Abstract
Environmental contextSecondary organic aerosols formed from the oxidation of volatile organic compounds make a significant contribution to atmospheric particulate matter, which in turn affects both global climate change and human health. We investigate the mechanisms of formation and the chemical properties of secondary organic aerosols derived from isoprene, the most abundant non-methane-based, volatile organic compound emitted into the Earth’s atmosphere. However, the exact manner in which these aerosols are formed, and how they are affected by environmental conditions, remains unclear. AbstractSecondary organic aerosol (SOA) formation from the photooxidation of methacrolein (MACR) was examined in a dual outdoor smog chamber under varied initial nitric oxide (NO) levels, relative humidities (RHs) and seed aerosol acidities. Aerosol sizing measurements and off-line chemical analyses by gas chromatography/mass spectrometry and ultra performance liquid chromatography/electrospray ionisation high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-Q-TOFMS) were used to characterise MACR SOA formation. Results indicate that both SOA mass and chemical composition largely depend on the initial MACR/NO ratio and RH conditions. Specifically, at lower initial NO levels (MACR/NO = ~2.7) more substantial SOA is formed under dry conditions (5–20 % RH) compared to wet conditions (30–80 % RH). However, at higher initial NO levels (MACR/NO = ~0.9), the maximum SOA formation was marginally higher under wet conditions. Furthermore, UPLC/ESI-HR-Q-TOFMS data suggest that most particle-phase oligomers, which have been previously observed to form from the oxidation of methacryloylperoxynitrate, were enhanced under dry conditions. In addition to 2-methylglyceric acid and organosulfates derived from MACR oxidation, a nitrogen-containing organic tracer compound was found to form substantially in both chamber-generated and ambient aerosol samples collected from downtown Atlanta, GA, during the 2008 August Mini-Intensive Gas and Aerosol Study (AMIGAS). Moreover, increasing aerosol acidity because of additional sulfuric acid appears to have a negligible effect on both SOA mass and most SOA constituents. Nevertheless, increased RH and aerosol acidity were both observed to enhance organosulfate formation; however, elevating RH mediates organosulfate formation, suggesting that wet sulfate aerosols are necessary to form organosulfates in atmospheric aerosols.
Published Version
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