June 22, 2020

Ninety-one percent of the world’s population lives in a region where air pollution levels exceed World Health Organization guidelines and every year, an approximate 4.2 million deaths are due to exposure from outdoor air pollution (WHO, 2018). Currently, the COVID-19 pandemic has led to 467,000 deaths, about 9 times fewer deaths caused annually by air pollution. Air pollution can be measured by satellites in space. Nitrogen dioxide (NO₂) is a natural and anthropogenic greenhouse gas regulated by the Environmental Protection Agency and primarily emitted from burning fossil fuels such as diesel, gasoline, and coal. Chemical reactions and weather also determine the amount of NO₂ in our atmosphere. Calculating ground NO₂ emissions requires more scientific analysis; however, NO₂ changes in the troposphere (8-14 km from surface) can be used as a proxy for NO₂ levels at Earth’s surface.

Spectral radiance measurements collected by satellite sensors can be converted to NO₂ concentrations. Recently, air quality scientists from the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) have found significant reductions in NO₂ corresponding to the COVID-19 pandemic, with some cities seeing levels fall by 50% compared to the same time last year (NASA, 2020; ESA, 2020). This interactive map allows users to observe monthly NO₂ changes that were measured by the TROPOspheric Monitoring Instrument (TROPOMI), onboard ESA’s Sentinel-5P satellite. 

Stanford University Professor, Marshal Burke, decided to look into whether the lives saved from air pollution reduction caused by COVID-19 shut down exceeds the death toll from the virus itself. In a recent blog, Burke used estimates of Chinese COVID-19 deaths and determined that the number of lives saved due to pollution reductions was roughly 20x the number of lives directly lost to the virus. 

We’ve also been hearing stories of how water clarity is improving. Waterways in Venice, Italy have turned noticeably more clear from less boat traffic stirring up sediment. While a change of water clarity is just one component of water quality, a change in air quality could actually impact water quality through a process called atmospheric deposition. Wet atmospheric deposition occurs when air pollutants such as NO₂ dissolve in water vapor and fall as precipitation. Dry atmospheric deposition is the free fall of atmospheric pollutants to the Earth’s surface. NO₂ deposition directly adds to nutrient pollution and in turn can promote harmful algal blooms and ultimately, loss of oxygen in the water. Studies show that atmospheric deposition of nitrate has the same gram‐for‐gram impact on oxygen depletion as terrestrial nitrate entering via rivers (Da et al., 2018). This interactive map allows users to map and plot a time series of surface chlorophyll at different locations. Surface chlorophyll-a concentration provides an estimate of live phytoplankton biomass and is used as an indicator of water quality. Chlorophyll concentration is calculated by using remote sensing reflectance measurements collected by the MODerate Imaging Spectroradiometer (MODIS) sensor onboard NASA’s Aqua satellite. Note that MODIS’s chlorophyll algorithm does not perform well in coastal waters, but provides information on chlorophyll trends. 

While we can all agree that the pandemic is not good for human health, it may lead to improvements in air and water quality, which have advantages. So now that we know anthropogenic greenhouse gas emissions can drop, how do we keep them down? We can support policies supporting renewable energy, cleaner transportation, energy-efficient homes, power generation, industry and better municipal waste management. We can take the necessary steps to reduce our carbon footprints, and most importantly, we can vote. Find your representative here and write a letter to let them know you want them to take action to reduce fossil fuel emissions in order to save lives. 

References:

Da, F., Friedrichs, M. A., & St‐Laurent, P. (2018). Impacts of atmospheric nitrogen deposition and coastal nitrogen fluxes on oxygen concentrations in Chesapeake Bay. Journal of Geophysical Research: Oceans, 123(7), 5004-5025.

European Space Agency (ESA). 2020. Air pollution remains low as Europeans stay at home. Available online at https://www.esa.int/Applications/Observing_the_Earth/Copernicus/Sentinel-5P/Air_pollution_remains_low_as_Europeans_stay_at_home

NASA. 2020. Reductions in Pollution Associated with Decreased Fossil Fuel Use Resulting from COVID-19 Mitigation. Available online at https://svs.gsfc.nasa.gov/4810

World Health Organization (WHO). 2018. Ambient (outdoor) air pollution. Available online at https://www.who.int/news-room/fact-sheets/detail/ambient-(outdoor)-air-quality-and-health