The air air pollution impression of the primary lockdown was exaggerated, as our examine reveals – Watts Up With That?
The sunset is shining over the streets of New York City during the Coronavirus lockdown in Midtown Manhattan, NYC 2020
Zongbo Shi, University of Birmingham and William Bloss, University of Birmingham
The pandemic prompted governments around the world to introduce lockdowns in early 2020, temporarily close workplaces, and empty streets and public spaces. As economic activity slowed, so did air pollutant emissions. Almost a year later, it becomes clear how this affects the air we breathe.
The easiest way to determine the impact of the lockdown on air quality is to compare the measurements taken before and after the date the lockdown started. Previous studies used this approach and reported large reductions in some pollutants such as nitrogen dioxide (NO₂). One study claimed that NO₂ emissions in Wuhan (the Chinese city where COVID-19 is believed to have occurred) fell by up to 90% at the height of the outbreak.
However, this comparison is misleading. Weather also affects pollution, for example by distributing emissions from cities. In winter, compared to spring, more fossil fuels are burned for heating, and the pollutants formed react differently in the atmosphere under different solar and temperature conditions, which means that air pollution varies between seasons. These factors obscure the impact of any single event on air pollutant concentrations.
Our new analysis looked at air pollution in the northern hemisphere in spring 2020 and adjusted it to remove the effects of weather and seasonal changes. This allowed us to isolate the effects of lockdowns on air quality alone in 11 cities: Beijing, Wuhan, Milan, Rome, Madrid, London, Paris, Berlin, New York, Los Angeles, and Delhi.
This is important because if people overestimate the benefits of the Air Quality Lockdown, they could underestimate the magnitude of the air pollution challenge in the world’s cities and fail to take the radical measures needed to bring urban air quality within healthy limits. Globally, air pollution is linked to nearly seven million premature deaths each year.
Ozone up, NO down
Our study examined the levels of NO₂, ozone (O₃) and fine particles such as soot (smaller than 2.5 micrometers; also known as PM2.5). NO₂ is emitted from vehicle exhaust, power plant chimneys and gas boilers. In contrast to the protective layer in the stratosphere 20 km above the earth, ozone close to the ground is an air pollutant that is created when hydrocarbons and nitrogen oxides (NOₓ) react in sunlight. Fine particles are emitted from various sources in industry, transport and agriculture and are small enough to be inhaled directly into the lungs. They can also be formed in the atmosphere from gaseous pollutants. All of these pollutants are harmful to health and cause a range of heart and lung diseases.
In all of the cities we examined, NO₂ levels fell during the closure, but the effect was less than what the values measured before and after would suggest. In Wuhan, for example, the measured NO₂ concentrations fell by 47% between the second and fifth week of the closure. However, some of this was due to weather and seasonal changes that would have occurred anyway. The lockdown alone accounted for 34%.
The measured NO₂ changes were closest to locations that are closest to the roads. However, the NO₂ values fell by less than the overall change in traffic suggests. This is because the number of high-emitting vehicles on roads, such as diesel-powered trucks, decreased only slightly compared to commuter traffic.
In fact, ozone levels rose in most places during the lockdown by only 2% in some places and up to 30% in others. This was mainly because traffic emissions of nitrogen oxides would normally have removed some of this ozone through reaction.
By removing seasonal and weather effects, the lockdown’s impact on air pollution can be isolated. Shi et al. (2021), author provided
The lockdown decreased PM2.5 levels in most of the cities we studied as primary emissions from road traffic and other sources decreased. However, high levels of PM2.5 were still recorded during the lockdown, particularly in Beijing, London and Paris. One possible reason is that weather patterns caused pollution to drift across cities from regions with a lot of heavy industry. Another reason is that the changing chemical nature of the atmosphere during the blockage resulted in more gaseous compounds in the air converting into these fine particles.
A window into the future
The lockdowns were an accidental global experiment that created cleaner air for many millions of people. Just lowering NO₂ levels would have brought far-reaching health benefits and, if continued, would enable most cities to meet the air quality guidelines set by the World Health Organization. However, this will have been offset by increases in ozone, and many of the changes are smaller than originally thought. This shows how big the challenge is in cleaning our air. A systematic approach to air pollution control that is tailored to each city and takes into account all types of pollutants would bring the greatest health benefits.
In a way, locks allow us to see into the future. The NO₂ changes in UK cities during the lockdown reflect expectations between 2027 and 2030 as emissions from fossil fuel vehicles phased out through electric alternatives.
While carbon dioxide (CO₂) mixes in the atmosphere around the world and can last for several hundred years, pollutants such as NO₂ stay in the air for about a day and stay close to their source. The lesson from the lockdown is that aggressive action to eliminate CO₂ sources – an international effort to solve a global problem – will also bring immediate benefits to air quality and health in your neighborhood.
Zongbo Shi, Professor of Atmospheric Biogeochemistry, University of Birmingham and William Bloss, Professor of Atmospheric Science, University of Birmingham
This article is republished by The Conversation under a Creative Commons license. Read the original article.
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