Retrieving the aerosol single-scattering albedo from the NO2 photolysis rate coefficient

Abstract

A method for retrieving the aerosol single-scattering albedo ω0 in the ultraviolet region close to 400 nm within the atmospheric column is proposed based on a comparison between both NO2 photolysis rate coefficients JNO2 measured by an optical actinometer under cloud-free conditions and JNO2 calculated using a radiative transfer model. This methodology is based on the sensitivity of JNO2 to ω0 variations as quantified using the radiative transfer model with particular emphasis on the aerosol extinction vertical profile. The method has been applied to assess ω0 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, Ångstróm coefficient, asymmetry factor, surface albedo) and the model used, resulted in a total absolute accuracy on ω0 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 ω0 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 ω0 are in very good agreement with results obtained by other methods within the framework of the same campaign.