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.

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.

Biomass burning organic aerosol (BBOA) in the atmosphere contains many compounds that absorb solar radiation, called brown carbon (BrC). While BBOA is in the atmosphere, BrC can undergo reactions with oxidants such as ozone which decrease absorbance, or whiten. The effect of temperature and relative humidity (RH) on whitening has not been well constrained, leading to uncertainties when predicting the direct radiative effect of BrC on climate. Using an aerosol flow-tube reactor, we show that the whitening of BBOA by oxidation with ozone is strongly dependent on RH and temperature. Using a poke-flow technique, we show that the viscosity of BBOA also depends strongly on these conditions. The measured whitening rate of BrC is described well with the viscosity data, assuming that the whitening is due to oxidation occurring in the bulk of the BBOA, within a thin shell beneath the surface. Using our combined datasets, we developed a kinetic model of this whitening process, and we show that the lifetime of BrC is 1 d or less below ∼1 km in altitude in the atmosphere but is often much longer than 1 d above this altitude. Including this altitude dependence of the whitening rate in a chemical transport model causes a large change in the predicted warming effect of BBOA on climate. Overall, the results illustrate that RH and temperature need to be considered to understand the role of BBOA in the atmosphere.

The pure rotational, high-resolution spectrum of the benzoic acid-water complex was measured in the range of 4-14 GHz, using a cavity-based molecular beam Fourier-transform microwave spectrometer. In all, 40 a-type transitions and 2 b-type transitions were measured for benzoic acid-water, and 12 a-type transitions were measured for benzoic acid-D2O. The equilibrium geometry of benzoic acid-water was determined with ab initio calculations, at the B3LYP, M06-2X, and MP2 levels of theory, with the 6-311++G(2df,2pd) basis set. The experimental rotational spectrum is most consistent with the B3LYP-predicted geometry. Narrow splittings were observed in the b-type transitions, and possible tunnelling motions were investigated using the B3LYP/6-311++G(d,p) level of theory. Rotation of the water moiety about the lone electron pair hydrogen-bonded to benzoic acid, across a barrier of 7.0 kJ mol(-1), is the most likely cause for the splitting. Wagging of the unbound hydrogen atom of water is barrier-less, and this large amplitude motion results in the absence of c-type transitions. The interaction and spectroscopic dissociation energies calculated using B3LYP and MP2 are in good agreement, but those calculated using M06-2X indicate excess stabilization, possibly due to dispersive interactions being over-estimated. The equilibrium constant of hydration was calculated by statistical thermodynamics, using ab initio results and the experimental rotational constants. This allowed us to estimate the changes in percentage of hydrated benzoic acid with variations in the altitude, region, and season. Using monitoring data from Calgary, Alberta, and the MP2-predicted dissociation energy, a yearly average of 1% of benzoic acid is expected to be present in the form of benzoic acid-water. However, this percentage depends sensitively on the dissociation energy. For example, when using the M06-2X-predicted dissociation energy, we find it increases to 18%.

Restructuring of monodisperse soot aggregates due to coatings of secondary organic aerosol (SOA) derived from hydroxyl radical-initiated oxidation of toluene, p-xylene, ethylbenzene, and benzene was investigated in a series of photo-oxidation (smog) chamber experiments. Soot aggregates were generated by combustion of ethylene using a McKenna burner, treated by denuding, size-selected by a differential mobility analyzer, and injected into a smog chamber, where they were exposed to low vapor pressure products of aromatic hydrocarbon oxidation, which formed SOA coatings. Aggregate restructuring began once a threshold coating mass was reached, and the degree of the subsequent restructuring increased with mass growth factor. Although significantly compacted, fully processed aggregates were not spherical, with a mass-mobility exponent of 2.78, so additional SOA was required to fill indentations between collapsed branches of the restructured aggregates before the dynamic shape factor of coated particles approached 1. Trends in diameter growth factor, effective density, and dynamic shape factor with increasing mass growth factor indicate distinct stages in soot aggregate processing by SOA coatings. The final degree and coating mass dependence of soot restructuring were found to be the same for SOA coatings from all four aromatic precursors, indicating that the surface tensions of the SOA coatings are similar.