A large organic aerosol source in the free troposphere missing from current models

TLDR

Current chemical transport models accurately reproduce sulfate and elemental carbon vertical profiles, showing sharp decreases from the boundary layer to the free troposphere due to wet scavenging. Aircraft measurements reveal unexpectedly high free‑tropospheric organic carbon concentrations—four times higher than model predictions—indicating a large, sustained source of secondary organic aerosol from long‑lived VOC oxidation that dominates aerosol mass and impacts intercontinental transport and climate forcing.

Abstract

Aircraft measurements of organic carbon (OC) aerosol by two independent methods over the NW Pacific during the ACE‐Asia campaign reveal unexpectedly high concentrations in the free troposphere (FT). Concentrations average 4 μg sm −3 in the 2–6.5 km column with little vertical gradient. These values are 10–100 times higher than computed with a global chemical transport model (CTM) including a standard 2‐product simulation of secondary organic aerosol (SOA) formation based on empirical fits to smog chamber data. The same CTM reproduces the observed vertical profiles of sulfate and elemental carbon aerosols, which indicate sharp decreases from the boundary layer to the FT due to wet scavenging. Our results suggest a large, sustained source of SOA in the FT from oxidation of long‐lived volatile organic compounds. We find that this SOA is the dominant component of aerosol mass in the FT, with implications for intercontinental pollution transport and radiative forcing of climate.