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 (sigma(ext,in-situ)) measurements were made aboard the NASA P3-B aircraft coincident with remote-sensing observations by the High-Spectral Resolution Lidar (HSRL; sigma(ext,HSRL)) aboard the NASA UC-12 aircraft. A statistical comparison revealed good agreement within instrumental uncertainty (sigma(ext,in-situ)=1.1(sigma ext,HSRL) - 3.2Mm(-1), r(2)=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 remote-sensing retrievals. Citation: Ziemba L. D., et al. (2013), Airborne observations of aerosol extinction by in situ and remote-sensing techniques: Evaluation of particle hygroscopicity, Geophys. Res. Lett., 40, 417-422, doi:10.1029/2012GL054428.