Skip to Content

Brown Carbon Production and Loss in Atmospheric Clouds: Dynamic Changes due to Droplet Drying, pH, and Photolysis

Active Dates 8/15/2021-8/14/2024
Program Area Atmospheric System Research
Project Description
This work directly address U.S. Department of Energy Atmospheric System Research priority research objectives related to both aerosol-cloud interactions and aerosol processes. The proposal addresses critical knowledge gaps about the life cycle and climate impacts of BrC. The proposal focuses on processes in clouds because these are difficult to test with atmospheric models, at present, at least in part due to a lack of observational data. The objectives address BrC formation and loss processes identified as important by laboratory studies conducted with aerosol and cloud water mimics; however, recent research from our group has demonstrated that these mimics may not be adequate to represent actual processes that occur in the atmosphere due to simplifications in matrix composition, unrealistic concentrations, or both. Thus, this work represents a significant innovation in that all experiments will be conducted with atmospheric cloud water.  The cloud samples will come from different sites, collected across multiple years, and will be selected to provide diverse conditions and source influences to enable the broad translation of these results to other locations. 
Light-absorbing organic compounds present in aerosols, collectively called brown carbon (BrC), contribute important but highly uncertain effects on climate. Among absorbing species present in aerosols, BrC is unique because it is both emitted directly into the atmosphere (called primary BrC) and formed from gas- and aqueous-phase reactions (called secondary BrC). It is also unique because reactions initiated by oxidants and ultraviolet (UV) light can rapidly transform molecules into non-absorbing (or more weakly absorbing) in a process called bleaching. Clouds likely represent a significant medium for the production of secondary BrC and for a variety of bleaching reactions, though the relative importance of formation and loss processes in clouds is not known.The acidity (or pH) of atmospheric particles and clouds affects the optical properties of BrC and bleaching rates, although the link between pH and BrC is yet another uncertainty in attempts to constrain its climate forcing effects. Given the wide variability of pH in the atmosphere (pH in particles and clouds ranges from -1 to 8), the optical properties of BrC and its bleaching behavior are expected to vary significantly in the atmosphere as well. The objective of this proposal is to enable better representations of the life cycle and optical properties of BrC in atmospheric models. The specific aims of this proposal are to: 1) characterize BrC formation in atmospheric cloud droplets that undergo drying, 2) characterize the pH-dependence of BrC absorbance in atmospheric cloud water, and 3) quantify the effects of photolysis on the evolution of BrC in atmospheric cloud water.These objectives will be accomplished through experiments using real cloud water samples collected from Whiteface Mountain (New York) and Schmücke Mountain (Germany)--two well established sites for cloud chemistry research.The behavior of BrC in each experiment will be linked to specific sources using cloud water composition and airmass trajectory analysis.Most clouds do not precipitate, so modeling will be conducted to understand how pH-dependent optical properties of BrC in cloud droplet residuals change under a variety of air mass trajectories. Complementing the laboratory experiments and modeling, evidence for all three processes in Amazonia will be probed through analysis of GoAmazon 2014/2015 field campaign data. The Amazon represents an ideal location to study these processes, as well, given the rich dataset associated with GoAmazon and the distinct seasonal characteristics of emissions and meteorology in the region.
Award Recipient(s)
  • University of Maryland Baltimore County (PI: Hennigan, Christopher)