{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,3,13]],"date-time":"2026-03-13T00:30:05Z","timestamp":1773361805079,"version":"3.50.1"},"reference-count":54,"publisher":"MDPI AG","issue":"7","license":[{"start":{"date-parts":[[2022,3,28]],"date-time":"2022-03-28T00:00:00Z","timestamp":1648425600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Remote Sensing"],"abstract":"<jats:p>We present tropospheric nitrogen dioxide (NO2) changes observed by the Canadian Pandora measurement program in the Greater Toronto Area (GTA), Canada, and compare the results with surface NO2 concentrations measured via in situ instruments to assess the local emission changes during the first two years of the COVID-19 pandemic. In the City of Toronto, the first lockdown period started on 15 March 2020, and continued until 24 June 2020. ECMWF Reanalysis v5 (ERA-5) wind information was used to facilitate the data analysis and reveal detailed local emission changes from different areas of the City of Toronto. Evaluating seven years of Pandora observations, a clear NO2 reduction was found, especially from the more polluted downtown Toronto and airport areas (e.g., declined by 35% to 40% in 2020 compared to the 5-year mean value from these areas) during the first two years of the pandemic. Compared to the sharp decline in NO2 emissions in 2020, the atmospheric NO2 levels in 2021 started to recover, but are still below the mean values in pre-pandemic time. For some sites, the pre-pandemic NO2 local morning rush hour peak has still not returned in 2021, indicating a change in local traffic and commuter patterns. The long-term (12 years) surface air quality record shows a statistically significant decline in NO2 with and without April to September 2020 observations (trend of \u22124.1%\/yr and \u22123.9%\/yr, respectively). Even considering this long-term negative trend in NO2, the observed NO2 reduction (from both Pandora and in situ) in the early stage of the pandemic is still statistically significant. By implementing the new wind-based validation method, the high-resolution satellite instrument (TROPOMI) can also capture the local NO2 emission pattern changes to a good level of agreement with the ground-based observations. The bias between ground-based and satellite observations during the pandemic was found to have a positive shift (5\u201312%) than the bias during the pre-pandemic period.<\/jats:p>","DOI":"10.3390\/rs14071625","type":"journal-article","created":{"date-parts":[[2022,3,29]],"date-time":"2022-03-29T21:45:51Z","timestamp":1648590351000},"page":"1625","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":13,"title":["Tropospheric and Surface Nitrogen Dioxide Changes in the Greater Toronto Area during the First Two Years of the COVID-19 Pandemic"],"prefix":"10.3390","volume":"14","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-4784-4502","authenticated-orcid":false,"given":"Xiaoyi","family":"Zhao","sequence":"first","affiliation":[{"name":"Air Quality Research Division, Environment and Climate Change Canada, Toronto, ON M3H 5T4, Canada"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2731-5956","authenticated-orcid":false,"given":"Vitali","family":"Fioletov","sequence":"additional","affiliation":[{"name":"Air Quality Research Division, Environment and Climate Change Canada, Toronto, ON M3H 5T4, Canada"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Ramina","family":"Alwarda","sequence":"additional","affiliation":[{"name":"Air Quality Research Division, Environment and Climate Change Canada, Toronto, ON M3H 5T4, Canada"},{"name":"Department of Physics, University of Toronto, Toronto, ON M5S 1A7, Canada"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Yushan","family":"Su","sequence":"additional","affiliation":[{"name":"Ontario Ministry of the Environment, Conservation and Parks, Toronto, ON M9P 3V6, Canada"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Debora","family":"Griffin","sequence":"additional","affiliation":[{"name":"Air Quality Research Division, Environment and Climate Change Canada, Toronto, ON M3H 5T4, Canada"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Dan","family":"Weaver","sequence":"additional","affiliation":[{"name":"Department of Physical and Environmental Sciences, University of Toronto at Scarborough, Toronto, ON M1C 1A4, Canada"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-9947-1053","authenticated-orcid":false,"given":"Kimberly","family":"Strong","sequence":"additional","affiliation":[{"name":"Department of Physics, University of Toronto, Toronto, ON M5S 1A7, Canada"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Alexander","family":"Cede","sequence":"additional","affiliation":[{"name":"NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA"},{"name":"LuftBlick, Fritz-Konzert-Stra\u00dfe 4, 6020 Innsbruck, Austria"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Thomas","family":"Hanisco","sequence":"additional","affiliation":[{"name":"NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Martin","family":"Tiefengraber","sequence":"additional","affiliation":[{"name":"LuftBlick, Fritz-Konzert-Stra\u00dfe 4, 6020 Innsbruck, Austria"},{"name":"Department of Atmospheric and Cryospheric Sciences, University of Innsbruck, 6020 Innsbruck, Austria"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Chris","family":"McLinden","sequence":"additional","affiliation":[{"name":"Air Quality Research Division, Environment and Climate Change Canada, Toronto, ON M3H 5T4, Canada"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Henk","family":"Eskes","sequence":"additional","affiliation":[{"name":"Royal Netherlands Meteorological Institute (KNMI), 3731 De Bilt, The Netherlands"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Jonathan","family":"Davies","sequence":"additional","affiliation":[{"name":"Air Quality Research Division, Environment and Climate Change Canada, Toronto, ON M3H 5T4, Canada"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Akira","family":"Ogyu","sequence":"additional","affiliation":[{"name":"Air Quality Research Division, Environment and Climate Change Canada, Toronto, ON M3H 5T4, Canada"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Reno","family":"Sit","sequence":"additional","affiliation":[{"name":"Air Quality Research Division, Environment and Climate Change Canada, Toronto, ON M3H 5T4, Canada"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Ihab","family":"Abboud","sequence":"additional","affiliation":[{"name":"Air Quality Research Division, Environment and Climate Change Canada, Toronto, ON M3H 5T4, Canada"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Sum Chi","family":"Lee","sequence":"additional","affiliation":[{"name":"Air Quality Research Division, Environment and Climate Change Canada, Toronto, ON M3H 5T4, Canada"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2022,3,28]]},"reference":[{"key":"ref_1","unstructured":"(2014). 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