Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Nitrogen dioxide (NO<span class="inline-formula"><sub>2</sub></span>) is mainly affected by local emission and meteorology rather than long-range transport. Accurate knowledge of its long-term variabilities and drivers is significant for understanding the evolution of economic and social development, anthropogenic emission, and the effectiveness of pollution control measures on a regional scale. In this study, we quantity the long-term variabilities and the underlying drivers of NO<span class="inline-formula"><sub>2</sub></span> from 2005–2020 over the Yangtze River Delta (YRD), one of the most densely populated and highly industrialized city clusters in China, using OMI spaceborne observations and the multiple linear regression (MLR) model. We have compared the spaceborne tropospheric results to surface in situ data, yielding correlation coefficients of 0.8 to 0.9 over all megacities within the YRD. As a result, the tropospheric NO<span class="inline-formula"><sub>2</sub></span> column measurements can be taken as representative of near-surface conditions, and we thus only use ground-level meteorological data for MLR. The inter-annual variabilities of tropospheric NO<span class="inline-formula"><sub>2</sub></span> vertical column density (NO<span class="inline-formula"><sub>2</sub></span> VCD<span class="inline-formula"><sub>trop</sub></span>) from 2005–2020 over the YRD can be divided into two stages. The first stage was from 2005–2011, which showed overall increasing trends with a wide range of (1.91 <span class="inline-formula">±</span> 1.50) to (6.70 <span class="inline-formula">±</span> 0.10) <span class="inline-formula">×</span> 10<span class="inline-formula"><sup>14</sup></span> molec. cm<span class="inline-formula"><sup>−2</sup></span> yr<span class="inline-formula"><sup>−1</sup></span> (<span class="inline-formula"><i>p</i><i>&lt;</i>0.01</span>) over the YRD. The second stage was from 2011–2020, which showed overall decreasing trends of (<span class="inline-formula">−</span>6.31 <span class="inline-formula">±</span> 0.71) to (<span class="inline-formula">−</span>11.01 <span class="inline-formula">±</span> 0.90) <span class="inline-formula">×</span> 10<span class="inline-formula"><sup>14</sup></span> molec. cm<span class="inline-formula"><sup>−2</sup></span> yr<span class="inline-formula"><sup>−1</sup></span> (<span class="inline-formula"><i>p</i><i>&lt;</i>0.01</span>) over each of the megacities. The seasonal cycles of NO<span class="inline-formula"><sub>2</sub></span> VCD<span class="inline-formula"><sub>trop</sub></span> over the YRD are mainly driven by meteorology (81.01 %–83.91 %), except during winter when anthropogenic emission contributions are pronounced (16.09 %–18.99 %). The inter-annual variabilities of NO<span class="inline-formula"><sub>2</sub></span> VCD<span class="inline-formula"><sub>trop</sub></span> are mainly driven by anthropogenic emission (69.18 %–81.34 %), except for a few years such as 2018 which are partly attributed to meteorology anomalies (39.07 %–91.51 %). The increasing trends in NO<span class="inline-formula"><sub>2</sub></span> VCD<span class="inline-formula"><sub>trop</sub></span> from 2005–2011 over the YRD are mainly attributed to high energy consumption associated with rapid economic growth, which causes significant increases in anthropogenic NO<span class="inline-formula"><sub>2</sub></span> emission. The decreasing trends in NO<span class="inline-formula"><sub>2</sub></span> VCD<span class="inline-formula"><sub>trop</sub></span> from 2011–2020 over the YRD are mainly attributed to the stringent clean air measures which either adjust high-energy industrial structure toward low-energy industrial structure or directly reduce pollutant emissions from different industrial sectors.

Highlights

  • As a major tropospheric pollutant, nitrogen dioxide (NO2) threatens human health and crop growth and involves in a series of atmospheric photochemical reactions (Yin et al., 2019;Wang et al, 2011;Geddes et al, 2012)

  • The inter annual variabilities of tropospheric NO2 vertical column densities (VCDs) are mainly driven by anthropogenic emission

  • We establish a multiple linear regression (MLR) model to quantify the contributions of meteorology and anthropogenic emission to the long-term variabilities of tropospheric NO2 VCDs during 2005-2020 over the Yangtze River Delta (YRD)

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Summary

Introduction

As a major tropospheric pollutant, nitrogen dioxide (NO2) threatens human health and crop growth and involves in a series of atmospheric photochemical reactions (Yin et al., 2019;Wang et al, 2011;Geddes et al, 2012). Typical space borne instruments include the SCIAMACHY, GOME, OMI, and TROPOMI, which have been widely used in scientific investigations of global nitrogen cycle, O3 formation regime, and regional pollution & transport, quantification of NO2 emissions from biomass burning regions, megacities, and industrial facilities, and validation of shipborne observations and atmospheric chemical transport models (CTMs) (Richter et al, 2005;Bechle et al, 2013;Boersma et al, 2011;Ghude et al, 2009;Lamsal et al, 2008). Using space borne observations to derive long term trends of NO2 and their drivers provides valuable information for evaluation of regional emissions, and improves our understanding of atmospheric evolutions. Ghude et al, (2009) found the same phenomenon as those of Richter et al, (2005) with GOME and SCIAMACHY observations from to 2006, which disclosed that tropospheric NO2 VCDs showed increasing trends over the rapid developing regions

OMI NO2 product
Ground level NO2 data
Meteorological fields
Multiple linear regression (MLR) model
Variabilities at provincial level
Variabilities at megacity level
Comparisons with the CNMEC data
Drivers of seasonal cycles of tropospheric NO2 VCDs
Drivers of inter annual variabilities of tropospheric NO2 VCDs
Findings
Conclusions

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