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.

Leaf Stomatal Uptake of Alkyl Nitrates

“…Assuming uptake initially proceeds through dissolution, then bulk liquid reactivities on the time scale of seconds to minutes would be required to explain the maximum V d rates we measured, which are much faster than the neutral hydrolysis lifetimes of MBN and EHN (∼7–200 days). Given that the gas-phase photoreactivities of these nitrates also follow the observed trend in deposition (i.e., IPN < MBN < EHN), it is possible that gas-phase or surface reactions in the inner leaf could be responsible for driving deposition. , Enzymatic processing within leaves has also been proposed as an uptake mechanism in other deposition studies investigating organic nitrate deposition. , More detailed leaf-level experiments are needed to elucidate the mechanism of uptake of RONO 2 in vegetation.…”

“…The experimentally determined MBN and EHN mesophilic uptake rates (190 ± 30 cm –1 s and 17 ± 4 cm –1 s) were prescribed in the model to simulate canopy-level deposition, and these rates were assumed to scale linearly with the canopy leaf area index. Photochemical losses for MBN and EHN were prescribed following observations and trends in RONO 2 photochemical lifetimes reported by Abeleira et al (2018) . A complete description of the model and simulation conditions can be found in the Supporting Information (SI-2).…”

“…The RONO 2 family consists of organic nitrates with a range of functionalities and properties leading to different atmospheric fates of these molecules. , Monofunctional alkyl nitrates have long chemical lifetimes in the atmosphere (τ > 1 day) and can undergo long-range transport and rerelease NO x far away from source regions. − The chemical formation and degradation of these RONO 2 compounds have been shown to have noticeable impacts on regional nitrogen and ozone budgets. − Atmospheric observations of multifunctional nitrates, on the other hand, have shown that these nitrates typically react and deposit closer to source regions but have also been shown to export regionally. ,, The exact physiochemical properties and processes that govern the atmospheric fate of RONO 2 and the relative role of deposition versus chemistry in the atmospheric loss of RONO 2 remain uncertain.…”

“…To gain a better understanding of the processes, properties, and rates governing RONO 2 deposition to vegetation, we investigated the branch-level deposition of isopropyl nitrate (IPN), methylbutyl nitrate (MBN), and ethylhexyl nitrate (EHN) using a dynamic branch chamber. − The compounds chosen for the study span a range of monofunctional RONO 2 formed from anthropogenic precursors and a range of gas and liquid phase reactivity, allowing us to gain more mechanistic insight into the process of RONO 2 deposition. − ,− In addition, MBN was chosen for this study as a representative surrogate for the multifunctional RONO 2 formed from the oxidation of isoprene and to give a comparison with previous deposition experiments reported in the literature . Finally, using the mechanistic information we gained from our chamber study we scaled our leaf-level measurements to the canopy-level to determine if the depositional loss of these nitrates was competitive with their respective photochemical losses.…”

Measurements of Atmosphere–Biosphere Exchange of Oxidized Nitrogen and Implications for the Chemistry of Atmospheric NOx

“…Ecosystem-scale flux towers provide a more localized representation than observations from space. Such flux tower observations incorporate differences between species, individual plants, and individual leaves and can be used to infer atmosphere–biosphere exchange rates. ,− However, deconvoluting chemistry, deposition, and emission processes beneath a canopy can be challenging. Efforts to do so are confounded by uncertainties in soil NO emission rates and the rates of chemical transformations.…”