Aerosol size distribution and calculated light-scattering efficiency in the Sacramento plume during CARES
Authors
Larry Kleinman — Brookhaven National Laboratory
Chongai Kuang — Brookhaven National Laboratory
Arthur J Sedlacek — Brookhaven National Laboratory
Gunnar I. Senum — Brookhaven National Laboratory
Stephen R. Springston — Brookhaven National Laboratory
Jian Wang — Washington University in St. Louis
Jerome D Fast — Pacific Northwest National Laboratory
John Hubbe — Pacific Northwest National Laboratory
John E Shilling — Pacific Northwest National Laboratory
Jason Tomlinson — Pacific Northwest National Laboratory
Rahul Zaveri — Pacific Northwest National Laboratory
Category
Aerosol Properties
Description
In this study we consider aerosol light scattering, using aircraft data collected upwind, above, and downwind of Sacramento, California during the DOE-sponsored Carbonaceous Aerosol and Radiative Effects Study (CARES) field campaign. As found in other urban studies, aerosol concentration normalized to a conservative tracer such as CO increased downwind of Sacramento, primarily from secondary organic aerosol (SOA) formation. Optical and radiative effects depend on whether the newly formed aerosol mass is added to small or large particles. The size range and wavelengths of interest intersect the Rayleigh regime in which scattering is proportional to DP6, and scattering efficiency (scattering per unit mass or volume) is proportional to DP3. The emission or formation of small particles in an urban environment could cause a competition for condensables, thereby reducing the aerosol mass that becomes an effective scatterer.
Aerosol size distributions measured with a UHSAS (60–1000 nm) showed accumulation mode(s) with DP < 500 nm and a coarse mode with DP > 650 nm. Although the coarse mode can dominate scattering, particularly at long wavelengths, it is not of primary interest in the CARES campaign as it is unlikely to be associated with anthropogenic emissions, though it could be a function of land use. In the accumulation mode size range it was common for there to be two modes with geometric mean diameters of approximately 100 and 250 nm. There were flight transects where one or the other mode dominated and other transects where the two modes made roughly equal contributions to accumulation mode volume.
Scattering efficiency has been calculated for transects upwind, over, and downwind of Sacramento. We consider the 14 flights with SW flow. For dry particles in the boundary layer, the scattering efficiency of aerosol smaller than 500 nm at a wavelength of 550 nm averaged 3.8 and 3.1 Mm-1 per um3/cm3 for the morning and afternoon, respectively. Trends from upwind to downwind locations were small. Day to day, the scattering efficiency, averaged over individual transects, varied between 1 and 8, according to whether the accumulation size range was dominated by the small or large mode. Taking the morning measurements as a starting point, we consider the dependence of scattering on growth conditions, specifically the size-dependent sink for condensables. Radiative effects are in the process of being calculated, with consideration of upscatter fraction, RH dependence of size, and aerosol layer depth.