In situ measurements of water uptake by black carbon-containing aerosol in wildfire plumes

Water uptake by black carbon (BC)-containing aerosol was quantified in North American wildfire plumes of varying age (1 to similar to 40 h old) sampled during the SEAC(4)RS mission (2013). A Humidified Dual SP2 (HD-SP2) is used to optically size BC-containing particles under dry and humid conditions from which we extract the hygroscopicity parameter, kappa , of materials internally mixed with BC. Instrumental variability and the uncertainty of the technique are briefly discussed. An ensemble average kappa of 0.04 is found for the set of plumes sampled, consistent with previous estimates of bulk aerosol hygroscopicity from biomass burning sources. The temporal evolution of kappa in the Yosemite Rim Fire plume is explored to constrain the rate of conversion of BC-containing aerosol from hydrophobic to more hydrophilic modes in these emissions. A BC-specific kappa increase of similar to 0.06 over 40 h is found, fit well with an exponential curve corresponding to a transition from a kappa of 0 to a kappa of similar to 0.09 with an e-folding time of 29 h. Although only a few percent of wildfire particles contain BC, a similar kappa increase is estimated for bulk aerosol and the measured aerosol composition is used to infer that the observed kappa change is driven by a combination of incorporation of ammonium sulfate and oxidation of existing organic materials. Finally, a substantial fraction of wildfire-generated BC-containing aerosol is calculated to be active as cloud condensation nuclei shortly after emission likely indicating efficient wet removal. These results can constrain model treatment of BC from wildfire sources.