This study presents the assessment of nitrogen dioxide (NO 2 ) pollution within the twin cities of Islamabad and Rawalpindi by using car MAX-DOAS (Multi Axis-Differential Optical Absorption Spectroscopy) instrument during two different scenarios of with and without the availability of CNG (compressed natural gas) fuel. Information perceived from this study can be used to get an idea about recent air quality conditions of twin cities and its repercussions on ecological and human health. International air quality monitoring field campaigns were conducted during November 2012 and December 2013. Results showed different concentrations of NO 2 (68.2, 74.25 and 93.65 ppb) at various locations of Islamabad and Rawalpindi cities, exceeded the Pak-NEQS levels of 42.5 ppb. High NO 2 concentrations can be attributed to emissions from an oil refinery, traffic congestion and solid waste dump site along IJP road, and due to non-availability of CNG during December 2013. Results compared with OMI satellite observations exhibited that NO 2 columns from OMI observations are largely underestimated.

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.