Girl on a climbing frame in sunlight filtered through thick bushfire smoke
During the latest Australian bushfire season, the particulate matter – microscopic inhalable particles in the air – reached a maximum of 160 μg m−3 (more than four times the level associated with good air quality). Photo: Shutterstock

Research carried out at the height of the 2019/2020 Australian bushfire season demonstrates how the relationships between extreme pollution, heatwaves and drought exacerbate the impact of bushfires on urban outdoor livability.

The environmental and air quality monitoring campaign, a joint study from UNSW Sydney and the University of Sydney, found that specific combinations of air temperature and relative humidity were conducive to higher and lower levels of air pollution. 

Air pollution was generally higher during the night and in the morning, especially after daytime temperatures above 35°C. Intense rain was found to cause the highest and most persistent dust levels. Despite their acknowledged air-cleansing properties, heavy raindrops trigger a mechanism that produces solid particles from soil, which can substantially raise local pollution levels.

Additionally, dense layers of particulate matter – microscopic solid particles and liquid droplets found in air – were found to strongly reduce UV radiation. 

The research, titled Experimental evidence of the multiple microclimatic impacts of bushfires in affected urban areas: the case of Sydney during the 2019/2020 Australian season, is the first to identify these interrelationships on a local scale under weather anomalies, such as bushfires. The study offers a new holistic approach to environmental monitoring.

“The size and destructive potential of the 2019–20 Australian bushfires shocked the world,” says senior co-author Scientia Professor Mat Santamouris from UNSW Built Environment. “In the duration of our study, some 186,000km2 was burnt – that’s almost the size of England. 

“The fires claimed the lives of at least 34 people, destroyed nearly 5900 buildings and decimated the native wildlife. 

“It is crucial we have a better understanding of the potential consequences of an increased rate and extension of bushfires, especially to improve our risk preparedness and subsequent coping strategies.”

Conditions monitored in Sydney’s Inner West

A compact meteorological station in Petersham, in Sydney’s Inner West, monitored conditions from 20 December 2019 to 13 January 2020, when hundreds of fires were burning at Sydney's borders. 

It measured air temperature, relative humidity, barometric pressure, rainfall, wind patterns, solar and UV radiation, UV index, and particulate matter (PM). 

During this time, extreme pollution, heatwaves and drought were recorded simultaneously. The PM10 content, that is microscopic inhalable particles, reached a maximum of 160 μg m−3 (more than four times the level associated with good air quality), the temperature peaked at 46.2°C, and the accumulated rain was just 13.6mm (compared to 91.1mm, the mean rainfall in January over the last 20 years). 

Wind gusts of more than 100km/h also resulted in unpredictable fire paths and extremely poor air quality in the nearby areas. 

“The kind of expansive burning we saw during this season’s bushfires triggers an especially health-threatening vicious circle: extreme pollution suppresses rain, which prolongs drought, which sustains heat waves and bushfires’ persistence,” says Prof. Santamouris.

High temperatures combined with humidity exacerbates air pollution

While long-distance transport and winds made it difficult to establish definitive correlations, the research identified strong dependencies between temperature and humidity levels that produced detrimental effects.

Lead author Dr Giulia Ulpiani, from the University of Sydney, says: “Interestingly, a strong interconnection between environmental parameters could be identified under the combined effect of bushfires, heatwaves and intense droughts. 

“Particle air pollution tended to accumulate under specific thermohygrometric conditions [alignments in temperatures and humidity levels], especially after overheating episodes, as well as in response to strong sea breeze and heavy rain due to the dislodgement of fine particles from the soil.” 

Correlations between UV radiation and concentrations of air pollution 

Solar and UV radiation were also measured to quantify the degree of reduction that occurred under the thick layer of bushfire smoke that frequently blanketed the city during the observation period. 

The research established a link between UV index and the PM concentration in the air. The study identified several PM thresholds above which UV radiation was strongly blocked out and below which the UV index was likely to surpass moderate levels. 

“This finding is especially relevant as intense ultraviolet radiation causes erythema, premature aging, skin cancer, cataracts and other ophthalmohelioses [eye disorders caused by, or related to, sunlight exposure],” senior co-author Professor Gianluca Ranzi, from the University of Sydney, says.

The research further demonstrated this relationship using evolutionary AI algorithms, and supports previous scientific evidence of the ability for smoke particles suspended in the air to reduce UV irradiance.

Increases in urban heat island intensity and decreases in cool island events

The study also compared the urban heat island intensity during the bushfires to that recorded during the same period over the previous 20 years. 

Urban areas are significantly warmer than their surroundings due to human activity, known as the urban heat island effect. When surrounding non-urban areas face extreme dry conditions, moisture in urban areas evaporates creating a cool island event.

Prof. Santamouris stressed that “data from several BoM meteorological stations indicated an additional effect ascribable to persisting bushfires: the disappearance of cool island events and the exacerbation of the heat island intensity over the median. 

“The results of our research are further confirmation that holistic approaches are evermore required in urban sciences, to discern the way the built environment evolves under climate changes and weather extremes.

“The associations we discovered would be extremely valuable in building up a cohesive national health protection strategy and encouraging better responsiveness from governments and city planners, the report found.”

The study was selected by the American Institute of Physics as an exceptional work and is published in the IOP Publishing journal Environmental Research.