Pushan Shah, South Australian Environment Protection Authority
Jason Choi, Environment Protection Authority Victoria
The beginning of 2020 brought COVID-19, an unexpected global pandemic that caused normal activities to cease with government directives to stay at home. These directives were termed ‘lockdowns’, with many businesses forced to close and everyday movements restricted. The restrictions manifested themselves in different ways in Australia’s capital cities, but all impacted airborne emissions. Here, the evidence of emissions reductions is examined through ambient pollutant concentrations measured in Adelaide, Brisbane, Melbourne and Sydney during these lockdown periods.
Adelaide
On 12 March 2020, the South Australian Government implemented lockdown measures, lasting until the end of April. The measures were expected to reduce air pollution due to a decrease in noncommercial vehicle traffic, and in industrial and commercial activity. The average reduction in traffic volume was 40% during the peak of the restrictions, before returning to near-normal levels at the end of July (Figure 1).
There are no roadside monitoring stations in Adelaide to observe the impacts of these reductions directly, but the reduction in people travelling to Adelaide central business district (CBD) is observed from the centrally positioned station in the CBD. In Adelaide CBD, levels of carbon monoxide (CO) decreased by 40% in accordance with the traffic volumes, compared with measurements taken during the same period (March–April) in the 5 years before the COVID-19 lockdowns (Figure 2). Evidence of decreases in nitrogen oxides (NOx) and fine particulate matter (PM2.5) is harder to observe. Five years of data were used for the pre-pandemic period to reduce the influence that large-scale events, such as bushfires and land burns, can have on estimates of ambient pollutant concentrations. Although there was a small reduction in NOx levels, PM2.5 levels continued to be influenced by bushfire smoke in the early part of 2020 and remained very similar to past averages. Overall, an improvement in air quality was recorded, with 20–40% reduction in all pollutants during March and April 2020.
Brisbane
Numbers of coronavirus cases in Brisbane were much lower than elsewhere in Australia in early 2020, so restrictions were less strict. Still, many people chose to work from home, and this is reflected in a 13% decrease in nitrogen dioxide (NO2) levels in Brisbane compared with the 10-year average (Welchman et al. 2020). However, the Katestone report notes that the impact of meteorology (e.g. stagnant, pollutant-trapping conditions) may have masked the level of reductions that might otherwise have occurred in Brisbane.
Melbourne
Lockdown measures in Melbourne were introduced on 17 March 2020. Traffic flows began decreasing immediately, to a minimum of 30% in the week of 12 April 2020, compared with pre-lockdown flows. Traffic began resuming steadily and by the beginning of June had reached 80% of normal levels. This was curbed again in August when very strict lockdown measures were in place for more than 100 days. The impact of the traffic reductions is most easily seen in the NO2 columns (Figure 3). Comparing the 2020 profile on 10 April with the profiles calculated from the preceding 5 years shows a 24% reduction in NO2 in the area surrounding the city of Melbourne.
Air quality data collected at all Environment Protection Authority Victoria monitoring stations in Melbourne between 17 March and 12 May for 6 years (2015–20) were examined, separating out 2020 to assess the impacts of the lockdowns. The average diurnal columns over these periods also show a 25% decrease in NO2 during the lockdowns (Figure 4), similar to the NO2 reduction calculated by Ryan et al. (2021). At the same time, there was a 3% increase in ozone. This increasing ozone effect has been observed all over the world and tends to be greater in cities where the lockdown measures were strictest (Gkatzelis et al. 2021). Although NOx compounds are involved in the chemical production and removal of ozone, a direct link between decreasing NO2 and increased ozone cannot be made without considering other ozone-forming chemicals such as volatile organic compounds (VOCs), and the impact of temperature.
Reductions in other pollutants such as carbon monoxide (CO) and PM2.5 during the lockdown period in Melbourne were more difficult to observe, as reductions were also observed in February and early March. This might be caused by reduced tourism to the area following the severe summer 2019–20 bushfires. Post-lockdown, there were increases in CO levels during 2020 beyond those from the previous 5 years. This was caused by smoke from wood heaters being used more prolifically when evening temperatures decreased well below 10 °C.
Sydney
Traffic volumes decreased by 19–44% across Sydney as the lockdowns started to take effect from 16 March 2020 (Duc et al. 2021). Duc et al. (2021) examined the raw air quality data and observed decreases in concentrations of NO2 (–18%), CO (–13%) and PM2.5 (–13%), while ozone levels increased (+1.5%). However, by detrending the air quality data for meteorological effects, Ryan et al. (2021) estimated that NO2 levels decreased by 8% as a result of lockdowns, but concentrations of ozone and PM2.5 increased by 20% and 24%, respectively.
Conclusions
Chemical reactions in the atmosphere are highly nonlinear, and unexpected changes in secondary pollutants (e.g. ozone increases) can occur when primary pollutants such as NO2 are reduced. However, reducing NOx emissions in Australia will almost always lead to better overall air quality in our urban areas. Although there is some nonlinearity in the chemistry, and there are occasions when ozone levels may be more related to VOCs than NOx, policies that reduce primary emissions of NOx and other air pollutants and their precursors will almost always succeed in improving air quality.