AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
Sensitivity of Aerosol Optical Depth to Aerosol and Meteorological Parameters in the Summertime Continental Boundary Layer
CHARLES BROCK, Nick Wagner, Bruce Anderson, Andreas Beyersdorf, Pedro Campuzano-Jost, Annmarie Carlton, Douglas Day, Glenn Diskin, Timothy Gordon, Jose-Luis Jimenez, Daniel Lack, Jin Liao, Ann M. Middlebrook, Mathews Richardson, Rebecca Washenfelder, Andre Welti, Luke Ziemba, Daniel Murphy, NOAA Earth System Research Laboratory, Boulder, CO
Abstract Number: 30 Working Group: Aerosols, Clouds, and Climate
Abstract Earth system models that simulate aerosol processes are typically evaluated by comparing model-calculated aerosol optical depth (AOD) with satellite and ground-based remote sensing observations. However, the relationship between the primary modeled aerosol parameter, particulate mass, and AOD is a complex function of aerosol composition, size, and hygroscopicity, convolved with meteorology. To examine which factors are most important in controlling AOD, aircraft observations of meteorological, trace gas, and aerosol microphysical, chemical, and optical properties were made in summer 2013 in the southeastern United States. Representative vertical profiles were produced by aggregating data from 37 individual aircraft profiles made in the afternoon under conditions of a well-mixed boundary layer with typical fair-weather cumulus. A simple aerosol model based on these observations was used to calculate the sensitivity of AOD to changes in specific aerosol and meteorological parameters. These observationally based sensitivity calculations show that the AOD produced for a fixed column abundance of dry aerosol mass in these conditions can be highly variable, and is especially sensitive to meteorological factors such as relative humidity and the depth of the planetary boundary layer. For example, calculated AOD ranged from 0.137 to 0.305, a factor of 2.2, as the relative humidity was varied between the observed 10th and 90th percentile profiles with all other parameters, including dry aerosol mass, held constant. This range in AOD associated with variable RH may be compared with the variation in AOD of 0.082 to 0.253, a factor of 3.1, due to the observed range in dry aerosol mass holding all other parameters constant. Calculated AOD was also sensitive to the boundary layer depth, less sensitive to the observed range in aerosol hygroscopicity, mean size, and size distribution width, and least sensitive to observed variations in dry and wet aerosol refractive index. These sensitivity studies emphasize that, in addition to predicting aerosol mass, models must accurately simulate relative humidity and boundary layer structure, as well as aerosol hygroscopicity and size distribution, to properly calculate AOD and aerosol direct radiative effects.