Spatiotemporal changes in aerosol properties by hygroscopicgrowth and impacts on radiative forcing and heating ratesduring DISCOVER-AQ 2011

Abstract. This work focuses on the characterization of vertically-resolved aerosol hygroscopicity properties and their direct radiative effects through a unique combination of ground-based and airborne remote sensing measurements during the DISCOVER-AQ 2011 field campaign in the Washington D.C. – Baltimore metropolitan area. To that end, we combined measurements from a multiwavelength Raman lidar located at NASA Goddard Space Flight Center and the airborne NASA Langley HSRL-1 lidar system. In-situ measurements on board the P-3B airplane and ground-based AERONET-DRAGON served to validate and complement quantifications of aerosol hygroscopicity from lidar measurements and also to extend the study both temporally and spatially. The focus here is on the 22nd and 29th of July, 2011 which were very humid days and characterized by a stable atmosphere and increasing relative humidity with height in the planetary boundary layer (PBL). Combined lidar and radiosonde measurements allowed the retrieval of the Hanel hygroscopic growth factor which agreed with that obtained from airborne in-situ measurements, and also explained the significant increase of extinction and backscattering with height. Airborne measurements also confirmed aerosol hygroscopicity throughout the entire day in the PBL and identified sulfates and water soluble organic carbon as the main species of aerosol particles. The combined Raman and HSRL-1 measurements permitted the inversion for aerosol microphysical properties revealing an increase of particle radius with altitude consistent with hygroscopic growth. Aerosol hygroscopicity was identified as the main reason to explain aerosol optical depth increases during the day, particularly for fine mode particles. Lidar measurements were used as input to the libRadtram radiative transfer code to obtain vertically-resolved aerosol radiative effects and heating rates under dry and humid conditions, and the results reveal that aerosol hygroscopicity is responsible for larger cooling effects in the shortwave range (7–10 W/m2 depending on aerosol load) near the ground, while heating rates produced a warming of 0.12 K/day near the top of PBL where aerosol hygroscopic growth was highest.