Exploring the temporal trends and seasonal behaviour of tropospheric trace gases over Pakistan by exploiting satellite observations

Air pollution has emerged as a key environmental issue of Pakistan as its major cities are ranked among top polluted cities in the South Asian region. Due to lack of adequate ground-based monitoring facilities, satellite observations are the only viable source for long-term assessment of air quality over the region. This study analyses the atmospheric concentrations and decadal changes derived from multiple satellite instruments of four important tropospheric trace gases over Pakistan: surface concentrations of carbon monoxide (CO), tropospheric columns of formaldehyde (HCHO), nitrogen dioxide (NO2) and ozone (O3). High levels of all these trace gases were observed over Punjab region, which may be attributed to its metropolitan importance. It is the major agricultural, industrialized and urbanized region of the country. Seasonal Mann-Kendall (SMK) test is used to calculate the statistical significance of temporal trend during the last decade (2005–2014). Surface CO showed a significant decrease of about 13 ± 1% over the decade. Tropospheric column ozone (O3) exhibited a significant increasing trend with a temporal increase of 10.4 ± 1.2%/decade whereas NO2 showed a significant temporal increase of about 28 ± 2%/decade. However, an insignificant increase of about 8.3 ± 0.3%/decade is estimated for HCHO over the decade. A distinct seasonal behaviour is observed among trace gases. The long-term relationship among trace gases is further explored by statistical tests. It is found that O3, NO2 and HCHO have a significant impact on the long-term decrease of CO whereas; the trend in O3 critically depends on NO2 levels along with CO over the region. A statistical model is developed to estimate the long-term association of trace gases over the region. The model indicates that O3 production may have major influence on decrease of CO over Pakistan in a (volatile organic compound) VOC-limited regime