Atmospheric aerosol variations at Okinawa Island in Japan observed by MAX-DOAS using a new cloud-screening method

Atmospheric aerosol profile observations using Multi Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) have been conducted at Cape Hedo (26.87°N, 128.25°E), the northernmost point of Okinawa Island in Japan, since 30 March 2007. Comparisons of aerosol extinction at 476 nm by MAX-DOAS with ground-based lidar measurements for cloud-free conditions over more than 1 year showed good agreement on both seasonal and intraseasonal time scales, with differences of less than 30% on average for 0-1 km. Agreement between aerosol optical depths retrieved by MAX-DOAS and the sky radiometer was also observed during the same period, with differences of less than 30% in most cases. A cloud-screening method using MAX-DOAS data based on the physical properties of clouds was developed to evaluate aerosol variations, focused primarily on transboundary air pollution below low-level clouds, using relative humidity derived from MAX-DOAS H₂O measurements and the MAX-DOAS color index, defined as the ratio of the intensities at 500 and 380 nm. This method consists of two steps: cloud screening in the troposphere using the color index and cloud-base height determination from the relative humidity. The former was consistent with lidar cloud screening at 0-6 km, and for the latter, a strong negative correlation between the lidar cloud-base height and the relative humidity was found. Using this unique cloud-screening method, we investigated aerosol variations at 0-1 km. A clear annual minimum was found in August-September, with low variability in relation to oceanic sources of clean air masses, whereas the maximum was found in November-May, with large variability in relation to continental sources of polluted air masses. This new cloud-screening method can be useful for evaluating aerosols below clouds, particularly during the northern winter at Cape Hedo, when and where transboundary air pollution from the Asian continent below low-level clouds frequently occurs in conjunction with strong westerly winds.