Impacts of Aging\nand Relative Humidity on Properties\nof Biomass Burning Smoke Particles

Quantifying changes in the properties of smoke aerosols\nunder varying\nconditions is important for understanding the health and environmental\nimpacts of exposure to smoke. Smoke composition, aerosol liquid water\ncontent, effective density (ρ<sub>eff</sub>), and other properties\ncan change significantly as smoke travels through areas under different\nambient conditions and over time. During this study, we measured changes\nin smoke composition and physical properties due to oxidative aging\nand exposure to humidity. We found that smoke aging led to SOA formation\nand increases in ratios of organic carbon to elemental carbon. Aerosol\nliquid water content increased with increasing relative humidity (RH),\nand aged smoke took up more water than fresh smoke at all humidity\nlevels, likely due to a combination of changes in aerosol surface\npolarity at low and medium RH and increases in surface area with aging\nat high RH. Growth factors ranged from 1.06 ± 0.08 for fresh\nsmoke at low RH to 1.32 ± 0.08 for aged smoke at high RH. Oxidative\naging and exposure to humidity led to increases in ρ<sub>eff</sub>. For 100 nm particles, ρ<sub>eff</sub> ranged from ∼1.2\nfor fresh smoke at low RH to ∼1.6 for aged smoke at high RH.\nResults from these experiments suggest that exposure to humidity leads\nto smoke restructuring and compaction and/or changes in surface chemistry.