Investigation of vertical column densities and profiles of water vapor using look-up table method with MAX-DOAS

Abstract: Water vapor is one of the crucial greenhouse gases in the atmosphere, and the accurate determination of its concentration and vertical profiles is essential for investigating regional water cycling, and climate-environmental changes. This study investigated a method for retrieving atmospheric water vapor vertical column densities (VCDs) and profiles using the Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) technique. The approach involves a two-step inversion: first, aerosol extinction profiles are inverted, followed by the inversion of water vapor profile information. A parameterized look-up table method is employed during profile inversion, minimizing reliance on prior information. The study focused on the Huaibei region, using MAX-DOAS to retrieve O₄ absorption aerosol profiles. By minimizing the cost function and employing the look-up table method, water vapor profiles are inverted, including parameters such as H₂O VCD, water vapor weighting factors in the boundary layer (ω), and boundary layer height (h). Results indicated that, during the observation period in the Huaibei region, water vapor was  primarily concentrated below 1 km, gradually decreasing with altitude. The comparison of the H₂O VCDs retrieved using the look-up table method with the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 model and geometric approximation demonstrated a strong agreement in diurnal trends (correlation coefficients: 0.93 and 0.98, respectively). In order to understand the sources of water vapor in different vertical layers in the Huaibei region, a 24-hour backward trajectory clustering analysis was conducted using the HYSPLIT model based on the observed wind fields during the monitoring period. The results indicated that at 500 meters altitude, water vapor primarily originated from the southeast direction, while at 1 km and 2 km altitude, the dominant source of water vapor was from the southwest direction. The study demonstrates the successful inversion of tropospheric water vapor vertical column densities and profiles, providing reliable technical support for obtaining regional water vapor information.