The short answer: Yes, but it’s complicated

Some cities that have implemented stay-at-home measures to reduce the spread of COVID-19 are enjoying cleaner air (particularly lower NO2 levels), but not all of them.

Pollution can still be high even when everyone is staying home because there are lots of different pollutants in the air that come from many different sources. Some types of pollution also travel long distances (ex: particulate matter) while others stay in the atmosphere for long periods of time (ex: ground level ozone). This means that, even if everyone has stopped driving around in the city, there are other sources and types of pollution that can cause poor air quality. 

The medium answer: more details on those pollutants

Good or bad air quality is determined by how many pollutants are in the air and how concentrated they are. There are several types of pollutants, they come from different sources, and they behave differently. For example: 

Nitrogen Dioxide (NO₂) is a pollutant that has been the most analyzed pollutant during the COVID-19 stay-at-home measures. This gas is closely linked to human activity in cities (car exhaust, heating, etc.) and has a high dispersion rate – it clears from the air very quickly and doesn’t travel far. 

Ground-level ozone (O₃) is also a gas and is closely linked with NO₂. The production of ground level ozone involves a reaction between exhaust gases (NO₂) and UV rays. This is particularly interesting in the context of the stay-at-home measure being implemented across the world. Not only is ozone produced by NO₂, it is also destroyed by it. Broadly speaking, there exists an equilibrium relationship between the two pollutants. Lower NO₂ emissions means lower ozone production – but less ozone destruction, thus a longer lifetime in the atmosphere. Since NO₂ is the pollutant most reduced by confinement measures, we have been seeing some significant ozone spikes over the last few weeks.

Particulate matter (PM1, PM2.5, PM10), on the other hand, comes from things burning (internal combustion engines, wood stoves, toasters, etc.) but also from sources like agricultural processes, construction, etc. Particulate matter can travel for incredible distances in the wind and can cause pollution peaks even when you would think there should be clean air. 

Our Plume Air Quality Index is designed to give you an indication of the overall air pollution level and is driven by the pollutant most harmful to your health at that time, so if NO₂ is down, but ozone is up, the AQI will still show high pollution. 

The Plume AQI has seven levels of pollution, or thresholds. These thresholds are linked to the exposure limits outlined by the World Health Organisation. Each category represents the amount of time it is safe to spend in that level of pollution. For example: one year (PAQI < 20), one day (PAQI <50), one hour (PAQI <100). 

In practice, this means that if my average daily exposure exceeds 50 Plume AQI, I will start to affect my health. 

The seven levels of pollution are:

Plume Labs AQI	Description	Colour
0-20	Clean Air
21-50	Moderate Pollution
51-100	High Pollution
101-150	Very High Pollution
151-200	Excessive Pollution
201-300	Extreme Pollution
301+	Very Extreme Pollution

The long answer: how we figured it out 

Pollution levels in Europe

We took a look at two cities in Europe that have taken stay-at-home measures as part of their efforts to slow the spread of COVID-19: Madrid and Paris. We focused specifically on NO₂ which is the urban pollutant most directly linked with people’s movement in the city as a major source of this gas is car and truck exhaust. In all three cities we observed a significant drop in NO₂ levels following the implementation of stay-at-home and social distancing rules.

Check out the interactive maps below to see the street-by-street differences.

However, these drops in NO₂ do not necessarily mean that the air is always cleaner. For example, in Paris we saw some serious peaks of ground-level ozone and particulate matter during the ongoing, city-wide shut-down. There are several reasons for this. In the case of particulate matter, wind can carry this ultra-fine dust for miles and miles. In this instance, the pollution transference came from agricultural activities outside of the city. Ground-level ozone is an even more interesting and complicated case (More detailed info on ozone is found below) because of its link to NO₂.

What about levels in the USA?

We then did the same analysis for three cities in the USA: Chicago, Los Angeles, and New York.

The difference between the European cities we looked at, and those in the US is quite pronounced. In New York we found a very small drop in NO₂ emissions. In Chicago there is a more pronounced reduction, but still nothing compared to those cities in Europe. In LA, interestingly we found slightly higher levels of NO₂ in our ‘after’ analysis – particularly around Saint James Park and Chinatown. One explanation for this increase is the share-ratio of emissions from cars vs trucks which is 9% for cars and 50% for trucks. This means that only 9% of emissions are caused by individuals driving in their cars, while 50% of emissions come from trucking traffic. Therefore a decrease in car traffic, as a result of stay-at-home measures, can easily be offset by a slight increase in deliveries and truck traffic.

What else is contributing to unexpected pollution levels?

Sunny days: This when ground-level ozone (O3) really drives pollution levels into the danger-zone. If you missed the bit above about how it is produced, have a look, it’s pretty interesting.

The inversion layer: Generally speaking, the atmosphere gets colder as altitude increases. However in some cases, temperatures can actually increase at higher altitudes. This can occur when, for example, a warmer, less-dense air mass moves over a cooler, denser air mass. This heat creates a “lid” of warm air in the atmosphere, called an inversion layer. The inversion layer prevents pollutants from dispersing, meaning they stay trapped closer to the ground.

Wind: Wind helps increase the dispersion of gasses like NO₂ coming from car and truck exhaust. Particulate matter is also carried by the wind. This can be good or bad, depending on which way the wind is blowing. For example, in Paris, we saw a dramatic reduction of NO₂ since the stay-at-home measures have been enacted, but there was a significant spike in particulate matter that was carried from agricultural operations into the city by the wind.

Rain: Precipitation can clear the air of particulate matter. As the rain falls, the water droplets bind with the particles floating in the air and bring them down to the ground where we won’t be breathing them in.

Conclusion

One thing remains clear as we continue our mission to clear the air: cleaning up air pollution is a complicated problem and understanding how it’s created and moves around the world is critical if we’re going to build on the improvements we’ve seen over the last little while.