To clarify three-dimensional (3-D) spatiotemporal variations and horizontal–vertical transport processes in nitrogen dioxide (NO₂) over urban areas, combined NO₂ profile observations by multiple Multi AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) and direct wind observations by 3-D coherent Doppler lidar were made over an urban area in Japan. MAX-DOAS measurements were conducted at Yakuin and Fukuoka University in Fukuoka urban area with high temporal resolution of four minutes.

Enhanced NO₂ concentrations were often observed over the city center. We conducted a case study on 29 November 2018 under clear sky conditions and NO₂ profiles were well retrieved. Using MAX-DOAS at two locations, high NO₂ concentrations were observed near the surface of the city center in the morning. Higher NO₂ concentrations appearing gradually at higher altitudes over the city center and disappearing in the afternoon are explained using direct evidence of a 3-D wind field: an airmass with a high NO₂ concentration was transported upward over the city center. Then, the airmass was advected landward by a sea breeze. Multiple MAX-DOAS combined with 3-D Doppler lidar constitutes a powerful tool for elucidating horizontal–vertical transport processes. It can contribute to the improvement of data retrieval by satellites for urban areas.