Rapid methods for inversion of MAXDOAS elevation profiles to surface-associated box concentrations, visibility, and heights: application to analysis of Arctic BrO events

Abstract. Multiple Axis Differential Optical Absorption Spectroscopy (MAXDOAS) is a remote sensing technique that measures surface-associated trace gas profiles using simple automated instrumentation that requires very low power and is deployable at remote sites. However, the analysis of MAXDOAS data is complex and often cannot be applied rapidly or consistently over long measurement periods. Here we present three transparent methods to analyze MAXDOAS data. The box profile method finds the best trace gas layer height and surface-associated vertical column density (VCD) to simultaneously fit oxygen collisional dimer (O₄) and trace gas differential slant column density (dSCD) observations. The elevated viewing method estimates the surface-associated VCD from observations at high view elevations, such as 10° and 20°. The horizon viewing method estimates the surface concentration of a trace gas by using near-horizon view trace gas and O₄ data. We apply these methods to a two-month data set and show that the methods retrieve information 80% of the time and provides a consistent time series. Surface-associated trace gas VCD observations by the elevated viewing method correlate (r² > 0.93) with the box profile method with slopes within 15% of unity. Surface-associated concentration observations from the horizon viewing method correlate well (r² > 0.90) with the box profile method and a slope within 4% of unity. Application of these retrieval methods to UV-absorbing trace gases other than BrO is straightforward, and application in other spectral regions is discussed. These methods provide rapid and comprehensive inversions of MAXDOAS spectral data that are useful during field campaigns, as well as, verification of more complex (e.g. optimal estimate inversion) methods.