Long-term trends and trend reversal detection in two decades of tropospheric NO2 satellite observations

In this work, observations from four different satellite sensors GOME/ERS-2, SCIAMACHY/ENVISAT, GOME-2/Metop-A and GOME-2/Metop-B are used to study the tropospheric NO2 patterns and trends during the period 4/1996-9/2017. The GOME and GOME-2 data are "corrected" according to the SCIAMACHY observations to compile a self-consistent global dataset. The highest concentrations are observed over urban, industrialized and highly populated areas and over ship tracks in the oceans. It is shown that tropospheric NO2 has generally decreased during the last two decades over the industrialized-highly populated regions of the so-called "Western World" (average decrease of the order of 49% over the U.S., the Netherlands and the U.K., 36% over Italy and Japan and 32% over Germany and France) and increased over developing regions (average increase of 160% over China and 33% over India). Tropospheric NO2 is very sensitive to socioeconomic changes (e.g. environmental protection policies, economic recession, warfare, etc.) which may cause either short term changes or even a reversal of the trends. The application of a method that detects the year when a trend reversal happened reveals that tropospheric NO2 trends switched from positive to negative and vice versa over several regions around the globe. A country-level analysis revealed clusters of countries that exhibit similar positive-to-negative and negative-to-positive reversals. In addition, 29 out of 64 examined megacities and large urban agglomerations experienced a trend reversal at some point within the last two decades.