ABSTRACT
Nitrogen oxides (NOx) are major air pollutants that play a crucial role in atmospheric chemistry. We compare Ozone Measuring Instrument's (OMI) NO2 records with the in situ surface measurements from the Air Quality System of the US Environmental Protection Agency and the Texas Commission on Environmental Quality network in the state of Texas with the goal of understanding the correspondence of satellite and in situ surface observations and identifying the potential synergies between the two observing systems. Our analysis of over 40 in situ daily surface site observations, mostly from urban areas, and OMI daily observed data suggests a correlation (r) ranging between 0.2 and 0.8. The correlation improves considerably (r > 0.5) for monthly average data. Weekly variation of surface NO2 with a Sunday minimum is well captured by OMI tropospheric NO2 column observations with similar weekend reductions. NO2 trend in Texas during 2005–2019 is characterized by significant reductions of 20%–36% in highly populated cities and urban centers. However, a significant (up to 80%) increase was observed in oil and gas producing regions of the Permian and Eagle Ford Basins over the same period. In March–April of 2020, like the other US and global cities, Texas experienced up to 60% reduction in NO2 levels in major cities due to travel restrictions imposed at local and national levels to contain the spread of COVID-19. Though such reduction is temporary, these reductions were significantly larger than those achieved in the past 16 years of OMI record suggesting that technological advancement can curtail NOx emissions. © 2023. American Geophysical Union. All Rights Reserved.
ABSTRACT
Nitrogen oxides detected in urban regions are primarily emitted by transportation methods and are crucial precursors for air pollutants and climate forcers such as ozone and fine particulate matter. We investigate the trends of nitrogen dioxide (NO2) obtained from a satellite instrument and surface monitors over the megacity, Seoul, South Korea, from 2005 to 2019. Both satellite Ozone Monitoring Instrument NO2 and surface in situ concentrations decreased by up to 30% between 2015 and 2019 while significant trends were not observed between 2005 and 2015. Further analysis shows the continual reduction of NO2 concentrations prior to and during the COVID-19 pandemic in 2020. This study highlights the efficacy of South Korean pollution control policies targeting vehicular emissions. However, this study also found inconsistencies between trends observed in the official bottom-up emission inventory and data collected from space and surface sites. Further research will be urgently needed to understand the causes for the discrepancies.