Aqueous Photoreactions of Wood Smoke Brown Carbon

Abstract

A dominant source of light-absorbing aerosol particles, brown carbon (BrC), to the atmosphere is smoke from biomass burning. Aqueous aging of biomass burning organic aerosol can increase BrC absorbance, which may extend its atmospheric lifetime in aerosol particles, cloud droplets, and fog droplets. This study investigates the aqueous aging of biomass burning BrC and the connection between absorbance and chemical composition. The water-soluble component of laboratory-generated wood smoke BrC was analyzed using aerosol-chemical ionization mass spectrometry, liquid chromatography–mass spectrometry (electrospray ionization), UV–vis spectroscopy, and NMR spectroscopy as it was exposed to UV-B light and OH oxidation to simulate photo-oxidation in the atmosphere. During UV-B light exposure, absorbance at 400 nm increased by greater than a factor of 2 and remained high for the 6 h exposure period. A similar increase in absorbance was observed during OH oxidation, up to an OH exposure of 4 × 10–10 M·s. At a cloud water OH concentration of 1 × 10–14 M, this OH exposure corresponds to ∼11 h of aqueous OH oxidation. Further OH oxidation led to a net loss of absorbance after an OH exposure of 1.5 × 10–9 M·s (∼42 h of aqueous OH oxidation). The increase in absorbance in both cases was linked to the formation of aromatic dimer compounds and functionalized products only during OH oxidation. The loss of absorbance with extended OH oxidation correlated with a loss of aromatic compounds and breakdown to smaller molecules. These results show that aqueous aging of the biomass burning material through photo-oxidation primarily increases the absorbance of BrC and may result in longer-lived BrC in the atmosphere.

Aqueous production of secondary organic aerosol from fossil-fuel emissions in winter Beijing haze

“…The decreased light absorption by aq-SOA is consistent with loss in PAHs and aromaticity (Fig. 2 A-E) (54).…”

Diel Cycle Impacts on the Chemical and Light Absorption Properties of Organic Carbon Aerosol from Wildfires in the Western United States

“…As expected, 290 EMAC(λ) for these experiments nominally showed light absorption diminishment for most experiments, however, EMAC(λ) exceeded unity between 3.79 ± 0.05 and 17.41 ± 4.04 days. This effect of initial darkening and subsequent lightening has been reported previously (Sumlin et al, 2017a;Hems et al, 2020) extracting with purified water. Furthermore, the OH• oxidation was carried out by adding hydrogen peroxide to the extract solution and exposing it to UV lamps.…”

“…Where a through f are fit coefficients tabulated in Table S2, O3 is the ozone mixing ratio measured at the exit of the OFR (molec cm -3 ), OHRext is the external OH• reactivity (s -1 ), which was calculated from the summed products of 220 ambient VOC concentrations (measured with Vocus) and their and their OH• rate coefficients (see Data Availability section), H2O is the ambient water vapor mixing ratio (%), and τ is the residence time in the OFR. Corresponding calculated OH• exposures ranged from 3.91×10 11 to 2.53×10 12…”

Aging of Atmospheric Brown Carbon Aerosol

“…220 This sequential enhancement and bleaching suggests that the primary BrC is well-mixed even at 15 % RH, and it closely parallels behaviour observed during aqueous OH oxidation of BrC, as discussed in Section 3.1. 161,163 Enhancement may involve the formation of highly functionalized or oligomeric species from phenolic compounds like catechol, 266 which are prevalent biomass burning emissions. 277 For heated primary BrC generated under the same conditions, no initial enhancement occurs, as shown in Figure 12, suggesting that the remaining, lower volatility constituents have less capacity for absorption enhancement.…”