Has there been an actual reduction of aerosols, is there some measurement data supporting this? Where does the 20% come from?
I would imagine there has been a large reduction of aerosols based on most of the world was in Lockdown for 3 months, that speaks for itself without any need for evidence that this would reduce aerosols by a large amount, the evidence of how much will be published in time. Below are some articles i found on this.
The 20% is just a rough estimate based on reduced industrial activity,GDP, flights grounded, reduction in car travel, holidays, hotels, restaurants and bars shut etc.
Which is partly ongoing the reduction of aerosols and the effects will be continuing for a long time if activity/jobs/travel/consumption/tourism is reduced into next year.
The International Air Transport Association said it did not expect 2019 levels to be exceeded until 2023.
https://www.breakingtravelnews.com/news/article/no-full-aviation-recovery-until-at-least-2023-iata-predicts/Some 1.3 million passengers flew into the UK last month compared with 11.1 million in July 2019, the Home Office said.
https://www.msn.com/en-gb/news/uknews/flight-arrivals-down-89-in-july-despite-travel-corridors/ar-BB18qjfmResearchers around the world are now trying to parse out how Coronavirus lockdown measures have affected global emissions. In the Nature Climate Change study published last month, an international team of scientists found that daily CO2 emissions dropped by 17% at the peak of the coronavirus shutdown.
Another study published in Geophysical Research Letters in May found that nitrogen dioxide pollution over China, Western Europe and the United States decreased by as much as 60% in early 2020 compared to the same time last year. And a study focused on northern China found that levels of particulate matter known as PM 2.5, a notable human health hazard, decreased by approximately 60 percent in January and February.
https://insideclimatenews.org/news/08062020/sulfate-emissions-coronavirus-arctic-heatwavesThe COVID-19 pandemic elicited a global response to limit associated mortality, with social distancing and lockdowns being imposed. In India, human activities were restricted from late March 2020. This ‘anthropogenic emissions switch-off’ presented an opportunity to investigate impacts of COVID-19 mitigation measures on ambient air quality in five Indian cities (Chennai, Delhi, Hyderabad, Kolkata, and Mumbai), using in-situ measurements from 2015 to 2020. For each year, we isolated, analysed and compared fine particulate matter (PM2.5) concentration data from 25 March to 11 May, to elucidate the effects of the lockdown. Like other global cities, we observed substantial reductions in PM2.5 concentrations, from 19 to 43% (Chennai), 41–53% (Delhi), 26–54% (Hyderabad), 24–36% (Kolkata), and 10–39% (Mumbai). Generally, cities with larger traffic volumes showed greater reductions. Aerosol loading decreased by 29% (Chennai), 11% (Delhi), 4% (Kolkata), and 1% (Mumbai) against 2019 data.
https://www.sciencedirect.com/science/article/pii/S221067072030603XConcentrations of sulfur dioxide in polluted areas in India have decreased by around 40% between April 2019 and April 2020. Using data from the Copernicus Sentinel-5P satellite, from the European Union Copernicus program, scientists have produced new maps that show the drop in concentrations across the country in times of COVID-19.
https://scitechdaily.com/sulfur-dioxide-concentrations-drop-over-india-during-covid-19/https://www.sciencedaily.com/releases/2020/07/200709141538.htm"We are experiencing the worst economic shock since the Great Depression, while at the same time we have experienced the greatest drop in greenhouse gas emissions since the burning of fossil fuels began," Dr Malik said.
This study focuses on 'live' data to 22 May (with the exception of air travel, for which only a 12-month forecast exists), differing from most assessments of the economic impacts of the pandemic based on scenario analyses and/or projections -- and it is the first to provide an overview of the combined economic, social and environmental impacts, including indirect effects, of the coronavirus.
Key Reductions
Consumption: US$3.8 trillion (4.2 percent ~ GDP of Germany)
Jobs: 147m (4.2 percent of the global workforce)
Income from wages and salaries: $2.1 trillion (6 percent)
Most directly hit: US, China (mainland), air transport and related tourism
Greenhouse gas emissions: 2.5Gt (4.6 percent) -- larger than any drop in human history*
Other atmospheric emissions -- PM2.5: Dangerously fine particulate matter emissions fall 0.6 Mt (3.8 percent); SO2 & NOx: Sulfur dioxide emissions from burning fossil fuels -- which has been linked to asthma and chest tightness -- and emissions from nitrogen oxide -- from fuel combustion, for example, driving cars -- fall 5.1 Mt (2.9 percent).
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0235654