Beating the eucalypt blues – new ways to model air quality

By Mary O'CallaghanApril 5th, 2016

Australia is obliged to report on pollution levels. How we model these levels can be improved.
The Blue Mountains NSW
A blue sky day in the Blue Mountains NSW.

Eucalyptus are one of the highest emitters of biogenic volatile organic compounds which is what gives the Blue Mountains that blue haze. Image: Peter Asquith / Flikr

The Blue Mountains west of Sydney are not blue. But the air around them is often seen as a blue haze, especially on hot summer days.

This bluish haze is caused by a chemical reaction in the atmosphere, driven by a compound called isoprene.

Isoprene is one of scores of compounds in the atmosphere that contribute to air pollution and the creation of particles that are harmful to our health. Most of these volatile organic compounds, or VOCs, are gases that are emitted from everyday substances such as paint stripper, cleaning products, furniture, even cosmetics.

“What is interesting about isoprene is that it is emitted from trees, possibly as a form of defence against pest insects”, says Dr Kathryn Emmerson, a senior research scientist at CSIRO.

“We refer to these natural emissions of VOCs as ‘biogenic’ VOCs, or BVOCs. Biogenic means they are produced by living organisms or biological processes.”

Eucalypts the natural polluters

Of all the species of trees, one of the highest emitters of BVOCs is a genus that covers not just the Blue Mountains but most of south-eastern Australia—Eucalyptus. Because of this, the region is considered a global hotspot for BVOCs.

“Emission rates for eucalypts vary hugely, even for different plants of the same species of eucalypt”, says Kathryn.

Yet, the world’s state-of-the-art model for predicting BVOC emissions from nature—MEGAN (the Model of Emissions of Gases and Aerosols from Nature)—has classified eucalypts as a high emitter across the board.

MEGAN, which is used by the US and Europe to project the concentrations of BVOCs in the atmosphere, uses meteorological data, land-use data, and emission factors based on observations.

Up to recently, the CSIRO air quality team had only a basic way of predicting BVOCs for Australia. “We’ve now incorporated MEGAN into CSIRO’s air quality model, CTM”, says Kathryn. “We use the CTM for modelling air pollution and for forecasting smoke from bushfires.”

machine in a lab used to monitor air pollution
CSIRO spectrometer

Air pollution is monitored using a mass spectrometer. Photo: CSIRO

Modelling throws light on emission rates

The team also measured the actual rate of emissions at four sites across the southeast, at different times of the year.

When Kathryn compared the model projections with the observed measurements, she was surprised at how much they differed. For isoprene, the predictions were overestimated by a factor of six.

“It seems that eucalypts may not emit as much isoprene as was previously thought”, says Kathryn. “The high BVOC emission rates used in the model are based on young eucalypt trees, less than seven years old, and it may be that these young trees emit more isoprene than adult trees.”

Kathryn also found that no single emissions factor suits all seasons and all conditions. “We can tweak the emission factors for eucalypts in the model, but each site needs to be treated individually.

The implications for Australia’s air quality

Australia must monitor air quality under the National Environment Protection Measures which aim to reduce our exposure to harmful smogs.

“Sydney is surrounded by eucalypt forests so we need to understand these BVOCs to better control pollution”, says Kathryn.

“Isoprene contributes to that chemical mix. So what we need to do next is remap emission rates for the whole southeast region.

“The region will still be a global hotspot, but better BVOC measurements for the region will make our calculations for the continent more accurate.

“Isoprene is so reactive that it’s a hot topic; our research will also inform the global chemistry models.”

Taking to the skies to improve the model

In an exciting prospect for Australia, researchers from the US National Center for Atmospheric Research (NCAR) in Colorado, who built MEGAN, are keen to come to Australia and measure the region’s emission rates from the air. A similar offer from Britain is being discussed.

“All our BVOC measurements have been taken at ground level”, says Kathryn. “NCAR would bring their own Hercules aircraft, which is fitted out like a flying laboratory, with particle counters and chemical analysers on board. The aircraft zig-zags across the canopy, continuously taking in air and giving us real-time measurements.”

Kathryn is coordinating an international bid to the US National Science Foundation to fund the NCAR aircraft time. Several Australian universities are also interested in participating.

“We’re aiming for summer 2019”, she says, “Summer is peak emissions time.”


  1. Fantastic! Good luck

  2. BVOC could also act as nuclei in cloud condensation. Are you also able to investigate BVOC emissions and rainfall patterns?

    1. Generally BVOCs are gases (*), so need to undergo chemical transformations in the atmosphere to form secondary organic aerosol (SOA). If the SOA particles are small enough they could act as cloud condensation nuclei, though usually the transformed BVOCs will condense onto existing nuclei making them larger and “oily” (i.e. less water soluble). Currently our chemical transport model does not have the capability to allow the SOA to feedback on the meteorology. Perhaps something for the future!
      * there are some BVOCs which have such a high molecular weight that they can nucleate straight out of the gas phase without the need to undergo chemistry, but these species are in much lower concentrations.

  3. Any idea of when the results might be in for the Isoprene/BVOC aircraft study, and it ‘s influence on the remapping of emission rates for the whole southeast region? And whether it is to be repeated in any other regions around Australia?

    1. Hi Iina,
      Dr Kathryn Emmerson has replied: “Thank you for your question. Sadly we didn’t get funding for the full aircraft study, but a ground based study did go ahead earlier this year (Jan – March 2020). I wrote a short piece for the Atmospheric Composition and Chemistry webpage as the experiments were underway: The results are still being investigated.”
      Many thanks, ECOS.

  4. Just studying road crashes – does eucalypts vapour settle out overnight and could it be one of the ingredients in grime on the road that make them slippery after rain, dew or in th epeak of summer make roads slippery. I m a civil engineer so Im out of my chemistry depth here – does anyone have an opinion on this.

    1. Hi John,
      Dr Kathryn Emmerson has replied: “Thank you for your question. Unfortunately, this is outside of my area of expertise. To answer such a research question, we would need to measure the chemical composition of grime on the roads to determine what it is made of.”
      Many thanks, ECOS.

    2. John, ‘natural’ or background deposition of small soil, salt and other airborne particles is around one g/m2/month, and this would be higher on and near roads as vehicles themselves create dust emissions. In terms of what makes roads slippery I think this deposition of dust particles would far outweigh any deposition of gaseous VOCs from Eucalypts. (If the VOC deposition on road surfaces was significant you’d probably be able to smell it, and be able to distinguish it from the tar smell).

Commenting on this post has been disabled.