Retrieval of Glyoxal during MAD-CAT Campaign in Mainz, Germany and IESE-NUST, Islamabad, Pakistan

Abstract

Glyoxal being the tiniest α-dicarbonyl forms during VOC oxidation reactions. It has residence time of few hours determined by photolysis and hydroxyl radical reactions during day time. In the presence of NOx, glyoxal photolysis results in the formation of secondary organic pollutants along with the tropospheric ozone. This is a kind of pioneer study for glyoxal retrieval in Pakistan. It presents a comparison of glyoxal retrieval from mini MAX-DOAS measurements at two different monitoring sites in Pakistan and Germany. First data set was obtained during MAD-CAT (Multi Axis DOAS-Comparison Campaign for Aerosols and Trace gases) campaign held from 18 June–13 July, 2013 in Mainz, Germany. Second data set was generated from ground based MAX-DOAS observations at IESE-NUST (Institute of Environmental Sciences and Engineering - National University of Sciences and Technology), Islamabad, Pakistan. Glyoxal differential slant column densities (DSCDs) were retrieved using DOAS (differential optical absorption spectroscopy) technique. And by using geometric air mass factor approach, tropospheric vertical column densities (VCDs) were derived from measured DSCDs. Firstly, retrieved glyoxal was compared for the month of June and July over both measurement sites in respective years. Covariant of solar irradiation, ambient air temperature and relative humidity were investigated. A significant correlation (r > 0.8) was observed for actinic flux and glyoxal diurnal variabilities at both study sites. A quantitative difference in glyoxal VCDs observed was mainly due to difference in actinic flux and vegetation profiles of both monitoring sites. Seasonal cycle of glyoxal was investigated for data set of IESE from October-2014 to September 2015. It exhibited good correlation with metrological parameters (solar radiation, ambient temperature and relative humidity).