Direct aqueous photochemistry of isoprene high-NOx secondary organic aerosol

Abstract

Secondary organic aerosol (SOA) generated from the high-NO(x) photooxidation of isoprene was dissolved in water and irradiated with λ > 290 nm radiation to simulate direct photolytic processing of organics in atmospheric water droplets. High-resolution mass spectrometry was used to characterize the composition at four time intervals (0, 1, 2, and 4 h). Photolysis resulted in the decomposition of high molecular weight (MW) oligomers, reducing the average length of organics by 2 carbon units. The average molecular composition changed significantly after irradiation (C(12)H(19)O(9)N(0.08) + hν → C(10)H(16)O(8)N(0.40)). Approximately 65% by count of SOA molecules decomposed during photolysis, accompanied by the formation of new products. An average of 30% of the organic mass was modified after 4 h of direct photolysis. In contrast, only a small fraction of the mass (<2%), belonging primarily to organic nitrates, decomposed in the absence of irradiation by hydrolysis. Furthermore, the concentration of aromatic compounds increased significantly during photolysis. Approximately 10% (lower limit) of photodegraded compounds and 50% (upper limit) of the photoproducts contain nitrogen. Organic nitrates and multifunctional oligomers were identified as compounds degraded by photolysis. Low-MW 0N (compounds with 0 nitrogen atoms in their structure) and 2N compounds were the dominant photoproducts. Fragmentation experiments using tandem mass spectrometry (MS(n), n = 2-3) indicate that the 2N products are likely heterocyclic/aromatic and are tentatively identified as furoxans. Although the exact mechanism is unclear, these 2N heterocyclic compounds are produced by reactions between photochemically-formed aqueous NO(x) species and SOA organics.

Molecular Characterization of Organosulfates in Organic Aerosols from Shanghai and Los Angeles Urban Areas by Nanospray-Desorption Electrospray Ionization High-Resolution Mass Spectrometry

“…For example, Nguyen et al saw no evidence of photodegradation stopping after 0, 1, 2, 3, and 4 h of the aqueous photolysis of isoprene/NO x SOA. 19 Using actinometry to estimate the photon flux from the lamp used in experiments, we have concluded that 1 h of photolysis under our lamp is approximately equivalent to ∼4 h of photolysis under the overhead sun in the case of GUA/NO x SOA and ∼8 h in the case of APIN/O 3 SOA. The lifetime of droplets in cumulus clouds is typically in the range of 10−40 min, so the times probed here correspond to photolysis that may be experienced during several cloud cycles.…”

“…The oxidants include ozone, which controls the oxidation of many unsaturated organics in clean air, and OH in the presence of NO x (=NO + NO 2 ) concentration, which is representative of an urban environment. In combination with previous studies of aqueous photolysis of limonene, isoprene, and naphthalene SOA, 15,19,20 this study provides a database for making general conclusions about the role of aqueous photochemistry in the atmospheric processing of SOA.…”

“…17,18 In contrast, the photolysis of aqueous solutions of model SOA prepared by the oxidation of isoprene, limonene, and naphthalene was found to lead to photodegradation of larger organics into smaller products. 15,19,20 Direct and indirect photolysis processes occur simultaneously, but their relative importance can be predicted only for the simplest organic compounds, which are not representative of SOA composition. 21 While much attention has been given to the aqueous oxidation of organic compounds with the OH radical, 7 there is limited knowledge about the direct photolysis of atmospherically relevant organics in water, especially for processes involving multifunctional organic compounds.…”

“…This significant decrease in the N/C ratio is different from the trend observed in direct aqueous photolysis of isoprene/NO x SOA where the number of nitrogen-containing compounds increased substantially during photolysis. 19 This suggests that nitrogen-containing molecules in isoprene SOA may be a unique case that may require further investigation. Our results indicate that for a majority of SOA, the N/C ratio is an additional useful indicator of aerosol age, complementary to the commonly used O/C ratio.…”

Dithiothreitol activity by particulate oxidizers of SOA produced from photooxidation of hydrocarbons under varied NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; levels

“…The reaction of aldehydes with OH radicals followed by the reaction with molecular oxygen yields peroxy acyl radicals (RC(O)OO; Finlayson-Pitts and Pitts, 2000). RC(O)OO can react with NO 2 to form PANs and react with HO 2 radicals to form RC(O)OOH (Finlayson-Pitts and Pitts, 2000;Nguyen et al, 2012;Xu et al, 2014). In this study, the nanomoles of organic hydroperoxides per microgram of SOA were quantified using the NPBA assay, represented by [OHP] m (nmol µg-SOA −1 ).…”