Observational Insights into Aerosol Formation from Isoprene

Abstract

Atmospheric photooxidation of isoprene is an important source of secondary organic aerosol (SOA) and there is increasing evidence that anthropogenic oxidant emissions can enhance this SOA formation. In this work, we use ambient observations of organosulfates formed from isoprene epoxydiols (IEPOX) and methacrylic acid epoxide (MAE) and a broad suite of chemical measurements to investigate the relative importance of nitrogen oxide (NO/NO₂) and hydroperoxyl (HO₂) SOA formation pathways from isoprene at a forested site in California. In contrast to IEPOX, the calculated production rate of MAE was observed to be independent of temperature. This is the result of the very fast thermolysis of MPAN at high temperatures that affects the distribution of the MPAN reservoir (MPAN / MPA radical) reducing the fraction that can react with OH to form MAE and subsequently SOA (FMAE formation). The strong temperature dependence of FMAE formation helps to explain our observations of similar concentrations of IEPOX-derived organosulfates (IEPOX-OS; ∼1 ng m⁻³) and MAE-derived organosulfates (MAE-OS; ∼1 ng m⁻³) under cooler conditions (lower isoprene concentrations) and much higher IEPOX-OS (∼20 ng m⁻³) relative to MAE-OS (<0.0005 ng m⁻³) at higher temperatures (higher isoprene concentrations). A kinetic model of IEPOX and MAE loss showed that MAE forms 10−100 times more ring-opening products than IEPOX and that both are strongly dependent on aerosol water content when aerosol pH is constant. However, the higher fraction of MAE ring opening products does not compensate for the lower MAE production under warmer conditions (higher isoprene concentrations) resulting in lower formation of MAE-derived products relative to IEPOX at the surface. In regions of high NOx, high isoprene emissions and strong vertical mixing the slower MPAN thermolysis rate aloft could increase the fraction of MPAN that forms MAE resulting in a vertically varying isoprene SOA source.