Isoprene carries approximately half of the flux of non-methane volatile organic carbon emitted to the atmosphere by the biosphere. Accurate representation of its oxidation rate and products is essential for quantifying its influence on the abundance of the hydroxyl radical (OH), nitrogen oxide free radicals (NO ), ozone (O), and, via the formation of highly oxygenated compounds, aerosol. We present a review of recent laboratory and theoretical studies of the oxidation pathways of isoprene initiated by addition of OH, O, the nitrate radical (NO), and the chlorine atom. From this review, a recommendation for a nearly complete gas-phase oxidation mechanism of isoprene and its major products is developed. The mechanism is compiled with the aims of providing an accurate representation of the flow of carbon while allowing quantification of the impact of isoprene emissions on HO and NO free radical concentrations and of the yields of products known to be involved in condensed-phase processes. Finally, a simplified (reduced) mechanism is developed for use in chemical transport models that retains the essential chemistry required to accurately simulate isoprene oxidation under conditions where it occurs in the atmosphere-above forested regions remote from large NO emissions.

Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO3) represents one of the important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere. This interaction has been recognized for more than 3 decades, during which time a large body of research has emerged from laboratory, field, and modeling studies. NO3-BVOC reactions influence air quality, climate and visibility through regional and global budgets for reactive nitrogen (particularly organic nitrates), ozone, and organic aerosol. Despite its long history of research and the significance of this topic in atmospheric chemistry, a number of important uncertainties remain. These include an incomplete understanding of the rates, mechanisms, and organic aerosol yields for NO3-BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO3 radical, the difficulty of characterizing the spatial distributions of BVOC and NO3 within the poorly mixed nocturnal atmosphere, and the challenge of constructing appropriate boundary layer schemes and non-photochemical mechanisms for use in state-of-the-art chemical transport and chemistry–climate models.This review is the result of a workshop of the same title held at the Georgia Institute of Technology in June 2015. The first half of the review summarizes the current literature on NO3-BVOC chemistry, with a particular focus on recent advances in instrumentation and models, and in organic nitrate and secondary organic aerosol (SOA) formation chemistry. Building on this current understanding, the second half of the review outlines impacts of NO3-BVOC chemistry on air quality and climate, and suggests critical research needs to better constrain this interaction to improve the predictive capabilities of atmospheric models.

Abstract. The reactive partitioning of cis and trans β-IEPOX was investigated on hydrated inorganic seed particles, without the addition of acids. No organic aerosol (OA) formation was observed on dry ammonium sulfate (AS); however, prompt and efficient OA growth was observed for the cis and trans β-IEPOX on AS seeds at liquid water contents of 40–75% of the total particle mass. OA formation from IEPOX is a kinetically limited process, thus the OA growth continues if there is a reservoir of gas-phase IEPOX. There appears to be no differences, within error, in the OA growth or composition attributable to the cis / trans isomeric structures. Reactive uptake of IEPOX onto hydrated AS seeds with added base (NaOH) also produced high OA loadings, suggesting the pH dependence for OA formation from IEPOX is weak for AS particles. No OA formation, after particle drying, was observed on seed particles where Na+ was substituted for NH4+. The Henry's Law partitioning of IEPOX was measured on NaCl particles (ionic strength ~9 M) to be 3 × 107 M atm−1 (−50 / +100%). A small quantity of OA was produced when NH4+ was present in the particles, but the chloride (Cl-) anion was substituted for sulfate (SO42-), possibly suggesting differences in nucleophilic strength of the anions. Online time-of-flight aerosol mass spectrometry and offline filter analysis provide evidence of oxygenated hydrocarbons, organosulfates, and amines in the particle organic composition. The results are consistent with weak correlations between IEPOX-derived OA and particle acidity or liquid water observed in field studies, as the chemical system is nucleophile-limited and not limited in water or catalyst activity.

S1.0 KINETIC MECHANISM DEVELOPMENT.A kinetic mechanism is formulated to simulate the reaction conditions of these experiments.The reactions included are listed in the Appendix (Table SA2, SA3, and SA4). Table SA5 contains a list of the abbreviations used. Rate constants for most of the reactions included in the mechanism are based on recommendations from JPL 1 , IUPAC 2-3 , or MCM v3.2 4 . However, some rate constants and branching ratios are not known. For these, we use our best judgement based on available data; explanations of the assumptions on which these estimates are based are included in this section. Some branching ratios and rate constants are estimated based on the experimental results presented here. Many of these branching ratios depend on the fraction of δ-and β-isomers that form (Table 3 and 5), which will likely depend on the lifetime of the RO 2 radical (Section 4.1). Thus, the reaction products and rates presented here are most consistent with the experimental results for this study in which the overall RO 2 lifetime was ~ 30 s. The kinetic mechanism developed here represents our current level of understanding, and deviations from the experimental results highlight areas for future study. S1.1. Basic Reactions in Kinetic Mechanism. HO 2 was constrained in the kinetic mechanism by the measured H 2 O 2 production rate. Prior to photooxidation, H 2 O 2 is predominantly formed from HO 2 + HO 2 reactions. To match the observed H 2 O 2 production rate in experiments 5, 6, and 8, we arbitrarily increased the reaction rate constant for CH 2 O + NO 3 by a factor of 2.5-3 in the kinetic mechanism above that recommended by IUPAC. Although not perfect when correcting for the missing HO 2 in this manner, the H 2 O 2 curves for the kinetic mechanism and the experimental results were fairly consistent. Under-prediction of HO 2 could be caused by other S3 missing chemistry including unaccounted for surface chemistry, later generation chemistry not incorporated into the kinetic mechanism, or many other possibilities. Here, we are confident that there is a missing source of HO 2 , but are agnostic about the mechanism responsible.Because the predominant loss of isoprene is due to reaction with NO 3 , the measured isoprene decay rate was used to constrain the amount of NO 3 present. Cantrell et al. 5 proposed that N 2 O 5would react with water present on the wall surface to form nitric acid even under dry conditions.We included a wall loss rate for N 2 O 5 (i.e., NO 3 loss rate) such that the isoprene decay in the kinetic mechanism matched with experimental results. This rate constant is chamber/experiment specific. For experiment 5 (24 m 3 , 2.2 ppm CH 2 O), 6 (24 m 3 , 4.7 ppm CH 2 O), 7 (1 m 3 , 2 ppm CH 2 O) and 8 (1 m 3 , 4 ppm CH 2 O), N 2 O 5 wall loss rate constants that best fit experimental conditions were 1.5 x 10 -4 , 12 x 10 -4 , 6 x 10 -4 and 12 x 10 -4 s -1 , respectively. We observe that the N 2 O 5 loss rate appears to be sensitive to both the mixing ratio of CH 2 O and the chamber.However, it s...

We use a large laboratory, modeling, and field dataset to investigate the isoprene + O3 reaction, with the goal of better understanding the fates of the C1 and C4 Criegee intermediates in the atmosphere.