Composite study of aerosol export events from East Asia and North America Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA
01 Feb 2013
Received: 30 Jul 2012 – Published in Atmos. Chem. Phys. Discuss.: 28 Aug 2012Abstract. We use satellite observations of aerosol optical depth (AOD) from the
Moderate Resolution Imaging Spectrometer (MODIS) together with the GEOS-Chem
global chemical transport model to contrast export of aerosols from East
Asia and North America during 2004–2010. The GEOS-Chem model reproduces the
spatial distribution and temporal variations of Asian aerosol outflow
generally well, although a low bias (−30%) is found in the model fine
mode AOD, particularly during summer. We use the model to identify 244
aerosol pollution export events from E. Asia and 251 export events from N.
America over our 7-year study period. When these events are composited by
season, we find that the AOD in the outflow is enhanced by 50–100%
relative to seasonal mean values. The composite Asian plume splits into one
branch going poleward to the Arctic in 3–4 days, with the other crossing the
Pacific Ocean in 6–8 days. A fraction of the aerosols is trapped in the
subtropical Pacific High during spring and summer. The N. American plume
travels to the northeast Atlantic, reaching Europe after 4–5 days. Part of
the composite plume turns anticyclonically in the Azores High, where it
slowly decays. Both the Asian and N. American export events are favored by a
dipole structure in sea-level pressure anomalies, associated with
mid-latitude cyclone activity over the respective source regions. This
dipole structure during outflow events is a strong feature for all seasons
except summer, when convection becomes more important. The observed AOD in
the E. Asian outflow exhibits stronger seasonality, with a spring maximum,
than the N. American outflow, with a broad spring/summer maximum. The large
spring AOD in the Asian outflow is the result of enhanced sulfate and dust
aerosol concentrations, but is also due to a larger export efficiency of
sulfate and SO2 from the Asian boundary layer relative to the N.
American boundary layer. While the N. American sulfate outflow is mostly
found in the lower troposphere (1–3 km altitude), the Asian sulfate outflow
occurs at higher altitudes (2–6 km). In the Asian outflow 42–59% of the
sulfate column is present above 2 km altitude, with only 24–35% in the N.
American outflow. We link this to the factor of 2–5 lower precipitation in
the warm conveyor belts (WCB) of midlatitude cyclones over E. Asia compared
to N. America. This relative lack of precipitation makes Asian WCB very
efficient for injecting aerosols in the middle troposphere.
Revised: 25 Dec 2012 – Accepted: 09 Jan 2013 – Published: 01 Feb 2013
Citation: Luan, Y. and Jaeglé, L.: Composite study of aerosol export events from East Asia and North America, Atmos. Chem. Phys., 13, 1221-1242, doi:10.5194/acp-13-1221-2013, 2013.