Water-Soluble Organic Aerosol material and the light-absorption characteristics of aqueous extracts measured over the Southeastern United States

Abstract

Light absorption of fine particle (PM<sub>2.5</sub>) aqueous extracts between wavelengths of 200 and 800 nm were investigated from two data sets: 24-h Federal Reference Method (FRM) filter extracts from 15 Southeastern US monitoring sites over the year of 2007 (900 filters), and online measurements from a Particle-Into-Liquid Sampler deployed from July to mid-August 2009 in Atlanta, Georgia. Three main sources of soluble chromophores were identified: biomass burning, mobile source emissions, and compounds linked to secondary organic aerosol (SOA) formation. Absorption spectra of aerosol solutions from filter extracts were similar for different sources. Angstrom exponents were ~7±1 for biomass burning and non-biomass burning-impacted 24-h filter samples (delineated by a levoglucosan concentration of 50 ng m<sup>−3</sup>) at both rural and urban sites. The absorption coefficient from measurements averaged between wavelength 360 and 370 nm (Abs<sub>365</sub>, in units m<sup>−1</sup>) was used as a measure of overall brown carbon light absorption. Biomass-burning-impacted samples were highest during winter months and Abs<sub>365</sub> was correlated with levoglucosan at all sites. During periods of little biomass burning in summer, light absorbing compounds were still ubiquitous and correlated with fine particle Water-Soluble Organic Carbon (WSOC), but comprised a much smaller fraction of the WSOC, where Abs<sub>365</sub>/WSOC (i.e., mass absorption efficiency) was typically ~3 times higher in biomass burning-impacted samples. Factor analysis attributed 50% of the yearly average Abs<sub>365</sub> to biomass burning sources. Brown carbon from primary urban emissions (mobile sources) was also observed and accounted for ~10% of the regional yearly average Abs<sub>365</sub>. Summertime diurnal profiles of Abs<sub>365</sub> and WSOC showed that morning to midday increases in WSOC from photochemical production were associated with a decrease in Abs<sub>365</sub>/WSOC. After noon, this ratio substantially increased, indicating that either some fraction of the non-light absorbing fresh SOA was rapidly (within hours) converted to chromophores heterogeneously, or that SOA from gas-particle partitioning later in the day was more light-absorbing. Factor analysis on the 24-h integrated filter data associated ~20 to 30% of Abs<sub>365</sub> over 2007 with a secondary source that was highest in summer and also the main source for oxalate, suggesting that aqueous phase reactions may account for the light-absorbing fraction of WSOC observed throughout the Southeastern US in summer

Development of a sensitive long path absorption photometer to quantify peroxides in aerosol particles (Peroxide-LOPAP)

“…benzoyl peroxide, lauroyl peroxide) are extracted and converted in the test reaction. Some extracted aerosol components may absorb visible or UV-light as well, as shown by Hecobian et al (2010). To account for this interference of absorbing aerosol components on the photometric peroxide measurements, the Teflon filters were cut in two halves and one half was analysed without potassium iodide.…”

Urban impacts on regional carbonaceous aerosols: Case study in central Texas

“…BrC absorbs light from the ultraviolet (UV) to lower visible wavelengths, with absorption greatly increasing as wavelength decreases. Sources of BrC include primary emissions (e.g., incomplete and smoldering combustion), as well as products of photochemical reactions in the atmosphere (Hecobian et al, 2010;Zhang et al, 2013). The presence of BrC in atmospheric particulate matter is often determined through the spectral properties of water-soluble organic carbon (WSOC) in aqueous solutions; however, not all WSOC is BrC (Hoffer, 2006).…”

“…Absorption coefficient for WSOC absorption (b abs ) and mass absorption cross-section measurements for the United States have been made in the Southeast and on the West Coast, including Atlanta, GA (b abs ¼ 0.61 AE 0.38 Mm À1 ), and Pasadena, CA (b abs ¼ 0.88 AE 0.71 Mm À1 ), but are needed for the central United States, particularly for BrC (Hecobian et al, 2010;. Aerosol absorption measurements are needed in this region to improve regional accuracy and reduce uncertainty in global climate models (Bond and Bergstrom, 2006;Cheng et al, 2011;Thompson et al, 2012).…”

Evidence for pyrazine-based chromophores in cloudwater mimics containing methylglyoxal and ammonium sulfate

“…The solution mass absorption coefficient MAC soln as a function of wavelength was determined by UV/visible absorbance and 15 total organic carbon (TOC) measurements following Hecobian et al (2010) and Zhang et al (2011Zhang et al ( , 2013. The spectroscopic setup includes a World Precision Instruments 3000 Series Liquid Waveguide Capillary Cell (LWCC-3100) with a 94 cm optical path length and an internal volume of 250 µL, an Ocean Optics lamp (DT-Mini-2), and an Ocean Optics detector (USB 4000-UV-Vis).…”

“…Where A ref is the absorbance in the reference region of 650-710 nm where no BrC absorbance is observed and is used to correct for changes in the total light transmittance due to movement of the fiber optic cables (Hecobian et al, 2010;Zhang et al, 2011Zhang et al, , 2013 Figure 1 shows the measured absorptivity for the products of methylglyoxal/AS under all four pH starting conditions studied including pH 2, 5, 7, and 9 in both dried and capped samples ( Figure 1a) as well as the log-log plot (representing the absorption Ångström exponent, AAE) for each sample ( Figure 1b). All spectra are characterized by an absorption maximum at or near 300 nm with tails of varying intensity into the visible region.…”