Light Absorbing Properties of Primary and Secondary Brown Carbon in a Tropical Urban Environment.

Brown carbon (BrC) has significant climatic impact, but its emission sources and formation processes remain under-represented in climate models. However, there are only limited field studies to quantify the light absorption properties of specific types of primary and secondary organic aerosols (POAs and SOAs) in different environments. This work investigates the light absorption properties of the major OA components in Singapore, a well-developed city in the tropical region, where air quality can be influenced by multiple local urban sources and regional biomass burning events. The source-specific mass absorption cross-section (MAC) and wavelength dependence of different BrC components were quantified based on highly time-resolved aerosol chemical composition and absorption measurements. In particular, the combustion-related emission sources were the primary contributors to BrC light absorption and they were moderately absorbing. The SOA materials, which were freshly formed under atmospheric conditions with industrial influences, were also moderately light absorptive. The aged SOA components that were composed of aged regional emissions, including biomass burning and coal combustion emissions from nearby regions, were weakly light absorbing, highlighting the possibility of photobleaching of BrC during their atmospheric aging and dispersion. Lastly, our estimations illustrate that typical urban POAs and SOAs can contribute up to approximately 36-58% of the BrC absorption, even in some urban locations that are influenced by biomass burning emissions.

Effect of Relative Humidity on Secondary Brown Carbon Formation in Aqueous Droplets.

Atmospheric brown carbon (BrC) is a significant contributor to particulate light absorption. Reactions between small aldehydes and reduced nitrogen species have been shown to produce secondary BrC in atmospheric droplets. These reactions can be substantially accelerated upon droplet evaporation. Despite aqueous droplets undergoing continuous water evaporation and uptake in response to the surrounding relative humidity (RH), secondary BrC formation in these droplets under various RH conditions remains poorly understood. In this work, we investigate BrC formation from reactions of two aqueous-phase precursors, glyoxal and methylglyoxal, with ammonium sulfate or glycine in aqueous droplets after drying at a range of RH (30-90%). Our results illustrate, for the first time, that BrC production varies as a function of RH. For all four chemical reaction systems being investigated, mass absorption efficiencies (MAE, m2/g C) of aqueous aerosol products (from 270 to 512 nm wavelength range) generally increase with reducing RH to reach a maximum at ∼55-65% RH and subsequently decrease, caused by further drying. Chemical characterization using high-resolution aerosol mass spectrometry shows that the formation of nitrogen-containing organic species also follows a similar variation with RH. Our observations reveal that the acceleration of BrC production from evaporation of water may be diminished by other factors, such as limited particle-phase water content, phase transition, and volatility of reactants and products. Overall, our results highlight that intermediate RH conditions in the atmosphere may be more efficient in secondary BrC formation, indicating that the effect of RH needs to be included in atmospheric models for a more accurate representation of light-absorbing aerosol formation in aqueous droplets.