We describe observations of C1–C5 alkyl nitrates made at the Boulder Atmospheric Observatory in Northern Colorado in winter 2011, spring 2015, and summer 2015. Average mixing ratios of the alkyl nitrates are similar across the seasons, but increased diel variability in summer suggests increased production balanced by increased loss relative to winter and spring. We use a sequential production-destruction model based on ratios of alkyl nitrates to their parent alkanes to investigate seasonal sources and sinks of C1–C5 alkyl nitrates. We explore the role of uncertainties in the production and loss kinetic parameters on the interpretation of local atmospheric photochemical aging through the use of a photochemical clock based on the evolution of the ratios of alkyl nitrates to their parent alkanes over time. Photochemical age is typically consistent with hours from sunrise, suggesting that the site experiences well-mixed air masses dominated by daily photochemistry with little carry-over from the previous day or from other locations. Contrary to studies in other locations, we obtain good model-measurement agreement using a newer upper-bound ethyl nitrate branching ratios. This suggests that the efficiency of ethyl nitrate production from ethane oxidation has previously been underestimated, and decreases the relative importance of alkoxy radical decomposition versus ethane photochemistry on ethyl nitrate production. We estimate the dry deposition velocity of methyl nitrates is small and consistent with previous estimates, and that deposition velocities increase with carbon number for the C2–C5 RONO2. Dry deposition is a small daytime sink relative to photolysis and reaction with OH for the alkyl nitrates, but improves the model-measurement comparison for methyl nitrate.

Short-chain alkyl nitrates (C1–C5 RONO2) typically account for only a small fraction of both the organic nitrate (RONO2) and NOy budget (NOy = NO + NO2 + HNO3 + HONO + 2N2O5 + HO2NO2 + RO2NO2 + NO3 + RONO2). However, the production and loss of C1–C5 RONO2 still impacts tropospheric ozone, HOx (RO2 + HO2), and NOx (NO + NO2) budgets. Ozone and RONO2 are produced simultaneously in the atmosphere (Figure 1a, b), and the ratio of C1–C5 RONO2 to their parent C1–C5 alkane can be used to determine the photochemical age of air masses (Bertman et al., 1995). However, uncertainties in the sources and sinks of RONO2 are substantial, and can lead to uncertainties in photochemical clock analyses (Simpson et al., 2003).

Figure 1

Coupled HOx–NOx reactions. (a) Ozone production cycle. (b) RONO2 formation reactions. (c) RONO2 sinks. DOI: https://doi.org/10.1525/elementa.299.f1

Figure 1

Coupled HOx–NOx reactions. (a) Ozone production cycle. (b) RONO2 formation reactions. (c) RONO2 sinks. DOI: https://doi.org/10.1525/elementa.299.f1

While direct emissions of C1–C2 RONO2 have been observed from both the ocean (e.g. Atlas et al., 1993; Blake et al., 2003b; Blake et al., 1999; Chuck et al., 2002) and biomass burning (Simpson et al., 2011; Simpson et al., 2002), the dominant source of C1–C5 RONO2 at continental mid-latitude sites is the photooxidation of anthropogenic precursors (R1–R3) (e.g. Bertman et al., 1995; Flocke et al., 1998b; Roberts, 1990; Roberts et al., 1998; Russo et al., 2010; Simpson et al., 2003; Worton et al., 2010) (Figure 1a, b):

R1
R2
R3a
R3b
R4

The fraction of proton abstraction (H-atom abstraction branching ratio) from the parent alkane (RH) at a particular carbon atom is α1 (R1). Values for α1 calculated from structural activity relationship studies are presented in Table S1 (Kwok and Atkinson, 1995). The formation of the peroxy (RO2) radical is fast. In the presence of NOx, RO2 reacts with NO (R3); the minor pathway forms monofunctional C1–C5 RONO2 (R3b, α3). Thus, the fraction of reactions between the parent alkane and OH in the presence of NO that lead to RONO2 formation is the product of α1 and α3 (β = α1α3), which we differentiate from the RONO2 formation branching ratio as the integrated RONO2 branching ratio.

Reaction with OH (R4) and photolysis (R5) are typically considered the major sinks of C1–C5 RONO2 (Bertman et al., 1995; Perring et al., 2013; Roberts, 1990; Talukdar et al., 1997a; Talukdar et al., 1997b), although deposition may also be important (Russo et al., 2010) (Figure 1c). The products of RONO2 + OH depend on the size and structure of the RONO2. The major (>50%) products from proton abstraction of C3–C4 linear and branched RONO2 by OH are NO2 and either aldehydes or ketones via the decomposition of intermediate alkoxy radicals (Aschmann et al., 2011). For example, the reaction of OH with linear 2-hexyl and 3-hexyl RONO2 produces a variety of aldehydes and ketones including 2-hexanone, 3-hexanone, propanal, and butanal. A fraction of those reactions retain the nitrate functionality to produce multifunctional RONO2, such as C6-carbonyl nitrates, hydroxycarbonyl nitrates, and dinitrates (Aschmann et al., 2012):

R4

Photolysis of C1–C5 RONO2 releases the corresponding alkoxy radical and NO2 (R5) (Roberts, 1990; Roberts and Fajer, 1989; Talukdar et al., 1997b):

R5

Photolysis rates vary with RONO2 structure, pressure (i.e. altitude), spectral radiance (intensity as a function of wavelength), and temperature. These photolysis rates are on the order of 10–7–10–6 s–1 for C1–C5 RONO2 at mid-latitude surface sites through all seasons, corresponding to lifetimes against photolysis of 6 for 2-butyl nitrate to 125 days for methyl nitrate (Bertman et al., 1995; Clemitshaw et al., 1997; Roberts, 1990; Roberts and Fajer, 1989; Simpson et al., 2003; Talukdar et al., 1997b; Wang et al., 2013; Worton et al., 2010).

Deposition and aerosol uptake are typically ignored as sinks of C1–C5 RONO2 due to their low Henry’s law constants (2.64 M atm–1 for MeONO2; decreasing with increasing carbon number for mono-functional RONO2 (Kames and Schurath, 1992)) and high vapor pressures (>3 torr (Fischer and Ballschmiter, 1998; Lim and Ziemann, 2005; Roberts, 1990)). Solubility of monofunctional RONO2 is low, and hydrolysis is slow (10–5–10–3 s–1) (Robertson et al., 1982); water uptake is thus likely a small sink for C1–C5 RONO2. However, the sum of all RONO2 deposition accounts for ~3% of annual global nitrogen deposition of 92.9 Tg (Neff et al., 2002). Speciated oxidized nitrogen deposition rates are essential for accurate modeling of oxidized nitrogen deposition (Neff et al., 2002). Russo et al. (2010) report a dry deposition velocity of 0.13 cm s–1 for methyl nitrate (MeONO2), which reduces the estimated summer lifetime of MeONO2. The modeled global distribution of MeONO2 is thus sensitive to the inclusion of dry deposition, which reduces the impact of long range transport of a HOx + NOx source to remote regions of the globe (Williams et al., 2014). To the best of our knowledge, Russo et al. (2010) provide the only observational estimate of speciated C1–C5 RONO2 dry deposition.

Using measurements of speciated C1–C5 RONO2, hydrocarbons, ozone, and other trace gases, we explore seasonal trends in C1–C5 RONO2 observed at the Boulder Atmospheric Observatory in the Front Range of Northern Colorado from winter, spring, and summer measurement campaigns. The Front Range is an interesting region to study C1–C5 RONO2 because the C2–C5 alkanes are abundant due to the high density of oil and natural gas operations (Abeleira et al., 2017; Gilman et al., 2013; McDuffie et al., 2016; Pétron et al., 2012; Pétron et al., 2014; Swarthout et al., 2013). Specifically, C2–C5 alkane mixing ratios are 5–300× higher than most other ground sites where speciated C1–C5 RONO2 measurements have been reported (e.g. Bertman et al., 1995; Lyu et al., 2015; Russo et al., 2010; Swanson et al., 2003; Wang et al., 2010; Wang et al., 2013). The Front Range includes densely populated urban areas and high traffic interstate highways, and the region violates the National Ambient Air Quality Standard for ozone. Outside of Denver, the Front Range appears to be transitioning from a NOx-saturated ozone production regime to peak production (Abeleira and Farmer, 2017). Here, we use a simple analytical model to explore the importance of – and uncertainties in – sources and sinks of C1–C5 RONO2, and their impact on estimating the photochemical age of sampled air masses. This analysis includes estimates of dry deposition velocities of C1–C5 RONO2, allowing us to investigate the relative importance of short-chain RONO2 sinks.

### 2.1. Campaigns and site description

The C1–C5 alkyl nitrates, their parent alkane precursors, and other trace gases were measured at the NOAA Boulder Atmospheric Observatory (BAO) in Northern Colorado during winter 2011, spring 2015, and summer 2015. The winter 2011 measurements were part of the Nitrogen, Aerosol Composition, and Halogens on a Tall Tower (NACHTT) study from 18 February 2011 to 13 March 2011 (Brown et al., 2013; Swarthout et al., 2013). The spring 2015 measurements were associated with the Shale Oil and Natural Gas Nexus (SONGNEX) study from 20 March 2015 to 17 May 2015 (Abeleira et al., 2017; NOAA, 2017). The summer 2015 measurements occurred from 24 July 2015 to 29 August 2015 (Abeleira et al., 2017). BAO was in a semirural region with major urban centers to the south (Denver, 35 km), west (Boulder, 30 km), north (Fort Collins, 65 km), and northeast (Greeley, 65 km), but has since been decommissioned. The site is located on the edge of the Wattenberg natural gas field in the Denver-Julesberg basin, an area of extensive oil and natural gas exploration and extraction. (Abeleira et al., 2017; Brown et al., 2013; Gilman et al., 2013; McDuffie et al., 2016; Swarthout et al., 2013).

### 2.2. Measurements

We measured methyl nitrate (MeONO2), ethyl nitrate (EtONO2), 1-propyl nitrate (1-PrONO2), 2-propyl nitrate (2-PrONO2), 2-butyl nitrate (2-BuONO2), 2-pentyl nitrate (2-PeONO2), and 3-pentyl nitrate (3-PeONO2) along with their parent alkanes (methane, ethane, propane, n-butane, and n-pentane) during all three campaigns, with the exception of methane, which was not measured during winter 2011. During winter 2011, the whole air samples were collected hourly by a canister sampling system, and analyzed off-line with a multi-channel gas chromatography system (Swarthout et al., 2013). The analytical precision was 1–8% for the parent alkanes and 3–8% for the alkyl nitrates. During spring and summer 2015, the alkyl nitrates and parent alkanes (except methane) were measured with a similar 4-channel online chromatography system, but instead utilized a cryogen-free system to pre-concentrate ambient samples on 1 mm silica beads at –180°C for in situ measurement. The inlet was 22 m above ground level (agl) for the 2011 measurements, and 6 m agl for the 2015 measurements. The measurements, calibrations, and 4-channel online chromatography system are detailed in (Abeleira et al., 2017); the analytical precision for the parent alkanes and alkyl nitrates was 1–10%. A Picarro 6401 commercial Cavity Ring-Down Spectrometer measured methane during spring and summer 2015 (6% precision).

Fixed-height temperature and wind-speed measurements were located on the 300 m BAO tower at 10, 100, and 300 m agl (NOAA, 2017). Average daytime 10 m height temperatures were 5°C (winter 2011), 15°C (spring 2015), and 25°C (summer 2015). Vertically resolved temperature and wind-speed measurements between 0–270 m agl were made during the winter 2011 study from an instrument enclosure mounted to a carriage on the tower (Brown et al., 2013).

### 2.3. Estimating photochemical age from RONO2/RH

The evolution of RONO2 with air mass age can be described by a sequential reaction system in which the reaction of A to B represents the production of RONO2, and B to C represents the loss pathways (R6).

$A→kAB→kBC$
R6

Bertman et al. (1995) described the evolution of RONO2/RH as a function of air mass photochemical age (E1).

$[RONO2][RH]=βkA(kB−kA)(1−e(kA−kB)t)+[RONO2]0[RH]0e(kA−kB)t$
E1

The ratio of RONO2/RH is calculated as a function of time (seconds) using laboratory kinetic parameters. Because OH must be assumed, this time can be considered an ‘OH equivalent’ photochemical age – the time at an average OH concentration required to reach a given RONO2/RH ratio. The production (or source) term (kA) is k1[OH] and, in conjunction with β, represents the production of RONO2. Seasonal temperature and pressure adjusted integrated RONO2 branching ratios (β = α1α3) are in Table 1. Details of those calculations can be found in supplemental section 2. The destruction (or sink) term (kB) is k4[OH] + J5, and represents the loss of RONO2 via oxidation and photolysis. The units of kA and kB are s–1. Calculated values of kA and kB for each season are reported in Tables 2 and 3 respectively. Modeling RONO2/RH with E1 assumes the following: (1) The OH + RH reaction is the rate limiting step in the production of RONO2; (2) The evolution of RONO2/RH is a result of gas phase hydrocarbon chemistry only (Bertman et al., 1995); (3) RO2 reacts with NO only (i.e. no RO2 self-reactions), which is expected in an urban/suburban ‘high-NOx’ environment with NO > 0.1 ppbv (Flocke et al., 1991; Roberts et al., 1998); and (4) The only sinks of RONO2 are reaction with OH and photolysis (Bertman et al., 1995; Roberts, 1990; Talukdar et al., 1997a; Talukdar et al., 1997b).

Table 1

Seasonal integrated RONO2 branching ratios accounting for pressure and temperature dependencea. DOI: https://doi.org/10.1525/elementa.299.t1

RONO2Seasonal integrated RONO2 branching ratios (β = α1α3)

winter 2011spring 2015summer 2015

MeONO2 0.0093 ± 0.0005
EtONO2 0.038 0.034 0.028
1-PrONO2 0.005 0.005 0.004
2-PrONO2 0.40 0.036 0.030
2-BuONO2 0.086 0.078 0.065
2-PeONO2 0.010 0.086 0.068
3-PeONO2 0.060 0.053 0.042
RONO2Seasonal integrated RONO2 branching ratios (β = α1α3)

winter 2011spring 2015summer 2015

MeONO2 0.0093 ± 0.0005
EtONO2 0.038 0.034 0.028
1-PrONO2 0.005 0.005 0.004
2-PrONO2 0.40 0.036 0.030
2-BuONO2 0.086 0.078 0.065
2-PeONO2 0.010 0.086 0.068
3-PeONO2 0.060 0.053 0.042

a Integrated RONO2 branching ratios are calculated from H-atom abstraction values in Table S1 and RONO2 formation branching ratios in Tables S2 and S3. Details of the RONO2 formation branching ratio temperature and pressure dependent calculations can be found in the section 2 of the supplemental. We determine uncertainty in MeONO2 β by propagating the stated error in SI E5 in quadrature. Equation SI E4 used to calculate the EtONO2 formation branching ratios did not have errors associated with it. Errors have not been reported with calculated RONO2 formation branching ratios from SI E1–E3 (Arey et al., 2001; Carter and Atkinson, 1989), but the two least-squares standard deviations of the Δ[RONO2]/Δ[n-alkane] relationships used to derive the falloff expression parameters for C3–C5 RONO2 are typically <20%. We use 20% as a conservative error estimate for these integrated RONO2 branching ratios.

Table 2

Kinetic parameters for the formation of RONO2 at BAO. DOI: https://doi.org/10.1525/elementa.299.t2

RONO2kA

k1awinter 2011spring 2015summer 2015

×10–12 cm3 molec.–1 s–1×10–6 s–1

MeONO2 0.0064 0.004–0.014 0.010–0.024 0.026–0.051
EtONO2 0.248 0.189–0.566 0.420–1.05 0.99–1.98
1-PrONO2 1.1 0.93–2.80 1.98–4.94 4.98–8.75
2-PrONO2 1.1 0.93–2.80 1.98–4.94 4.98–8.75
2-BuONO2 2.36 2.11–6.32 4.37–10.97 9.43–18.89
2-PeONO2 3.8 3.44–10.31 7.13–17.82 15.21–30.42
3-PeONO2 3.8 3.44–10.31 7.13–17.82 15.21–30.42
RONO2kA

k1awinter 2011spring 2015summer 2015

×10–12 cm3 molec.–1 s–1×10–6 s–1

MeONO2 0.0064 0.004–0.014 0.010–0.024 0.026–0.051
EtONO2 0.248 0.189–0.566 0.420–1.05 0.99–1.98
1-PrONO2 1.1 0.93–2.80 1.98–4.94 4.98–8.75
2-PrONO2 1.1 0.93–2.80 1.98–4.94 4.98–8.75
2-BuONO2 2.36 2.11–6.32 4.37–10.97 9.43–18.89
2-PeONO2 3.8 3.44–10.31 7.13–17.82 15.21–30.42
3-PeONO2 3.8 3.44–10.31 7.13–17.82 15.21–30.42

a (Atkinson and Arey, 2003). The values of k1 are for 298 k and 760 torr. Units of k1 are ×10–12 cm3 molecule–1 s–1. Values of kA are calculated as k1[OH] with temperature dependent parameters from Atkinson and Arey (2003) using average daytime (08:00–18:00) 10 m agl temperatures (5°C, 12°C, and 25°C for winter 2011, spring 2015, and summer 2015) and seasonal Northern Colorado OH concentration ranges (details in section 3.2.1). Units of kA are ×10–6 s–1.

Table 3

Kinetic parameters for the loss of RONO2 at BAO. DOI: https://doi.org/10.1525/elementa.299.t3

RONO2J5kB

k4winter 2011spring 2015summer 2015winter 2011spring 2015summer 2015

×10–12 cm3 molec.–1 s–1×10–6 s–1

MeONO2 0.04a 0.07–0.50 0.19–0.79 0.30–0.94 0.11–0.62 0.27–0.99 0.46–1.26
EtONO2 0.218b 0.11–0.82 0.32–1.32 0.49–1.52 0.33–1.47 0.76–2.41 1.36–3.26
1-PrONO2 0.5972 0.17–1.10 0.47–1.75 0.70–1.99 0.77–2.89 1.66–4.74 3.09–6.68
2-PrONO2 0.302c 0.13–0.92 0.30–1.30 0.53–1.60 0.47–1.83 0.90–2.91 1.74–4.02
2-BuONO2 0.86b 0.12–0.98 0.31–1.55 0.48–1.99 0.98–3.56 2.03–5.85 3.92–8.87
2-PeONO2 1.84d 0.07–0.62 0.18–1.00 0.75–1.60 1.95–6.14 3.86–10.20 7.58–16.30
3-PeONO2 1.13d 0.07–0.62 0.18–1.00 0.75–1.60 1.18–4.00 2.44–6.66 4.74–10.68
RONO2J5kB

k4winter 2011spring 2015summer 2015winter 2011spring 2015summer 2015

×10–12 cm3 molec.–1 s–1×10–6 s–1

MeONO2 0.04a 0.07–0.50 0.19–0.79 0.30–0.94 0.11–0.62 0.27–0.99 0.46–1.26
EtONO2 0.218b 0.11–0.82 0.32–1.32 0.49–1.52 0.33–1.47 0.76–2.41 1.36–3.26
1-PrONO2 0.5972 0.17–1.10 0.47–1.75 0.70–1.99 0.77–2.89 1.66–4.74 3.09–6.68
2-PrONO2 0.302c 0.13–0.92 0.30–1.30 0.53–1.60 0.47–1.83 0.90–2.91 1.74–4.02
2-BuONO2 0.86b 0.12–0.98 0.31–1.55 0.48–1.99 0.98–3.56 2.03–5.85 3.92–8.87
2-PeONO2 1.84d 0.07–0.62 0.18–1.00 0.75–1.60 1.95–6.14 3.86–10.20 7.58–16.30
3-PeONO2 1.13d 0.07–0.62 0.18–1.00 0.75–1.60 1.18–4.00 2.44–6.66 4.74–10.68

a Preferred values from the Master Chemical Mechanism, MCM v3.3 ((Jenkin et al., 1997; Saunders et al., 2003)) via http://mcm.leeds.ac.uk/MCM, b(Morin et al., 2016), c(Romanias et al., 2015), d(Atkinson, 1990). The values of k4 are for 298 k and 760 torr. Units of k4 are ×10–12 cm3 molecule–1 s–1. Selection and calculation of J5 is detailed in the SI. Values of kB are calculated as k4[OH] + J5 with temperature dependent parameters from Atkinson and Arey (2003) using average daytime (08:00–18:00) 10 m agl temperatures (5°C, 12°C, and 25°C for winter 2011, spring 2015, and summer 2015) and seasonal Northern Colorado OH concentration ranges (details in section 3.2.1).

RONO2/RH ratios for a given alkyl nitrate/parent hydrocarbon are typically plotted against 2-BuONO2/n-butane because 2-BuONO2 often exhibits the highest mixing ratios in polluted areas, is dominated by photochemical production (Bertman et al., 1995; Lyu et al., 2015; Wang et al., 2013; Worton et al., 2010), and has no reported primary sources (Atlas et al., 1993; Simpson et al., 2002). Additionally, in the first 24 hours of photochemical aging, 95% of 2-BuONO2 is produced via oxidation of n-butane (Sommariva et al., 2008). Plotting RONO2/RH against 2-BuONO2/n-butane enables comparison of ambient data to these models without a priori knowledge of the absolute aging timescale of the ambient data (Bertman et al., 1995). This approach provides not only a quantitative metric of photochemical age, but also useful context for exploring the sources and sinks of alkyl nitrates.

### 3.1. Seasonal and diel trends in C1–C5 RONO2

Despite the relatively high hydrocarbon mixing ratios at BAO, average MeONO2, EtONO2, and 1-PrONO2 mixing ratios at BAO are similar to previous measurements from rural and remote sites (i.e. Roberts et al., 1998; Russo et al., 2010; Swanson et al., 2003), while 2-PrONO2 and the C4–C5 RONO2 were more representative of polluted urban sites (i.e. Lyu et al., 2015; Wang et al., 2013; Worton et al., 2010) (Table 4).

Table 4

Average alkyl nitrate mixing ratios (ppbtv) at BAO for winter 2011, spring 2015, and summer 2015. DOI: https://doi.org/10.1525/elementa.299.t4

RONO2winter 2011spring 2015summer 2015

Average mixing ratios (standard deviation), pptv

MeONO2 4 (1) 4 (1) 3 (1)
EtONO2 4 (1) 4 (1) 3 (2)
1-PrONO2 2 (1) 2 (1) 1.3 (0.9)
2-PrONO2 16 (8) 13 (6) 11 (6)
2-BuONO2 30 (20) 20 (10) 20 (10)
2-PeONO2 10 (10) 6 (5) 8 (7)
3-PeONO2 7 (6) 4 (4) 5 (5)
RONO2winter 2011spring 2015summer 2015

Average mixing ratios (standard deviation), pptv

MeONO2 4 (1) 4 (1) 3 (1)
EtONO2 4 (1) 4 (1) 3 (2)
1-PrONO2 2 (1) 2 (1) 1.3 (0.9)
2-PrONO2 16 (8) 13 (6) 11 (6)
2-BuONO2 30 (20) 20 (10) 20 (10)
2-PeONO2 10 (10) 6 (5) 8 (7)
3-PeONO2 7 (6) 4 (4) 5 (5)

Seasonal averages of 2-PrONO2 and C4–C5 RONO2 deviate little from winter 2011 to summer 2015 at BAO (Table 4). For example, 2-BuONO2 decreases from 30 pptv during winter 2011 to 20 pptv during summer 2015, but remains within the average ± standard deviation for all seasons. Average mixing ratios of MeONO2, EtONO2, and 1-PrONO2 do not change seasonally. Measurements at other ground sites and during flight campaigns have shown greater contrasts in seasonal variation for C1–C5 RONO2, and these differences have been attributed to meteorology, transport, and OH abundances (Beine et al., 1996; Blake et al., 2003a; Lyu et al., 2015; Russo et al., 2010; Simpson et al., 2004; Swanson et al., 2003; Wang et al., 2013). Maxima in C1–C5 RONO2 in the winter and minima in the summer are typically observed at remote sites (Beine et al., 1996; Blake et al., 2003a; Swanson et al., 2003) resulting from increased photochemical removal of RONO2 during summertime without an increase in production because of low precursor abundances (Swanson et al., 2003). In contrast, summer maxima were observed in polluted air masses outside of Freiburg, Germany (Flocke et al., 1998b).

The diel cycles of 2-PrONO2 and C4–C5 RONO2 have more pronounced diel variability in summer 2015 compared to the winter or spring campaigns (Figure 2b, 2c). During winter 2011, average 2-BuONO2 increases by a factor of 1.3 from 24 to 31 pptv between 04:00 and 14:00 (local time); during summer 2015, 2-BuONO2 increases by a factor of 2.5 from 13 to 33 pptv (Figure 2e). A distinct summer maxima occurs earlier in the day between 10:00–12:00, unlike the broad afternoon maxima observed during winter and spring. In contrast, MeONO2 exhibits little diel variability in all seasons, while EtONO2, and 1-PrONO2 exhibit small increases from 10:00 to 14:00 during summer 2015 (Figure 2a). Although daytime winter 2011 and summer 2015 mixing ratios were similar for 2-PrONO2 and C4–C5 RONO2, the rapid decrease to lower background mixing ratios in the summer is consistent with increased removal of the more reactive RONO2. The diel cycles of 2-PrONO2 and C4–C5 RONO2 are thus consistent with both increased summer photochemical sources from higher OH and increased summer losses from higher OH and photolysis rates. Diel cycles for the parent alkanes are presented and discussed in-depth in Swarthout et al. (2013) for winter 2011 and Abeleira et al. (2017) for spring and summer 2015.

Figure 2

Average seasonal C1–C5 RONO2 diel cycles. Average diel cycles in mixing ratio for (a) MeONO2, (b) EtONO2, (c) 1-PrONO2, (d) 2-PrONO2, (e) 2-BuONO2, (f) 2-PeONO2 and (g) 3-PeONO2 for the winter 2011 (blue circles), spring 2015 (red squares), and summer 2015 (black triangles) measurement campaigns. Error bars represent ±1 standard deviation from the mean of each 2-hour time bin. DOI: https://doi.org/10.1525/elementa.299.f2

Figure 2

Average seasonal C1–C5 RONO2 diel cycles. Average diel cycles in mixing ratio for (a) MeONO2, (b) EtONO2, (c) 1-PrONO2, (d) 2-PrONO2, (e) 2-BuONO2, (f) 2-PeONO2 and (g) 3-PeONO2 for the winter 2011 (blue circles), spring 2015 (red squares), and summer 2015 (black triangles) measurement campaigns. Error bars represent ±1 standard deviation from the mean of each 2-hour time bin. DOI: https://doi.org/10.1525/elementa.299.f2

### 3.2. Uncertainties in the RONO2/RH model

Uncertainties in RONO2 formation branching ratios and the choice of OH concentration are the main sources of uncertainty that affect the agreement between modeled and measured RONO2/RH and the estimation of the photochemical age from RONO2/RH. The use of an initial RONO2/RH value when modeling RONO2/RH also impacts the agreement, particularly at lower photochemical ages. We investigate these uncertainties by comparing modeled and observed MeONO2/methane, EtONO2/ethane, and 2-BuONO2/n-butane ratios for the spring 2015 campaign. These uncertainties also impact the interpretation of RONO2 sources and RONO2 sinks.

#### 3.2.1. OH concentration

The chosen OH concentration used for photochemical age estimation from the RONO2/RH model should accurately represent the average OH encountered by an air mass. A limitation of this technique is that the use of a single OH concentration to model RONO2/RH does not capture fluctuations in OH concentration that an air mass may experience. OH is responsible for not only initiating RONO2 production via oxidation of the parent alkanes (i.e. the source, kA), but also removing RONO2 (i.e. a key sink, kB). This interplay between source and sink dictates the rate at which RONO2 increases and RH decreases, and subsequently controls the rate of increase of RONO2/RH (Figure 3). Tables 2 and 3 list kA and kB calculated for a range of OH concentrations. We use an OH range of (1–3) × 106 molecule cm–3 for winter 2011 based on co-located, simultaneous OH measurements (Kim et al., 2014). We use an average daytime range of (4–8) × 106 molecule cm–3 for the summer 2015 campaign based on aircraft OH measurements from summer 2014 in the Northern Colorado Front Range (Ebben et al., 2017). We use (2–5) × 106 OH molecule cm–3 for the spring 2015 campaign.

Figure 3

Impact of OH concentration on modeled EtONO2/ethane. Modeled RONO2/RH ratios for (a) EtONO2/ethane and (b) 2-BuONO2/n-butane vary with the assumed concentration of OH. The rate of production from OH + RH (kA) and rate of destruction from OH and photolysis (kB) are calculated for each OH case. The high, low and base OH cases are modeled with OH values of 5, 2 and 3.5 × 106 molecule cm–3 respectively. Photolysis rates are spring averages (Table 3). Branching ratios are 0.034 (EtONO2) and 0.078 (2-BuONO2). (c) Observed (grey circles) and modeled (line, for each OH case) EtONO2/ethane ratios are plotted against 2-BuONO2/n-butane for the daytime (08:00–18:00) spring data (grey circles, c). Markers for 1 and 48 hours of photochemical aging for the three OH cases are overlaid on the modeled curves. DOI: https://doi.org/10.1525/elementa.299.f3

Figure 3

Impact of OH concentration on modeled EtONO2/ethane. Modeled RONO2/RH ratios for (a) EtONO2/ethane and (b) 2-BuONO2/n-butane vary with the assumed concentration of OH. The rate of production from OH + RH (kA) and rate of destruction from OH and photolysis (kB) are calculated for each OH case. The high, low and base OH cases are modeled with OH values of 5, 2 and 3.5 × 106 molecule cm–3 respectively. Photolysis rates are spring averages (Table 3). Branching ratios are 0.034 (EtONO2) and 0.078 (2-BuONO2). (c) Observed (grey circles) and modeled (line, for each OH case) EtONO2/ethane ratios are plotted against 2-BuONO2/n-butane for the daytime (08:00–18:00) spring data (grey circles, c). Markers for 1 and 48 hours of photochemical aging for the three OH cases are overlaid on the modeled curves. DOI: https://doi.org/10.1525/elementa.299.f3

We illustrate the impact of OH concentration on RONO2/RH with a high, low and base OH case (OH = 5, 2 and 3.5 × 106 molecule cm–3) for spring 2015 (Figure 3). The rate of RONO2 production and RH consumption increases with increasing OH concentration (Figure 3a, b); overestimating OH causes an under-estimation of photochemical age. The reactivity of n-butane to OH (kOH + n-butane = 2.36 × 10–12 cm3 molecule–1 s–1) is greater than 2-BuONO2 to OH (kOH + 2-BuONO2 = 0.86 × 10–12 cm3 molecule–1 s–1), causing a higher rate of increase in RONO2/RH in the high OH versus low OH case. The opposite is true for EtONO2 and ethane, where the reactivity of EtONO2 with OH (kOH + EtONO2 = 0.218 × 10–12 cm3 molecule) is greater than that of ethane (kOH + Ethane = 0.248 × 10–12 cm3 molecule). The ratio of RONO2 destruction to production (kB/kA) is a useful metric for predicting how RONO2/RH will increase or decrease over time. For example, this ratio for 2-BuONO2 is 0.70 for low OH, and decreases to 0.49 for high OH. This corresponds to a faster rate of consumption of the parent alkane and production of RONO2, which is balanced by a smaller increase in the rate of removal of RONO2 in the high OH case because 2-BuONO2 has a lower reactivity than n-butane. Increasing OH causes higher RONO2/RH ratio values to occur with less aging. For example, the EtONO2/ethane versus 2-BuONO2/n-butane value at 2 days of aging in the high OH case is not reached until 5 days of aging in the low OH case.

#### 3.2.2. Selection of RONO2 branching ratio

Branching ratios for the formation of some RONO2 from RO2 + NO are poorly constrained (Tables 5, 6; Supplemental section 1), and the uncertainties propagate to the integrated RONO2 branching ratio (β). For instance, the sole experimentally-derived formation branching ratio of MeONO2 of 0.0107 ± 0.0014 (298 K, 760 torr) (Butkovskaya et al., 2012) is an order of magnitude higher than the commonly used value of 0.001 (Master Chemical Mechanism; (Jenkin et al., 1997) and almost two orders of magnitude greater than the observationally constrained estimate of 1.5 × 10–4 (Flocke et al., 1998a). EtONO2 formation branching ratios at 298 K and 760 torr vary from 0.009 (Master Chemical Mechanism) to 0.03 ± 0.01 (direct experiment; (Butkovskaya et al., 2010a)). Similarly, 2-PeONO2 has a reported experimental range of 0.096 ± 0.009 (Aschmann et al., 2006) to 0.134 ± 0.016 (Carter and Atkinson, 1985). In contrast, 2-PrONO2 branching ratios have a very narrow range of experimental values at 298–300 K and 735–760 torr: 0.038 ± 0.002–0.043 ± 0.002 (Arey et al., 2001; Atkinson et al., 1987; Butkovskaya et al., 2010a; Carter and Atkinson, 1985). The selection or calculation of the formation branching ratios for these RONO2 is important for interpreting the agreement or disagreement between modeled and observed RONO2/n-alkane. Disagreement between modeled and observed RONO2/n-alkane has typically been attributed to alkoxy radical chemistry that is not captured by these relatively simple models (Bertman et al., 1995; Reeves et al., 2007; Simpson, 2003).

Table 5

C3–C5 RONO2 formation branching ratios. DOI: https://doi.org/10.1525/elementa.298.t5

ReferenceTypeaRONO2 formation αConditions

1-PrONO22-PrONO22-BuONO22-PeONO23-PeONO2Temp. (k)Press. (torr)

Atkinson 19821,b exp.  0.036 ± 0.005 0.077 ± 0.009  0.117 ± 0.019 299 ± 2 735
Atkinson 19832,b exp. 0.036 ± 0.005 0.077 ± 0.009  0.125 ± 0.003 299 ± 2 735
calc.  0.039 0.07  0.121
Atkinson 19843 exp. 0.020 ± 0.009 0.043 ± 0.003 0.090 ± 0.009 0.129 ± 0.014 0.118 ± 0.016 299 ± 2 735
Carter 19854 exp. 0.020 ± 0.009 0.042 ± 0.003 0.090 ± 0.008 0.129 ± 0.016 0.131 ± 0.016 299 ± 2 735
0.134 ± 0.016 0.146 ± 0.016
Atkinson 19875 calc. 0.018 0.046 0.083 0.135  299–300 735–740
Carter 19896 calc. 0.019 0.047 0.083 0.126 0.126–0.128 299–300 735–740
Arey 20017 exp. 0.016 0.039 ± 0.002 0.084 ± 0.009 0.106 ± 0.018 0.126 ± 0.018 296–300 735–740
Chow 20038 exp.  0.006    298 100
Aschmann 20069 exp.    0.096 ± 0.009 0.116 ± 0.009 297 ± 1 737 ± 4
Cassanelli 200710 exp.     0.11 ± 0.02 298 750
Butkovskaya 201011 exp.  0.038 ± 0.002    298 760
Jenkin 199712 MCM 0.020 0.042 0.090 0.129 0.131 298 760
ReferenceTypeaRONO2 formation αConditions

1-PrONO22-PrONO22-BuONO22-PeONO23-PeONO2Temp. (k)Press. (torr)

Atkinson 19821,b exp.  0.036 ± 0.005 0.077 ± 0.009  0.117 ± 0.019 299 ± 2 735
Atkinson 19832,b exp. 0.036 ± 0.005 0.077 ± 0.009  0.125 ± 0.003 299 ± 2 735
calc.  0.039 0.07  0.121
Atkinson 19843 exp. 0.020 ± 0.009 0.043 ± 0.003 0.090 ± 0.009 0.129 ± 0.014 0.118 ± 0.016 299 ± 2 735
Carter 19854 exp. 0.020 ± 0.009 0.042 ± 0.003 0.090 ± 0.008 0.129 ± 0.016 0.131 ± 0.016 299 ± 2 735
0.134 ± 0.016 0.146 ± 0.016
Atkinson 19875 calc. 0.018 0.046 0.083 0.135  299–300 735–740
Carter 19896 calc. 0.019 0.047 0.083 0.126 0.126–0.128 299–300 735–740
Arey 20017 exp. 0.016 0.039 ± 0.002 0.084 ± 0.009 0.106 ± 0.018 0.126 ± 0.018 296–300 735–740
Chow 20038 exp.  0.006    298 100
Aschmann 20069 exp.    0.096 ± 0.009 0.116 ± 0.009 297 ± 1 737 ± 4
Cassanelli 200710 exp.     0.11 ± 0.02 298 750
Butkovskaya 201011 exp.  0.038 ± 0.002    298 760
Jenkin 199712 MCM 0.020 0.042 0.090 0.129 0.131 298 760

1 (Atkinson et al., 1982), 2(Atkinson et al., 1983), 3(Atkinson et al., 1984), 4(Carter and Atkinson, 1985), 5(Atkinson et al., 1987), 6(Carter and Atkinson, 1989), 7(Arey et al., 2001), 8(Chow et al., 2003), 9(Aschmann et al., 2006), 10(Cassanelli et al., 2007), 11(Butkovskaya et al., 2010b), 12(Jenkin et al., 1997). aThe type designations are experimental for “exp.”, estimated for “est.”, preferred values at 298 k and 760 torr from the Master Chemical Mechanism for MCM, and “calc” for calculated isomeric branching ratios from a generalized falloff expression fitted to experimental branching ratios as reported in the individual references. bThe branching ratios are derived from the combined yields of 1-RONO2 + 2-RONO2 determined from Δ[RONO2 + 2-RONO2]/Δ[n-alkane] in the reported kinetics studies. Errors are typically two least-squares standard deviations of Δ[RONO2]/Δ[n-alkane] or Δ[RONO2]/Δ[reference product] relationship for experimental data. Details of the other referenced experiments and calculations will be discussed in the text (section 2.3.).

Table 6

MeONO2 and EtONO2 formation branching ratios. DOI: https://doi.org/10.1525/elementa.299.t6

ReferenceTypehRONO2 formation αConditions

MeONO2EtONO2Temp. (k)Press. (torr)

Atkinson 1982a exp.  <0.014 299 ± 2 735
Flocke 1998b est. 1.5–3.0 × 10–4  Lower tropospherek
Scholtens 1999c est. <0.03  295 100
Butkovskaya 2012d exp. 0.0107 ± 0.0014i  298 760
Ranschaert 2000e exp.  0.007 ± 0.003 298 100
Butkovskaya 2010f exp.  0.03 ± 0.01j 298 760
Jenkin 1997g MCM 0.001 0.009 298 760
ReferenceTypehRONO2 formation αConditions

MeONO2EtONO2Temp. (k)Press. (torr)

Atkinson 1982a exp.  <0.014 299 ± 2 735
Flocke 1998b est. 1.5–3.0 × 10–4  Lower tropospherek
Scholtens 1999c est. <0.03  295 100
Butkovskaya 2012d exp. 0.0107 ± 0.0014i  298 760
Ranschaert 2000e exp.  0.007 ± 0.003 298 100
Butkovskaya 2010f exp.  0.03 ± 0.01j 298 760
Jenkin 1997g MCM 0.001 0.009 298 760

a (Atkinson et al., 1984), b(Flocke et al., 1998a), c(Scholtens et al., 1999), d(Butkovskaya et al., 2012), e(Ranschaert et al., 2000), f(Butkovskaya et al., 2010a), g(Jenkin et al., 1997). hThe type designations are experimental for “exp.”, estimated for “est.”, and preferred values at 298 k and 760 torr from the Master Chemical Mechanism for MCM. i,jCalculated from the temperature and pressure dependent branching ratio equations for 298 k and 760 torr reported within each reference. kEstimated branching ratio for the lower troposphere from lower stratospheric MeONO2 observations. Errors are typically two least-squares standard deviations of Δ[RONO2]/Δ[n-alkane] or Δ[RONO2]/Δ[reference product] relationship for experimental data.

In Figure 4, we compare the modeled and observed EtONO2/ethane versus 2-BuONO2/n-butane using seasonal temperature and pressure dependent integrated RONO2 branching ratios, a commonly used value of 0.014 from Atkinson et al. (1982), and a value of 0.009 from the Master Chemical Mechanism (Jenkin et al., 1997). The temperature and pressure dependent EtONO2 formation branching ratio calculations yield integrated branching ratio values (β) of 0.038, 0.034, and 0.028 for winter 2011, spring 2015, and summer 2015 respectively (Table 1). Seasonal temperature and pressure integrated RONO2 branching ratios for 2-BuONO2 are calculated with values of 0.086, 0.078, and 0.065 for winter 2011, spring 2015, and summer 2015 respectively (Table 1).

Figure 4

Seasonal EtONO2/ethane model-measurement comparisons. Daytime (08:00–18:00) EtONO2/ethane ratios are plotted against 2-BuONO2/n-butane (grey circles) for (a) winter 2011, (b) spring 2015 and (c) summer 2015. Modeled ratios are calculated using branching ratios adjusted for seasonal pressure and temperature (solid blue line; see text for description), with an older value (β = 0.014) at 298 K and 760 torr 1(Atkinson et al., 1982), and with the preferred MCM value (β = 0.009) at 298 K and 760 torr 2(Jenkin et al., 1997). Error bars represent one standard deviation around the bin averages. Modeled/observed ratios are shown for (d) winter 2011, spring 2015 (e) and summer 2015 (f) for the temperature-adjusted branching ratio versus average branching ratio scenarios. Photochemical ages derived from E2 for the model scenarios are listed above the models. DOI: https://doi.org/10.1525/elementa.299.f4

Figure 4

Seasonal EtONO2/ethane model-measurement comparisons. Daytime (08:00–18:00) EtONO2/ethane ratios are plotted against 2-BuONO2/n-butane (grey circles) for (a) winter 2011, (b) spring 2015 and (c) summer 2015. Modeled ratios are calculated using branching ratios adjusted for seasonal pressure and temperature (solid blue line; see text for description), with an older value (β = 0.014) at 298 K and 760 torr 1(Atkinson et al., 1982), and with the preferred MCM value (β = 0.009) at 298 K and 760 torr 2(Jenkin et al., 1997). Error bars represent one standard deviation around the bin averages. Modeled/observed ratios are shown for (d) winter 2011, spring 2015 (e) and summer 2015 (f) for the temperature-adjusted branching ratio versus average branching ratio scenarios. Photochemical ages derived from E2 for the model scenarios are listed above the models. DOI: https://doi.org/10.1525/elementa.299.f4

In winter 2011, the temperature and pressure dependent EtONO2 branching ratio (β = 0.038) agrees well with observed values with a small underprediction of 9–13% (Figure 4a, d). In contrast, the branching ratio values of 0.009 and 0.014 lead to model underpredictions of 39–76%, with larger underpredictions at higher photochemical aging (Figure 4a, d). However, in spring 2015 the temperature and pressure dependent branching ratio (β = 0.034) overpredicts observations by 37–111%, with greater overpredictions at longer photochemical aging (Figure 4b, e). Better agreement is found with the 0.014 branching ratio (model underprediction of 11–31%), especially at higher photochemical ages. Similarly, during summer 2015 the temperature and pressure dependent branching ratio (β = 0.028) overpredicts observations by 11–83%. As the photochemical age increases past 3 hours, we find better agreement between modeled and observed values with the 0.014 branching ratio. In contrast, we note that previous studies often use lower branching values for EtONO2 of 0.006 to 0.014, and report model underpredictions for EtONO2/ethane of >200% (Bertman et al., 1995; Russo et al., 2010; Simpson et al., 2003; Wang et al., 2013; Worton et al., 2010).

Similar model overpredictions are observed with 3-PeONO2, though the reported range of the 3-PeONO2 formation branching ratio value is much narrower than EtONO2. The winter 2011 comparison (β = 0.060) is the best of the three seasons with overpredictions of 7–63%, with larger overpredictions after 3 hours of aging. The spring and summer 2015 (β = 0.053, 0.042) measurements are severely overpredicted with values of 91% to 320% for both seasons. The same temperature and pressure calculation method has been reported with parameters that lead to lower C5-RONO2 branching ratios (spring β = 0.044, summer β = 0.036) (Aschmann et al., 2006), but these values produce branching ratios that still severely overpredict the spring and summer 3-PeONO2/n-pentane observations by 61–270%. Overpredictions of 3-PeONO2/n-pentane ratios have been previously reported (Bertman et al., 1995; Russo et al., 2010; Simpson et al., 2003; Stroud et al., 2001; Worton et al., 2010). The implications of these comparisons will be discussed later in section 3.4.

#### 3.2.3. Selection of initial conditions

RONO2 is often assumed to be absent if no photochemistry has occurred in an airmass (Bertman et al., 1995; Roberts, 1990), although observations suggest this is not always the case. A non-zero initial RONO2/RH ratio accounts for non-zero RONO2/RH ratios during periods with no photochemistry in air masses impacted by marine and biomass burning RONO2 emissions (Atlas et al., 1993; Blake et al., 1999; Simpson et al., 2002). Non-zero RONO2/RH ratios in the absence of photochemistry are also common for the less reactive C1–C2 RONO2 (Bertman et al., 1995; Russo et al., 2010). Continental ground sites removed from oceanic sources and biomass burning plumes, including the BAO site, exhibit small non-zero RONO2/RH values in the morning prior to sunrise, often attributed to carryover from the previous days’. Although the RONO2/RH values prior to sunrise are typically low, it has become common practice to use a non-zero initial RONO2/RH ratio when applying the RONO2/RH model (Russo et al., 2010; Wang et al., 2013). Accounting for initial RONO2 concentrations yields better model-measurement agreement, particularly at photochemical ages <6 hours (Figure S1). The one exception is MeONO2/methane, likely because the initial ratio is not only small due to high methane concentrations, but also consistent because of the low reactivity of both MeONO2 (kOH + MeONO2 = 4 × 10–14 cm3 molecule–1 s–1) and methane (kOH + methane = 6.4 × 10–15 cm3 molecule–1 s–1). Thus, despite accounting for an initial MeONO2/methane ratio, the model overpredicts observations >6 hours of photochemical aging. One explanation for this poor model-measurement comparison is a missing RONO2 loss process.

### 3.3. Dry deposition

MeONO2/methane exhibits the largest model-measurement discrepancy of all the measured RONO2/RH pairs (Figures 5, S3). The observed trends in spring and summer 2015 MeONO2/methane are not captured by modeled values when a pressure dependent MeONO2 branching ratio (β = 0.0093) is used (Figure 5). Butkovskaya et al. (2012) note that the calculated pressure dependent branching ratio at leads to calculated steady-state MeONO2 concentrations 2–5 higher than upper troposphere observations. Use of a smaller MeONO2 branching ratio (i.e. β = 1.5 × 10–4, Flocke et al. (1998a)) provides better agreement in spring and summer 2015, but the model predicts an increase in MeONO2/methane at higher photochemical ages. Observed MeONO2/methane does not increase with increasing 2-BuONO2/n-butane during either spring or summer 2015, and instead exhibits a constant ratio with averages of (2.0 ± 0.4) × 10–6 and (1.6 ± 0.6) × 10–6 ppbv/pbbv during spring and summer 2015, respectively (Figure 5).

Figure 5

Seasonal MeONO2/methane model-measurement comparisons. Observed MeONO2/methane was plotted against 2-BuONO2/n-butane for spring 2015 (grey circles, a) and summer 2015 (grey circles, b). Model ratios are shown that exclude loss by dry deposition (black and blue lines), and include dry deposition determined from nighttime data (red). Dry deposition loss rates of 1.7 × 10–6 s–1 and 0.87 × 10–6 s–1 were estimated for spring and summer 2015. 1A pressure dependent MeONO2 branching ratio at 298 K of 0.0093 was used for both spring and summer (black line). A low branching ratio estimate of 1.5 × 10–4 was used for spring and summer (red and black lines) (Flocke et al., 1998a). All models use upper JRONO2 values for each RONO2 (Table 2). Black diamonds are average MeONO2/methane versus 2-BuONO2/n-butane averaged into five bins of equal number of points (n = 110/bin for spring 2015 and n = 69 for summer 2015). Error bars represent one standard deviation around the bin averages. Modeled/observed ratios are plotted for spring 2015 (c) and summer 2015 (d) for each scenario. Photochemical ages derived from the model scenarios are indicated. DOI: https://doi.org/10.1525/elementa.299.f5

Figure 5

Seasonal MeONO2/methane model-measurement comparisons. Observed MeONO2/methane was plotted against 2-BuONO2/n-butane for spring 2015 (grey circles, a) and summer 2015 (grey circles, b). Model ratios are shown that exclude loss by dry deposition (black and blue lines), and include dry deposition determined from nighttime data (red). Dry deposition loss rates of 1.7 × 10–6 s–1 and 0.87 × 10–6 s–1 were estimated for spring and summer 2015. 1A pressure dependent MeONO2 branching ratio at 298 K of 0.0093 was used for both spring and summer (black line). A low branching ratio estimate of 1.5 × 10–4 was used for spring and summer (red and black lines) (Flocke et al., 1998a). All models use upper JRONO2 values for each RONO2 (Table 2). Black diamonds are average MeONO2/methane versus 2-BuONO2/n-butane averaged into five bins of equal number of points (n = 110/bin for spring 2015 and n = 69 for summer 2015). Error bars represent one standard deviation around the bin averages. Modeled/observed ratios are plotted for spring 2015 (c) and summer 2015 (d) for each scenario. Photochemical ages derived from the model scenarios are indicated. DOI: https://doi.org/10.1525/elementa.299.f5

Deposition is rarely considered for the alkyl nitrates. Russo et al. (2010) report dry deposition velocities (Vd) for MeONO2 from summer observations in rural New Hampshire of 0.13 ± 0.07 cm s–1. Following their approach, we estimate a similar Vd of 0.09 cm s–1 from a single night in winter 2011 Briefly, Russo et al. (2010) calculated dry deposition fluxes from the nighttime decay rate of RONO2 (change in concentration over a given time, pptv s–1) multiplied by the nocturnal boundary layer height (H). Dividing the flux by the average concentration and multiplying by –1 gives a deposition velocity (Vd, converted to cm s–1). A negative flux and positive deposition velocity indicate a flux from the atmosphere to a surface. The loss rate from dry deposition (kdep) is calculated as Vd/H, where H is the boundary layer height. See SI for detailed description of calculation, assumptions, and selection criteria.

Including dry deposition in the β = 1.5 × 10–4 case results in good model-measurement agreement, avoiding the modeled increase in MeONO2/methane at higher photochemical ages that was not observed. Due to the lack of statistics of our singular dry deposition value, we use the value of 0.13 cm s–1 from Russo et al. (2010) for further analysis. We estimate a dry deposition loss rate for MeONO2 in spring 2015 of 1.7 × 10–6 s–1 (assuming daytime boundary layer height of 750 m). The loss of MeONO2 (kB) for spring 2015 is calculated to be 0.62 × 10–6 s–1 from reaction with OH and photolysis (Table 3). Thus, the inclusion of dry deposition in spring 2015 increases the loss rate by nearly 4-fold, and improves the model-measurement agreement (Figure 5a, c). However, the inclusion of MeONO2 loss by dry deposition during summer 2015 only minimally improves the model-measurement agreement (Figure 5b, d). The estimated summer 2015 MeONO2 dry deposition loss rate for an estimated daytime 1500 m boundary layer is 0.86 × 10–6 s–1, and the calculated kB from OH + MeONO2 and photolysis is 0.86 × 10–6 s–1. However, we note that by increasing the loss rate to 4 × 10–6 s–1 we can generate a model that does not exhibit increasing MeONO2/methane at higher photochemical ages (not shown). This indicates that loss from dry deposition during summer is underestimated with this method, or that there is an additional major unaccounted for MeONO2 loss process. Loss from dry deposition has little or no impact on the model-measurement agreement of the C2–C5 RONO2, which is not surprising since loss of RONO2 from OH+RONO2 becomes more important as the number of carbon increase (Figure S4).

We note a trend of increasing deposition velocity with larger RONO2 species (Table S7); while the lack of statistically robust values prevents us from investigating this pattern in great detail, we note that the increasing deposition velocity follows increasing vapor pressure, suggesting that the observed removal could be due to a temporary night-time partitioning of the gases to the surface.

### 3.4. C2+ RONO2 model-measurement comparison

In section 3.2.2, we note discrepancies among the three seasons regarding which EtONO2 branching ratio provides the best model-measurement agreement. In winter 2011, the temperature and pressure dependent value of 0.038 provides a near 1:1 agreement with the observations. In spring and summer 2015, a value between the temperature and pressure dependent branching ratios (spring β = 0.034, summer β = 0.028) and 0.014 (Atkinson et al., 1982) provides the better model-measurement agreement at photochemical ages <24 hours. The model typically underpredicts observed EtONO2/ethane by factors of 2 or more (Bertman et al., 1995; Russo et al., 2010; Simpson et al., 2003; Wang et al., 2013; Worton et al., 2010). However, those studies used a lower branching ratio (β = 0.014) (Bertman et al., 1995; Russo et al., 2010; Simpson et al., 2003; Wang et al., 2013; Worton et al., 2010). This underprediction was attributed to additional sources of ethyl radicals from the decomposition of larger alkoxy radicals (Bertman et al., 1995; Roberts et al., 1998; Worton et al., 2010). For example, Sommariva et al. (2008), with the Master Chemical Mechanism EtONO2 branching ratio of 0.009, suggest that OH+ethane reaction only accounts for 15% of EtONO2 in the first 24 hours of processing, while decomposition of alkoxy radicals from larger alkanes account for the rest. This is a reasonable explanation since alkoxy radical decomposition is known to occur (Atkinson, 1997; Atkinson and Carter, 1991; Orlando et al., 2003). However, the Master Chemical Mechanism EtONO2 branching ratio value of 0.009 is much lower than the temperature and pressure dependent value range of 0.028–0.038 for the three seasons in this study. Alternately, the winter 2011 model-measurement agreement with β = 0.038 and the spring and summer 2015 agreement with β = 0.014–0.034 could be consistent with the original branching ratios (i.e. β = 0.009–0.014) being too low, thereby underestimating the efficiency of EtONO2 production from ethane in air masses with <24 hours of processing. Production of EtONO2 from the decomposition of larger alkoxy radicals could still be important at longer processing times. However, with the current analysis we cannot definitively evaluate these arguments, and further work using a more detailed mechanistic model with a range of branching ratio values is warranted.

Modeled and observed 2-PrONO2/propane are in good agreement for the winter and summer campaigns (7–29% underprediction, within the standard deviation of averaged observations), and fair agreement in spring 2015 with overpredictions of 34–59%. As noted in section 3.2.2, winter 2011 model-measurement comparisons are within the standard deviation of observed values for 3-PeONO2, while the spring and summer 2015 models severely overpredict observed 3-PeONO2/n-pentane. The same pattern is observed for 2-PeONO2 for the three seasons. The use of the lowest calculated 2-PeONO2 branching ratio, as described in section 3.2.2, slightly reduces the severe model overprediction for 2-PeONO2 during spring and summer 2015. The lower C5-RONO2 branching ratios provide excellent model-measurement agreement for winter 2011, especially at photochemical ages <12 hours. The lack of model-measurement agreement in the spring and summer seasons is consistent with previous studies (e.g. Russo et al., 2010; Simpson et al., 2003; Sommariva et al., 2008; Worton et al., 2010). Reeves et al. (2007) attributed comparable model over-prediction to decomposition of C5 peroxy radicals before reacting with NO, though no mechanism or evidence was put forth. As discussed previously, alkoxy radical decomposition of larger alkoxy radicals to smaller alkyl radicals that form peroxy radicals is well established. However, this pathway would not reduce the available pool of peroxy radicals as those alkoxy radicals would have already been converted from the pool of peroxy radicals before decomposition, and would be captured by the formation branching ratio. Alternatively, the formation branching ratios of the C5 RONO2 may be too high. However, without better constraints on the RONO2 formation branching ratios it is difficult to reconcile these model-measurement discrepancies.

### 3.5. Photochemical age at BAO

We use the 2-PrONO2/propane and 2-BuONO2/n-butane ratios to derive photochemical ages during summer 2015 at BAO (OH = 6 × 106 molecules cm–3, β2-PrONO2 = 0.030, β2-BuONO2= 0.065, and average summer kA and kB values from Tables 2 and 3). The alkyl nitrate photochemical clock captures daily photochemistry: i.e., 89% of daytime data at BAO exhibit photochemical ages <12 hours and align with hours since sunrise during summer months (Figure S4). This consistency between hours since sunrise and derived photochemical age suggest that BAO typically experiences a fresh, well-mixed airmass with little influence from long-term transport or day-to-day carryover, and that 2-PrONO2 and 2-BuONO2 sources and sinks are typically well-described by the model. However, there are exceptional days when the assumption that photochemical age increases with simultaneously increasing RONO2 and decreasing RH is not met (Figure S5). For example, on 21 August 2015, calculated photochemical age increases through the afternoon despite decreasing 2-PrONO2 resulting from a rapid decrease in propane relative to 2-PrONO2 (Figure S5c). In Figure S5b photochemical age increases on 22 August 2015 between 06:00 and 10:00, and begins decreasing after 10:00 as a result of increasing propane concentrations that decrease RONO2/RH. Because this event fails to meet the model assumptions, the subsequent calculation results in a decreasing photochemical age through the day. These examples from late August 2015 coincide with smoke intrusion into the Front Range from wildfires in the Washington and Idaho (Lindaas et al., 2017) – although we note that most other summer 2015 days designated as smoke-impacted do not show this anomalous 2-PrONO2/propane trend.

Uncertainties in selection of OH concentration, RONO2 branching ratios, and rates of RONO2 photolysis and oxidation all impact modeled RONO2/RH and thus photochemical age. However, OH concentration is the most sensitive factor when estimating photochemical age of a sampled air mass using the RONO2/RH model. Incorporation of seasonal temperature and pressure dependent branching ratios reduces model-measurement discrepancies for EtONO2/ethane.

Dry deposition is a loss process for RONO2, but has little effect on photochemical models of these compounds with the exception of methyl nitrate. However, because reaction of RONO2 with OH releases NO2 and RO2 radicals, their production and export can both hinder local ozone production and contribute to downwind production of ozone, and potentially HNO3 or aerosol nitrate. Thus, deposition mitigates the impact on ozone of long-range transport of MeONO2 out of the source region (Williams et al., 2014). Further work investigating the mechanisms of deposition for the volatile RONO2 species is thus warranted.

The data used in this manuscript can be found in a repository hosted by the National Oceanic and Atmospheric Administration for SONGNEX (http://esrl.noaa.gov/csd/groups/csd7/measurements/2015songnex/) and NACHTT (https://www.esrl.noaa.gov/csd/groups/csd7/measurements/2011NACHTT/).

We thank Dan Wolfe, Bruce Bartram, and Gerhard Hübler for support at the BAO site during the SONGNEX campaigns.

We acknowledge the National Oceanic and Atmospheric Administration for funding the SONGNEX work (Award# NA14OAR4310148). We also acknowledge the National Science Foundation for funding the portion of data collection during NACHTT that was used in this manuscript (Award# ANT-1127774).

The authors have no competing interests to declare.

• Contributed to conception and design: AA, DFK, EF

• Contributed to acquisition of data: AA, DFK, EF, BS, BS, YZ

• Contributed to analysis and interpretation of data: AA, DFK, EF

• Drafted and/or revised the article: AA, DFK, EF

• Approved the submitted version for publication: AA, DFK, EF, BS, BS, YZ

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