Nitrogen dioxide (NO2) is one of the six common air pollutants that the U.S. Environmental Protection Agency (EPA) classifies as ‘criteria air pollutants’. Excessive exposure to NO2 can exacerbate respiratory diseases such as asthma, causing symptoms such as coughing and wheezing that can damage the lungs. In addition, the interaction of nitrogen oxides with other chemicals in the atmosphere produces acid rain, particulate matter, and ozone, which are major contributors to deteriorating air quality in urban and industrial areas. To map NO2 from space and on the ground, the authors used measurements from the European Space Agency (ESA) TROPOMI (TROPOspheric Monitoring Instrument) satellite and the U.S. EPA. These measurements were used to examine how reductions in activity related to COVID-19 impacted tropospheric NO2 in Houston, Texas.
By Mallika Gyawali, Thien Ngo, Sujan Neupane, Joshua Tickell, Antonio Gutierrez, Marissa Goncalves, Lauren Heckler, Madeline Lacamu, and Rudra Aryal
Introduction
Nitrogen dioxide (NO2) is a trace gas associated with various respiratory and environmental health issues.3 Anthropogenic and natural processes contribute to NO2 pollution in the atmosphere. Among the most significant anthropogenic processes is burning fossil fuels (64% of total emissions), while natural processes include vegetation fires, lightning, and soil erosion.
Power plants, motor vehicles, construction industries, and food manufacturing are among Texas’s most prevalent sources of tropospheric NO2 emissions. Oil & gas extraction is a highly booming industry in Texas, and all of these sources involve burning fossil fuels. According to Accounts1, Texas contributed significantly to the nation’s economy by producing the most oil (43%) and natural gas (25%). In addition, these booming industries have adversely affected the environment, human health, and natural ecosystems. Due to Texas’s migration destination status, that state experiences constant population growth, leading to increased pollution in Houston. By understanding NOx sources and atmospheric processes, air pollution could be mitigated without harming industrial and business activities.
The World Health Organization (WHO) declared a global pandemic on March 11, 2020, due to a worldwide outbreak of new Coronaviruses (COVID-19). Texas relies heavily on energy production, so its economy was at risk when COVID-19 struck. Lockdowns, quarantines, and travel restrictions limited the use of fossil fuels in daily life during this period, reducing NO2 pollution. As a result of the pandemic, atmospheric scientists had an unprecedented opportunity to study and track the amount of pollution caused by fewer human interactions with the environment.
Methodology
Authors obtained NO2 column density measurements from the European Space Agency’s (ESA) TROPOMI (TROPOspheric Monitoring Instrument) on board the Copernicus Sentinel-5 Precursor satellite. TROPOMI orbits the Earth every hundred minutes at an altitude of 817 km with a local overpass time around 13:30 and provides daily near full-surface coverage.2 With a spatial resolution of 3.5 x 3.5 km2, TROPOMI’s sensor is much more precise than other existing satellites, allowing for more accurate identification of individual NO2 plumes from space with unprecedented clarity. The data used in this study is from the TROPOMI NO2 standard product from the NASA Goddard Earth Sciences Data Active Archive Center.
Similarly, the U.S. Environmental Protection Agency (EPA) operates an air quality system (AQS) that provides ground-based continuous hourly NO2 measurements in certain locations. The EPA NO2 concentrations are expressed in PPB (parts per billion) and measured using chemiluminescence techniques. The data and measurements from the EPA are publicly available on their website.
After collecting the data, a monthly average was calculated using available data from 14 individual NO2 monitors within Houston. Then the two measurement systems were analyzed and examined to see how the COVID-19 lockdowns impacted NO2 trends and pollution in Houston.
Results and Discussion
The lockdown included a variety of restrictions, such as limiting the number of people who could gather, closing schools, and restricting travel. Businesses were permitted to reopen starting May 1st, 2020, but due to an increase in COVID-19 cases, the state of Texas suspended further reopening on June 25th.
The upper panel of Figure 1 illustrates the NO2 vertical column density (VCD) in Houston, as captured by high-resolution TROPOMI satellite data, presented in units of molecules/cm2. The data for March and April of 2019, 2020, and 2021 during the lockdown period of COVID-19 have been compared to evaluate the impact of the lockdown on NO2 emissions.
As shown in Figure 1 (top left panel), Houston, Baytown, and Pasadena were the centers of high NO2 pollution in 2019, with values as high as 6.5 x 10^15 molecules/cm^2. The Bolivar Peninsula and Alvin-Pearland, located in the southwest, had a measure of 5.2-5.8 x 10^15 molecules/cm^2. The northeastern region, like Woodville and Warren, and the southerly region, Palacios, have relatively low NO2 levels.
As shown in Figure 1, middle panel, COVID-19 significantly reduced NO2 levels in 2020, particularly in the core area of Houston city. Additionally, the darker blue and light blue regions compared to the right panel reflect the low NO2 levels compared to 2019. According to Figure 1, NO2 levels were almost returning to pre-covid levels in 2021. There is a distinct increase in NO2 in 2021 in some areas (like Sugarland and Richmond) compared to 2019, while the regions south of Angleton remain relatively clean. In Figure 1, the bottom panel shows the average VCD NO2 for March and April of 2019-2021 with box and whisker plots, which also explains the meaning of various data levels for the box and whisker plots. NO2 VCD decreased by 40% and 15% in 2020 and 2021 compared to 2019.
Figure 2 shows similar trends in the data collected from TROPOMI satellite and EPA monitors during the COVID-19 lockdown period in Texas. The upper panel of the figure presents a box-and-whisker time series of the average monthly TROPOMI tropospheric NO2 VCD data, averaged over a 0.30 x 0.30 box over Houston. The lower panel presents a similar plot for the average NO2 values in parts per billion, averaged over 14 monitors over Houston. The shaded regions in the figure denote the months of March and April, which corresponds to the COVID-19 lockdown period in Texas. The box-and-whisker plot uses the median (horizontal line) to divide the data into two sections, the middlebox represents the central 50% of the data, the lower and upper end of the whisker represents the low and high data values, respectively, and the blue dots on the box and whisker plot represent the mean values of the data points.
On average, the TROPOMI NO2 level reached its highest point in March 2019 at 0.73 x 1016 molecules/cm2, but by March 2020, it had decreased to 0.27 x 1016 molecules/cm2, a decrease of 63%. Additionally, the average EPA NO2 value was around 9 ppb in March 2019 and 6 ppb in March 2020, a decrease of 33%. This decrease can be attributed to the lockdowns, as most businesses and transportation operations were halted, showing a correlation between lockdowns and NO2 pollution.
In March 2021, the NO2 value increased to around 0.33 x 10^16 molecules/ cm^2, but it was still lower than in 2019. According to measurements by the EPA, pollution levels in 2019 were lower than in 2020 just before the lockdown but gradually decreased in March. In March 2019, the value was about 3ppb higher than in March 2020. Additionally, the peak mean in 2020 remained lower than in 2019, showing a consistent trend.
It is undeniable that the pandemic has resulted in a large number of deaths and had a negative impact on the global economy. However, the findings suggest that there have been some positive effects as well. The implementation of lockdowns during the crisis has led to improved air quality and significant environmental benefits.
References
- Accounts, T. C. of P. (n.d.). Texas’ Energy Profile. Retrieved January 3, 2023, from https://comptroller. texas.gov/economy/fiscal-notes/2022/sep/energy.php
- Griffin, D., Zhao, X., McLinden, C. A., Boersma, F., Bourassa, A., Dammers, E., et al. (2019). High-Resolution Mapping of Nitrogen Dioxide With TROPOMI: First Results and Validation Over the Canadian Oil Sands. Geophysical Research Letters, 46(2), 1049–1060. https:// doi.org/10.1029/2018GL081095
- Lamsal, L. N., Martin, R. V., Donkelaar, A. van, Steinbacher, M., Celarier, E. A., Bucsela, E., et al. (2008). Ground-level nitrogen dioxide concentrations inferred from the satellite-borne Ozone Monitoring Instrument. Journal of Geophysical Research: Atmospheres, 113(D16). https://doi.org/10.1029/2007JD009235
About the Authors
