Optical closure of multispectral aerosol optical properties: Effect of aerosol liquid water during the Two-Column Aerosol Project

Abstract:

Aerosols remain a major source of uncertainty in the global radiative budget. Studies that combine vertically-resolved measurements of aerosol size distributions and refractive index (inferred from aerosol composition measurements) are needed to assess our understanding of multispectral aerosol optical closure. A complication arises from the exchange of water and partitioning of semivolatile and soluble gases and aerosols to the surrounding air. Since aerosols are often sampled in vacuum and after drying, aerosol liquid water is often unmeasured, yet provides a partitioning volume for soluble gases to form aqueous secondary organic aerosol (aqSOA). An additional fundamental sampling challenge in assessing optical closure exists in the different spatial, spectral, and temporal scales at which remote sensing and in situ instruments characterize aerosols. A step towards overcoming this sampling challenge is the development of High Spectral Resolution Lidar (HSRL). Here we use data from the NASA airborne HSRL-2 instrument, which retrieves aerosol extinction profiles at 355, 532, and 1064 nm from backscatter measurements. The dataset we use is from the Two-Column Aerosol Project (TCAP) in July 2012, which tackled the sampling challenge by deploying two research aircraft above the DoE ARM mobile facility at Cape Cod, MA. The following facilities and instruments were deployed: 1) DoE G1 aircraft equipped with in situ measurements of aerosol size distributions (harmonized from UHSAS, PCASP, CAS sensors; Kassianov et al., 2015) and aerosol chemical composition to constrain refractive index (AMS, SP2); 2) the NASA King Air aircraft equipped with nadir-looking HSRL-2 (Müller et al., 2014). Both aircraft profiled over 3) the DoE ARM surface site equipped with: a) the University of Colorado 2D-MAX-DOAS (Ortega et al., 2016a, b), b) an AERONET sun photometer, and c) a Multifilter Rotating Shadowband Radiometer. This dataset is reanalyzed here to investigate the effects of aerosol water on dry aerosol size and composition, and our ability to constrain Mie calculations to obtain multispectral optical closure. In particular, the vertically-resolved Angstrom exponent is examined, and aerosol water effects are compared with model simulations that include pathways to form aqSOA.