AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Diurnal Variations and the Effect of Atmospheric Transport on Black Carbon Mixing State: Observations from the 2010 Carbonaceous Aerosols and Radiative Effects Study (CARES)
R. SUBRAMANIAN, Arthur Sedlacek, Rahul Zaveri, RTI International
Abstract Number: 582 Working Group: Carbonaceous Aerosols in the Atmosphere
Abstract Direct radiative forcing by BC-containing aerosols (BCA) is affected by the relative fraction of non-BC material and the particle morphology – collectively referred to as “mixing state”. In June 2010, as part of the CARES field campaign, DMT single particle soot photometers (SP2) were deployed at two ground sites. T0 was located inside the Sacramento, CA urban area, while T1 was 40 km away in Cool, CA. The SP2 provides BC mass per particle, as well as information on the mixing state at the single-particle-level for a narrow range of aerosol sizes. The SP2s deployed at CARES reported mixing state information for BCA larger than ~170 nm. The incandescent lag time, defined as the time gap between the scattering signal peak and the incandescent peak for a given particle, is used as a coating indicator. An incandescent lag time of 1.5 microseconds divides thinly-coated and thickly-coated BCA, with larger values indicating thickly-coated BC cores. Meteorological conditions favored transport from T0 to T1 over certain periods, including June 23. On this day at T0, 10.5±2.1% of the morning rush-hour (7 AM – 10 AM) BCA was thickly-coated, compared to 16.9±2.0% in the afternoon (2 PM to 6 PM). However, no such variability was observed at T1, where 17.0±1.9% of the BC cores sampled from 7 AM to 7 PM were thickly-coated. The mixing state of BC at T0 was influenced by local sources as well as by a build-up of aged aerosols in the region over June 22-28, when the thickly-coated BCA number fraction reached almost 30%. Preliminary results using Gao et al. (2007)’s Leading-Edge Optimization technique indicate that at T0, BC cores smaller than 100 nm mass-equivalent diameter are thickly-coated, with a coating thickness between 60-120 nm; little coating is observed for BC cores larger than 200 nm.