Night-time aerosol optical depths retrievals using starphotometry in Canada

Aerosol optical depth (AOD) is the most important (aerosol) radiative forcing parameter. During the day, it is traditionally measured using the well-known sunphotometry technique. Night-time AOD measurements on the other hand are sparse and historically the byproduct of astronomical observations where the AOD plays a secondary role of observation contaminant. Recently developed starphotometry techniques based on extinction measurements of bright-star radiation help to mitigate the lack of consistent and regular night-time measurements. Two starphotometers (denoted as SPSTAR03 and SPSTAR04) have been operating in Canada in intermittent fashion since 2007. SPSTAR03 was installed at a rural mid-latitude location near Egbert, Ontario (44°14’N, 79° 45’W) while SPSTAR04 was deployed in high Canadian Arctic at Eureka, Nunavut (79°59’N, 85°56’W). Several series of night-time AODs were acquired at both locations since these instruments saw first light. Sunphotometry parameters such as AOD, Angstrom exponent and fine (sub-micron) and coarse (super-micron) aerosol optical depth were derived from the star extinction measurements. Starphotometry AOD data were, on occasion, accompanied by zenith-pointing lidar backscatter data; this enabled optical consistency checks between the two types of instruments. In the case of Egbert, day-night (sunphotometry-starphotometry) AOD continuity analysis was also performed. Our presentation will summarize the findings with regard to the starphotometers’ performance and will elaborate on the principal operational difficulties difficulties which were encountered.