Research Notes #1: How did COVID-19 change air quality in major cities across North America?
A look at how has nitrogen dioxide changed in major cities during the pandemic.
This letter is part of a series focused on summarizing COVID-19 research that has used earth observation data to understand air quality and public health. I take fascinating research and summarize it into short, easy-to-read pieces.
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In the first letter, we look at a study by Goldberg et. al., published in Geophysical Research Letters. This is one of my favourite papers to come out of the pandemic and I highly recommend reading the paper in its entirety. The link for the paper can be found here.
In this study, the Goldberg et. al look at nitrogen dioxide columns major cities across North America, with the goal of understanding how much the pollutant concentration changed during the first major set of COVID-19 related precautions (Goldberg et al., 2020).
In case I haven’t mentioned this enough, nitrogen dioxide is a traffic-related pollutant that can result in adverse health effects for both long-term and short-term exposure times.
When conducting this research, they also developed strategies to account for the meteorological impacts on nitrogen dioxide columns. This is important to consider because the seasonal influence over nitrogen dioxide can be significant. For example, nitrogen dioxide has a shorter lifetime in the summer season than the winter season (where it lasts in the atmosphere for twice as long). This is despite the fact that emissions are typically higher in the summer season because of increased traffic and air conditioning loads (Goldberg et al., 2020). So it’s not enough to simply look at the concentration, because a supposed ‘pandemic-related’ reduction might actually be attributed to a warmer winter or spring season.
When accounting for this natural variability, the research team used three different variables: wind speed, wind direction and solar zenith angle. For those who may be unaware, the solar zenith angle is the angle between the sun’s rays and the vertical direction of the location being studied.
The paper provides information on where the data for these variables came from. The researchers also looked at the relationship between nitrogen dioxide and the climate variables over time, with interesting results. These relationships are summarized in the following points (Goldberg et al., 2020):
The nitrogen dioxide column density is higher during the cold season (where there is a higher solar zenith angle) than the warm season, which has a lower solar zenith angle .
The higher the wind speed, the lower the column density in the city (because there is more dispersion)
The impact of wind direction varies depending on the geography, terrain, and surrounding landscape.
Goldberg et. al then look at the pandemic-related changes in column density. To analyze the data, the researchers created four different methods to compare the data.
In Method 0, the researchers compare nitrogen dioxide columns during two time periods in 2020, which we can call “before” (January 1 - February 29) and “during” (March 15 to April 30) the first major set of COVID-19 precautions (Goldberg et al., 2020).
In Method 1, they compare the columns over during the “during” time period for 2019 and 2020. This was done to account for the solar zenith angle (Goldberg et al., 2020).
Method 2 is interesting. They normalize the day-to-day nitrogen dioxide columns by the four factors: solar zenith angle, wind speed, wind direction and day-of-week using a “standard meteorology,” which was represented by the climate on April 15 for each city (Goldberg et al., 2020).
Finally, in Method 3, the research team ran a GEM-MACH model with a ‘business as normal’ scenario, and compared it to the TROPOMI columns (Goldberg et al., 2020).
The paper provides a table that compares the results for each method and city. We summarize the results as follows:
Major Takeaways
Climate conditions were suitable for low nitrogen oxide and nitrogen dioxide columns (Goldberg et al., 2020).
For Method 1, when comparing the ‘during’ time frame in 2019 and 2020, the biggest reductions were in major cities across North America, with smaller to no reductions in other areas (Goldberg et al., 2020)..
The central and northwestern areas of United Stations have seen little change in nitrogen dioxide due to biogenic sources (Goldberg et al., 2020).
New York City - there was a 20% drop in nitrogen dioxide due to COVID-19 precautions (Goldberg et al., 2020)..
Los Angeles and Toronto had some of the biggest drops in nitrogen dioxide (Goldberg et al., 2020), and some of the lowest decreases were in Miami and Dallas.
I hope you enjoyed this short summary. There will be many more to come! If you have questions or comments on articles, programming, or earth observation data, feel free to leave a comment below.
References:
Goldberg, D. L., Anenberg, S. C., Griffin, D., McLinden, C. A., Lu, Z., & Streets, D. G. (2020). Disentangling the impact of the COVID-19 lockdowns on urban NO2 from natural variability. Geophysical Research Letters, 47, e2020GL089269. https://doi.org/10.1029/2020GL089269