Volume 48, Issue 5 e2020GL091987
Research Letter
Open Access

COVID-19 Crisis Reduces Free Tropospheric Ozone Across the Northern Hemisphere

Wolfgang Steinbrecht,

Corresponding Author

Wolfgang Steinbrecht

Deutscher Wetterdienst, Hohenpeißenberg, Germany

Correspondence to:

W. Steinbrecht,

wolfgang.steinbrecht@dwd.de

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Dagmar Kubistin,

Dagmar Kubistin

Deutscher Wetterdienst, Hohenpeißenberg, Germany

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Christian Plass-Dülmer,

Christian Plass-Dülmer

Deutscher Wetterdienst, Hohenpeißenberg, Germany

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Jonathan Davies,

Jonathan Davies

Environment and Climate Change Canada, Toronto, ONT, Canada

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David W. Tarasick,

David W. Tarasick

Environment and Climate Change Canada, Toronto, ONT, Canada

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Peter von der Gathen,

Peter von der Gathen

Alfred Wegener Institut, Helmholtz-Zentrum für Polar- und Meeresforschung, Potsdam, Germany

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Holger Deckelmann,

Holger Deckelmann

Alfred Wegener Institut, Helmholtz-Zentrum für Polar- und Meeresforschung, Potsdam, Germany

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Nis Jepsen,

Nis Jepsen

Danish Meteorological Institute, Copenhagen, Denmark

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Rigel Kivi,

Rigel Kivi

Finnish Meteorological Institute, Sodankylä, Finland

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Norrie Lyall,

Norrie Lyall

British Meteorological Service, Lerwick, UK

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Matthias Palm,

Matthias Palm

University of Bremen, Bremen, Germany

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Justus Notholt,

Justus Notholt

University of Bremen, Bremen, Germany

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Bogumil Kois,

Bogumil Kois

Institute of Meteorology and Water Management, Legionowo, Poland

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Peter Oelsner,

Peter Oelsner

Deutscher Wetterdienst, Lindenberg, Germany

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Marc Allaart,

Marc Allaart

Royal Netherlands Meteorological Institute, DeBilt, The Netherlands

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Ankie Piters,

Ankie Piters

Royal Netherlands Meteorological Institute, DeBilt, The Netherlands

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Michael Gill,

Michael Gill

Met Éireann (Irish Met. Service), Valentia, Ireland

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Roeland Van Malderen,

Roeland Van Malderen

Royal Meteorological Institute of Belgium, Uccle, Belgium

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Andy W. Delcloo,

Andy W. Delcloo

Royal Meteorological Institute of Belgium, Uccle, Belgium

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Ralf Sussmann,

Ralf Sussmann

Karlsruhe Institute of Technology, IMK-IFU, Garmisch-Partenkirchen, Germany

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Emmanuel Mahieu,

Emmanuel Mahieu

Institute of Astrophysics and Geophysics, University of Liège, Liège, Belgium

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Christian Servais,

Christian Servais

Institute of Astrophysics and Geophysics, University of Liège, Liège, Belgium

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Gonzague Romanens,

Gonzague Romanens

Federal Office of Meteorology and Climatology, MeteoSwiss, Payerne, Switzerland

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Rene Stübi,

Rene Stübi

Federal Office of Meteorology and Climatology, MeteoSwiss, Payerne, Switzerland

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Gerard Ancellet,

Gerard Ancellet

LATMOS, Sorbonne Université-UVSQ-CNRS/INSU, Paris, France

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Sophie Godin-Beekmann,

Sophie Godin-Beekmann

LATMOS, Sorbonne Université-UVSQ-CNRS/INSU, Paris, France

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Shoma Yamanouchi,

Shoma Yamanouchi

University of Toronto, Toronto, ONT, Canada

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Kimberly Strong,

Kimberly Strong

University of Toronto, Toronto, ONT, Canada

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Bryan Johnson,

Bryan Johnson

NOAA ESRL Global Monitoring Laboratory, Boulder, CO, USA

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Patrick Cullis,

Patrick Cullis

NOAA ESRL Global Monitoring Laboratory, Boulder, CO, USA

Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO, USA

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Irina Petropavlovskikh,

Irina Petropavlovskikh

NOAA ESRL Global Monitoring Laboratory, Boulder, CO, USA

Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO, USA

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James W. Hannigan,

James W. Hannigan

National Center for Atmospheric Research, Boulder, CO, USA

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Jose-Luis Hernandez,

Jose-Luis Hernandez

State Meteorological Agency (AEMET), Madrid, Spain

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Ana Diaz Rodriguez,

Ana Diaz Rodriguez

State Meteorological Agency (AEMET), Madrid, Spain

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Tatsumi Nakano,

Tatsumi Nakano

Meteorological Research Institute, Tsukuba, Japan

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Fernando Chouza,

Fernando Chouza

Jet Propulsion Laboratory, California Institute of Technology, Table Mountain Facility, Wrightwood, CA, USA

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Thierry Leblanc,

Thierry Leblanc

Jet Propulsion Laboratory, California Institute of Technology, Table Mountain Facility, Wrightwood, CA, USA

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Carlos Torres,

Carlos Torres

Izaña Atmospheric Research Center, AEMET, Tenerife, Spain

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Omaira Garcia,

Omaira Garcia

Izaña Atmospheric Research Center, AEMET, Tenerife, Spain

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Amelie N. Röhling,

Amelie N. Röhling

Karlsruhe Institute of Technology, IMK-ASF, Karlsruhe, Germany

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Matthias Schneider,

Matthias Schneider

Karlsruhe Institute of Technology, IMK-ASF, Karlsruhe, Germany

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Thomas Blumenstock,

Thomas Blumenstock

Karlsruhe Institute of Technology, IMK-ASF, Karlsruhe, Germany

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Matt Tully,

Matt Tully

Bureau of Meteorology, Melbourne, Australia

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Clare Paton-Walsh,

Clare Paton-Walsh

Centre for Atmospheric Chemistry, University of Wollongong, Wollongong, Australia

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Nicholas Jones,

Nicholas Jones

Centre for Atmospheric Chemistry, University of Wollongong, Wollongong, Australia

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Richard Querel,

Richard Querel

National Institute of Water and Atmospheric Research, Lauder, New Zealand

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Susan Strahan,

Susan Strahan

NASA Goddard Space Flight Center, Earth Sciences Division, Greenbelt, MD, USA

Universities Space Research Association, Columbia, MD, USA

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Ryan M. Stauffer,

Ryan M. Stauffer

NASA Goddard Space Flight Center, Earth Sciences Division, Greenbelt, MD, USA

Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD, USA

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Anne M. Thompson,

Anne M. Thompson

NASA Goddard Space Flight Center, Earth Sciences Division, Greenbelt, MD, USA

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Antje Inness,

Antje Inness

European Centre for Medium-Range Weather Forecasts, Reading, UK

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Richard Engelen,

Richard Engelen

European Centre for Medium-Range Weather Forecasts, Reading, UK

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Kai-Lan Chang,

Kai-Lan Chang

Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO, USA

NOAA Chemical Sciences Laboratory, Boulder, CO, USA

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Owen R. Cooper,

Owen R. Cooper

Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO, USA

NOAA Chemical Sciences Laboratory, Boulder, CO, USA

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First published: 09 February 2021
Citations: 16

Abstract

Throughout spring and summer 2020, ozone stations in the northern extratropics recorded unusually low ozone in the free troposphere. From April to August, and from 1 to 8 kilometers altitude, ozone was on average 7% (≈4 nmol/mol) below the 2000–2020 climatological mean. Such low ozone, over several months, and at so many stations, has not been observed in any previous year since at least 2000. Atmospheric composition analyses from the Copernicus Atmosphere Monitoring Service and simulations from the NASA GMI model indicate that the large 2020 springtime ozone depletion in the Arctic stratosphere contributed less than one-quarter of the observed tropospheric anomaly. The observed anomaly is consistent with recent chemistry-climate model simulations, which assume emissions reductions similar to those caused by the COVID-19 crisis. COVID-19 related emissions reductions appear to be the major cause for the observed reduced free tropospheric ozone in 2020.

Plain Language Summary

Worldwide actions to contain the COVID-19 virus have closed factories, grounded airplanes, and have generally reduced travel and transportation. Less fuel was burnt, and less exhaust was emitted into the atmosphere. Due to these measures, the concentration of nitrogen oxides and volatile organic compounds (VOCs) decreased in the atmosphere. These substances are important for photochemical production and destruction of ozone in the atmosphere. In clean or mildly polluted air, reducing nitrogen oxides and/or VOCs will reduce the photochemical production of ozone and result in less ozone. In heavily polluted air, in contrast, reducing nitrogen oxides can increase ozone concentrations, because less nitrogen oxide is available to destroy ozone. In this study, we use data from three types of ozone instruments, but mostly from ozonesondes on weather balloons. The sondes fly from the ground up to 30 kilometers altitude. In the first 8 km, we find significantly reduced ozone concentrations in the northern extratropics during spring and summer of 2020, less than in any other year since at least 2000. We suggest that reduced emissions due to the COVID-19 crisis have lowered photochemical ozone production and have caused the observed ozone reductions in the troposphere.

1 Introduction

Widespread measures to contain the COVID-19 pandemic have slowed, or even closed down, industries, businesses, and transportation activities, and have reduced anthropogenic emissions substantially throughout the year 2020. Guevara et al. (2020), or Barré et al. (2020) report European emissions reductions up to 60% for NOx, and up to 15% for Non-Methane Volatile Organic Compounds (NMVOC) in March/April 2020. Based on satellite observations of NO2 columns (Bouwens et al., 2020), comparable NOx emissions reductions are reported for Chinese cities in February 2020 (Ding et al., 2020; Feng et al., 2020). Globally averaged CO2 emissions decreased by 8.8% during the first half of 2020 (Z. Liu et al., 2020), consistent in timing and magnitude with the aforementioned NO2 emission reductions. The largest relative reductions occurred for air traffic, where emissions decreased by ≈40%, on average, in the first half of 2020 (Le Quéré et al., 2020a; Z. Liu et al., 2020), and remained low during the second half of 2020 (Le Quéré et al., 2020b).

These COVID-19 emissions reductions are large enough to affect ozone levels in the troposphere (Dentener et al., 2011). Tropospheric O3-NOx-VOC-HOx chemistry is, however, complex and nonlinear. The net effect of emission changes depends on NOx and VOC concentrations (e.g., Kroll et al., 2020; Sillman, 1999; Thornton et al., 2002). In polluted regions, at high NOx concentrations (>> 1pbb), reducing NOx concentrations can increase ozone, because ozone titration by NO is reduced (e.g., Sicard et al., 2020). At low concentrations (NOx < 1 nmol/mol), however, in the clean or mildly polluted free troposphere, reducing NOx lowers photochemical ozone production (e.g., Bozem et al., 2017), and results in less ozone.

Indeed, for many polluted regions, studies report increased near-surface ozone after COVID-19 lockdowns (e.g., Collivignarelli et al., 2020; Lee et al., 2020; Shi & Brasseur, 2020; Siciliano et al., 2020; Venter et al., 2020). Reduced surface ozone is reported for some rural areas, e.g., in the US and Western Europe (Chen et al., 2020; Menut et al., 2020). Meteorological conditions complicate matters, as they play an important role as well (Goldberg et al., 2020; Keller et al., 2021; Ordóñez et al., 2020; Shi & Brasseur, 2020).

In the free troposphere, ozone is an important greenhouse gas, and plays a key role in tropospheric chemical reactions, controlling the oxidizing capacity (e.g. Archibald et al., 2020; Cooper et al., 2014; Gaudel et al., 2018). The Northern Hemisphere free troposphere is dominated by net photochemical ozone production, proportional (albeit nonlinearly) to the availability of ozone precursor gases (e.g., Zhang et al., 2020). In contrast to increases of surface ozone in polluted urban areas after the COVID-19 emissions reductions, we find significant reductions of ozone in the northern extratropical free troposphere. These large-scale reductions occurred in late spring and summer 2020, following the widespread COVID-19 slowdowns, and are unique within the last two decades.

2 Instruments and Data

Regular observations of ozone in the free troposphere are sparse: Only around 50 ozone sounding stations worldwide (e.g. Tarasick et al., 2019), a handful of tropospheric LIDARs (Gaudel et al., 2015; Leblanc et al., 2018), and about twenty Fourier Transform Infrared Spectrometers (FTIRs, Vigouroux et al., 2015). In-Service Aircraft for a Global Observing System (IAGOS, Nédélec et al., 2015) are another important source of tropospheric ozone data. Due to the COVID-19 slowdowns, however, few IAGOS aircraft were flying in 2020, and IAGOS data became quite sparse, with only about 20 flights per month since April 2020, compared to more than 200 flights per month in 2019. The information content of satellite measurements on ozone in the free troposphere is limited, and accuracy is modest, 10%–30% (Hurtmans et al., 2012; Liu et al., 2010; Oetjen et al., 2014). The recent Tropospheric Ozone Assessment Report found large differences in tropospheric ozone trends derived from different satellite instruments, and even different signs in some regions (Gaudel et al., 2018).

Ozonesondes measure profiles with high vertical resolution, about 100 m, and good accuracy, 5%–15% in the troposphere, 5% in the stratosphere (Smit et al., 2007; Sterling et al., 2018; Tarasick et al., 2016; Van Malderen et al., 2016; Witte et al., 2017; WMO, 2014). This is adequate to detect ozone anomalies of several percent. We use stations with regular soundings, at least once per month since the year 2000, and with data available until at least July 2020. Soundings with obvious deficiencies were rejected (i.e. large data gaps, integrated ozone column from the sounding deviating by more than 30% from ground- or satellite-based spectrometer measurement). Table 1 provides information on stations, and public data archives.

Table 1. Stations in This Study, Mostly Ozonesonde Stations
Station Latitude ( N) Longitude (E) Data source (see caption) Data until Profiles/spectra per month in 2020
Alert, Canadaaa Due to COVID-19 restrictions, most Canadian ozonesonde data were available only up to March or April 2020.
, cc Stations affected by a drop-off in ECC sonde sensitivity >3% in the stratosphere, after 2015 (see Stauffer et al., 2020). The drop-off is much smaller (<<1%) in the troposphere, and should be negligible here. At many of the affected stations, ECC sondes behaved normally again in 2019/2020.
82.50 −62.34 W 4/2020 3.75
Eureka, Canadacc Stations affected by a drop-off in ECC sonde sensitivity >3% in the stratosphere, after 2015 (see Stauffer et al., 2020). The drop-off is much smaller (<<1%) in the troposphere, and should be negligible here. At many of the affected stations, ECC sondes behaved normally again in 2019/2020.
80.05 −86.42 W, E 9/2020 4.89
Ny-Ålesund, Norway 78.92 11.92 W, E 10/2020 7.10
Ny-Ålesund FTIR, Norway 78.92 11.92 N 7/2020 12.86
Thule FTIR, Greenland 76.53 −68.74 N 9/2020 73
Resolute, Canadaaa Due to COVID-19 restrictions, most Canadian ozonesonde data were available only up to March or April 2020.
74.72 −94.98 W 4/2020 5.50
Scoresbysund, Greenland 70.48 −21.95 E 11/2020 4.00
Kiruna FTIR, Sweden 67.41 20.41 N 7/2020 46
Sodankylä, Finland 67.36 26.63 W, E 12/2020 2.83
Lerwick, United Kingdom 60.13 −1.18 W, E 12/2020 3.92
Churchill, Canadaaa Due to COVID-19 restrictions, most Canadian ozonesonde data were available only up to March or April 2020.
, cc Stations affected by a drop-off in ECC sonde sensitivity >3% in the stratosphere, after 2015 (see Stauffer et al., 2020). The drop-off is much smaller (<<1%) in the troposphere, and should be negligible here. At many of the affected stations, ECC sondes behaved normally again in 2019/2020.
58.74 −93.82 W 3/2020 3.33
Edmonton, Canadaaa Due to COVID-19 restrictions, most Canadian ozonesonde data were available only up to March or April 2020.
, cc Stations affected by a drop-off in ECC sonde sensitivity >3% in the stratosphere, after 2015 (see Stauffer et al., 2020). The drop-off is much smaller (<<1%) in the troposphere, and should be negligible here. At many of the affected stations, ECC sondes behaved normally again in 2019/2020.
53.55 −114.10 W 3/2020 3.67
Goose Bay, Canadaaa Due to COVID-19 restrictions, most Canadian ozonesonde data were available only up to March or April 2020.
53.29 −60.39 W 3/2020 2.67
Bremen FTIR, Germany 53.13 8.85 N 10/2020 5.27
Legionowo, Poland 52.40 20.97 W 10/2020 4.00
Lindenberg, Germany 52.22 14.12 W 11/2020 4.73
DeBilt, Netherlands 52.10 5.18 W, E 12/2020 4.33
Valentia, Ireland 51.94 −10.25 W, E 12/2020 2.50
Uccle, Belgium 50.80 4.36 W, E 12/2020 12.00
Hohenpeissenberg, Germany 47.80 11.01 W 12/2020 10.50
Zugspitze FTIR, Germany 47.42 10.98 N 9/2020 73
Jungfraujoch FTIR, Switzerland 46.55 7.98 N 12/2020 46
Payerne, Switzerland 46.81 6.94 W 10/2020 11.10
Haute Provence, France 43.92 5.71 N 8/2020 2.50
Haute Provence LIDAR, France 43.92 5.71 N 8/2020 3.50
Toronto FTIR, Canada 43.66 −79.40 N 10/2020 59
Trinidad Head, California, USA 41.05 −124.15 G 12/2020 3.58
Madrid, Spain 40.45 −3.72 W 11/2020 4.09
Boulder, Colorado, USA 39.99 −105.26 G 12/2020 4.83
Boulder FTIR, Colorado, USA 39.99 −105.26 N 10/2020 56
Tateno (Tsukuba), Japanbb Tateno data were corrected for the change from Carbon Iodine to ECC ozonesondes in December 2009.
36.05 140.13 W 10/2020 2.70
Table Mountain LIDAR, California, USA 34.40 −117.70 N 8/2020 19
Izana, Tenerife, Spain 28.41 −16.53 W 8/2020 2.00
Izana FTIR, Tenerife, Spain 28.30 −16.48 N 9/2020 28
Hong Kong, China 22.31 114.17 W 9/2020 4.11
Hilo, Hawaii, USAcc Stations affected by a drop-off in ECC sonde sensitivity >3% in the stratosphere, after 2015 (see Stauffer et al., 2020). The drop-off is much smaller (<<1%) in the troposphere, and should be negligible here. At many of the affected stations, ECC sondes behaved normally again in 2019/2020.
19.72 −155.07 G 12/2020 4.08
Mauna Loa FTIR, Hawaii, USA 19.54 −155.58 N 10/2020 36
Paramaribo, Suriname 5.81 −55.21 N, E 10/2020 3.60
Pago Pago, American Samoacc Stations affected by a drop-off in ECC sonde sensitivity >3% in the stratosphere, after 2015 (see Stauffer et al., 2020). The drop-off is much smaller (<<1%) in the troposphere, and should be negligible here. At many of the affected stations, ECC sondes behaved normally again in 2019/2020.
−14.25 −170.56 G 12/2020 3.08
Suva, Fijicc Stations affected by a drop-off in ECC sonde sensitivity >3% in the stratosphere, after 2015 (see Stauffer et al., 2020). The drop-off is much smaller (<<1%) in the troposphere, and should be negligible here. At many of the affected stations, ECC sondes behaved normally again in 2019/2020.
−18.13 178.32 G 9/2020 1.44
Wollongong FTIR, Australia −34.41 150.88 N 10/2020 43
Broadmeadows, Australia −37.69 144.95 W 7/2020 4.29
Lauder, New Zealand −45.04 169.68 W 10/2020 4.40
Lauder FTIR, New Zealand −45.04 169.68 N 10/2020 99
Macquarie Island, Australia −54.50 158.94 W 7/2020 4.29

Apart from the sondes, FTIR spectrometers from the Network for the Detection of Atmospheric Composition Change (NDACC, De Mazière et al., 2018) provide independent information, based on a completely different method (ground-based solar-infrared absorption spectrometry). The altitude resolution of FTIR ozone profiles in the troposphere is much coarser (5–10 km) than that of the sondes, while accuracy is similar, 5%–10% (Vigouroux et al., 2015). Finally, we use data from tropospheric lidars (Gaudel et al., 2015; Granados-Muñoz & Leblanc, 2016), which provide ozone profiles from ≈3 to 12 km altitude, with accuracy comparable to the sondes (5%–10%; Leblanc et al., 2018), and slightly coarser altitude resolution (100 m–2 km).

We also use global atmospheric composition re-analyses from the Copernicus Atmosphere Monitoring Service for the years 2003–2019, and operational analyses for the year 2020 (CAMS, Inness et al., 2019; see also Park et al., 2020). The CAMS data are taken at the grid-points closest to the stations in Table 1. The analyses (in 2020) are adjusted for the small average difference to the re-analyses in 2018 and 2019. CAMS (re-)analyses are based on meteorological fields, and assimilation of satellite observations of ozone and NO2. However, for NO2 the impact of the assimilation is small and frequently insignificant, so that tropospheric NOx in CAMS is essentially controlled by the prescribed emissions (Inness et al., 2019). Similarly, the limited information content of current satellite measurements of tropospheric ozone means that tropospheric ozone in CAMS is also driven largely by the prescribed emissions (and the chemistry module). Stratospheric ozone, however, is constrained well by the assimilated satellite data. Thus, CAMS analyses account for the large Arctic stratospheric depletion in spring of 2020 (Manney et al., 2020; Wohltmann et al., 2020), for 2020 meteorological conditions, and for ozone transport, e.g. from the stratosphere to the troposphere (Neu et al., 2014). However, since they rely on “business as usual” emissions for 2020, the CAMS analyses do not account for the effects of COVID-19 emissions reductions in 2020 on tropospheric ozone (and NOx).

3 Results

For selected stations, Figure 1 presents the annual cycles of tropospheric ozone over the last 20 years, at 6 km, a representative altitude for the free troposphere. Monthly means (over 1 km wide layers) reduce synoptic meteorological variability and measurement noise, and focus on longer-term, larger-scale variations.

Details are in the caption following the image

Observed ozone monthly means at four typical stations. Results are for 6 km altitude. The thick red line highlights the year 2020. Climatological averages, and standard deviations over the years 2000–2020 are indicated by the thick black lines. Payerne (a) and Trinidad Head (c) are sonde stations. Jungfraujoch (b) is an FTIR station. Table Mountain (d) is a lidar station. Dark blue lines and scale on the right: CO2 emission reduction (in percent) from Le Quéré et al. (2020b), as a proxy for ozone precursor reductions in 2020. FTIR, Fourier Transform Infrared Spectrometer.

Payerne, Jungfraujoch, and Trinidad Head show an annual cycle with low ozone in winter and high ozone in summer. This is the case for most stations in the northern extratropics (Cooper et al., 2014; Gaudel et al., 2018; Parrish et al., 2020). Increased photochemical production due to more sunlight and warmer temperatures is the main driver for the summer ozone maximum in the northern extratropics (Archibald et al., 2020; Wu et al., 2007).

Figure 1 shows substantial yearly variability, but ozone levels are notably below average in 2020, at all four stations (thick red lines in Figure 1). At Payerne and Jungfraujoch, and a number of other stations, monthly means in spring and summer 2020 were actually the lowest, or close to the lowest, since 2000. For context, the dark blue lines in Figure 1 provide global CO2 emission reductions due to the COVD-19 pandemic (Le Quéré et al., 2020b). Comparable reductions apply to global ozone precursor emissions (NOx and VOCs). The (daily) emission reductions in Figure 1 indicate that the largest effect for ozone might be expected after March 2020. However, Figure 1 does not show any clear or close correspondence between unusual ozone monthly means in 2020 (red lines) and the emission reductions (dark blue lines).

Annual cycles of ozone anomalies, averaged over all northern extratropical stations (stations north of 15°N), are shown in Figure 2. Anomalies were defined as the relative deviation (in percent) from the 2000 to 2020 climatological mean of each calendar month at each station. As for the single stations in Figure 1, the observed northern extratropical average shows exceptionally low ozone throughout spring and summer 2020 (red line in Figure 2a). This is not reproduced by the CAMS analyses, which do not account for COVID-19 related emissions reductions, and simulate ozone in the usual range in 2020 (red line in Figure 2b). Again, there is no close temporal correspondence between the unusual behavior of observed ozone in 2020 (red line in Figure 2a), and the emission reductions (dark blue line in Figure 2a).

Details are in the caption following the image

Annual cycles of monthly mean northern extratropical ozone anomalies at 6 km altitude. Anomalies are in percent, relative to the climatological monthly mean calculated for each station/ instrument, and for the period 2000–2020 (all Januaries, all Februaries, …, all Decembers). These single station/instrument anomalies are then averaged over all northern extratropical stations/instruments (north of 15°N). Panel (a) Results from the station observations. Panel (b) Results for CAMS atmospheric composition (re-)analyses at grid points nearest the stations. The CAMS data do not account for COVID-19 related emissions reductions in 2020. Gray lines: individual years from 2000 to 2019. Thick red line: year 2020. Thick black lines: average anomaly, ±1 standard deviation over the years. Dark blue lines and scale on the right in panel (a): Global CO2 emission reduction in 2020 (in percent) from Le Quéré et al. (2020b), as in Figure 1. CAMS, Copernicus Atmosphere Monitoring Service.

Figures 1 and 2 show large negative anomalies from April to August 2020. Figure 3 compares anomaly profiles averaged over those five calendar months, between the years 2011 and 2020. Both years saw unusually large springtime ozone depletion in the Arctic stratosphere (Manney et al., 2020; Wohltmann et al., 2020). In the stratosphere, above ≈10 km, the Arctic depletion appears as low ozone, both in observations and CAMS results (particularly for stations north of 50°N). In both the stratosphere and the troposphere, the observed profiles show more variability than the smoother CAMS profiles. In 2020, most observed single station anomaly profiles (Figure 3b) are negative throughout the northern extratropical troposphere (between 1 and 10 km). This is not the case in 2011 (Figures 3a and 3c), nor in the CAMS data in 2020 (Figure 3d).

Details are in the caption following the image

Ozone anomaly profiles (in percent), averaged over April to August. Stations are excluded in years where their data cover less than three of these five months. Panel (a) for the year 2011. Panel (b) for the year 2020. Light blue lines: northern extratropical stations (north of 15°N). Light orange lines: remaining stations, south of 15°N. Thick dark blue line: mean of the northern extratropical stations. Thin dark blue lines: 95% confidence interval of the mean of the northern extratropical stations. Red line in panel (b): simulated ozone change at 40°N from Weber et al. (2020; Figure S4, scenario A3). Panels (c), (d): Same as (a), (b), but for CAMS (re-)analyses at the grid-points closest to the stations. CAMS, Copernicus Atmosphere Monitoring Service.

The 2020 anomaly is even clearer for the northern extratropical mean profile (dark blue lines in Figure 3). The observed 2020 mean anomaly profile is large, −6% to −9%, and statistically significant at the 95% level (more than 99% in fact) from 1 to 8 km (Figure 3b), whereas the corresponding CAMS profile is close to zero (Figure 3d). Figure 3 indicates that Arctic stratospheric springtime ozone depletion did not have a large effect on tropospheric ozone below 8 km in 2011 and 2020 (see also model simulations in Figure S1, based on Gelaro et al., 2017, and Strahan et al., 2019), and that the CAMS “business as usual” simulation does not account for the observed large negative tropospheric anomaly in 2020.

Figure 3b also shows a simulated profile of tropospheric ozone reduction from a recent chemistry-climate modeling study of COVID-like emissions decreases by Weber et al. (2020). This simulated profile (red line in our Figure 3b) matches the observed northern extratropical ozone reduction (dark blue line), from the ground up to about 8 km. Above 8 km, the simulated profile deviates by ≈10% from the observed profile, because it assumes fixed 2012 to 2014 meteorological conditions. The CAMS analyses (Figure 3d) show that 2020 meteorological conditions and springtime Arctic stratospheric ozone depletion resulted in ozone reductions of 5%–10% above 9 km, consistent with the observations.

Time series of the tropospheric anomaly (averaged from April to August, and from 1 to 8 km altitude) are shown in Figure 4. In the observations (left panel), the year 2020 stands out with large negative anomalies, not seen in the CAMS data. Across the 20 previous years, ozone anomalies at individual stations (thin lines) are scattered around zero. The northern extratropical average anomaly (dark blue line) is usually smaller than ±3%. The only other observed exception is the positive anomaly related to the (European) heat-wave summer of 2003 (Vautard et al., 2007). The large negative northern extratropical anomaly in the observations in 2020, ≈−7%, is clearly outside of the ±2σ range of the previous 20 years (thin dark blue lines). It is not reproduced by the CAMS “emissions as usual” analysis.

Details are in the caption following the image

Tropospheric ozone anomaly, averaged over April to August and from 1 to 8 km, for the years 2000–2020. Panel (a) Observations. Panel (b) CAMS atmospheric composition (re-)analyses. Light blue lines: northern extratropical stations (north of 15°N). Light orange lines: stations south of 15°N. Thick dark blue line: Average over all stations north of 15°N. Thin dark blue lines: ±2 standard deviations over all years of this average. CAMS, Copernicus Atmosphere Monitoring Service.

The geographic distribution of the average tropospheric ozone anomalies is shown for 2011 and 2020 in Figure 5. 2020 stands out in the observations with large negative anomalies at nearly all northern extratropical stations, and a fairly uniform geographical distribution (see Table S1 of the supplement for the numerical values). CAMS does show negative anomalies in 2020, but only north of 50°N, and not as large as the observations. In the Southern Hemisphere in 2020, agreement between observations and CAMS is quite good, typically within 2.5% or better (see also Table S1). In 2011, some stations show positive anomalies, negative anomalies are not as large as in 2020, and the geographical distribution is less uniform. Agreement between observations and CAMS is reasonable in 2011, usually within a few percent.

Details are in the caption following the image

Geographic distribution of observed tropospheric ozone anomalies (averaged over the months April to August, and over altitudes from 1 to 8 km) for the years (a) 2011 and (b) 2020. Panels (c) and (d): same, but for CAMS results at the station locations. Colored circles give the anomaly at the ozonesonde stations. Squares are for FTIR and lidar stations. See Table S1 of the supplement for the numerical values. Black filling indicates insufficient data in the given year. CAMS, Copernicus Atmosphere Monitoring Service; FTIR, Fourier Transform Infrared Spectrometers.

4 Discussion and Conclusions

Ozone stations in the northern extratropics indicate exceptionally low ozone in the free troposphere (1–8 km) in spring and summer 2020. Compared to the 2000–2020 climatology, ozone was reduced by 7% (≈4 nmol/mol). Such widespread low tropospheric ozone, across so many stations and over several months has not been observed in any previous year since 2000. The observed 7% ozone reduction in the free troposphere stands in contrast to increases of surface ozone by 10%–30%, reported for many polluted urban areas after the COVID-19 related emissions reductions in 2020 (e.g., Collivignarelli et al., 2020; Lee et al., 2020; Shi & Brasseur, 2020; Siciliano et al., 2020; Venter et al., 2020). However, the chemical regime for ozone in the free troposphere is different (e.g., Kroll et al., 2020; Sillman, 1999; Thornton et al., 2002), and free tropospheric ozone reductions are expected after the substantial decrease of precursor emissions due to the COVID-19 pandemic (e.g. Guevara et al., 2020; Zhang et al., 2020).

Recent model simulations of COVID-like emissions decreases (Weber et al., 2020) find tropospheric ozone reductions very similar to our observational results. From our results, and the simulations by Weber et al., 2020, it appears that the total tropospheric ozone burden of the northern extratropics decreased by about 7% for April–August 2020. The contribution from ozone increases in polluted urban areas to the total burden is opposite, but very small.

The Weber et al. (2020) simulations indicate that the major causes of tropospheric ozone reduction come from reduced surface transportation (ozone decrease throughout most of the northern extratropical troposphere), and from reduced aviation (ozone decrease mostly between 10 and 12 km altitude and north of 30°N, see also Grewe et al., 2017). While the simulations are qualitatively consistent with the observations, they consider only March to May. New simulations using more recent and extended emissions estimates (Le Quéré et al., 2020b), and further comparison with our station observations would be worthwhile.

The observed large and fairly uniform 7% reduction of ozone in the northern extratropical troposphere in spring and summer 2020 provides a far reaching test case for the response of tropospheric ozone to emission changes. Further quantification of this anomaly will be possible, when observations from commercial aircraft (IAGOS), and satellite instruments become available. Additional modeling studies will improve our understanding of the contributions from different sectors such as air traffic, and surface transportation.

Acknowledgments

The authors greatly acknowledge the know-how and the hard work of station personnel launching the ozonesondes and taking the ground-based measurements. Without their dedicated efforts over many years, and especially during the COVID-19 lockdowns in 2020, investigations like this one are not possible!

Deutscher Wetterdienst funds the ozone program at Hohenpeißenberg and makes research like this possible.

NOAA GML supported additional launches in Boulder and Trinidad Head in April and May 2020. NOAA and NASA's Upper Atmosphere Composition Observations (UACO) Program support the SHADOZ ozone soundings at Hilo, Pago-Pago (American Samoa) and Suva (Fiji). UACO also provides partial support for the Boulder FTIR and the Table Mountain Lidar.

The NDACC FTIR stations Bremen, Ny-Ålesund, Izaña, Kiruna, and Zugspitze have been supported by the German Bundesministerium für Wirtschaft und Energie (BMWi) via DLR under grants 50EE1711A, 50EE1711B, and 50EE1711D. Izaña, Kiruna, and Zugspitze have also been supported by the Helmholtz Society via the research program ATMO.

The FTIR measurements in Bremen and Ny-Ålesund receive additional support by the Senate of Bremen, the FTIR measurements in Ny-Ålesund also by AWI Bremerhaven. The University of Bremen further acknowledges funding by DFG (German research foundation) TRR 172, Project Number 268020496, within the Transregional Collaborative Research Center “ArctiC Amplification: Climate Relevant Atmospheric and SurfaCe Processes, and Feedback Mechanisms (AC)3.”

The University of Liège contribution has been supported primarily by the Fonds de la Recherche Scientifique, FNRS under grant J.0147.18, as well as by the CAMS project. EM is a senior research associate of the F.R.S.-FNRS.

The Toronto FTIR measurements were supported by Environment and Climate Change Canada, the Natural Sciences and Engineering Research Council of Canada (NSERC), and the NSERC CREATE Training Program in Technologies for Exo-Planetary Science.

The University of the Wollongong thanks the Australian Research Council that has provided significant support over the years for the NDACC site at Wollongong, most recently as part of project DP160101598.

Part of this research work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D004).

The National Center for Atmospheric Research is sponsored by the National Science Foundation. The NCAR FTS observation programs at Thule, GR and Boulder, CO are supported under contract by the National Aeronautics and Space Administration (NASA). The Thule work is also supported by the NSF Office of Polar Programs (OPP). We wish to thank the Danish Meteorological Institute for support at the Thule site and NOAA for support of the MLO site.

Key results for this manuscript were generated using Copernicus Atmosphere Monitoring Service Information from the European Community.

No author reports a financial (or other) conflict of interest.

    Data Availability Statement

    Most of the ozonesonde data used in this study are freely available from the World Ozone and UV Data Center (https://woudc.org) at Environment Canada (https://exp-studies.tor.ec.gc.ca/), and are downloadable at https://woudc.org/archive/Archive-NewFormat/OzoneSonde_1.0_1/).

    Some ozonesonde data for 2020 were not yet available at the WOUDC. Instead, rapid delivery data were obtained from ftp://zardoz.nilu.no/nadir/projects/vintersol/data/o3sondes (requires registration), at the Nadir database of the Norwegian Institute for Air Quality (NILU, https://projects.nilu.no/nadir/obs.html). Registration information, and the same data in a different format, are available from the European Space Agency Validation Data Center (https://evdc.esa.int/).

    For Boulder, Trinidad Head, Hilo, Fiji, and Samoa, stations operated by the US National Oceanic and Atmospheric Administration, Global Monitoring Laboratory (https://www.esrl.noaa.gov/gmd/ozwv/), data can be obtained freely from ftp://aftp.cmdl.noaa.gov/data/ozwv/Ozonesonde/.

    FTIR and lidar data, as well as some ozonesonde data, are from the Network for the Detection of Atmospheric Composition Change (https://ndacc.org), and are freely available at ftp://ftp.cpc.ncep.noaa.gov/ndacc/station/ and ftp://ftp.cpc.ncep.noaa.gov/ndacc/RD/.

    Copernicus Atmosphere Monitoring Service (CAMS) global chemical weather EAC4 re-analyses are available at https://atmosphere.copernicus.eu/data. CAMS operational global analyses and forecasts are available at https://apps.ecmwf.int/datasets/data/cams-nrealtime/.