Abstract View
Using Constrained Source Apportionment to Characterize Water Soluble Brown Carbon in Los Angeles Summertime Organic Aerosol
LELIA HAWKINS, Sarah Kavassalis, Christopher Wright, Jason Casar, Benjamin Moul, Harvey Mudd College
Abstract Number: 336
Working Group: Source Apportionment
Abstract
Water soluble brown carbon (WS-BrC) and non-refractory aerosol chemical composition were quantified across two summers in Claremont, CA using collocated instruments. Claremont is located approximately 30 miles east of downtown Los Angeles in the foothills of the San Gabriel mountains. A Brechtel Particle-Into-Liquid Sampler (PILS) in series with a UV/visible wave-guide spectrometer (LWCC) and GE Analytical Total Organic Carbon Analyzer (TOC) provides continuous measurements of WS-BrC in PM1 at one minute resolution while an Aerodyne quadrupole Aerosol Chemical Speciation Monitor provides non-refractory PM1 organic and inorganic aerosol composition and loading every 10-15 minutes. Organic aerosol concentration obtained from the ACSM is well correlated to water-soluble OC and source apportionment of the OM show that a large fraction is secondary (OOA type I or II). WS-BrC was characterized by wavelength-dependent absorbance measurements from 300-700 nm, allowing more detailed analysis than a single-wavelength approach. Aerosol absorptivity at 365 nm is greatest in the morning, like HOA, with afternoon SOA formation appearing to dilute the absorptivity by the addition of non-absorbing OOA. However, absorptivity is only mildly correlated to HOA fraction of OM, indicating that a more complex set of processes governs its formation during these periods in Los Angeles. To deconvolute the observed absorbance, ACSM-derived organic aerosol spectra were first processed using the multi-linear engine (ME-2) in SoFi (Datalystica). We explored a range of model solutions from 3-6 factors. These model solutions were then used as constraints on a second factor analysis, targeting the wavelength-dependent absorbance time series. With some variation, most solutions indicate that we observed factors characterized by strong UV and weak visible absorbance (“non-absorbing”), strong UV absorbance with a tail of absorbance extending into the visible region (“typical BrC”), and a wavelength-independent BrC factor, with modest, constant absorbance extending beyond 400nm. Preliminary analysis suggests that although most WS-BrC is connected to rapidly forming chromophores associated with HOA type aerosol, other sources of WS-BrC contribute to the total observed signal in the Los Angeles basin.