Oral or Poster

IAMAS

M02 - Atmospheric Composition and the Asian Monsoon

NO2 and HCHO Vertical Column Densities in two Polluted Megacities, Bangkok (Thailand) and Dhaka (Bangladesh), from Pandora Measurements

In this study, NO2 and HCHO vertical column densities (VCDs) in Bangkok (Thailand) and Dhaka (Bangladesh), two densely populated megacities, were quantitatively assessed through year-round ground-based hyperspectrometer Pandora observations as part of the Pandonia Global Network (PGN). Although NO2 and HCHO VCDs in Bangkok and Dhaka exhibited similar seasonal variations, the annual mean NO2 VCD in Dhaka (1.16 DU) was twice as high as in Bangkok (0.58 DU). We noted that the NO2 VCDs observed at background clean sites in South Asia were below 0.2 DU (e.g., Nainital: 0.17 DU, Agam: 0.08 DU). The diurnal variation of NO2 VCDs in Bangkok exhibited a peak around 10:00 LST, followed by a decrease until 14:00 LST, and then a secondary increase appeared from the late afternoon until sunset due to traffic emissions. In contrast, Dhaka showed a delayed peak around 12:00 LST, attributed to longer durations of traffic congestion. Elevated NO2 VCDs during the dry winter season at both sites, peaking especially in December, are thought to be result from increased local emissions due to brick firing in brick kilns and crop residue burning. Contrary to NO2, HCHO VCDs showed similar values in the two megacities (0.64 DU in Dhaka and 0.57 DU in Bangkok), exceeding levels at other sites by approximately 1.5-2 times (Bandung: 0.47 DU, Singapore: 0.37 DU, Nainital: 0.29 DU, and Agam: 0.24 DU). HCHO VCDs in Bangkok started to steadily increase after sunrise, peaked around 12:00 LST, and then gradually decreased throughout the afternoon. In contrast, no distinct diurnal variation of HCHO VCDs was apparent in Dhaka. Peak concentrations of HCHO VCDs appearing in April can be explained not only by increased emissions due to vegetation activity but also by transport from forest fires.