Comparative analysis of atmospheric glyoxal column densities retrieved from MAX-DOAS observations in Pakistan and during MAD-CAT field campaign in Mainz, Germany

Abstract

Photolysis of glyoxal (CHOCHO) and other volatile organic compounds (VOC) in the presence of NOx results in tropospheric ozone and secondary organic pollutants formation. Glyoxal, with a relatively short lifetime, plays an important role in VOC formation in the planetary boundary layer. This study presents a comparative analysis of CHOCHO retrieval from mini MAX-DOAS observations at two different monitoring sites in Germany and Pakistan. Firstly, CHOCHO differential slant column densities (DSCDs) were retrieved by using differential optical absorption spectroscopy (DOAS) technique during a field campaign called MAD-CAT (Multi Axis DOAS-Comparison Campaign for Aerosols and Trace gases) from 18 June to 17 July 2013 in Mainz, Germany (49.965387°N, 8.242531°E). A second dataset was acquired from 18 June to 17 July 2015 at ground-based measurements taken with mini MAX-DOAS at IESE (Institute of Environmental Sciences and Engineering), NUST (National University of Sciences and Technology) Islamabad (33.6416°N, 72.9835°E), Pakistan. Tropospheric vertical column densities (VCDs) of CHOCHO were derived from measured DSCDs by using geometric air mass factor approach. Results show that CHOCHO emissions from biogenic sources are largely driven by actinic flux. Covariance of ambient temperature and relative humidity was also investigated at both sites. Significant correlation between actinic flux and CHOCHO VCDs (r > 0.8) along with similar diurnal variation was observed at both monitoring sites. Quantitative difference observed in CHOCHO VCDs is primarily triggered by the difference in actinic flux and vegetation profiles of both monitoring sites.