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During the NASA DISCOVER-AQ campaign over the US Baltimore, MD–Washington, D.C.,metropolitan area in July 2011, the NASA P-3B aircraft performed extensiveprofiling of aerosol optical, chemical, and microphysical properties. Thesein situ profiles were coincident with ground-based remote sensing (AERONET)and in situ (PM2.5) measurements. Here, we use this data set to studythe correlation between the PM2.5 observations at the surface and thecolumn integrated measurements. Aerosol optical depth(AOD550 nm) calculated with the extinction (550 nm) measuredduring the in situ profiles was found to be strongly correlated with thevolume of aerosols present in the boundary layer (BL). Despite the strongcorrelation, some variability remains, and we find that the presence ofaerosol layers above the BL (in the buffer layer – BuL) introducessignificant uncertainties in PM2.5 estimates based on column-integratedmeasurements (overestimation of PM2.5 by a factor of 5). This suggeststhat the use of active remote sensing techniques would dramatically improveair quality retrievals. Indeed, the relationship between theAOD550 nm and the PM2.5 is strongly improved byaccounting for the aerosol present in and above the BL (i.e., integrating theaerosol loading from the surface to the top of the BuL). Since more than15% of the AOD values observed during DISCOVER-AQ are dominated by aerosolwater uptake, the f(RH)amb (ratio of scattering coefficient atambient relative humidity (RH) to scattering coefficient at low RH; seeSect. 3.2) is used to study the impact of the aerosol hygroscopicity on thePM2.5 retrievals. Theresults indicate that PM2.5 can be predicted within a factor up to 2even when the vertical variability of the f(RH)amb is assumedto be negligible. Moreover, f(RH = 80%) and RH measurements performedat the ground may be used to estimate the f(RH)amb during dryconditions (RHBL < 55%)

atmospheric chemistry and physics

Factors that influence surface PM&amp;lt;sub&amp;gt;2.5&amp;lt;/sub&amp;gt; values inferred from satellite observations: perspective gained for the US Baltimore–Washington metropolitan area during DISCOVER-AQ