Analysis of long-term (2005–2018) trends in tropospheric NO2 percentiles over Northeast Asia

Abstract

Atmospheric nitrogen dioxide (NO2), which has adverse effects on human health, has significantly changed due to rapid economic growth in Northeast Asia. Trends in tropospheric NO2 vertical column densities (VCDs) can be an indicator of the effectiveness of national environmental policies to reduce NO2 emissions. A number of studies have focused on long-terms trends in NO2 VCDs, mostly relying on arithmetic means obtained from various satellite sensors. The results of such studies may provide limited insights due to the skewed measurement distributions. To analyze these whole data, this study used the lowest 5th, 25th, 50th, 75th, and 95th percentiles (hereinafter referred to as B05, Q1, MED, Q3, and T95, respectively), as well as the arithmetic means, of NO2 VCDs over Northeast Asia (Northeast China, Southeast China, Republic of Korea, and Japan) retrieved from the Ozone Monitoring Instrument (OMI) from 2005 to 2018. This paper presents in priority the trends for the means, B05 (background concentrations), and T95 (high concentrations) in tropospheric NO2. All three categories showed a reversal in their trends from positive to negative at about 2011. However, the absolute results from 2005 to 2018 show the following differences: the mean and T95 decreased (NE China: −0.04 and −0.34, SE China: −0.05 and −0.32, Korea: −0.02 and −0.16, and Japan: −0.01 and −0.24 × 1015 molecules cm−2 yr−1, respectively) while B05 increased (NE China: +0.07, SE China: +0.06, Korea: +0.03 × 1015 molecules cm−2 yr−1), except in Japan (−0.01 × 1015 molecules cm−2 yr−1). We ascribe this discrepancy to a significant increase in the background concentrations (B05) of tropospheric NO2 until 2011 due to intensive industrial activities in Northeast Asia. High concentration (T95) emissions, on the other hand, have substantially decreased since 2011 partly due to strict environmental regulations in the region. These findings require further elaboration in the future using diurnal variations of tropospheric NO2 VCDs from the Geostationary Environment Monitoring Spectrometer (GEMS), as well as the consideration of socioeconomic factors (e.g., economic growth/recession, environmental regulations, etc.).