Airborne observations of aerosol extinction by in situ and remote-sensing techniques: Evaluation of particle hygroscopicity

Extensive profiling of aerosol optical, chemical, and microphysical properties was performed in the Washington DC/Baltimore MD region in July 2011 during NASA DISCOVER-AQ. In situ extinction coefficient (sext,in-situ) measurements were made aboard the NASA P3-B aircraft coincident with remote-sensing observations by the High-Spectral Resolution Lidar (HSRL; sext,HSRL) aboard the NASA UC-12 aircraft. A statistical comparison revealed good agreement within instrumental uncertainty (sext,in-situ = 1.1 sext,HSRL -3.2 Mm-1, r2 = 0.88) and demonstrated the robust nature of hygroscopicity measurements (f(RH)) necessary to correct observations at dry relative humidity (RH) to ambient conditions. The average liquid-water contribution to ambient visible-light extinction was as much as 43% in this urban region. f(RH) values were observed to vary significantly from 1.1 to 2.1 on a day-to-day basis suggesting influence from both local and transported sources. Results emphasize the importance of accounting for the RH dependence of optical- and mass-based aerosol air-quality measurements (e.g., of PM2.5), especially in relation to satellite and remotesensing retrievals.