxidation of SO₂ there. The resulting production of particulate SO₄ reduced visibility and increased acidity in the urban air around Los Angeles. Particulate SO₄ pollution extended into the California Central Valley and the desert-montane Southwest with air mass transport and in-situ production. This case is a unique regional “disturbance” comparable to the appearance of severe smoke pollution in industrialized Europe and England.

Data in Table 3 lists emissions and ambient SO_x in the Los Angeles area, or more broadly in the California South Coast Air Basin (SCAB), after the 1960s. Most of the pre-1990s data derive from local or state monitoring in the basin. The recent SO₄ concentrations shown in the table come from an IMPROVE site and a CSN site east of the urban area. The daily emissions are substantial across the basin through 1974, and are later followed by major emission reductions. For example, between 1980 and 1995 reductions in (assumed) annual averages of daily SO₂ emissions of ∼5%/y resulted in the same annual reductions in SO₂, while SO₄ reductions were ∼4%/y. Comparison of early emissions with more recent estimates is problematic because local authorities combined the Los Angeles Air Pollution Control District with five counties in the region to form the South Coast Air Quality Management District (AQMD) in 1976. Qualitatively, both ambient SO₂ and SO₄ have declined in the SCAB with SO₂ emission reductions, but perhaps not proportionally (e.g., Kurosaka, 1976). Levels reported into the early 21^st century suggest a leveling of ambient SO₂ concentrations basin-wide along with urban SO₄. The rural concentrations at least 100 km from downtown Los Angeles approach ambient levels seen elsewhere in the inland West or further northwest along the Pacific Coast.

The SCAB (and the Pacific coastal lands) experiences some influence of SO_x from intercontinental transport across the Pacific Ocean from Asia. The influence of this source of SO₄, for example, is estimated annually to be ∼0.1–0.5 µg/m³ (e.g. Van Curen, 2003; Hidy and Blanchard, 2005; Heald et al., 2006). The sea salt SO₄ component from evaporated, windblown spray from near coastal waters or surf is estimated to be of order 0.2 µg/m³ on average for particles ∼<35 µm diameter and < 30 ng/m³ for 1.1 µm diameter particles.(e.g., Miller et al., 1972; Hidy et al., 1974; Quinn and Bates, 2005 ). These “unmanageable” sources of SO₄ along the coast, combined with a mesoscale offshore air circulation (e.g., Hidy et al., 1974), create a potentially high baseline for the lower troposphere over California.

The Perraud et al. (2015) study of SCAB air quality provides insight into baseline SO_x concentrations occurring in the absence of local SO₂ emissions. The modeling analysis compared results with and without local anthropogenic SO₂ emissions, taking into account oxidized organosulfur compounds from natural sources and from anthropogenic sources such as urban and agricultural activity. The calculations indicate a decrease in (SO₄) particle formation of two orders of magnitude compared with current levels. However, particulate SO₄ will still be generated in southern California through intermediate reaction pathways associated with sulfuric acid and methanesulfonic acid.

Like ambient air conditions, SO₄ concentrations in western precipitation show complex trends reflecting regional scale scavenging of airborne material. Shown in Figure 7 are examples of SO₄ in precipitation chemistry along the Pacific Coast (inland to ∼200 km). In this region, precipitation is influenced by mountain (orographic) effects, and by atmospheric processes occurring across the eastern Pacific Ocean. The site near the SCAB (Tanbark Flat) indicates high levels of aqueous SO₄ relative to the other examples, but reflects a steady decline in SO₄ concentration until the mid-1990s when ambient SO₄ levels tend to level out after local SO₂ emissions reductions have taken place. By 2014, SO₄ in precipitation at Tanbark Flat had steadied at ∼ 0.4 mg/L. Another California site in the Sierra Nevada is located inland at Sequoia National Park NNE of Los Angeles, and east of the San Joaquin Valley. A large areal source of SO₂ exists from agriculture and oil and gas operations in the San Joaquin Valley. At these California precipitation monitoring locations, a recorded decline in SO₄ concentration began in 1980, but leveled off after the mid-1990s to about 0.3 mg/L. Farther north and west near the coast, rural sites in Oregon and Washington show lower SO₄ concentrations with some evidence of downward trends from before 1990 and ending in 2014 at 0.1–0.2 mg/L. These sites are likely to represent SO₄ in precipitation heavily influenced by local weather over the eastern Pacific Ocean and orographic effects from the mountains to the east of the Coast. Increased fossil fuel combustion in Asia over the past two decades does not appear to appreciably affect precipitation SO₄. Thus one could interpret the current SO₄ concentrations in Oregon and Washington as a coarse measure of a continental baseline for precipitation SO₄.

Added perspective on precipitation SO₄ trends is found further eastward towards the Great Plains (Figure S5). The precipitation SO₄ levels range from a high in eastern Kansas to low values in Montana and Wyoming. The higher levels in Kansas approach concentrations found in the more populated and industrialized midwestern and eastern parts of the country; as in the East and Midwest, the Kansas measurements show a steadily declining trend. The lower levels in Wyoming and Montana indicate essentially constant conditions after the mid-1980s to about 0.2 mg/L. Two additional sites are included for trends, one in Colorado and one in North Dakota near the Canadian border. Both show steadily declining SO₄ trends since 1980, approaching the 2014 concentrations seen in Wyoming and Montana. The minimum level of ∼0.2 mg/L appears to reflect transport from the Pacific Coast to inland areas having isolated population centers that only minimally add to the wet SO₄ loading.

In summary, the contemporary record of annual average ambient SO_x concentrations and precipitation SO₄ in the US shows a direct response to SO_x emissions on spatial scales ranging from continental to sub-regional. Changes in SO₂ concentrations tend to be 1:1 proportional to emissions, while changes in SO₄ concentrations are less than 1:1 relative to emissions changes. Global modeling consistently shows that ambient SO₄ changes in the US, Europe, and Asia are proportional to SO_x emission changes, but the proportionality constant varies by location (Manktelow et al., 2007). Between 1985 and 2000, the US and European column burden of SO₄ changed 0.65% for every 1% change in SO_x emissions. However, for Asia, the change in SO₄ was estimated to be 0.88% for a 1% change in emissions. These authors concluded that as emissions tended to move southward between 1985 and 2000, in-cloud oxidation tended to be less oxidant limited. After 1985, a 12% reduction in global SO₂ emissions resulted in a 3% reduction in SO₄ burden.

The previous sections show the effects of growth and decline of SO_x emissions on ambient SO_x concentrations, precipitation SO₄, and S deposition. As noted, the deposition rate is proportional to ambient concentrations. These components of the flux of atmospheric S affect the S cycle. Changes in natural tropospheric chemistry that result from changing SO_x concentrations also modify the S system. The impact of SO_x on atmospheric chemistry affects not only emissions-concentration trends, but also influences the complexities of tropospheric chemical interactions themselves. Sulfate production from SO₂ either in ambient air or in hydrometeors depends on interactions involving gas-phase oxidation coupled with oxidant formation, and including aqueous chemistry. The latter involves dissolved oxidants, reactions catalyzed by metals (e.g., iron and manganese) or sooty carbon (e.g., Larson, 1980; Calvert and Stockwell, 1984). Multiphase interactions in the troposphere have introduced potentially important evolutionary changes in air chemistry with SO_x emissions. Three examples of SO_x multi-pollutant reactions include interactions with photochemically induced oxidant chemistry, SO₄ with nitrate, and SO_x with organic vapors and particulate carbon.

The first case involves the oxidation of S gases in the troposphere by OH free radical in the gas phase, by H₂O₂, especially in aqueous reactions, and, to a lesser degree, ozone (O₃) (e.g., National Acad Sci - NAS, 1983; Seinfeld and Pandis, 1998). It is widely known that the SO_x chemistry is coupled to the photochemical cycle of oxidant production through a complex chain of nitrogen oxide and organic vapor reactions that form smog. S gas oxidation through an intermediate step of SO₃ formation tends to syphon away OH and H₂O₂ from the oxidant production cycle (e.g., Kleinman, 1985; Stockwell et al., 1988). This effect is generally thought to be of minor importance for O₃ production, even in highly reactive urban smog conditions. As SO₂ levels increased across the continent, there would have been a tendency to reduce the intensity of the photochemical cycle, moderated by reductions in solar radiation by haze formed in part by sulfate particles. With sustained reduction of SO_x in the troposphere, the oxidant forming cycle would tend to be more active without OH and H₂O₂ scavenging by SO₂ (e.g., Stockwell, 1994).

The complexities of combined gas phase and condensed phase SO₂ interactions in tropospheric chemistry led some investigators to hypothesize that the SO₂ oxidation process should be non-linear when coupled with the oxidant cycle (e.g., NAS, 1975; Calvert and Stockwell, 1984; Stockwell et al.,1988). The 1980s estimates of long-term SO₄ concentration and deposition trends predicted by models assumed a seasonally “constant” linear oxidation rate proportional to SO₂ (e.g., Hidy, 1994). However, field experiments showed a range of more than an order of magnitude in apparent oxidation rates (e.g., Larson, 1980). Additionally, comparison of early ambient SO₂ and SO₄ concentration changes with SO_x emissions yielded mixed results regarding non-linearity (e.g., Kurosaka, 1976; Frank, 1974). Study of the observations of SO₄ in rainfall spatially averaged over > 100,000 km² across the eastern US also showed apparent non-linearity relative to SO_x emission changes (e.g., Hilst, 1992). Non-linearity could appear empirically as a curvature in observed SO₂–SO₄ trends relative to SO_x emission changes (e.g. NAS, 1975). Possibly, multidimensional diagrams would relate SO₄ to oxidant chemistry, analogous to an Empirical Kinetics Modeling Approach (EKMA) result relating O₃ to its precursors (volatile organic compounds--VOCs and nitrogen oxides--NO_x) (e.g., Lloyd et al., 1981).

Modeling studies have qualified the apparent linearity in SO_x change with SO₂ emissions. Stockwell (1994), for example, found enhanced linearity of emission source-SO_x chemistry through the coupling of gas phase and aqueous oxidation processes, specifically, through the linkage of the H₂O₂ production rate with interactions of SO₂ and HO₂ radicals. Benkowitz (1991) found that modeled linearity depended on interactions aloft, which in turn depended on the vertical distribution of SO₂ as influenced by stratoform clouds compared with convective systems. The relations of emissions to ambient SO_x and rainwater SO₄ tend to become more linear after the 1990s, as seen in the observational examples above. Apparent ambiguities in the linearity of the SO₂ and SO₄ responses to emission reductions, however, have implications for the future rate of reduction of SO_x and for its chemical interactions in the presence of oxidants and organosulfur compounds (see also Perraud et al., 2015).

Another case involves the multiphase aerosol chemistry of SO₄ and nitrate. In the troposphere, SO_x is oxidized through a pathway of sulfuric acid to salts of ammonia in suspended particles. If sufficient NH₃ is present, nitrate (as NH₄NO₃) tends to displace SO₄ salts as particulate (NH₄)_xH_ySO₄ declines with decreasing ambient SO₄ concentrations. This process depends on nitrate equilibrium with gaseous nitric acid in the presence of water and other species, as well as on temperature (e.g., Seinfeld and Pandis, 1998). Study of SO₄ and NO₃ in the US indicates that the aerosol particle chemistry varies at different locations from a small displacement of sulfate by nitrate at current SO_x levels to a larger one (e.g. Blanchard and Hidy, 2005). The direction of anthropogenic SO_x perturbation in aerosols is likely to focus more on the NO₃ component, with the caveat of the importance of organic carbon in aerosols (e.g., Hallquist et al., 2009).

This brings us to the last example of the chemistry of acid SO₄ and organic carbon vapors. Their interaction has a role in organic aerosol production in the troposphere, which could be changing along with changing ambient concentrations of SO_x. In the 1990s, increased interest in acid-based reactions of certain organic vapors, including naturally occurring isoprene and terpenes, arose based on laboratory experiments and increasing evidence from field studies (e.g., Hallquist et al., 2009). These reactions are closely related to photochemical processes that form oxidants. This interest has stimulated a variety of contemporary studies characterizing and quantifying this class of chemical reactions. Field studies have been especially aimed at conditions of high natural VOC concentrations found in the forested regions of the Southeast (e.g., Surratt et al., 2007; Weber et al., 2007; Hallquist et al., 2009; Carlton et al., 2014; Hidy et al., 2014; Liao et al., 2015). The findings from the Southeastern Oxidant and Aerosol Study (SOAS) experiments and other studies in the SEARCH region and in Tennessee indicate that adducts of VOC-SO₄ reactions are found particularly in summer even with greatly reduced contemporary levels of SO_x. One can imagine that this route to organic carbon particle formation was increasingly important in the 20^th century with elevated SO_x and acid SO₄ present. Their presence with natural VOCs, including isoprene and terpenes, could have contributed to the known degradation of visibility by haze across much of the Southeast after the early 20^th century.

Sulfur oxides (SO_x) are an example of short-lived species in the troposphere that are relevant to the Anthropocene concept. They have been emitted into the air, with smoke, as a byproduct of the fossil fuel combustion that accompanied the industrial revolution, which expanded from 19^th century Europe and North America to the world in the 20^th century. Anthropogenic SO_x represent a disturbance to the biogeochemical sulfur cycle, affecting not only the troposphere, but also terrestrial and aquatic ecosystems. Sulfur oxide levels exceeding natural emissions have been reported in precipitation in the contiguous US since the 1920s and in ambient air post-World War II. Sulfur oxide concentrations peaked in the US in the mid-1970s, while later being curtailed with emission reductions resulting from air pollution regulation. Since the 1990s, SO_x emissions have declined, and ambient annual average SO_x and precipitation SO₄ have decreased linearly with annual emissions, with somewhat different proportionalities. The current concentrations in ambient air and in precipitation in the US tend toward 0.5–2 µg/m³ SO₄, ∼ 1 ppbv SO₂ and 0.1–0.5 SO₄ mg/L in precipitation. These levels could begin to approach preindustrial levels in the US West, but remain elevated in the East. Despite the decreases, SO_x concentrations remain well above a hypothetical background; the declines will eventually level off to a steady baseline condition in the foreseeable future, reflecting natural emission sources, reduced domestic anthropogenic impact following further emission reductions, and continued transcontinental influence. The major anthropogenic SO_x influence on the lower troposphere is estimated to begin roughly with industrialization in 1850. The US anthropogenic SO_x contribution has spanned over 160 years, with its decline associated with evolving technology and a regulatory response intended to address known toxicity to humans and ecosystems.

The relatively short lifetime of SO_x of a few days in the troposphere is associated with reactivity and solubility in hydrometeors. Spatial gradients in ambient concentrations occur on a scale of 400–1000 km, thus constraining most of the influence of US emission sources to domestic receptor locations. Most of the higher concentrations of SO_x remain in the industrialized and densely populated eastern US. Here the SO₂ concentrations are generally about 1 ppbv or less, and SO₄ concentrations are less than 3 µg/m³. The western regions are spatially heterogeneous, with isolated emission sources surrounded by agricultural activity and remote regions, comprised of mountain ranges, prairie, arid deserts and forested lands. The west coast of the US is affected by a transcontinental residue of SO_x from Asian and marine sources, which create annual average SO₂ levels less than 1 ppbv and SO₄ concentrations near 0.1 µg/m³.

The rise and fall of SO_x in the lower troposphere over perhaps two centuries affects not only the sulfur cycle, but also tropospheric chemistry. The oxidation of S gases to form SO₄ involves coupling with the photochemical oxidant cycle through OH radical and H₂O₂ oxidation pathways. Non-linear atmospheric chemical reactions result in changes in ambient or precipitation SO₄ that are less than 1:1 proportional to changes in SO₂ emissions. Sulfate change also affects the nitrate component of suspended particles as a result of ammonium-nitrogen oxide-SO₄ interactions. Moreover, acid SO₄ influences the production of organic aerosols through reactions recently shown to involve oxidation in the presence of acids of volatile organic species, including isoprene and terpenes.

Recognition of the rise of SO_x emissions with urbanization and industrialization in the 19^th and 20th centuries and the consequent air quality effects resulted in a socio-economic response sometimes identified with environmentalism. Initial pressure for managing public health and returning to a natural state motivated the public to demand improvements, first in smoke levels, then in local ambient SO_x concentrations. The local initiatives appear to have had little effect on pollution reduction until after World War II (e.g., Brimblecomb, 1987; Mosley, 2014). After a century of increasing public pressure, regional and national lawmakers acted to develop laws for environmental protection (e.g., Stradling and Thorsheim, 1999; Mosley, 2014). Through the 20^th century, the framework for these laws relied heavily on science and engineering to supply guidance and solutions for pollution reduction. These included development of efficient and clean combustion technologies, emission control devices, low S fuel switching, and conservation. By the late 20^th century, SO_x pollution and other contamination reached international proportions, leading to a series of agreements to address acid rain, trans-international boundary pollution and other impacts. Sulfur oxide pollution is well under control in the developed world, with reduction in emissions following major investments in technology and energy resource changes. However, the global distribution of SO_x emissions is changing. SO_x emissions began declining in North America and Europe after 1970, but global emissions were relatively constant between 1970 and 2005 as global industrialization proceeded and populations continued to grow.

The changes in ambient SO_x and precipitation SO₄ with emission reductions confirm that humans can affect the levels of short-lived species in the atmosphere. The impact of such constraints is difficult to measure in the Earth system. There is little evidence of substantial influence of anthropogenic S emissions on the global S cycle; however, local or regional ecological elements of this cycle are modified (e.g., Clair et al., 2011). Epidemiological studies of direct human health response to SO_x reductions are rare; two examples of population health improvement with reduced SO₄ have appeared in the recent literature (e.g., Pope et al., 2007, 2009). These studies document improvement in population health with reduction in SO₄, but are controversial, particularly in relation to the air quality data used by the authors. Aqueous ecosystems have shown a response in eastern North America (Clair et al., 2011), but terrestrial ecosystem response to SO_x deposition appears to be slow and weakly documented relative to atmospheric change. Both the human and ecosystem responses are likely to be slow to emerge and difficult to establish in proportion to SO_x reductions.

In the 21^st century, the evolution of environmental protection has begun to address projected changes due to climate forcing by long-lived and short-lived pollutants. Concerns about climate change have taken at least a half century to emerge from the initial debates over scientific findings to increasing public attention. An international agreement has now been established to moderate the risk of potential climatic effects, which will likely require major changes in primary energy resources. Ironically, continental, ambient SO₄ concentrations act in an opposing direction to greenhouse gases in climate forcing, so the reductions in SO_x concentrations that have been achieved will likely reinforce the warming effects of greenhouse gases.

Measurements and data cited are from peer-reviewed journals, books, sponsored reports and US government data or summaries.

© 2016 Hidy and Blanchard. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

We are indebted to Peter Mueller for providing access to some of the historic literature we adopted and to his long standing friendship as a co-investigator on many of the SO_x experiments and studies involving GH. We appreciate the access provided by J. Schwab to observations from Whiteface Mountain and the graphics supplied by J. Hand. We also recognize the valuable measurements of the SEARCH project, as well as the knowledge of SO_x afforded by the IMPROVE and CSN data base. Our thanks to the library of the California Air Resources Board for assistance in finding copies of old reports describing SO_x studies in southern California.