key: cord-311827-jfdlb2g8
authors: Chen, L.-W. Antony; Chien, Lung-Chang; Li, Yi; Lin, Ge
title: Nonuniform impacts of COVID-19 lockdown on air quality over the United States
date: 2020-07-21
journal: Sci Total Environ
DOI: 10.1016/j.scitotenv.2020.141105
sha: 
doc_id: 311827
cord_uid: jfdlb2g8

Abstract Most of the state governments in United States (U.S.) issued lockdown or business restrictions amid the COVID-19 pandemic in March 2020, which created a unique opportunity to evaluate the air quality response to reduced economic activities. Data acquired from 28 long-term air quality stations across the U.S. revealed widespread but nonuniform reductions of nitrogen dioxide (NO2) and carbon monoxide (CO) during the first phase of lockdown (March 15–April 25, 2020) relative to a pre-lockdown reference period and historical baselines established in 2017–2019. The reductions, up to 49% for NO2 and 37% for CO, are statistically significant at two thirds of the sites and tend to increase with local population density. Significant reductions of particulate matter (PM2.5 and PM10) only occurred in the Northeast and California/Nevada metropolises where NO2 declined the most, while the changes in ozone (O3) were mixed and relatively minor. These findings are consistent with lower transportation and utility demands that dominate NO2 and CO emissions, especially in major urban areas, due to the lockdown. This study provides an insight into potential public health benefits with more aggressive air quality management, which should be factored into strategies to reopen the U.S. and global economy.

The ongoing pandemic of coronavirus disease has created challenges for governments around the world to balance public safety and economy. In the U.S., following the national emergency declared by President Trump on March 13, 2020, states and municipalities have issued various degrees of lockdown and/or stay-at-home policies suiting local specific conditions (Lin et al., 2020) . Such policies impact air quality through, most notably, declined "non-essential" transportation and energy consumption (Le Quéré et al., 2020) . Among the criteria air pollutants (CAPs), the U.S. Environmental Protection Agency (EPA)'s national emissions inventory attributes 74% of nitrogen oxides (NO x , sum of nitrogen dioxide [NO 2 ] and nitric oxide [NO] ) and 59% of carbon monoxide (CO) emissions to on-and off-road traffic and electric generation (U.S. EPA, 2016) . Ambient levels of the two pollutants might be most affected by the lockdown, compared with primary PM 2.5 and PM 10 (particulate matter with aerodynamic diameters below 2.5 and 10 micrometers, respectively) of which only 10% and 4% result from traffic and electric generation. Ozone (O 3 ) is formed in the atmosphere through photochemical reaction of NO x and volatile organic compounds (VOCs). Reduced NO x and VOCs emissions could either lower or lift O 3 concentrations depending on the local photochemical regime (Sillman and He, 2002) .

The unprecedented situation of COVID-19 pandemic creates an opportunity to assess the contribution of transportation and commercial activities to local air quality and the potential outcome of more stringent emission regulations. Such assessments have been carried out for many large cities around the world (Kerimary et al., 2020; Nakada and Urban, 2020; Sharma Journal Pre-proof J o u r n a l P r e -p r o o f 4 et al., 2020; Tobías et al., 2020) , but the evidence in the US is lacking. This paper analyzed data from long-term air quality monitoring stations across the U.S. and estimated reductions of CAPs during the first phase of extensive lockdown. Findings can inform future modeling studies that attempt to capture policy outcomes by simulating state-wise emission reductions.

Such information is also important for the post-pandemic air quality management.

The EPA National Core (NCore) network tracks long-term trends of CAPs across the U.S. (Scheffe et al., 2009) . Daily NCore data for January 1, 2020 -April 30, 2020 were acquired from Airnowtech (https://www.airnowtech.org/) and cross-verified with those reported to the U.S. EPA AirData website (https://www.epa.gov/outdoor-air-quality-data). The six weeks or 42 days between March 15 and April 25, 2020 was designated as the first-phase lockdown period (P1), as many states began restricting businesses and schools in the week of March 15 but relaxed the restrictions somewhat coming into May 2020 (Lin et al., 2020; Raifman et al., 2020) . A reference period deemed business as usual between January 25 and March 7, 2020 (P0) was also selected, and the relative concentration of a pollutant I, i.e., [I] year should not affect the inter-annual comparison because those effects are mostly canceled in the P1/P0 ratio. P1 and P0 with more than one third of missing data (i.e., >14 days out of the 42-day period) were excluded. For the 28 sites selected, [ ] ′ resulted from at least 2 years of valid data. To estimate the confidence interval of %, a bootstrapping procedure (Mooney and Duval, 1993 ) based on 12,000 resampling/recalculation of the data were carried out using the Matlab® Statistics Toolbox. The type I error was set to 5%.

Twenty-eight NCore sites with 2017-2020 NO 2 , O 3 , and PM 2.5 data mostly available through the end of April 2020 were identified for this analysis. These sites are in or proximate to 28 different metropolises among 23 states. CO and PM 10 were also reported from 21 and 13 of the 28 sites, respectively. There are also significant associations among ∆NO 2 %, ∆CO%, and ∆PM 2.5 % (see Table   S2 ). For the top 9 sites in Table 1 with the most reductions in NO 2 concentration, all the other pollutants except O 3 also declined. As a secondary pollutant, O 3 did not show a clear pattern across the country, with significant increases and decreases observed at 2 and 7 sites, respectively, for the lockdown period.

The lockdown appeared to lower NO 2 and CO more broadly and significantly than PM, consistent with declining mobile and power plant emissions and similar to observations in Europe (Sicard et al., 2020; Tobías et al., 2020) . On a national scale both PM 2.5 and PM 10 are (Komenda, 2020) . This is in contrary to states such as North Dakota and Wyoming where schools were closed but businesses remained open (no stay-at-home order). Significant CO reductions nonetheless were observed at Bismarck, ND and Cheyenne, WY during the lockdown (Table 1 ).

There could also be an urban-rural contrast in how the lockdown affects air quality, as fewer non-essential commercial activities occur in rural and suburban areas than in urban centers.

Enforcing such policies in rural areas is also more difficult. Population density of the zip code where a site is located serves as a surrogate of urbanization and is plotted against ∆NO 2 % in Population density does explain the different ∆NO 2 % between the two OH sites and between the two MD sites that bore a uniform lockdown policy within the respective state. In both cases, NO 2 reduction increased with the local population density (Figure 1) Sommer et al. (2020) .

The lockdown or stay-at-home orders issued by the U.S. government to counter the COVID-19 pandemic has nonuniformly impacted air pollution in the U.S. More consistent NO 2 and CO declines than other pollutants coincide with reduced transportation and utility demands, while inter-site differences reflect not only the local lockdown policy but also population density. The first phase of lockdown in general affected urban more than suburban air quality. Although these effects are temporary, public health benefits from more aggressive air quality management should be considered in the recovery efforts, such as accelerating the transition into cleaner fuels and mass transportation. 

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Utah governor asks, Salt Lake City mayor orders residents to stay home to slow the spread of the coronavirus. The Salt Lake Tribune

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National Emission Inventory (NEI)

16) OR1 Portland -11 (-33, 15) -21 (-19, 74) -6 (-12, 26) WA1 Seattle -11

The authors thank staffs from SailBri Cooper Inc. for collecting and organizing the NCore air quality data, and support from School of Public Health, University of Nevada, Las Vegas for publishing the paper.
