Development of new retrieval method of tropospheric ozone from MAX-DOAS measurements and its application to long-term MAX-DOAS measurements in China and Germany for the validation of satellite observations and model results of tropospheric ozone

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Tropospheric ozone (O3) plays an important role in controlling the tropospheric chemical composition through various reactions with OH radicals, CO, CH4, NOx, and hydrocarbons. It is a greenhouse gas and pollutant detrimental to human health and crop and ecosystem productivity. However the lack of observations of vertical profiles of tropospheric obscures studies on its impact on the environment, climate, health and ecosystems. Recently developed products of tropospheric ozone derived from satellite observations and chemical transport modelling have large uncertainties and urgently need credible measurements of vertical profiles of tropospheric ozone for validation. Multi axis DOAS (MAX-DOAS) is a state of the art ground-based remote sensing technique to retrieve tropospheric vertical profiles and columns of trace gases (e.g. NO2, BrO, SO2, HCHO, HONO, and CHOCHO) from UV and visible spectra of scattered sunlight. However due to the strong influence of stratospheric O3 on MAX-DOAS measurements, still no satisfactory algorithms for the retrieval of tropospheric ozone profiles from MAX-DOAS measurements exist. In our recent preliminary study, we found that this difficulty could be addressed by the use of external data of stratospheric ozone (Method 1) or by utilising the temperature dependence of the ozone absorption cross section (Method 2), and two corresponding retrieval algorithms of tropospheric ozone from MAX-DOAS measurements have been developed. In this study, we will further improve the Method 1 to make the retrieval independent from external data sets by the use of stratospheric ozone also derived from the MAX-DOAS measurements at high solar zenith angle. The improved method will be validated during two campaigns in China and Germany by comparing them with results from ozone sondes, O3 lidars, Fourier transform spectrometry as well as in-situ O3 measurements. The new method will be evaluated and further improved for situations of strong haze pollutions (low visibility) in China. Then the new method will be adapted and applied for the analysis of long-term measurements of several MAX-DOAS instruments located in Germany and China. A data base of tropospheric ozone profiles will be generated based on the MAX-DOAS measurements in the North China Plain region and Yangtze River delta region, China and in Mainz, Germany. The data base will be used to validate satellite products and model simulations of tropospheric ozone, and characterize its temporal-spatial variation, the effects of regional transport, and to identify sources of tropospheric ozone. In addition to method 1, also method 2 (utilising the temperature dependence of O3 absorptions) will be further improved.