Water-Soluble Secondary Organic Aerosol, Brown Carbon and Oxalic Acid in the Southeastern United States

RODNEY J. WEBER (1), Arsineh Hecobian (1), Xiaolu Zhang (1), Mei Zheng (1), Neil Frank (2)

(1) Department of Earth and Atmospheric Sciences, Georgia Institute of Technology, 30332, Atlanta, USA (2) Office of Air Quality Planning & Standards, US EPA, Research Triangle Park, NC, USA

     Abstract Number: 330
     Last modified: May 7, 2010

Abstract
One possible route to forming secondary organic aerosol (SOA) is thought to involve partitioning of semi-volatile organic compounds to liquid water, followed by heterogeneous chemical transformation. In laboratory experiments, such as those with isoprene, heterogeneous aerosol formation and transformation pathways that may occur in clouds or fog drops have been shown to form oligomers and various organic acids, with oxalic acid being a predominant product. Other studies demonstrate that aqueous phase reactions of dicarbonyls (an isoprene oxidation product) produced oligomers and colored organics (i.e., brown carbon).

Ambient measurements of the water-soluble organic carbon (WSOC) fraction of SOA, soluble brown carbon (e.g., light absorption spectra of aqueous extracts), organic acids and aerosol source tracers were consistent with this SOA formation mechanism. Based on 24-h integrated filter measurements at 15 sites throughout the Southeastern U.S., a region known for extensive biogenic and anthropogenic VOC emissions, a PMF analysis identified a factor that linked WSOC, brown carbon and oxalic acid. This factor was highest in summer and was the largest single contributor to WSOC, accounting for approximately 50 percent of the yearly (2007) regional average. Additionally, online measurements of WSOC and light absorption spectra using a Particle-Into-Liquid Sampler (PILS) suggest a chemical evolution to brown carbon: Diurnal averages showed that WSOC freshly formed before midday was not colored, but some fraction became brown later in the day. This paper will present results from studies where WSOC, soluble brown carbon and organic acids are used to investigate possible routes leading to the formation of SOA.