Differential Optical Absorption Spectroscopy (DOAS) is a remote sensing technique to detect different trace gas concentrations in the atmosphere. The Multi Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements by the Pandora instrument scan the sky at different elevation angles and main data products include near surface concentration, tropospheric column and vertical profile for different trace gases. It addresses an important gap in near surface air quality measurements that is difficult for in-situ, satellite and other remote sensing measurements to address. Different applications of the MAX-DOAS technique have been presented in this study for improving our understanding of tropospheric chemistry and near surface air quality monitoring.

Formaldehyde (HCHO) concentration retrieved from the DOAS technique exhibits significant variation depending on the fitting parameters used. This systematic variation stems from different factors such as uncertainty in molecular absorption cross section measurement, temperature dependence of trace gas absorption, correlation between trace gases and combination of absorbers used in the DOAS fitting. To investigate the sensitivity and systematic uncertainty of HCHO retrieval, different fitting scenarios were created where fitting parameters like wavelength range, polynomial order, offset order and molecular absorption cross section were varied. To minimize systematic uncertainty and provide steady variability, the fitting scenario that most closely resembles the median of the range is selected and recommended as base fitting scenario. In addition, a real time analytical method to calculate HCHO near surface volume mixing ratio is presented where radiative transfer modelling is not required. The HCHO near surface volume mixing ratio calculated by MAX-DOAS is compared with surface HCHO measured by a ground in-situ instrument. The Pandora MAX-DOAS agrees very well with the ground in-situ instrument for the whole campaign (R²= 0.83, slope= 0.92) and provides excellent agreement for clear days (R²= 0.83= 0.88, slope=0.95). Additionally, a methodology is presented for detecting the mixing layer height (MLH) by using Pandora MAX-DOAS vertical water vapor distribution measurements. The wavelet method is applied to detect sharp gradients in the water vapor vertical profiles for estimation of mixing layer height. The Pandora derived mixing layer depth is compared to the estimations from the collocated Ceilometer (Vaisala CL51, EPA) measurements. Pandora MAX-DOAS agrees well with Ceilometer measurements for different time intervals during the day with a correlation coefficient of 0.68 to 0.76. Nitrogen Dioxide (NO₂) and Formaldehyde (HCHO) tropospheric columns and vertical profiles measured at the Hartsfield-Jackson Atlanta International Airport are also presented. Even though anthropogenic emissions decreased severely all over the United States due to Covid lockdown restrictions in 2020, trace gas levels at airports remained relatively same due to continuing air traffic. MAX-DOAS measurements are performed at different azimuth angles which gives a three dimensional representation of NO₂ and HCHO vertical profiles and enables to observe and distinguish air pollution at different directions. These measurements further show the potential of MAX-DOAS measurements for near surface air quality monitoring.