Moreover, these direct emissions are considered ‘primary pollutants’, whereas the preponderance of pollutants that are deleterious to human health are ‘secondary’ - that is, resulting from chemical processing in the atmosphere. We might even anticipate an increase in certain emissions, for example of volatile chemical products 12 from increased household and workplace cleaning. Thus, the emissions of pollutants other than NO x - such as sulfur dioxide (SO 2), ammonia (NH 3), and various volatile organic compounds (VOCs) - are likely to exhibit changes that are quite different from what has been observed for NO 2, and these differences are likely to vary from location to location. Though emissions from passenger vehicles and aviation have undeniably dropped during the pandemic, emissions from other sectors (for example, power generation, agriculture) are largely unaffected by COVID-19, as of course are emissions from natural sources (for example, plants, wildfires, dust, volcanoes). While the decline in NO 2 undoubtedly mitigates its associated health burden, it is merely one air pollutant of many human activities emit a wide range of gases and particles into the atmosphere. Thus, any COVID-19-driven decline must be disentangled from this pre-existing trajectory, as well as from meteorologically driven variability (for example, refs. As a result, in many regions of the world NO 2 is no longer a pollutant of leading concern for example, in 2019 the whole of the United States complied with the air quality standards for NO 2 ( ). Furthermore, air quality regulations in North America, Europe and, more recently, China, and resulting reductions of emissions are responsible for long-term declines in NO x (for example, refs. For example, meteorology and emissions are seasonally variant, generally leading to a decline in NO 2 from winter to spring in the Northern Hemisphere 7. While the ease with which satellite images of NO 2 are generated has led many to focus on NO x emissions changes, attributing this decline to COVID-19 is complicated by a number of factors. Reductions have been particularly stark in regions dominated by diesel vehicles, which are higher NO x emitters than their gasoline counterparts 6. NO 2 is concentrated in urban regions (due to vehicle emissions and a short chemical lifetime) and is easily observed from space, thus, satellite images have provided clear evidence of declines in populated regions in recent months, spurring commentary on improving air quality 1, 2. Pervasive declines in atmospheric NO 2 (a pollutant associated with adverse respiratory effects) through the spring of 2020 exemplify the effect of these lowered emissions. Less travel (passenger vehicles, public transit, aircrafts) reduces emissions of nitrogen oxides (NO x = NO + NO 2), a major combustion by-product. The COVID-19 pandemic has undoubtedly altered emissions, as, for example, people spend more time in their homes and less time in transit 5. ![]() ![]() Because these linkages can be highly nonlinear, atmospheric chemistry complicates how pandemic-induced emission changes have and will continue to impact air quality overlooking this chemistry undermines public understanding of air pollution, and risks erroneous decision-making. ![]() However, atmospheric chemical reactions are essential links between emissions and atmospheric composition. Much of this discussion, both in the media and in the scientific literature, has neglected the central role of chemical reactions and transformations in dictating air quality, or has suggested that the role of this chemistry is unexpected. 1, 2, 3, 4 for an updated list of papers, see ). This has propelled efforts to measure and analyse changes to air quality (defined here as the abundance of gases and particles harmful to human health), and a rush to publish scientific studies characterizing the links between the pandemic and air pollution (for example, refs. Compelling satellite images of reductions in air pollutants as governments enforced quarantine and social-distancing policies have sparked suggestions that the COVID-19 pandemic has led to cleaner air.
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