Continuation of Tropospheric Nitrogen Dioxide (NO2) Retrievals by using measurements from MAX-DOAS and Validate with Satellite Observations

Abstract

The monitoring and evaluation of nitrogen dioxide (NO2) vertical column densities (VCDs) in urban areas is critical owing to the negative effects of trace gases on human health and the environment. The study presents an in-depth analysis into the temporal fluctuation of NO2 column densities in Islamabad, Pakistan, from January 2021 to February 2022. Advanced ground-based instrumentation Multi Axis-Differential Optical Absorption Spectroscopy (MAX-DOAS), as well as satellite data from OMI (Ozone Monitoring Instrument) and TROPOMI (TROPOspheric Monitoring Instrument), were analyzed. The diurnal observations of NO2 indicated that the higher values were seen during the late morning and afternoon hours, mostly as a result of traffic congestion, automobile emissions and peak industrial activity. The NO2 levels exhibited an increase over the winter months due to reduced solar irradiation and colder temperatures. For the time period of January 2021 – February 2022, daily MAX-DOAS measurements have shown a positive correlation of 69% and 79% with OMI and TROPOMI, respectively. Likewise, the average monthly correlation for OMI and TROPOMI with MAX-DOAS is 86% and 93%, respectively.

Analysis also presents the validation of satellite observation through ground-based MAX- DOAS measurements for OMI and TROPOMI from June 2018-February 2022 and April

2015-February 2022. Average daily and monthly correlation of MAX-DAOS was found to be R=0.73 and 0.84 for OMI and an improved correlation of R=0.81 and 0.95 for TROPOMI. Both OMI and TROPOMI underestimated the ground-based NO2 observations due to their low spatial resolution, however, TROPOMI showed a higher correlation with the MAX-DAOS observations.