On the correlation between the depth of the boundary layer and the columnar aerosol size distribution

Abstract

Most inversion algorithms used to retrieve the aerosol characteristics from ground-based solar radiation measurements, assume a vertically homogeneous atmosphere and constant aerosol optical characteristics over the whole air column. This assumption leads to correct results only if the extinction of the solar radiation incident on the earth surface depends on the mean optical characteristics of the entire atmospheric column, rather than on the details of the vertical distribution of air particles. The validity of this simplifying assumption is an issue of a certain importance, which requires a more definite answer based on observational evidence. The aerosol vertical profile is significantly affected by the extent of the atmospheric boundary layer, within which the physical properties of the suspended particulate matter show sharp variations. Therefore, if the above assumption were not correct, one should observe a significant correlation between the retrieved optical characteristics and the thickness of the boundary layer measured on the same site over time. In this paper we limit ourselves to study the possible correlations between the aerosol volume size distributions and the mixing height of the atmospheric boundary layer, in order to ascertain whether there exists any dependence between the volume size distributions and the vertical distribution of the aerosol, in a site where there are strong reasons to believe that most of the existing aerosol is confined below the mixing height level. The investigation is based on a dataset of measurements taken at the Southern Great Plains base station of the ARM Program, located in Oklahoma, USA. Measurements of the columnar aerosol characteristics are taken by means of a Cimel sun-sky radiometer of the Aeronet network, while vertical profiles of pressure and temperature, used for the determination of the height of the boundary layer, are taken by vertical sounding systems. In order to find a possible relationship between the columnar aerosol size distribution and the depth of the boundary layer, a statistical analysis has been carried out on this data. The analysis has not shown any significant correlation between those two parameters. This negative result supports the validity, for the Southern Great Plains site, of the assumption that the radiation incident on the earth surface depends only on the mean optical characteristics of the entire atmospheric column, rather than on the details of the aerosol vertical distribution.