UV-VIS Formaldehyde Retrieval Correction for Snow Covered Regions

Abstract:

Retrieval of trace gases in the ultraviolet and visible requires precise air mass factor (AMFs) calculations to convert slant columns to vertical column densities. One of the key parameters in AMFs calculations is surface reflectance. Traditionally, operational retrievals have relied in monthly surface albedo climatologies with modest spatial resolutions. These climatologies are unable to provide accurate a-priori surface reflectance information for regions covered by snow and ice.

Here we present a correction scheme for snowy scenes in the ultraviolet and the results of applying it to OMI formaldehyde observations over Canada and Europe. The methodology leverages different observations and techniques. After classifying each satellite pixel as either covered by snow/ice or not using the Interactive Multisensor Snow and Ice Mapping System (IMS) product, the MODIS Snow cover product and the Sea Ice Index to establish we derive its surface reflectance. The surface reflectance for each pixel is obtained by spatially averaging daily MODIS Bidirectional Reflectance Distribution Function (BRDF) retrievals. When daily retrievals are not available for a particular date we rely on a multiyear monthly BRDF climatology. Finally we use a probabilistic statistical method to extrapolate BRDF values from the spectral region covered by MODIS band 4 (459 – 479 nm) to the spectral region where AMFs for formaldehyde retrievals are calculated (340 nm).