Seasonality, sources and sinks of C1-C5 alkyl nitrates in the Colorado Front Range

Abstract

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.