Observations of the vertical and regional variability of aerosol optical properties over central and eastern North America

Aerosol optical properties were measured in situ from a research aircraft d uring three recent field experiments in the central and eastern parts of No rth America as well as over areas of the western Atlantic Ocean. Regional a nd vertical variability of the aerosol properties for boundary layer and fr ee tropospheric air were determined. In general, the differences between di stributions of aerosol properties measured at low (planetary boundary layer ) and high (free troposphere) altitudes were small but statistically signif icant at the 3% level or better. However, most of the aerosol optical thick ness of the layers studied (similar to 5 km down to similar to 100 m) was e ncountered in the lowest 1 km of each layer. As a result, the near-surface measurements of aerosol optical properties adequately represented the porti on of the lower column that dominates the radiative effects. The estimated error encountered by using near-surface aerosol measurements to calculate t he layer forcing was typically <10%.