Abstract
Abstract. 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.
Highlights
The long-range transport of aerosols from East Asia to the North Pacific Ocean and the North American west coast has been observed for several decades (e.g., Prospero, 1979; Duce et al, 1980; Andreae et al, 1988; Prospero et al, 2003; Jaffe et al, 1999, 2003; Arimoto et al, 1996, 1997; Husar et al, 2001; Clarke et al, 2001; VanCuren, 2003; Bertschi et al, 2004; Bertschi and Jaffe, 2005; Heald et al, 2006a; McKendry et al, 2008)
American aerosol outflow events based on dozens of individual cases for each season in order to examine the general features of these events
Lapina et al (2011) used a Goddard Earth Observing System (GEOS)-Chem simulation with land biogenic secondary organic aerosol (SOA) formation and still noted a systematic GEOS-Chem underestimate of Moderate Resolution Imaging Spectrometer (MODIS) aerosol optical depth (AOD) by 20 % over the remote oceans, which they attributed to a combination of satellite retrieval bias and a missing marine aerosol source
Summary
The long-range transport of aerosols from East Asia to the North Pacific Ocean and the North American west coast has been observed for several decades (e.g., Prospero, 1979; Duce et al, 1980; Andreae et al, 1988; Prospero et al, 2003; Jaffe et al, 1999, 2003; Arimoto et al, 1996, 1997; Husar et al, 2001; Clarke et al, 2001; VanCuren, 2003; Bertschi et al, 2004; Bertschi and Jaffe, 2005; Heald et al, 2006a; McKendry et al, 2008). A mechanistic understanding of the processes leading to export and long-range transport of pollution has come from studying individual case studies, in which specific transport plumes were observed by in situ instruments and/or by satellites over the N. American aerosol outflow events based on dozens of individual cases for each season in order to examine the general features of these events This approach provides more generality than individual long-range transport case studies, without losing useful information that is washed out by simple seasonal averaging. We apply this compositing methodology to the GEOS-Chem chemical transport model and to MODIS observations of aerosol optical depth (AOD) for the 2004– 2010 period.
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