JULY 2005: A large organic aerosol source in the free troposphere missing from current models
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 µ g sm-3 in the 2-7 km column with little vertical gradient. These values are 10-100 times higher than computed with the GEOS-Chem model including a standard simulation of secondary organic aerosol (SOA) formation [Chung and Seinfeld, 2002]. The same model reproduces successfully 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 the radiative budget of the Earth. A full description is given inHeald et al., 2005.
Figure Top Row: Mean vertical profiles of organic carbon (OC) aerosol
concentrations from the ACE-Asia aircraft mission off the coast of Japan.
Observations are shown in black, with individual measurements as crosses.
Results from the GEOS-Chem model are shown in red with standard deviations
for each altitude interval.
Figure Bottom Row: Mean vertical profiles of OC/sulfate mass ratio (left)
and sulfate concentrations (center) from the ACE-Asia campaign, and
elemental carbon (EC) aerosol concentrations from the TRACE-P campaign in
the same region one month earlier. Observations are shown in black.
GEOS-Chem model results are shown in red, with standard deviations of
concentrations for each altitude interval. Individual EC observations from
the ACE-Asia campaign are shown as blue symbols.