First-time remote sensing of NO2 vertical distributions in an urban street canyon using Topographic Target Light scattering Differential Optical Absorption Spectroscopy (ToTaL-DOAS)

Topographic Target Light scattering Differential Optical Absorption Spectroscopy (ToTaL-DOAS) measurements were performed in a street canyon in Gwangju, Korea (35°10′51.43″N, 126°52′53.74″E) during a period of 10 days in October 2010. NO2 slant volume mixing ratios (VMRs) at the lowest elevation angle (EL) of 1°, as obtained from ToTaL-DOAS measurements, were validated via comparisons with collocated in situ ground data, showing good agreement within a scatter range of 20% during the entire measurement period, and within a scatter range of 15% when cloudy days were excluded. For the first time, based on inversion calculations using ToTaL-DOAS data, we retrieved NO2 vertical distributions that consist of five layers (from 6 to 118 m) for three measurement days. We obtained generally decreasing NO2 VMRs with altitude, whereas increased NO2 VMRs at two elevated layers (6–19 m and 19–33 m) were observed on 26 and 27 October. We obtained a coefficient of determination (R2) of 0.87 (0.61) between slant VMRs at 1° EL (retrieved NO2 VMRs in the layer from 6 m to 19 m) and in situ data, with the scatter of the correlation being within the 15% (10%) range. The relation between retrieved NO2 VMRs at 6–19 m and slant VMRs at 1° EL yielded an R2 value of 0.97, with the scatter of the correlation being within the 5% range. The results demonstrate that the ToTaL-DOAS technique is a useful tool in identifying the temporal and vertical characteristics of NO2 VMRs in an urban street canyon with a complex geometry.