Contribution of Organic Nitrates to Organic Aerosol over South Korea during KORUS-AQ.

The role of anthropogenic NO _x emissions in secondary organic aerosol (SOA) production is not fully understood but is important for understanding the contribution of emissions to air quality. Here, we examine the role of organic nitrates (RONO ₂ ) in SOA formation over the Korean Peninsula during the Korea-United States Air Quality field study in Spring 2016 as a model for RONO ₂ aerosol in cities worldwide. We use aircraft-based measurements of the particle phase and total (gas + particle) RONO ₂ to explore RONO ₂ phase partitioning. These measurements show that, on average, one-fourth of RONO ₂ are in the condensed phase, and we estimate that ≈15% of the organic aerosol (OA) mass can be attributed to RONO ₂ . Furthermore, we observe that the fraction of RONO ₂ in the condensed phase increases with OA concentration, evidencing that equilibrium absorptive partitioning controls the RONO ₂ phase distribution. Lastly, we model RONO ₂ chemistry and phase partitioning in the Community Multiscale Air Quality modeling system. We find that known chemistry can account for one-third of the observed RONO ₂ , but there is a large missing source of semivolatile, anthropogenically derived RONO ₂ . We propose that this missing source may result from the oxidation of semi- and intermediate-volatility organic compounds and/or from anthropogenic molecules that undergo autoxidation or multiple generations of OH-initiated oxidation.