Seasonality of the lower tropospheric ozone over China observed by the Ozone Monitoring Instrument

We analyzed the ozone (O-3) profile product (PROFOZ) derived from Ozone Monitoring Instrument(OMI) ultraviolet (UV) spectra to reveal spatial and temporal variations in O-3 distributions over China. Although discriminating vertical O-3 in the lower troposphere is a challenge for satellite-borne measurements, previous research has confirmed the reliability of the lowermost layer O-3 of the PROFOZ product, corresponding to 0-3 km, under the high O-3 condition over China by comparison with the airborne measurements. In the present study, we focus on the seasonal variation of O-3 obtained in the lowermost layer over China. We track the O-3 enhancement under polluted conditions by using the ozone anomaly (Delta O-3), defined as the difference between the retrieval values and a priori values, because our focus is the temporally high O-3 level compared with the background level. We divide the 25-40 degrees N and 100-135 degrees E region into clusters according to the similarity of the seasonal variation in Delta O-3 at the lowermost layer corresponding to approximately 0-3 km in altitude. Using this cluster analysis, we distinguish the areas in which Delta O-3 has outstanding seasonality with high values in summer, particularly in June, and low values in winter over the North China Plain and Sichuan basin. The areas with these anomalous Delta O-3 values correspond to high NO2 emission areas. We compare the results with model simulations from the Meteorological Research Institute-Chemistry Climate Model (MRI-CCM2) and meteorological data. The areas showing outstanding seasonality also correspond to those of high chemical production rates in June. Along the coastal area, Delta O-3 values tend to drop to negative in August, which can be attributed to the inflow of clean oceanic air to inland regions. The results presented here again demonstrate the reliability of the lower tropospheric O-3 data from the OMI PROFOZ product and suggest good capability of UV satellite sensors for monitoring O-3 pollution that will be launched in the near future.