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Despite decades of progress in reducing nitrogen oxide (NO_x) emissions, ammonium nitrate (AN) remains the primary inorganic component of particulate matter (PM) in Los Angeles (LA). Using aerosol mass spectrometry over multiple years in LA illustrates the controlling dynamics of AN and their evolution over the past decades. These data suggest that much of the nitric acid (HNO₃) production required to produce AN in LA occurs during the nighttime via heterogeneous hydrolysis of N₂O₅. Further, we show that US Environmental Protection Agency–codified techniques for measuring total PM_2.5fail to quantify the AN component, while low-cost optical sensors demonstrate good agreement. While previous studies suggest that declining NO_xhas reduced AN, we show that HNO₃formation is still substantial and leads to the formation of many tens of micrograms per cubic meter of AN aerosol. Continued focus on reductions in NO_xwill help meet the PM_2.5standards in the LA basin and many other regions. 

We investigate the gas-phase photo-oxidation of 2-ethoxyethanol (2-EE) initiated by the OH radical with a focus on its autoxidation pathways. Gas-phase autoxidation intramolecular H-shifts followed by O2 additionhas recently been recognized as a major atmospheric chemical pathway that leads to the formation of highly oxygenated organic molecules (HOMs), which are important precursors for secondary organic aerosols (SOAs). Here, we examine the gas-phase oxidation pathways of 2-EE, a model compound for glycol ethers, an important class of volatile organic compounds (VOCs) used in volatile chemical products (VCPs). Both experimental and computational techniques are applied to analyze the photochemistry of the compound. We identify oxidation products from both bimolecular and autoxidation reactions from chamber experiments at varied HO2 levels and provide estimations of rate coefficients and product branching ratios for key reaction pathways. The H-shift processes of 2-EE peroxy radicals (RO2) are found to be sufficiently fast to compete with bimolecular reactions under modest NO/HO2 conditions. More than 30% of the produced RO2 are expected to undergo at least one H-shift for conditions typical of modern summer urban atmosphere, where RO2 bimolecular lifetime is becoming >10 s, which implies the potential for glycol ether oxidation to produce considerable amounts of HOMs at reduced NOx levels and elevated temperature. Understanding the gas-phase autoxidation of glycol ethers can help fill the knowledge gap in the formation of SOA derived from oxygenated VOCs emitted from VCP sources. 

Secondary organic aerosol (SOA) is ubiquitous in the atmosphere and plays a pivotal role in climate, air quality, and health. The production of low-volatility dimeric compounds through accretion reactions is a key aspect of SOA formation. However, despite extensive study, the structures and thus the formation mechanisms of dimers in SOA remain largely uncharacterized. In this work, we elucidate the structures of several major dimer esters in SOA from ozonolysis of α-pinene and β-pinene—substantial global SOA sources—through independent synthesis of authentic standards. We show that these dimer esters are formed in the particle phase and propose a mechanism of nucleophilic addition of alcohols to a cyclic acylperoxyhemiacetal. This chemistry likely represents a general pathway to dimeric compounds in ambient SOA. 

In this study we revisit one of the simplest RO2 + RO2 reactions: the self-reaction of the ethene derived hydroxyperoxy radical formed via sequential addition of ·OH and O2 to ethene. Previous studies of this reaction suggested that the branching to ‘accretion products,’ compounds containing the carbon backbone of both reactants, was minimal. Here, CF3O− GC-CIMS is used to quantify the yields of ethylene glycol, glycolaldehyde, a hydroxy hydroperoxide produced from RO2 + HO2, and a C4O4H10 accretion product. These experiments were performed in an environmental chamber at 993 hPa and 294 K. We provide evidence that the accretion product is likely dihydroxy diethyl peroxide (HOC2H4OOC2H4OH = ROOR) and forms in the gas-phase with a branching fraction of 23 ± 5%. We suggest a new channel in the RO2+RO2 chemistry leading directly to the formation of HO2 (together with glycolaldehyde and an alkoxy radical). Finally, by varying the ratio of the formation rate of RO2 and HO2 in our chamber, we constrain the ratio of the rate coefficient for the reaction of RO2 + RO2 to that of RO2 + HO2 and find that this ratio is .22±.07, consistent with previous flash photolysis studies. 

RO 2 + OH reactions at atmospheric conditions lead to widespread levels of the previously omitted strong oxidizing agent ROOOH. 

Abstract. Extensive airborne measurements of non-methane organic gases (NMOGs), methane, nitrogen oxides, reduced nitrogen species, and aerosol emissions from US wild and prescribed fires were conducted during the 2019 NOAA/NASA Fire Influence on Regional to Global Environments and Air Quality campaign (FIREX-AQ). Here, we report the atmospheric enhancement ratios (ERs) and inferred emission factors (EFs) for compounds measured on board the NASA DC-8 research aircraft for nine wildfires and one prescribed fire, which encompass a range of vegetation types. We use photochemical proxies to identify young smoke and reduce the effects of chemical degradation on our emissions calculations. ERs and EFs calculated from FIREX-AQ observations agree within a factor of 2, with values reported from previous laboratory and field studies for more than 80 % of the carbon- and nitrogen-containing species. Wildfire emissions are parameterized based on correlations of the sum of NMOGs with reactive nitrogen oxides (NOy) to modified combustion efficiency (MCE) as well as other chemical signatures indicative of flaming/smoldering combustion, including carbon monoxide (CO), nitrogen dioxide (NO2), and black carbon aerosol. The sum of primary NMOG EFs correlates to MCE with an R2 of 0.68 and a slope of −296 ± 51 g kg−1, consistent with previous studies. The sum of the NMOG mixing ratios correlates well with CO with an R2 of 0.98 and a slope of 137 ± 4 ppbv of NMOGs per parts per million by volume (ppmv) of CO, demonstrating that primary NMOG emissions can be estimated from CO. Individual nitrogen-containing species correlate better with NO2, NOy, and black carbon than with CO. More than half of the NOy in fresh plumes is NO2 with an R2 of 0.95 and a ratio of NO2 to NOy of 0.55 ± 0.05 ppbv ppbv−1, highlighting that fast photochemistry had already occurred in the sampled fire plumes. The ratio of NOy to the sum of NMOGs follows trends observed in laboratory experiments and increases exponentially with MCE, due to increased emission of key nitrogen species and reduced emission of NMOGs at higher MCE during flaming combustion. These parameterizations will provide more accurate boundary conditions for modeling and satellite studies of fire plume chemistry and evolution to predict the downwind formation of secondary pollutants, including ozone and secondary organic aerosol.