Long-term (2005–2015) trends analysis of OMI retrieved NO ₂ columns in Taiwan

In this study, the monthly data retrieved from the Ozone Monitoring Instrument (OMI) have been used to estimate long-term trends of NO ₂ (2005–2015) in the entire Taiwan as well as in five representative city areas of Taiwan. The long-term NO ₂ data have been used for trend analysis and evaluated to find the extent of the effects of NO _x emission reduction and related environmental regulations on the NO ₂ trend. Desirable policies and strategies for further reducing NO _x emission have been also discussed. The corresponding reductions are 15–37% for the five city areas and 24% for the entire Taiwan over the 10-yr span. Clearly, the governmental policies/measures enacted during this 10-yr period are responsible for the reduction of tropospheric NO ₂ column values. The OMI NO ₂ column values of four cities (i.e., Taipei, Taichung, Kaohsiung, and Douliu) are relatively highly correlated (r = 0.70–0.95), indicating the cities have similar emission sources. Furthermore, the correlations between observed OMI NO ₂ columns and their emission NO _x are all significant in the three metropolitan cities and the entire Taiwan (r = 0.66–0.89). The correlation of OMI-based NO ₂ VCDs and ground-based NO ₂ levels is high in Kaohsiung (r = 0.82) and moderate in Taipei and Taichung (r = 0.60 and 0.59, respectively). The satellite retrieved OMI NO ₂ data clearly indicate their usefulness in evaluating decreasing trend of the NO ₂ , a precursor gas of PM _2.5 during the 10-yr span for the entire region of Taiwan and for the selected five cities.