Trends in Measured PM2.5 Concentrations in the New York City Metropolitan Area During the COVID-19 Pandemic

June 1, 2020
Several recent news stories have reported significant decreases in nitrogen dioxide (NO2) levels in China1and parts of Europe2 due to emissions reductions during the COVID-19 pandemic. Concentrations of pollutants, such as NO2 and fine particulate matter (PM2.5) are often related to emissions associated with human activities such as vehicle traffic and industrial activity. As some of these activities have been reduced during the COVID-19 pandemic, associated emissions have likely been reduced. The question considered here is what, if any, changes in PM2.5 concentrations may have occurred during the pandemic in a major metropolitan area?

Preliminary ambient air quality monitoring data available from the U.S. Environmental Protection Agency (EPA) and the New York State Department of Environmental Conservation (NYSDEC) allow for an initial examination of how air quality, as represented by concentrations of PM2.5 from March 1, 2020 (the date of the first reported positive COVID-19 case in New York), onward compares to air quality in prior years in the New York City metropolitan area.

The EPA AirData website3 includes tools for generating multiyear tile plots of daily average PM2.5 Air Quality Index (AQI) categories for selected core based statistical areas (CBSA). These plots use color coding to represent broad categories of air quality for selected pollutants. Figure 1 shows tile plots for the New York-Newark-Jersey City, NY-NJ-PA CBSA for PM2.5. Green represents “Good” air quality (≤12 µg/m3), yellow represents “Moderate” (12.1–35.4 µg/m3), and orange represents “Unhealthy for Sensitive Groups” (35.5–55.4 µg/m3). The daily AQI value is based on the maximum 24-hour PM2.5 concentration measured over all monitors within the CBSA.
Figure 1
Figure 1: PM2.5 Daily AQI for New York-Newark-Jersey City CBSA (March 1–May 18)

The data for March through May 18 show a relatively greater proportion of green coding (Good) for 2020, suggesting an improvement in air quality relative to the same period in the five prior years.

Daily PM2.5 AQI data from AirData4 were extracted for the New York-Newark-Jersey City CBSA, converted to PM2.5 concentrations5, and then used to construct frequency distributions of PM2.5 values for the March 1–May 18 period for 2015 through 2020. The results in Figure 2 show that a far larger percentage of days from March 1 to May 18 in 2020 had PM2.5 concentrations of ≤12 µg/m3 (Good) compared to the average values in the same period over each of the prior five years. During this period in 2020, good air quality occurred on about 92% of the days. In comparison, good air quality occurred only on about 49% of the same calendar days in 2015, 70% in 2016, 60% in 2017, 55% in 2018, and 55% in 2019. Maximum daily PM2.5 concentrations in the CBSA during the pandemic are clearly lower in 2020 compared to the same period in the five prior years.

Figure 2
Figure 2: Distribution of Daily Maximum PM2.5 Concentrations in New York City-Newark-Jersey City CBSA (March 1–May 18)

The distributions shown in Figure 2 are based on the maximum daily PM2.5 concentrations measured at any monitor in the CBSA, so they represent the worst-case monitor for any individual day. A more representative assessment of regional PM2.5 levels can be determined based on the average of all measured concentrations in a region on each day. Daily average PM2.5 concentration data at 11 monitors in New York City, a subset of the CBSA, for March 1 to May 18 in the years 2015 to 2020 were calculated based on hourly data from NYSDEC6 and used to compute a mean concentration for each day. The resulting values were used to construct frequency distributions of mean citywide daily PM2.5 concentrations during this period in each year (Figure 3). The results in Figure 3 show that average daily PM2.5 values in New York City for these days in 2020 are noticeably lower than for the same period in each of the five prior years.

Figure 3
Figure 3: Distributions of Mean Daily Citywide PM2.5 Concentrations (March 1–May 18)

Finally, a comparison of daily average values at a single monitor was conducted using PM2.5 data from the City College of New York (CCNY) monitor in upper Manhattan.7 Figure 4 shows distributions of daily average PM2.5 concentrations at this monitor for March 1–May 18 for 2020 and for the same period in each of the prior five years. The lower concentrations in 2020 are readily apparent compared to other years.

Figure 4
Figure 4: Distributions of Daily PM2.5 Concentrations (March 1–May 18) at CCNY Monitor in Upper Manhattan

Our initial analysis of available PM2.5 data for the New York City metropolitan area examined maximum and average daily values over the area as well as concentrations at a single monitor in upper Manhattan. These analyses all show that levels are clearly lower starting March 1, 2020, than during the same period in earlier years. Although there are other potentially confounding factors that might account for some of the year-to-year variations, the magnitude and consistency of the differences clearly suggest that PM2.5 levels in this area are notably lower during the COVID-19 pandemic than in prior years.

Potential seasonal effects have been minimized by comparing the same calendar periods in various years. Another confounding effect could be a general downward trend in PM2.5 levels over time. However, as shown in Figure 5, a comparison of citywide average concentrations during pre-pandemic months (January and February) and pandemic months (March, April, and May) for 2019 and 2020 suggests that something more than a general downward concentration trend is in play. The distributions for the pre-pandemic months in 2019 and 2020 are remarkably similar, while the distributions for the pandemic months are quite different in 2019 and 2020. The lower concentrations in later months are likely associated with seasonal effects.

Figure 5
Figure 5: Distributions of NYC-Wide Average Daily PM2.5 Concentrations

The data presented here are still considered preliminary. They have not necessarily undergone full quality assurance by regulatory agencies, and could change in the future. As more data become available, it will be instructive to see whether this trend continues and examine whether similar changes can be seen in other areas.

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