Factors that influence surface PM2.5 values inferred from satellite observations: perspective gained for the Baltimore-Washington Area during DISCOVER-AQ

During the NASA DISCOVER-AQ campaign over the Washington D.C., - Baltimore, MD, metropolitan region in July 2011, the NASA P-3B aircraft performed extensive profiling of aerosol optical, chemical, and microphysical properties. These in-situ profiles were coincident with ground based remote sensing (AERONET) and in-situ (PM2.5) measurements. Here, we use this data set to study the correlation between the PM2.5 observations at the surface and the column integrated measurements. Aerosol optical depth (AOD) calculated with the extinction (532 nm) measured during the in-situ profiles was found to be strongly correlated with the volume of aerosols present in the boundary layer (BL). Despite the strong correlation, some variability remains, and we find that the presence of aerosol layers above the BL (in the buffer layer – BuL) introduces a significant uncertainties in PM2.5 estimates based on column-integrated measurements. This motivates the use of active remote sensing techniques to dramatically improve air quality retrievals. Since more than a quarter of the AOD values observed during DISCOVER-AQ are dominated by aerosol water uptake, the f(RH)amb (obtained from two nephelometers at different relative humidities – RHs) is used to study the impact of the aerosol hygroscopicity. The results indicate that PM2.5 can be predicted within a factor of 1.6 even when the vertical variability of the f(RH)amb is assumed to be negligible.