Monitoring of atmospheric nitrogen dioxide by long-path pulsed differential optical absorption spectroscopy using two different light paths

Abstract

Measurements of the local distribution of atmospheric nitrogen dioxide (NO₂) by long-path pulsed differential optical absorption spectroscopy (LP-PDOAS) in Tokyo during August 2008 are presented. Two LP-PDOAS systems simultaneously measured average NO₂ temporal mixing ratios along two different paths from a single observation point. Two flashing aviation obstruction lights, located 7.0 km north and 6.3 km east from the observation point, were used as light sources, allowing spatiotemporal variations of NO₂ in Tokyo to be inferred. The LP-PDOAS data were compared with ground-based data measured using chemiluminescence. Surface wind data indicated that large inhomogeneities were present in the spatial NO₂ distributions under southerly wind conditions, while northerly wind conditions displayed greater homogeneity between the two systems. The higher correlation in the NO₂ mixing ratio between the two LP-PDOAS systems was observed under northerly wind conditions with a correlation factor R² = 0.88. We demonstrated that the combined deployment of two LP-PDOAS systems oriented in different directions provides detailed information on the spatial distribution of NO₂.