Impacts of Traffic Reductions Associated With COVID-19
on Southern California Air Quality
H. A. Parker
1
, S. Hasheminassab
2
, J. D. Crounse
1
, C. M. Roehl
1
, and P. O. Wennberg
1,3
1
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA,
2
South Coast
Air Quality Management District, Diamond Bar, CA, USA,
3
Division of Engineering and Applied Science, California
Institute of Technology, Pasadena, CA, USA
Abstract
On 19 March 2020, California put in place Stay-At-Home orders to reduce the spread of
SARS-CoV-2. As a result, decreases up to 50% in traffic occurred across the South Coast Air Basin (SoCAB).
We report that, compared to the 19 March to 30 June period of the last 5 years, the 2020 concentrations of
PM
2.5
and NO
x
showed an overall reduction across the basin. O
3
concentrations decreased in the western
part of the basin and generally increased in the downwind areas. The NO
x
decline in 2020 (approximately
27% basin-wide) is in addition to ongoing declines over the last two decades (on average 4% less than the
−
6.8% per year afternoon NO
2
concentration decrease) and provides insight into how air quality may
respond over the next few years of continued vehicular reductions. The modest changes in O
3
suggests
additional mitigation will be necessary to comply with air quality standards.
Plain Language Summary
On 19 March 2020, California put in place Stay-At-Home orders to
reduce the spread of SARS-CoV-2. As a result, there was much less traffic in Southern California. Reduced
traffic along with a month-long stretch of unusually rainy weather at the beginning of the lockdown led
to significant reductions in PM
2.5
and NO
x
levels across the basin. Concentrations of O
3
, on the other
hand, showed inconsistent changes across the basin. The response of O
3
to these large changes in nitrogen
oxide concentrations suggests mitigation efforts beyond those associated with continuing vehicle
emission reductions will be important to meet clean air goals.
1. Introduction
As restrictions were enacted to slow the spread of SARS-CoV-2, the virus that causes COVID-19, the
decrease in human activity (traffic, industry, etc.) in major cities worldwide resulted in significant changes
in air quality. Cities in China, Italy, Germany, and the United States have shown decreases in atmo-
spheric nitrogen dioxide (NO
2
) concentrations (Bauwens et al., 2020; Goldberg et al., 2020; Naeger &
Murphy, 2020). In Pittsburgh, Pennsylvania, for example, significant decreases in concentrations of NO
2
, car-
bon monoxide (CO), and fine particulate matter (PM
2.5
) have been observed (Tanzer-Gruener et al., 2020).
Los Angeles (LA), known for its car culture and multidecadal fight with air pollution (Parrish et al., 2016;
Pollack et al., 2013), was reported to have some of the cleanest air in its history as a result of the sudden
drop in traffic emissions (https://www.latimes.com/opinion/story/2020-04-22/coronavirus-is-making-it-
clear-that-car-culture-is-its-own-kind-of-plague). For LA and the broader South Coast Air Basin (SoCAB),
however, the COVID-19 restrictions coincided with precipitation at least 3 times the historical aver-
age (supporting information Figure S1). As the anomalously rainy period ended in the SoCAB, the levels of
the secondary pollutant ozone (O
3
) returned to values comparable to or exceeding those of previous years
despite the sustained decrease in traffic flow (more than 20% below the values in January and February).
The influence of nitrogen oxide (NO
x
) pollution in the SoCAB on air quality has been the subject of a
decades-long study. Since the mid-twentieth century, NO
x
in the SoCAB has been decreasing by roughly 3%
per year on average (Parrish et al., 2016; Pollack et al., 2013). In the last decade, regulations of NO
x
have been
focused on reducing the emissions from heavy duty diesel vehicles (Final 2016 Air Quality Management
Plan, 2016). Historically, reductions in weekend NO
x
emissions have led to higher weekend O
3
levels. Higher
weekend O
3
levels are the result of the combination of increased photochemical production of oxidant
(O
x
=
NO
2
+
O
3
) from elevated OH levels due to the reduced loss of OH via its reaction with NO
2
and
an increased fraction of O
x
present as O
3
due to the reduced conversion to NO
2
via reaction with NO. This
RESEARCH LETTER
10.1029/2020GL090164
Special Section:
The COVID-19 Pandemic:
Linking Health, Society and
Environment
Key Points:
• SoCAB maximum 1-hr NO
x
and 24-hr PM
2.5
concentrations
decreased 27% and 29%,
respectively, between 19 March and
30 June of 2015–2019 and 2020
• The 8-hr daily maximum O
3
showed inconsistent changes across
the basin during the COVID-19
associated decrease of atmospheric
NO
x
concentrations
• During a shift to a NO
x
-limited
regime, a better understanding of
VOC emission sources is needed to
improve air quality in the SoCAB
Supporting Information:
• Supporting Information S1
Correspondence to:
S. Hasheminassab and P. O. Wennberg,
sina.hasheminassab@gmail.com;
wennberg@caltech.edu
Citation:
Parker, H. A., Hasheminassab, S.,
Crounse, J. D., Roehl, C. M.,
& Wennberg, P. O. (2020). Impacts
of traffic reductions associated with
COVID-19 on Southern California air
quality.
Geophysical Research Letters
,
47
, e2020GL090164. https://doi.org/
10.1029/2020GL090164
Received 1 AUG 2020
Accepted 10 NOV 2020
Accepted article online 23 NOV 2020
©2020. The Authors.
This is an open access article under the
terms of the Creative Commons
Attribution-NonCommercial-NoDerivs
License, which permits use and
distribution in any medium, provided
the original work is properly cited, the
use is non-commercial and no
modifications or adaptations are made.
PARKER ET AL.
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Geophysical Research Letters
10.1029/2020GL090164
phenomenon is known as the “weekend effect.” The weekend effect has been used to predict the effects of
future NO
x
emission reductions on air quality (Baidar et al., 2015). Changes in volatile organic compound
(VOC) emissions do not generally scale with NO
x
because these emissions are associated with many sec-
tors (and include biogenic emissions). On-road vehicle VOC emissions are now thought to account for only
about one fourth of the total emissions (CEPAM: 2016 SIP - Standard Emission Tool, 2019).
Both the weekend effect and the especially large reductions in vehicular emissions in 2020 provide evidence
for the continuing efficacy of mobile fleet emissions reductions on air quality. Given the long-term trends
in such emissions, the experience of spring 2020 provides a glimpse of what the air quality will look like
approximately 5 years into the future of vehicle targeted emission reductions.
2. Methods and Data
Basin-wide air pollutant data (O
3
,NO
2
,NO
x
, and PM
2.5
) were obtained from the California Air Resources
Board (CARB) Air Quality Data Query Tool (https://www.arb.ca.gov/aqmis2/aqdselect.php) (Figure 1). The
2020 air quality data are preliminary, unvalidated, and subject to change. Continuous measurements of
PM
2.5
along with trace gas measurements of CO, SO
2
,O
3
,NO,NO
2
, and NO
y
were conducted at the Cal-
tech campus by the Caltech air quality system (CITAQS) using Teledyne instrumentation (Text S1). While
the regulatory NO
2
chemiluminescence measurements are known to include contributions from other
nitrogen-containing species due to the non-selectivity of the molybdenum converter (Villena et al., 2012),
the chemiluminescence data from the South Coast Air Quality Management District (South Coast AQMD)
station in Pasadena (located approximately 400 m south of the CITAQS) agree within a few tenths of a ppb
with the optical NO
2
measurements from the CITAQS. Remotely sensed CH
2
O total column abundances are
provided by the Total Carbon Column
Observing Network
(TCCON) site in P
asadena (Wunch et al., 2011).
Temperature and precipitation data are taken from meteorological sensors located alongside the CH
2
O
measurement (https://tccon-weather.caltech.edu). Historical
observations of temperature, relative humid-
ity, and wind speed data across the basin were obtained from CARBs Meteorology Data Query Tool, and
precipitation data were acquired from the National Oceanic and Atmospheric Administration (NOAA). The
CITAQS,
TCCON
site, and meteorological
station are all located in or on the Linde
+
Robinson Laboratory
on the southwest corner of the Caltech campus roughly half a kilometer north of a regulatory air moni-
toring station in Pasadena, operated by the South Coast AQMD. Our analysis also makes use of O
3
,NO
2
,
CH
2
O, PM
2.5
, and weather data from the 2010 CalNex campaign ground site also located on the Caltech cam-
pus (https://www.esrl.noaa.gov/csl/projects/calnex/). Basin-wide daily traffic counts were obtained from
the Caltrans PeMS website (https://pems.dot.ca.gov/). TROPOMI tropospheric NO
2
columns are used for
illustrative purposes in Figure 2 and follow suggested data quality guidelines (Veefkind et al., 2012).
In our analysis, we use the sum of NO
2
and O
3
, also referred to as oxidant (O
x
), CH
2
O, and PM
2.5
as metrics
of air quality. O
x
is conserved with respect to the cycling of NO
2
photolysis to O
3
and NO, and O
3
reacting
with NO to reform NO
2
. This makes O
x
measurements useful as a diagnostic of air chemistry since it is
less sensitive to local effects on photochemistry (e.g., local NO emissions reacting with O
3
to form NO
2
,or
clouds changing the photolysis frequency of NO
2
) and is instead driven by overall emissions, losses, and net
photochemical O
x
production. CH
2
O is often used as a proxy for VOC reactivity, especially for the oxidation
of small alkenes from both anthropogenic and biogenic sources (Pollack et al., 2012; Wolfe et al., 2016; Zhu
et al., 2014). PM
2.5
is both directly emitted and produced within the atmosphere (secondary), with the latter
generally being dominant in the SoCAB. Secondary production of PM
2.5
arises from NH
4
,NO
3
, and sulfate
chemistry and the oxidation of gas-phase VOCs and is the main culprit for low visibility during smog events
(Schiferl et al., 2014).
In sections 3.1 and 3.4, we only consider the air monitoring sites that were active through the entire 2015
to 2020 period. For NO
x
and O
3
, we consider sites that measure both of these parameters, while for PM
2.5
we consider data from all the sites with PM
2.5
measurements (Figure S2). In these sections, we report 24-hr
PM
2.5
, 8-hr daily maximum (DM) O
3
, and 1-hr DM NO
x
that have regulatory relevance.
In section 3.2, we use data from 13 sites in the basin that have measurements of both NO
2
and O
3
for the
2000 to 2020 period. In section 3.3, we focus on data from Pasadena only. In the above mentioned sections,
we focus on data collected during the afternoon hours (12 p.m. to 4 p.m. local) since the afternoons are often
the times with maximum values of O
3
or O
x
and are therefore the most influential in terms of air quality
reporting, such as O
3
exceedances (Figure S3). For an accurate comparison from year to year, we define the
19 March to 30 June window as the COVID-19 (or simply COVID) period for all comparisons.
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3. Results and Discussion
3.1. The Confluence of Anomalous Weather and COVID-19 Restrictions
On 19 March 2020, the state of California enacted Stay-At-Home orders restricting all non-essential work
in order to reduce the spread of COVID-19 (https://www.gov.ca.gov/wp-content/uploads/2020/03/3.19.20-
attested-EO-N-33-20-COVID-19-HEALTH-ORDER.pdf). Eleven days before this order, on 8 March 2020,
mobility and traffic started decreasing everywhere in the SoCAB (Figure 1c). By April, SoCAB traffic and
mobility dropped to about 50% of the pre-COVID-19 period (January and February, 2020). SoCAB traffic
counts slowly recovered from late April through early June and stabilized at about 80% of pre-COVID-19
levels by the end of June (Figure 1c) despite different phases of restrictions. While the traffic flow decreased
in all areas of the basin, the average differences varied in different parts of the basin as the western and
eastern areas have returned close to pre-COVID-19 values (Figure 1c, right panel). Concurrently, the air
quality index (AQI) in the second half of March and beginning of April were consistently green, and
SoCAB citizens enjoyed clean air with high visibility (Figures S4–S7). Naturally, this led to the associa-
tion of the decrease in traffic with clean air and the condemnation of LA car culture as the culprit for
bad air quality (https://www.latimes.com/opinion/story/2020-04-22/coronavirus-is-making-it-clear-that-
car-culture-is-its-own-kind-of-plague).
The decrease in traffic and improvement in air quality was also coincident with frequent stormy conditions
and above-normal amounts of rainfall. The rainfall in the basin in 2020 was well above that of the past
decade with precipitation in March and April over 3 and 5 times the average values, respectively (Figure S1).
Rainfall affects air quality by removing pollutants such as nitric acid and PM
2.5
from the air through wet
deposition (Seinfeld & Pandis, 2006). In addition, rainy periods are associated with higher basin ventilation
rates, decreasing pollution buildup in the basin. Figure 1b shows the basin-maximum concentrations of
8-hr DM O
3
, 1-hr DM NO
x
, and 24-hr PM
2.5
for the pre-COVID-19 and post-COVID-19 periods in 2020
along with the average values for 2015 to 2019 with the 2020 rainy days shaded in blue. During the rainy
period in March and early April, temperatures dropped below the range observed over the previous 5 years
(Figure S1). During this drop in temperature, the 1-hr DM NO
x
and 24-hr PM
2.5
were consistently lower than
the lower limits of the 2015 to 2019 range. The 8-hr DM O
3
concentrations were consistently at the lower
end of the 2015 to 2019 range. After the rainy period, temperatures in late April and early May rose above
historical values (Figure S1) and 8-hr DM concentrations of O
3
were highly elevated. In fact, in May 2020,
SoCAB experienced 18 days of O
3
exceedance from the federal standard of 70 ppb—more than any other year
from 2015 to 2019. The spike in O
3
concentrations outside the range of the 2015 to 2019 values in late April
and early May is coincident with, and likely partially due to, a similar pattern of higher temperatures and
lower wind speeds in the basin (Figures 1 and S1). This return to higher O
3
levels occurred although traffic
remained at least 30% lower than pre-COVID levels. Aft
er May, however, the temperatures, wind speeds,
and O
3
concentrations in the basin returned to values within the range of values observed in 2015–2019.
NO
x
concentrations remained equal to or lower than the previous 5 years, and, in June, PM
2.5
concentrations
dropped lower than the lower end of the range of values from the past 5 years. To assess the impact of rainy
days on the observed trends, a sensitivity test was carried out. When the rainy days are excluded from the
analysis, the basin-maximum levels are comparable to the values for the entire window with only a 2.76%,
2.11%, and 0.64% difference between including rainy days and not for 24-hr PM
2.5
, 8-hr DM O
3
, and 1-hr
DM NO
x
, respectively (Figure S8).
The changes in 8-hr DM O
3
concentrations in 2020 were not consistent across the basin (Figure S9). Com-
pared to the same months in 2015–2019, sites in the western part of the basin generally experienced lower
8-hr DM O
3
concentrations (up to 9 ppb or 22% reduction) while most of downwind areas experienced an
overall increase (up to 8 ppb or 15% increase). The 24-hr PM
2.5
and 1-hr DM NO
x
showed an overall decrease
across the basin (10–45% and 13–40% reduction, respectively) (Figures S10 and S11). As discussed above,
while the COVID-19 countermeasures altered pollutant concentrations in LA, the anomalous weather
significantly contributed to the clean air observed in late March and early April.
3.2. Twenty Years of Reductions and COVID-19
In the context of the trends in air quality in the SoCAB over the past decade, the diverse response of sec-
ondary pollutants to the large drops in vehicular emissions during the COVID-19 period is not surprising.
Over the past 20 years, vehicular emissions, particularly heavy-duty diesel trucks, have been targeted by
regulation, and atmospheric concentrations of NO
x
have decreased substantially (Final 2016 Air Quality
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Figure 1.
(a) Box plot of the basin-maximum 24-hr PM
2.5
, 8-hr daily maximum O
3
, and 1-hr daily maximum NO
x
during the COVID-19 period (19 March to 30 June) in 2020 and in the past 5 years (2015–2019) in the South Coast Air
Basin. Horizontal lines inside boxes denote median values, edges of box denote the 25th and 75th percentiles, and the
whiskers denote
±
1.5
×
IQR. Dots are data points
>
1.5
×
IQR. The confidence diamond in each box contains the mean
and the upper and lower 95% of the mean. The means are reported to the right of the box plots with the standard
deviation in parenthesis. (b) The 7-day moving average of basin-maximum 24-hr PM
2.5
, 8-hr daily maximum O
3
,and
1-hr daily maximum NO
x
in 2020 and in the past 5 years in the South Coast Air Basin. (c) (left) Basin-wide daily
average traffic flow deviation from January to February in percent is plotted with the 7-day moving average represented
by the red line. (right) Average difference from January to February traffic levels for 19 March to 30 June period
separated by the source/receptor area for the South Coast Air Basin.
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