Abstract
In the current era, more than half of the world’s population lives in cities and this trend is on the rise after the industrial revolution since 1850 which led to the expansion of urban areas and gave rise to the formation of megacities. As a result of the enhanced industrial and power generation activities as well as traffic loads, megacities face severe problems of air pollution. Lahore, the second largest city of Pakistan with a population of more than 11 million is no exception to this. During the last few years, Lahore has been ranked among the topmost polluted cities in the world since last few years. In the current is thesis, results of simultaneous measurements of four important tropospheric trace gases: NO2, SO2, HCHO and CHOCHO by using the car Multi Axis Differential Optical Absorption Spectroscopy (car MAX-DOAS) in and around Lahore are presented. Measurements were performed on many days during 5 campaigns in different years and seasons. The spatial distribution of tropospheric vertical column densities (VCDs) is retrieved from the observations. By combining the measurements with wind information the total emissions/fluxes of NO2, SO2 and HCHO from the city of Lahore are estimated. In this study, the data analysis is based on previous studies, but also important improvements were developed and implemented. In particular, a modified approach was introduced to calculate the VCDs derived from the spectra obtained during campaigns when the instrument motor did not work properly. The new approach was tested based on data when the motor worked properly, and good consistency was found between the modified and the standard methods. Also a correction method is developed to deal with the problem of spectral interference of O3 with SO2 at high solar zenith angles (SZA). The new method improved the data quality and allowed to analyze data also at high SZA Both improvements are important for future studies. In the following the most important measurement results are summarized. In general, enhanced values of tropospheric VCDs of the trace gases were observed in the city center and along the areas of enhanced industrial activities. The derived emissions of NOx and the underlying NO2 VCDs were found comparable with those in other large cities like Shanghai, China and Delhi, India. For the other trace gases only the tropospheric VCDs could be compared to similar measurements at other locations. Here much higher SO2 VCDs were found compared to other large cities like Hefei, Nanjing or Shanghai (China). For HCHO similar values were observed as in other polluted regions, like e.g. Yangtze River Delta, China, (YRD). The CHOCHO VCDs around Lahore were found comparable to the glyoxal VCDs at other polluted regions like e.g. Beijing, China. No prominent seasonality was observed for the NOx emissions and HCHO fluxes. In contrast, a clear seasonal variability was observed for SO2 with emissions in spring approximately two times higher than in summer. The glyoxal VCDs were found higher in summer than in spring by about a factor of two. The car MAX-DOAS results were also compared with TROPOMI satellite observations and the bottom-up emission inventory EDGAR. A good qualitative agreement (R2 = 0.56 and 0.62) was observed with the tropospheric NO2 VCDs from TROPOMI. However, like in previous studies, the satellite data was found around 40 to 50% lower than the car MAX-DOAS results. For SO2 and HCHO the comparison of the car MAX-DOAS results to TROPOMI was not performed on an individual satellite pixel basis because the satellite results are close to or below the detection limit. For both trace gases a similar range of tropospheric VCDs was found in the averaged satellite and car MAX-DOAS data. The NOx emissions estimated from the car MAX-DOAS observations were also compared with the emissions estimated from the TROPOMI satellite observations. Here, consistent with the VCD comparison, an underestimation of 30-65% was found. In contrast, the comparison with the EDGAR emission inventory, showed very good agreement within 5-15%. For the SO2 emissions larger differences (36-47%) were found indicating that the SO2 emissions from Lahore in EDGAR data base probably need to be updated accordingly. This research has established an unprecedented insight into the spatio-temporal variability of the studied four trace gases over Lahore. The results of this thesis added new and significant information on the air quality and the related trace gas emissions for the megacity of Lahore. These new findings can help to improve the air quality of the city by devising better policies and the improved analysis techniques will also be useful for the execution of similar studies in other megacities
