Inhomogeneity of NO2 over Yokosuka, an urban site in Japan observed by MAX-DOAS

Since April 2007, continuous NO2 profile observations have been performed using ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) at Yokosuka (35.32°N, 139.65°E), an urban site in Japan. MAX-DOAS is a passive remote sensing technique using scatted visible and ultraviolet solar radiation at several elevation angles. It has been suggested that MAX-DOAS can allow us to retrieve vertical information on not only NO2 but also aerosol, H2 O, HCHO, and CHOCHO. In this study, we compare mean NO2 concentrations for 0-1 km layer derived from MAX-DOAS and surface values taken by in-situ measurements near the source region. Comparisons showed generally good agreement on seasonal, intraseasonal and diurnal time scales. However, MAX-DOAS NO2 was less than that of in-situ (surface) values particularly at higher surface values. Since the MAX-DOAS telescope used in this study was set in the azimuth direction of sea, it is expected that MAX-DOAS shows low values when the measured air masses contain information over ocean. To confirm this, MAX-DOAS NO2 was generally low when air was advected from sea. Also, for air masses measured by MAX-DOAS, the horizontal distance (depth) was estimated from the box air mass factor. We find that the concentration was generally higher when the horizontal distance was shorter, as expected. On the other hand, particularly low NO2 concentration was sometimes observed only in MAX-DOAS NO2 . For these cases, the air mass was likely vertically well mixed and the horizontal distance was long. Attention should be paid to consider such inhomogeneity to compare the MAX-DOAS measurement with in-situ surface measurement as well as to validate satellite-derived NO2 column amounts by MAX-DOAS over urban areas.