Retrieving of particulate matter from optical measurements: A semiparametric approach

The fine particle abundance, i.e., particle matter (PM) concentration, is one of the indicators of air quality and is therefore subject to ground-based measurements. Complementary satellite aerosol remote sensing techniques provide one with maps of the aerosol optical thickness (AOT), which is sensitive to particle abundance. This paper investigates the problem of retrieving the PM concentration from the AOT, both on daily average values, on the basis of a large data set where data from the air quality networks are combined with ground-based measurements of the AOTs. It is found that a linear model fails at explaining the data well but that the performance may be significantly improved when such a linear relationship is conditioned on auxiliary parameters, mainly meteorological variables. The proposed model is expressed as an additive varying coefficient model (AVCM), which is defined as a linear model where the coefficients are additive functions of the auxiliary parameters. The model is represented using penalized smoothing splines, allowing for a proper control of the overall number of degrees of freedom via multiple smoothness parameters selection. The methodology is applied to data collected around Lille (France). The PM10 concentrations are retrieved with an average uncertainty of less than 20%, leading to a correlation coefficient of 0.87 between fitted and expected PM10.