Hygroscopicity and cloud condensation nuclei (CCN) activity of fresh and aged biomass-burning aerosol.

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Description

Primary organic aerosol (POA) particles can act as CCN and indirectly affect climate by altering the microphysical properties of clouds. The hygroscopic nature and CCN activity of these aerosols vary with size and composition. Of the total POA emissions from combustion, 88% is estimated from biomass burning. Anoxic conditions in the core of a burning piece of biomass lead to emission of organic carbon (OC) as gas and subsequent condensation into aerosol particles. Although POA particles are often hydrophobic at emission, several atmospheric processes, collectively termed ‘aging’, change their hygroscopic nature. The purpose of this research is to identify and quantify the reactions that affect the climate-relevant properties of these ubiquitous aerosols so that their properties at emission can be explicitly linked to direct and indirect radiative forcing. We conducted bench-scale experiments on OC aerosol to determine its hygroscopicity and changes in CCN activity upon aging. OC aerosol was aged with controlled concentrations of anhydrous NH3 and O3, two common reactive trace gases in the lower atmosphere. The relative humidity (RH) during aging was controlled to simulate RH conditions in the lower atmosphere (5-85%), as water taken up by particles may participate in the aging reactions. A continuous flow streamwise thermal gradient CCN counter, a scanning mobility particle sizing system and a differential mobility analyzer (DMA) were used to obtain size-resolved activation curves for OC aged under the conditions described. We use the activation diameter (D50), the diameter at which 50% of the total aerosol particles are activated, as an indicator of CCN activity. A reduction in D50 implies greater CCN activity. We measured the size growth factors of fresh OC aerosol at RH ranging from 40 to 90%. Two hygroscopic tandem DMA tests with -Köhler model fits gave similar results of = 0.08±0.005 and = 0.07±0.018. All CCN measurements were done at a supersaturation of 0.3%. OC aged with 10 ppmv NH3 had minimal change in D50, less than 5% reduction, for either low and high RH during aging (5% and 85%, respectively). OC aged with high concentration of NH3 (875 ppmv) at 70% RH had a 23% decrease in D50. Aging with 100 ppbv O3 (RH < 5%) resulted in no change in D50 of the OC, although the slope of the activation curve was steeper than that of fresh OC. This finding indicates that activation behavior is more consistent across the particle population after aging.