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 degrees N, 128.25 degrees 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.