The lingering impact of bushfires

Australian scientists are examining how serious bushfires, such as those that devastated large areas of the country last summer, affect the urban microclimate.

Understanding the impact of fires on the weather and air quality in surrounding areas could help in preparing for the broader consequence of an expected increase in their number, size and severity, they say.

Recent US research, reported in Cosmos, showed that clouds created by fires in NSW and Victoria spewed 300,000–900,000 tonnes of smoke into the stratosphere, which is believed to be more than from any previous fire event.

For the Australian study, researchers from the University of Sydney (USyd) and the University of New South Wales (UNSW) set up a monitoring station at a site in Sydney for three weeks in December and January as hundreds of bushfires were ravaging nearby areas.

“Many studies have investigated and confirmed the health implications of bushfires, as biomass burning is a major source of ambient particulate matter, but underexplored is the impact on local urban microclimates, concerning not just temperature, but factors including solar and UV radiation, relative humidity, wind patterns and gusts, and urban heat island intensity,” says USyd’s Gianluca Ranzi, senior co-author of a paper in the journal Environmental Research Communications.

The station collected data on air temperature, relative humidity, barometric pressure, precipitation, wind speed and direction, solar radiation, UV radiation and UV index, as well as a range of particulate matter pollution (PM1, PM2.5 and PM10).

During the monitoring period, extreme pollution, heatwave and drought were recorded simultaneously. The temperature peaked at 46.2 degrees Celsius, accumulated rain was just 13.6 millimetres, and levels of PM2.5 exceeded the national and World Health Organisation standard on three days.

From this, a “suite of dependencies emerged between PM concentration and air temperature, relative humidity, wind speed and rain”, the researchers write in their paper.

The findings suggest that specific combinations of air temperature and relative humidity were conducive to higher/lower levels of pollutant accumulation, reflecting findings from previous studies.

“In general, higher PM concentration was recorded for night-time and early morning, especially after daytime overheating events, with temperatures above 35 degrees Celsius,” says USyd’s Giulia Ulpiani, the paper’s lead author.

“We also found that long-transport mechanisms and complex interactions between prevailing and local winds could have played a major role, making it difficult to establish definite correlations between PMs and single environmental parameters.”

Intense rain splashing was associated with the most intense concentration of dust, Ulpiani says, and their data confirms that, despite their acknowledged air-cleansing properties, heavy raindrops triggered a mechanism that produces solid particles from soil, which can substantially raise local pollution levels.

The study also established a link between ultraviolet index and PM concentration. It identified several PM thresholds above which UV radiation was strongly blocked, and below which the UVI was likely to surpass moderate levels.

Using evolutionary algorithms, the relationship was further demonstrated, and supports previous scientific evidence of the attenuating effects of smoke aerosols on UV irradiance.

“We also compared the urban heat island intensity during the bushfire event to that recorded during the same period over the previous 20 years,” says senior co-author Mat Santamouris, from UNSW.

“Data from several Bureau of Meteorology stations indicated an additional effect of the microclimatic perturbation caused by the bushfires: the disappearance of cool island events and the exacerbation of UHI events over the median.”

The researchers acknowledge that quantitative evaluations must be interpreted with caution, but suggest their study offers a new holistic approach to environmental monitoring.