Secondary organic aerosol formation from isoprene organic nitrates in a temperate forest

Abstract:

Organic nitrates contribute significantly to the secondary organic aerosol (SOA). However, large uncertainties still remain in the fate of isoprene organic nitrates and subsequent aerosol formation. In this study, a chemistry-canopy model is updated with the state-of-the-art gas-phase isoprene oxidation mechanism RCIM (the reduced Caltech isoprene mechanism) and coupled with a thermodynamic model for gas-particle partitioning to investigate the SOA formation from isoprene organic nitrates at the University of Michigan Biological Station (UMBS) during the 2016 AMOS field campaign. Model results suggest that formation of isoprene aerosols through aqueous pathways dominates over the vapor-pressure based partitioning to organic aerosol phase, with an average in-canopy SOA mass yield from isoprene of about 10%. The SOA formation from organic nitrates accounts for 31 % and 92 % of the total isoprene SOA under low- and high-NOx conditions, respectively. Under low NOx, isoprene SOA from IEPOX formation pathway dominates, yet tetrafunctional organic nitrates are the key precursors for isoprene SOA formation under high-NOx conditions. The modeled SOA formed from organic nitrates suggest large concentration gradients between in and above the forest canopy, with greater formation within the canopy by a factor of four under low NOx conditions. This highlights the need for evaluating these processes at a finer vertical scale than can be included in most regional models.