Retrieving the aerosol single-scattering albedo from the NO 2 photolysis rate coefficient

Abstract

A method for retrieving the aerosol single-scattering albedo ω 0 in the ultraviolet region close to 400 nm within the atmospheric column is proposed based on a comparison between both NO 2 photolysis rate coefficients J NO2 measured by an optical actinometer under cloud-free conditions and J NO2 calculated using a radiative transfer model. This methodology is based on the sensitivity of J NO2 to ω 0 variations as quantified using the radiative transfer model with particular emphasis on the aerosol extinction vertical profile. The method has been applied to assess ω 0 within the framework of the Indian Ocean Experiment (INDOEX) over the Goa area located on the west coast of India. In the first place, the period under study was between March 2 and March 13, 1999 when ground-based lidar measurements were available. It has been further extended between February 25 and March 23, 1999 by making appropriate assumptions on the mean altitude of the aerosol layer. Taking into account the uncertainties related to each parameter introduced in the calculation (optical thickness, Ångström coefficient, asymmetry factor, surface albedo) and the model used, resulted in a total absolute accuracy on ω 0 calculated to be 0.06 for observations close to local noon, and up to 0.07 for observations performed within 3 h before and after local noon. Results are less accurate for other times. For the 12 days under study, the values of ω 0 calculated at noon range from 0.89 to 0.93. They vary weakly throughout the day with further differences being observed from one day to another. Such small variations in ω 0 are in very good agreement with results obtained by other methods within the framework of the same campaign. DOI: 10.1111/j.1600-0889.2004.00093.x