The effects of isoprene and NO_<i>x</i> on secondary organic aerosols formed through reversible and irreversible uptake to aerosol water

• Main Authors: M. M. H. El-Sayed, D. L. Ortiz-Montalvo, C. J. Hennigan
• Format: Article
• Language: English
• Published: Copernicus Publications 2018-01-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/18/1171/2018/acp-18-1171-2018.pdf

Isoprene oxidation produces water-soluble organic gases capable of partitioning to aerosol liquid water. The formation of secondary organic aerosols through such aqueous pathways (aqSOA) can take place either reversibly or irreversibly; however, the split between these fractions in the atmosphere is highly uncertain. The aim of this study was to characterize the reversibility of aqSOA formed from isoprene at a location in the eastern United States under substantial influence from both anthropogenic and biogenic emissions. The reversible and irreversible uptake of water-soluble organic gases to aerosol water was characterized in Baltimore, Maryland, USA, using measurements of particulate water-soluble organic carbon (WSOC_p) in alternating dry and ambient configurations. WSOC_p evaporation with drying was observed systematically throughout the late spring and summer, indicating reversible aqSOA formation during these times. We show through time lag analyses that WSOC_p concentrations, including the WSOC_p that evaporates with drying, peak 6 to 11 h after isoprene concentrations, with maxima at a time lag of 9 h. The absolute reversible aqSOA concentrations, as well as the relative amount of reversible aqSOA, increased with decreasing NO_<i>x</i> ∕ isoprene ratios, suggesting that isoprene epoxydiol (IEPOX) or other low-NO_<i>x</i> oxidation products may be responsible for these effects. The observed relationships with NO_<i>x</i> and isoprene suggest that this process occurs widely in the atmosphere, and is likely more important in other locations characterized by higher isoprene and/or lower NO_<i>x</i> levels. This work underscores the importance of accounting for both reversible and irreversible uptake of isoprene oxidation products to aqueous particles.