Light absorption properties of water soluble organic aerosol from Residential Wood Burning in Fresno, CA: Results from 2013 NASA DISCOVER-AQ Campaign

Abstract:

Light absorption properties of water soluble organic aerosol were investigated at Fresno, CA from 13 January to 11 February, 2013 as part of the NASA DISCOVER-AQ campaign. The light absorption spectra of water soluble organic aerosol in PM_2.5 was measured using a UV/vis diode array detector (DAD) coupled with a particle into liquid sampler (PILS) that sampled downstream of a PM_2.5 cyclone (URG). The PILS was also coupled with two ion chromatographs (IC) to measure inorganic and organic ionic species in PM_2.5. In addition, an Aerodyne High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) was deployed at the same site to measure size-resolved chemical composition of submicrometer aerosol (PM₁) in real time during this study.

Light absorption at 365 nm (Abs₃₆₅), which is typically used as a proxy of water-soluble brown carbon (BrC), showed strong enhancement during night time and appeared to correlate well (r = 0.71) with biomass burning organic aerosol (BBOA) from residential wood burning for heating in the Fresno area. The tight correlations between Abs₃₆₅ and biomass burning relevant tracers such as acetonitrile (r = 0.69), AMS-signature ions for phenolic compounds (r = 0.52-0.71), PAH (r = 0.74), and potassium (r = 0.67) further confirm that biomass burning contributed significantly to water soluble brown carbon during this study. The absorption angstrom exponent (Å_a) values fitted between 300 and 700 nm wavelength were 3.3 ± 1.1, 2.0 ± 0.9 and 4.0 ± 0.8, respectively, in the morning, afternoon and nighttime, indicating that BrC is prevalent at night in Fresno during wintertime. However, there are also indications that small amount of BrC existed during the daytime as well, likely due to daytime wood burning and other sources such as the formation of light-absorbing secondary organic aerosol (SOA). Finally, light absorption at 300 nm, 330 nm, and 390 nm were found to correlate tightly with BBOA, which indicate that biomass burning also emits chromophores that are absorbing at these wavelengths. Literature reviews and analysis of the HR-ToF-AMS organic aerosol spectra suggest that nitrogen-containing organic compounds from biomass burning emissions seem to contribute to aerosol absorption at these wavelengths. However, the properties and potential impacts of these compounds remain unknown.