Measurements of volatile organic compounds (VOCs) have been ongoing for decades to track growth rates and assist in curbing emissions of these compounds into the atmosphere. To accurately establish mole fraction trends and assess the role of these gas-phase compounds in atmospheric chemistry it is essential to have good calibration standards. A necessity and precursor to accurate VOC gas standards are the gas cylinders and the internal wall treatments that aid in maintaining the stability of the mixtures over long periods of time, measured in years. This paper will discuss the stability of VOC gas mixtures in different types of gas cylinders and internal wall treatments. Stability data will be given for 85 VOCs studied in gas mixtures by National Metrology Institutes and other agency laboratories. This evaluation of cylinder treatment materials is the outcome of an activity of the VOC Expert Group within the framework of the World Meteorological Organization (WMO) Global Atmospheric Watch (GAW) program.

In the framework of the In Service Aircraft for Global Observing System (IAGOS) program, airborne in-situ O 3 and CO measurements are performed routinely using in-service aircraft, providing vertical profiles from the surface to about 10–12 km. Due to the specificity of IAGOS measurements (measurements around busy international airports), uncertainties exist on their representativeness in the lower troposphere as they may be impacted by emissions related to airport activities and/or other aircraft. In this study, we thus investigate how the IAGOS measurements in the lower troposphere compare with nearby surface stations (from the local Air Quality monitoring network (AQN)) and more distant regional surface stations (from the Global Atmospheric Watch (GAW) network). The study focuses on Frankfurt but some results at other European airports (Vienna, Paris) are also discussed. Results indicate that the IAGOS observations close to the surface do not appear to be strongly impacted by local emissions related to airport activities. In terms of mixing ratio distribution, seasonal variations and trends, the CO and O 3 mixing ratios measured by IAGOS in the first few hundred metres above the surface have similar characteristics to the mixing ratios measured at surrounding urban background stations. Higher in altitude, both the difference with data from the local AQN and the consistency with the GAW regional stations are higher, which indicates a larger representativeness of the IAGOS data. Despite few quantitative differences with Frankfurt, consistent results are obtained in the two other cities Vienna and Paris. Based on 11 years of data (2002–2012), this study thus demonstrates that IAGOS observations in the lowest troposphere can be used as a complement to surface stations to study the air quality in/around the agglomeration, providing important information on the vertical distribution of pollution.

We present a characterization of reactive gases (RG: O 3 , NO, NO 2 ,SO 2 , CO) and methane (CH 4 ) variability in the central Mediterranean basin,analyzing in situ measurements at three new permanent WMO/GAW Observatories in Southern Italy: Capo Granitola – CGR (Sicily), Lamezia Terme – LMT (Calabria) and Lecce – ECO (Apulia). At all the measurement sites, a combination of the breeze wind system (especially at CGR and LMT),PBL dynamics, anthropogenic/natural emissions, and photochemistry lead the appearance of well-defined diurnal cycles for the observed RG. According to O 3 /NO x variability, local emissions appeared to influence CGR and LMT (no NO x data were available for ECO during the period of study) in 4% and 20% of the hourly data, nearby sources in 39% and 40%, remote sources in 31% and 14%, while background O 3 /NO x were observed in 26% of cases for both the stations. Most of the background O 3 /NO x were observed during daytime, when offshore air masses usually affected the measurement sites. Local sources of CH 4 at CGR can be related to biogenic (oxic) emissions from biomasses along the coastline, while emissions from live stocks can represent a local source of CH 4 at LMT. Finally, we provide first hints about the export of O 3 from Sicily/Southern Italy to the Mediterranean Sea by comparing simultaneous observations at CGR and Lampedusa (LMP), a small island in the middle of the Strait of Sicily where a WMO/GAW Regional Station is located. In summer,O 3 increased by some 7 ppb for transport times lower than 48 h, while no statistical significant differences were observed for travel time longer than 48. This would suggest that photochemical O 3 production occurred within air-mass travelling from CGR to LMP, but also that the central Mediterranean MBL represents a O 3 sink for relatively aged air-masses.

Trace gas measurements from whole air samples collected weekly into glass flasks at background monitoring sites within the NOAA Global Greenhouse Gas Reference Network program (with most of the sites also being World Meteorological Organization (WMO) Global Atmospheric Watch (GAW) stations) were used to investigate the variability-lifetime relationship for site characterization and to estimate regional and seasonal OH concentrations. Chemical species considered include the atmospheric trace gases CO, H 2 , and CH 4 , as well as the non-methane hydrocarbons (NMHC) ethane (C 2 H 6 ), propane (C 3 H 8 ), i -butane ( i -C 4 H 10 ), and n -butane ( n -C 4 H 10 ). The correlation between atmospheric variability and lifetime was applied on a global scale spanning 42 sites with observations covering a period of 5 years. More than 50,000 individual flask measurement results were included in this analysis, making this the most extensive study of the variability-lifetime relationship to date. Regression variables calculated from the variability-lifetime relationship were used to assess the “remoteness” of sampling sites and to estimate the effect of local pollution on the measured distribution of atmospheric trace gases. It was found that this relationship yields reasonable results for description of the site remoteness and local pollution influences. Comparisons of seasonal calculated OH concentrations ([OH]) from the variability-lifetime relationships with six direct station measurements yielded variable agreement, with deviations ranging from ∼20% to a factor of ∼2–3 for locations where [OH] monitoring results had been reported. [OH] calculated from the variability-lifetime relationships was also compared to outputs from a global atmospheric model. Resutls were highly variable, with approximately half of the sites yielding agreement to within a factor of 2–3, while others showed deviations of up to an order of magnitude, especially during winter.

Ozone is generally assumed to have weak diurnal variations in the free troposphere due to lower production rates than in the boundary layer, in addition to a much lower NO titration and the absence of dry deposition at the surface. However, this hypothesis has not been proven due to a lack of high frequency observations at multiple times per day. For the first time, we take benefit from the frequent O 3 vertical profiles measured above Frankfurt in the framework of the MOZAIC-IAGOS program to investigate the diurnal variations of O 3 mixing ratios at multiple pressure levels throughout the troposphere. With about 21,000 aircraft profiles between 1994 and 2012 (98 per month on average), distributed throughout the day, this is the only dataset that can allow such a study. As expected, strong diurnal variations are observed close to the surface, in particular during spring and summer (enhanced photochemistry and surface deposition). Higher in altitude, our observations show a decrease of the diurnal cycle, with no diurnal cycle discernible above 750 hPa, whatever the season. Similar results are observed for the different percentiles of the O 3 distribution (5 th , 25 th , 50 th , 75 th , 95 th ). An insight of the changes of the diurnal cycles between 1994–2003 and 2004–2012 is also given. We found higher O 3 mixing ratios during the latter period, particularly on the lowest pressure levels, despite lower mixing ratios during summer.

Long-term observations of reactive gases in the troposphere are important for understanding trace gas cycles and the oxidation capacity of the atmosphere, assessing impacts of emission changes, verifying numerical model simulations, and quantifying the interactions between short-lived compounds and climate change. The World Meteorological Organization’s (WMO) Global Atmosphere Watch (GAW) program coordinates a global network of surface stations some of which have measured reactive gases for more than 40 years. Gas species included under this umbrella are ozone, carbon monoxide, nitrogen oxides, and volatile organic compounds (VOCs). There are many challenges involved in setting-up and maintaining such a network over many decades and to ensure that data are of high quality, regularly updated and made easily accessible to users. This overview describes the GAW surface station network of reactive gases, its unique quality management framework, and discusses the data that are available from the central archive. Highlights of data use from the published literature are reviewed, and a brief outlook into the future of GAW is given. This manuscript constitutes the overview of a special feature on GAW reactive gases observations with individual papers reporting on research and data analysis of particular substances being covered by the program.

Here we present high frequency long-term observations of ethane, benzene and methyl chloride from the AGAGE Ragged Point, Barbados, monitoring station made using a custom built GC-MS system. Our analysis focuses on the first three years of data (2005–2007) and on the interpretation of periodic episodes of high concentrations of these compounds. We focus specifically on an exemplar episode during September 2007 to assess if these measurements are impacted by long-range transport of biomass burning and biogenic emissions. We use the Lagrangian Particle Dispersion model, NAME, run forwards and backwards in time to identify transport of air masses from the North East of Brazil during these events. To assess whether biomass burning was the cause we used hot spots detected using the MODIS instrument to act as point sources for simulating the release of biomass burning plumes. Excellent agreement for the arrival time of the simulated biomass burning plumes and the observations of enhancements in the trace gases indicates that biomass burning strongly influenced these measurements. These modelling data were then used to determine the emissions required to match the observations and compared with bottom up estimates based on burnt area and literature emission factors. Good agreement was found between the two techniques highlight the important role of biomass burning. The modelling constrained by in situ observations suggests that the emission factors were representative of their known upper limits, with the in situ data suggesting slightly greater emissions of ethane than the literature emission factors account for. Further analysis was performed concluding only a small role for biogenic emissions of methyl chloride from South America impacting measurements at Ragged Point. These results highlight the importance of long-term high frequency measurements of NMHC and ODS and highlight how these data can be used to determine sources of emissions 1000’s km away.

A record spanning ten years of non-methane hydrocarbon (NMHC) data from the Pico Mountain Observatory (PMO), Pico Island, Azores, Portugal, was analyzed for seasonal NMHC behavior, atmospheric processing, and trends, focusing on ethane and propane. The location of this site in the central North Atlantic, at an elevation of 2225 m asl, allows these data to be used to investigate the background conditions and pollution transport events occurring in the lower free North Atlantic troposphere. The quantity ln([propane]/[ethane]) was used as an indicator of both photochemical processing and a marker for the occurrence of pollution transport events detected at the station. The Pico data were compared with three other continuous NMHC data sets from sites bordering the North Atlantic, i.e. the Global Atmospheric Watch (GAW) stations at Summit, Greenland, Hohenpeisssenberg, Germany, and Cape Verde, using ln([propane]/[ethane]) results as an indicator for the degree of photochemical processing (‘aging’) seen in the data. Comparisons of these three data sets showed some significant differences in the seasonal background and range of observed values. The statistical distribution of binned monthly data was determined, and individual sample events were then scaled to the monthly median observed value. Back trajectories, determined by the HYSPLIT model were used to investigate the geographic origin of the observed trace gases as a function of the degree of photochemical processing. Results show that PMO samples have been subjected to a diversity of air transport and aging, from highly processed air to freshly emitted air throughout the year, and in particular during summer months. The predominant air transport is from North America, with only occasional influence from continental areas located east and southeast (Europe and Africa). The available record was found to be too variable and still too short to allow deciphering NMHC trends from the data. Ethane and propane measurements at the PMO were compared with the MOZART-4 atmospheric chemistry and transport model at the appropriate time and location. The model was found to yield good agreement in the description of the lower range of atmospheric mole fractions observed, of the seasonal cycle, and the regional oxidation chemistry. However, ethane and propane enhancements in transport events were underestimated, indicating that after the ≥ 3 days of synoptic transport to PMO the spatial extent of plumes frequently is smaller than the 2.8°x2.8° (∼300 km) model grid resolution.

In situ atmospheric ozone measurements aboard the R/V Ronald H. Brown during the 2008 Gas-Ex and AMMA research cruises were compared with data from four island and coastal Global Atmospheric Watch stations in the Atlantic Ocean to examine ozone transport in the marine boundary layer (MBL). Ozone measurements made at Tudor Hill, Bermuda, were subjected to continental outflow from the east coast of the United States, which resulted in elevated ozone levels above 50 ppbv. Ozone measurements at Cape Verde, Republic of Cape Verde, approached 40 ppbv in springtime and were influenced by outflow from Northern Africa. At Ragged Point, Barbados, ozone levels were ∼ 21 ppbv; back trajectories showed the source region to be the middle of the Atlantic Ocean. Ozone measurements from Ushuaia, Argentina, indicated influence from the nearby city; however, the comparison of the daily maxima ozone mole fractions measured at Ushuaia and aboard the Gas-Ex cruise revealed that these were representative of background ozone in higher latitudes of the Southern Hemisphere. Diurnal ozone cycles in the shipborne data, frequently reaching 6–7 ppbv, were larger than most previous reports from coastal or island monitoring locations and simulations based on HO x photochemistry alone. However, these data show better agreement with recent ozone modeling that included ozone-halogen chemistry. The transport time between station and ship was estimated from HYSPLIT back trajectories, and the change of ozone mole fractions during transport in the MBL was estimated. Three comparisons showed declining ozone levels; in the subtropical and tropical North Atlantic Ocean the loss of ozone was < 1.5 ppbv day −1 . Back trajectories at Ushuaia were too inconsistent to allow for this determination. Comparisons between ship and station measurements showed that ozone behavior and large-scale (∼ 1000 km) multi-day transport features were well retained during transport in the MBL.