Emitted by numerous primary sources and formed by secondary sources, atmospheric brown carbon (BrC) aerosol is chemically complex. As BrC aerosol ages in the atmosphere via a variety of chemical and physical processes, its chemical composition and optical properties change significantly, altering its impacts on climate. Research in the past decade has considerably expanded our understanding of BrC reactions in both the gas and condensed phases. We review these recent advances in BrC aging chemistry with a focus on gas phase reactions leading to BrC formation, aqueous and in-cloud processes, and aerosol particle reactions. Connections are made between single component BrC proxies and more complex chemical mixtures, as well as between laboratory and field measurements of BrC chemistry. General conclusions are that chemical change can darken the BrC aerosol particles over short timescales of hours close to source and that considerable photobleaching and oxidative whitening will occur when BrC is a day or more removed from its source.

Nitrous acid (HONO) is an important component of indoor air as a photolabile precursor to hydroxyl radicals and has direct health effects. HONO concentrations are typically higher indoors than outdoors, although indoor concentrations have proved challenging to predict using box models. In this study, time-resolved measurements of HONO and NO2 in a residence showed that [HONO] varied relatively weakly over contiguous periods of hours, while [NO2] fluctuated in association with changes in outdoor [NO2]. Perturbation experiments were performed in which indoor HONO was depleted or elevated and were interpreted using a two-compartment box model. To reproduce the measurements, [HONO] had to be predicted using persistent source and sink processes that do not directly involve NO2, suggesting that HONO was in equilibrium with indoor surfaces. Production of gas phase HONO directly from conversion of NO2 on surfaces had a weak influence on indoor [HONO] during the time of the perturbations. Highly similar temporal responses of HONO and semivolatile carboxylic acids to ventilation of the residence along with the detection of nitrite on indoor surfaces support the concept that indoor HONO mixing ratios are controlled strongly by gas-surface equilibrium.

Always cite the published version, so the author(s) will receive recognition through services that track citation counts, e.g. Scopus. If you need to cite the page number of the author manuscript from TSpace because you cannot access the published version, then cite the TSpace version in addition to the published version using the permanent URI (handle) found on the record page. ABSTRACT 7Light absorbing organic aerosol particles, referred to as brown carbon, are geographically 8 widespread and can have an important climate impact through the absorption of solar radiation. 9Recent studies, both in the laboratory and the field, have shown that brown carbon aerosols can be 10 bleached of their color by direct photolysis and photo-oxidation reactions on the timescale of hours 11 to days. However, the photo-oxidation of nitrophenol molecules, which are colored compounds 12 often associated with biomass burning organic aerosol, show an enhancement in light absorption 13 before the color is lost. This study investigates the mechanism of color enhancement and the fate 14 of three nitrophenol compounds, specifically nitrocatechol, nitroguaiacol, and dinitrophenol, in 15 aqueous aerosol using online aerosol chemical ionization mass spectrometry (aerosol-CIMS). The 16 second order rate constants for the three nitrophenols with OH radicals in the aqueous phase at pH 17 2 7 (298 K), were determined to be 5 × 10 9 M -1 s -1 , 5.2 × 10 9 M -1 s -1 , and 3.7 × 10 9 M -1 s -1 for 18 nitrocatechol, nitroguaiacol, and dinitrophenol, respectively. For a representative aqueous OH 19 concentration, these rate constants correspond to an aqueous lifetime with respect to OH on the 20 order of hours. While the nitrophenol molecules react rapidly with OH, the initial products, which 21 are functionalized by additional electron-donating OH groups, likely lead to the observed 22 absorption increase in the visible range. Further photo-oxidation fragments the aromatic structure 23 to produce smaller, highly oxygenated molecules which no longer absorb strongly at visible 24 wavelengths. These products include furoic acid, glyoxylic acid, malonic acid, oxalic acid, and 25 isocyanic acid. All three nitrophenols investigated formed similar products during photo-26 oxidation, suggesting that these results could be generalized to this larger class of compounds. 27Keywords: brown carbon aerosol, biomass burning, photo-bleaching, aqueous oxidation, aerosol 28 aging 29 aldehydes, 8-10 as well as photo-oxidation of phenolic compounds. 11,12 Nitrophenols are a class of 40 light absorbing compounds often detected in significant concentrations (10s of ng m -3 ) in 41 biomass burning BrC. 13,14 These phenolic compounds are derived from pyrolysis of lignin, 14 but 42 can also be formed by photo-oxidation of anthropogenic pollutants such as xylene 15 and 43 toluene. 16 Nitrophenols are readily water soluble and have been measured in cloud, fog, and rain 44 water at concentrations of up to tens of ug L -1 . 13 45The magnitude of the climate impact of BrC d...

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.

The climate forcing of light-absorbing organic aerosol, or brown carbon (BrC), emitted from biomass burning may be significant but is currently poorly constrained, in part due to evolution during its...