Abstract
Abstract. We investigate the seasonality in aerosols over the Southeastern United States using observations from several satellite instruments (MODIS, MISR, CALIOP) and surface network sites (IMPROVE, SEARCH, AERONET). We find that the strong summertime enhancement in satellite-observed aerosol optical depth (AOD) (factor 2–3 enhancement over wintertime AOD) is not present in surface mass concentrations (25–55% summertime enhancement). Goldstein et al. (2009) previously attributed this seasonality in AOD to biogenic organic aerosol; however, surface observations show that organic aerosol only accounts for ∼35% of fine particulate matter (smaller than 2.5 μm in aerodynamic diameter, PM2.5) and exhibits similar seasonality to total surface PM2.5. The GEOS-Chem model generally reproduces these surface aerosol measurements, but underrepresents the AOD seasonality observed by satellites. We show that seasonal differences in water uptake cannot sufficiently explain the magnitude of AOD increase. As CALIOP profiles indicate the presence of additional aerosol in the lower troposphere (below 700 hPa), which cannot be explained by vertical mixing, we conclude that the discrepancy is due to a missing source of aerosols above the surface layer in summer.
Highlights
Fine particulate matter and exhibits similar seasonality to total sur
We show that seasonal differences in water uptake cannot sufficiently explain ter, PM2.5) in the region is dominated by ammonium sulfate and organic matter (OMH),ywdhriochlotoggyethaernadccount for 60
In order to determine whether the observed seasonality in aerosol optical depth (AOD) can be attributed to changes in mass loading or mass extinction efficiency, we explore the sensitivity of the mass extinction efficiency to the observed seasonal changes in relative humidity (Sect. 3.2)
Summary
Fine particulate matter (smaller than 2.5 μm in aerodynamic diameter, PM2.5) and exhibits similar seasonality to total sur-. Previous studies have shown that the fine parthese surface aerosol measurements, but underrepresents the ticulate matter We show that seasonal differences in water uptake cannot sufficiently explain ter, PM2.5) in the region is dominated by ammonium sulfate and organic matter (OMH),ywdhriochlotoggyethaernadccount for 60–. As CALIOP profiles indicate the presence of additional aerosol in the lower troposphere (below 700 hPa), which cannot be explained by verti-. Throughout most of the Syecari,eonrgcaneicscarbon is procal mixing, we conclude that the discrepancy is due to a miss- duced from wood combustion and diesel exhaust; while secing source of aerosols above the surface layer in summer
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