Files
Abstract
Brown carbon (BrC) have significant but poorly constrained impacts on the climate. The major source of BrC is incomplete combustion of biomass fuels, where the combustion conditions (temperature and air-to-fuel ratio) are poorly controlled, leading to large variations in reported BrC light-absorption properties. In order to investigate the dependence of BrC light-absorption properties on combustion conditions, we performed controlled combustion experiments with model fuels (toluene and benzene) and obtained the mass absorption cross-section (MAC) and absorption Ångström exponent (AAE) of the BrC emissions at different combustion conditions. We found that more efficient combustion (higher temperature and/or higher air-to-fuel ratio) generated more absorptive (darker) BrC. Furthermore, regardless of fuel type and combustion conditions, the emitted aerosols exhibited a unified continuum of light-absorption properties that can be characterized by MAC at 532 nm (MAC532) and AAE pairs. The pairs were well-correlated with the elemental carbon-to-organic carbon ratio (EC/OC), which is a proxy of combustion conditions.
BrC is subject to atmospheric processing by gas-phase oxidants such as the nitrate radical (NO3). We investigated the evolution of chemical composition and light-absorption properties of BrC due to heterogeneous reaction with NO3. We generated BrC from the controlled combustion of toluene and oxidized it with NO3 in an oxidation flow reactor. We found that the heterogeneous reaction of BrC with NO3 led to significantly enhance BrC light absorption by the addition of chromophoric functional groups (e.g., nitro group). However, condensation of gas-phase PAH+NO3 oxidation products that were less-absorbing than the particulate PAHs can counterbalance this enhancement.
Differences in online and offline optical measurement approaches could also lead to discrepancies between different studies. We generated BrC with variable light-absorption properties from the controlled combustion of toluene, isooctane, and cyclohexane, and compared the light-absorption properties of BrC aerosol retrieved from online method with those of their methanol and dichloromethane (DCM) extracts retrieved from offline method. We found that the DCM has higher extraction efficiency than methanol for the same BrC aerosols since DCM can extract more mass and darker BrC aerosol. Furthermore, we found that solvent-extraction techniques can substantially underestimate BrC light-absorption properties.
BrC is subject to atmospheric processing by gas-phase oxidants such as the nitrate radical (NO3). We investigated the evolution of chemical composition and light-absorption properties of BrC due to heterogeneous reaction with NO3. We generated BrC from the controlled combustion of toluene and oxidized it with NO3 in an oxidation flow reactor. We found that the heterogeneous reaction of BrC with NO3 led to significantly enhance BrC light absorption by the addition of chromophoric functional groups (e.g., nitro group). However, condensation of gas-phase PAH+NO3 oxidation products that were less-absorbing than the particulate PAHs can counterbalance this enhancement.
Differences in online and offline optical measurement approaches could also lead to discrepancies between different studies. We generated BrC with variable light-absorption properties from the controlled combustion of toluene, isooctane, and cyclohexane, and compared the light-absorption properties of BrC aerosol retrieved from online method with those of their methanol and dichloromethane (DCM) extracts retrieved from offline method. We found that the DCM has higher extraction efficiency than methanol for the same BrC aerosols since DCM can extract more mass and darker BrC aerosol. Furthermore, we found that solvent-extraction techniques can substantially underestimate BrC light-absorption properties.