Study of atmospheric glyoxal using multiple axis differential optical spectroscopy (MAX-DOAS) in India

Glyoxal (CHOCHO) is an important atmospheric trace gas formed by the oxidation of volatile organic compounds (VOCs). We present a study of atmospheric CHOCHO from three locations (Pune, Mahabaleshwar and Delhi) in India using Multi-Axis Differential Optical Spectroscopy (MAX-DOAS) observations. Pune and Delhi represent urban environments, whereas Mahabaleshwar represents a forested rural region. Differential slant column densities (dSCDs), vertical column densities (VCDs), surface volume mixing ratios (vmr) and vertical profiles of CHOCHO were measured and compared to satellite observations. Delhi showed the highest average CHOCHO VCD (1.57 ± 0.98 × 1015 molec. cm−2) and vmr (average of 0.43 ± 0.27 ppb), whereas Mahabaleshwar showed the lowest average VCD (0.66 ± 0.40 × 1015 molec. cm−2) and average vmr (0.08 ± 0.03 ppb) among the three sites. Average CHOCHO VCD (1.52 ± 0.66 × 1015 molec. cm−2) and vmr (0.33 ± 0.18 ppb) over Pune were close to but lower than Delhi. A large seasonal variation was observed in Pune, with the highest (1.82 ± 0.62 × 1015 molec. cm−2) and lowest (0.79 ± 0.45 × 1015 molec. cm−2) monthly average VCDs observed in January and May, respectively. This inter-annual pattern was observed in both ground-based and satellite observations. Satellite observations systematically underestimated the CHOCHO VCDs compared to ground-based observations. Comparison with previous studies shows that Pune and Delhi have similar levels of CHOCHO with respect to other urban regions, while Mahabaleshwar is comparable to other remote environments worldwide.