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

Abstract

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 118m) for three measurement days. We obtained generally decreasing NO2 VMRs with altitude, whereas increased NO2 VMRs at two elevated layers (6–19m and 19–33m) 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 6m to 19m) and in situ data, with the scatter of the correlation being within the 15% (10%) range. The relation between retrieved NO2 VMRs at 6–19m 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.