AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Changes in Visibility and Local Radiative Forcing in the Southeast U.S. Linked to Decreased Aerosol Sulfate Mass
ALEXIS ATTWOOD, Rebecca Washenfelder, Weiwei Hu, Nick Wagner, Allison McComiskey, Pedro Campuzano-Jost, Douglas Day, Brett Palm, Suzane Simoes de Sa, Charles Brock, Eric Edgerton, Karsten Baumann, Jose-Luis Jimenez, Steven G. Brown, CU CIRES- NOAA ESRL
Abstract Number: 252 Working Group: Air Quality and Climate in the Southeast US: Insights from Recent Measurement Campaigns
Abstract Sulfate aerosol is a major contributor to PM2.5 mass in the United States and impacts direct and indirect aerosol radiative forcing, visibility and health. The mass concentration of aerosol sulfate has decreased in the southeast U.S. by 3.9(±0.8)% yr-1 between 1992 – 2013, consistent with the 60% reduction in nationwide SO2 emissions from 1990 – 2010 following the Clean Air Act Amendment. During the Southeastern Oxidant and Aerosol Study (SOAS), which took place in the summer of 2013, we deployed a novel, broadband cavity enhanced spectrometer (BBCES) to measure relative humidity dependent aerosol extinction in the ultraviolet as a function of wavelength. The aerosol composition in this region is dominated by ammonium sulfate, which along with organics contributes 60 - 90% of surface PM2.5. Using submicron aerosol optical extinction and composition data, we determine a relationship between the enhancement in aerosol extinction at elevated relative humidity, γext, and the mass fraction of organics, which shows decreased water uptake and reduced optical extinction at lower sulfate and higher organic mass fraction. We calculate that the reduction in particle hygroscopicity has caused a -1.6(±0.3)% yr-1 change in aerosol optical extinction from 1998-2013 based on changes in chemical composition alone. This change in ambient extinction is separate and in addition to the change in extinction of -3.1(±0.8)% yr-1 that we calculate as a direct result of decreased aerosol mass over the same period. Direct measurements of visibility show an improvement between 1998 – 2013, which is consistent with these expectations. Radiative forcing calculations of the decreased aerosol extinction and resulting increased surface solar irradiance will also be presented.