Aerosol optical properties in the southeastern United States in summer ndash; Part 2: Sensitivity of aerosol optical depth to relative humidity and aerosol parameters

摘要

Aircraft observations of meteorological, trace gas, and aerosol propertieswere made between May and September 2013 in the southeastern United States(US). Regionally representative aggregate vertical profiles of median andinterdecile ranges of the measured parameters were constructed from 37individual aircraft profiles made in the afternoon when a well-mixed boundarylayer with typical fair-weather cumulus was present (Wagner et al., 2015). Weuse these 0–4 km aggregate profiles and a simple model to calculate thesensitivity of aerosol optical depth (AOD) to changes in dry aerosol mass,relative humidity, mixed-layer height, the central diameter and width of theparticle size distribution, hygroscopicity, and dry and wet refractive index,while holding the other parameters constant. The calculated sensitivity is aresult of both the intrinsic sensitivity and the observed range of variationin these parameters. These observationally based sensitivity studies indicatethat the relationship between AOD and dry aerosol mass in these conditions inthe southeastern US can be highly variable and is especially sensitive torelative humidity (RH). For example, calculated AOD ranged from 0.137 to0.305 as the RH was varied between the 10th and 90th percentile profiles withdry aerosol mass held constant. Calculated AOD was somewhat less sensitive toaerosol hygroscopicity, mean size, and geometric standard deviation,. However, some chemistry–climate models prescribe values of substantially larger than we or others observe, leading topotential high biases in model-calculated AOD of  ∼  25 %. Finally,AOD was least sensitive to observed variations in dry and wet aerosolrefractive index and to changes in the height of the well-mixed surfacelayer. We expect these findings to be applicable to other moderately pollutedand background continental air masses in which an accumulation mode between0.1–0.5 µm diameter dominates aerosol extinction.