Long-term Tropospheric ozone column data record from GOME to TROPOMI using CCD algorithm

long-term tropospheric ozone time series has been generated for the tropical band (20°S to 20°N) based on convective cloud differential algorithm (CCD). Tropical tropospheric ozone columns were retrieved from several European sensors starting with observations by GOME in 1995 and including data from SCIAMACHY, OMI, GOME-2A and GOME-2B. The algorithm was developed within the ESA CCI project has been updated regularly.The time series has now been extended by DLR with data from GOME-2C and TROPOMI and now encompasses 25 years. The tropospheric ozone retrieval for all data sets is based on the total columns retrieved with the GODFIT algorithm and associated cloud products.

There are however some differences between the different tropospheric columns from the different sensors which have to be corrected for. The operational TROPOMI tropical tropospheric ozone data have a much higher spatial and temporal resolution. Therefore the data are averaged to 1.25° x 2.5° and 1 month to match the resolution of the other sensors. For the CCD time series, we used SCIAMACHY data as reference and fitted an offset and a trend correction to the data of the other sensors. For the sensors that do not have a temporal overlap with SCIAMACHY we used the harmonized version of GOME-2A and OMI as reference. We estimated the trend based on the merged long-term time series. For the tropics an overall trend of +0.9 DU/decade was found in the data set until 2021, varying locally between -0.5 and 1.8 DU/decade. We also looked for seasonal trends, although the uncertainty in the trend analysis increases. The strongest increase in tropospheric ozone was found for March-April-May and the smallest trends occurred during Oct-Dec. Besides the long term trend the data might also be used to investigate the influence of single events like extreme bio mass burning seasons.