Significance of nitrogen dioxide absorption in estimating aerosol optical depth and size distributions

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Abstract
Concurrent measurements of spectral direct beam solar radiation and columnar nitrogen dioxide (NO2) content at a suburban site, McMaster University, in Hamilton, Ontario, were used to determine the effect of NO2 absorption upon aerosol optical depths, derived size distributions, and the columnar particle number density and mass. Aerosol optical depths were reduced by 22–47, 12–25, 3–6% and 1% at wavelengths of 400, 500, 610 and 670 nm, respectively, when the gaseous absorption was included. The smaller reductions refer to average NO2 amounts (3 × 10−3 atm‐cm) and the larger to high amounts (>6 × 10−3 atm‐cm). Columnar number density was reduced by 51–95% and mass by 13–26%. At high NO2 amounts the inclusion of NO2 absorption narrowed the size distribution derived from median optical depths and changed it from bimodal to unimodal.