Variability of Aerosol Optical Properties at Four North American Surface Monitoring Sites

Abstract

Aerosol optical properties measured over several years at surface monitoring stations located at Bondville, Illinois (BND); Lamont, Oklahoma (SGP); Sable Island, Nova Scotia (WSA); and Barrow, Alaska (BRW), have been analyzed to determine the importance of the variability in aerosol optical properties to direct aerosol radiative forcing calculations. The amount of aerosol present is of primary importance and the aerosol optical properties are of secondary importance to direct aerosol radiative forcing calculations. The mean aerosol light absorption coefficient (σap) is 10 times larger and the mean aerosol scattering coefficient (σsp) is 5 times larger at the anthropogenically influenced site at BND than at BRW. The aerosol optical properties of single scattering albedo (?o) and hemispheric backscatter fraction (b) have variability of approximately ±?3% and ±?8%, respectively, in mean values among the four stations. To assess the importance of the variability in ?o and b on top of the atmosphere aerosol radiative forcing calculations, the aerosol radiative forcing efficiency (?F/δ) is calculated. The ?F/δ is defined as the aerosol forcing (?F) per unit optical depth (δ) and does not depend explicitly on the amount of aerosol present. Based on measurements at four North American stations, radiative transfer calculations that assume fixed aerosol properties can have errors of 1%?6% in the annual average forcing at the top of the atmosphere due to variations in average single scattering albedo and backscatter fraction among the sites studied. The errors increase when shorter-term variations in aerosol properties are considered; for monthly and hourly timescales, errors are expected to be greater than 8% and 15%, respectively, approximately one-third of the time. Systematic relationships exist between various aerosol optical properties [σap, ?o, b, ?F/δ, and Ångström exponent (å)] and the amount of aerosol present (measured by σsp) that are qualitatively similar but quantitatively different among the four stations. These types of systematic relationships and the regional and temporal variations in aerosol optical properties should be considered when using ?climatological? averages.