The application of MAX-DOAS to the measurement of tropospheric gases and aerosols in marine and continental environments

Multi Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) is a passive DOAS technique that uses scattered sunlight to determine differential slant column densities (DSCDs) of trace gas absorbers along multiple axes. MAX-DOAS measurements probe long path lengths, and when coupled with a radiative transfer model (RTM) to determine the average light path travelled, MAX-DOAS has the potential to yield trace gas vertical column densities (VCDs), aerosol optical depths, and trace gas and aerosol layer heights. Determination of DSCDs and VCDs have advantages over point-source measurements in that they are more sensitive to the total atmospheric load of a pollutant, and are relatively insensitive to variations in the boundary layer height. Two Canadian field campaigns: one from Saturna Island, BC, located in the Strait of Georgia; the other from Ridgetown, ON, in southwestern Ontario; employed a MAX-DOAS instrument to obtain spectra that were analyzed to yield DSCDs of NO2, O4 and HCHO. The measured spectra from Ridgetown were also compared to RTM calculations to yield NO2 VCDs, aerosol optical depths, and gas and aerosol layer heights. The method of determining NO2 VCDs in this way was validated for the first time by comparison to composite VCDs derived from aircraft and ground-based measurements of NO2, satellites, and a chemical model. The usefulness of the MAX-DOAS technique was extended further, using both DSCDs and VCDs, to include the interpretation of pollutant transport at Saturna and Ridgetown, and to provide an example of fumigation of elevated industrial pollutants brought to the surface at a lake breeze front at Ridgetown.