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Presenter: Hisahiroo Takashima

Seminar Title: Atmospheric aerosol and gas variations at Okinawa Island in Japan by ground-based MAX-DOAS measurement

Since March 2007, we made a continuous aerosol and gas profile measurement in the lower troposphere using a new system based on the Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) at Cape Hedo, Okinawa, Japan (26.87N, 128.25E). Cape Hedo is a site of our planning network with the primary purpose of detecting changes of tropospheric composition, including both aerosols and trace gases. The MAX-DOAS instrument measures the ultraviolet/visible spectra of scattered sunlight at various elevation angles [e.g., Irie et al., ACP, 2008]. The measured spectra are analyzed by a DOAS method and subsequently by a sophisticated retrieval algorithm utilizing the optimal estimation method and a Monte Carlo radiative transfer model to retrieve the vertical profile information of aerosol extinction coefficients (354 and 476 nm) and several trace gases (NO2, SO2, HCHO, CHOCHO, H2O, and O3) simultaneously.

In this presentation, we mainly focus on atmospheric aerosol variation using a new cloud screening method. Comparison of the MAX-DOAS aerosol measurement with ground-based lidar and sky radiometer measurements for cloud free conditions over more than one year shows good agreement between the three observations on both seasonal and intraseasonal time scales, despite different geometries. A MAX-DOAS cloud screening method based on physical properties of cloud has been developed using the MAX-DOAS relative humidity and the MAX-DOAS color index, defined by the ratio of the intensities at 500 nm and 380 nm.

This method consists of two steps: cloud screening in the troposphere is applied using the color index and cloud height is judged by the relative humidity. The former is consistent with the lidar cloud screening at 0-6 km and the latter has a strong negative correlation between the lidar cloud height and the relative humidity. Using the screening method, we investigate aerosol variations at 0-1 km. A clear annual minimum is found in August-September with a low variability related to ocean source of clean air mass, whereas a maximum is found during November-May with a large variability related to continental source of polluted air mass. This new cloud screening method can be useful particularly during winter-spring, when transboundary air pollution frequently occurred below the homogenous low level cloud in association with strong westerly wind.

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