Role of aldehyde chemistry and NO<sub>x</sub> concentrations in secondary organic aerosol formation
Aldehydes are an important class of products from atmospheric oxidation of hydrocarbons. Isoprene (2-methyl-1,3-butadiene), the most abundantly emitted atmospheric non-methane hydrocarbon, produces a significant amount of secondary organic aerosol (SOA) via methacrolein (a C<sub>4</sub>-...
| Main Authors: | , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2010-08-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/10/7169/2010/acp-10-7169-2010.pdf |
| Summary: | Aldehydes are an important class of products from atmospheric oxidation of hydrocarbons. Isoprene (2-methyl-1,3-butadiene), the most abundantly emitted atmospheric non-methane hydrocarbon, produces a significant amount of secondary organic aerosol (SOA) via methacrolein (a C<sub>4</sub>-unsaturated aldehyde) under urban high-NO<sub>x</sub> conditions. Previously, we have identified peroxy methacryloyl nitrate (MPAN) as the important intermediate to isoprene and methacrolein SOA in this NO<sub>x</sub> regime. Here we show that as a result of this chemistry, NO<sub>2</sub> enhances SOA formation from methacrolein and two other α, β-unsaturated aldehydes, specifically acrolein and crotonaldehyde, a NO<sub>x</sub> effect on SOA formation previously unrecognized. Oligoesters of dihydroxycarboxylic acids and hydroxynitrooxycarboxylic acids are observed to increase with increasing NO<sub>2</sub>/NO ratio, and previous characterizations are confirmed by both online and offline high-resolution mass spectrometry techniques. Molecular structure also determines the amount of SOA formation, as the SOA mass yields are the highest for aldehydes that are α, β-unsaturated and contain an additional methyl group on the α-carbon. Aerosol formation from 2-methyl-3-buten-2-ol (MBO232) is insignificant, even under high-NO<sub>2</sub> conditions, as PAN (peroxy acyl nitrate, RC(O)OONO<sub>2</sub>) formation is structurally unfavorable. At atmospherically relevant NO<sub>2</sub>/NO ratios (3–8), the SOA yields from isoprene high-NO<sub>x</sub> photooxidation are 3 times greater than previously measured at lower NO<sub>2</sub>/NO ratios. At sufficiently high NO<sub>2</sub> concentrations, in systems of α, β-unsaturated aldehydes, SOA formation from subsequent oxidation of products from acyl peroxyl radicals+NO<sub>2</sub> can exceed that from RO<sub>2</sub>+HO<sub>2</sub> reactions under the same inorganic seed conditions, making RO<sub>2</sub>+NO<sub>2</sub> an important channel for SOA formation. |
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| ISSN: | 1680-7316 1680-7324 |