Enhanced NO2 at Okinawa Island, Japan caused by rapid air-mass transport from China as observed by MAX-DOAS

Since March 2007, continuous NO2 profile observations have been performed using ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) at Cape Hedo (26.87°N, 128.25°E) on Okinawa Island, Japan. NO2 observations show particularly low values of less than 0.3 ppbv at 0–1 km in most cases during the northern summer, whereas higher concentrations of more than 1 ppbv were occasionally observed during winter and spring. Trajectory analysis using meteorological analysis data showed that the high NO2 concentrations were caused mainly by rapid air-mass transport in the marine boundary layer, with strong westerly winds bringing the air mass from coastal China to the study site within ∼24 h. Such rapid transport is a major cause of NO2 variations at Cape Hedo, even though the region is located approximately 700 km from China.