First MAX-DOAS Observations of Formaldehyde and Glyoxal in Phimai, Thailand

Multiaxis differential optical absorption spectroscopy (MAX-DOAS) observations were performed for the first time in Thailand at the rural site of Phimai (15.18 degrees N, 102.56 degrees E). Vertical profiles of eight components were retrieved separately using the Japanese MAX-DOAS profile retrieval algorithm, version 2. Here we focused on formaldehyde (HCHO) and glyoxal (CHOCHO) and evaluated their volume mixing ratios in the lowest layer (0-1km) of the retrieved vertical profiles. We estimated the ratio of CHOCHO to HCHO concentrations (R-GF), which has been suggested to be an important tracer indicative of changes in volatile organic compound emissions. Higher HCHO and CHOCHO concentrations were observed during the dry season, reflecting the influence of biomass burning. The mean R-GF for the Phimai site was estimated to be 0.0320.005. The estimated R-GF for the dry season (0.0280.002) was lower than that during the wet season (0.0400.003), when biogenic emissions were dominated. While Phimai can be characterized as an environment with low nitrogen dioxide (NO2) concentrations (<1ppbv), the potential impact of biomass burning on the lower R-GF during the dry season was investigated by the occasional enhancement of NO2 concentrations as a tracer of biomass burning.