Tracking acidification in Australia and Asia
(Article reproduced from the Division's external
newsletter, Atmosphere, Issue 1, February, 1996)
Prior to the 1980s, very little was known about rainwater
quality in Australia. The vastness of our landscape presents a challenge
to anyone setting out to make measurements. Atmospheric conditions in
the tropical north are very different from those in Melbourne and Sydney,
and in our power generating regions.
During the past 15 years, CSIRO’s Dr Greg Ayers has been investigating
the impact on rainwater of our sprawling cities and growing industrialisation.
Recognised as Australia’s pre-eminent expert on atmospheric acidification,
Greg’s research and monitoring work are helping government and industry
safeguard our environment.
“If we don’t understand the problem or know its extent, there’s
little chance we can prevent damage,” he says.
During the early 1980s, Greg and fellow chemist Rob Gillett monitored
rainwater at 12 sites in Sydney. In metropolitan areas they found that
rainwater was more acidic than in less polluted parts of the city.
‘Industry and motor vehicles were clearly making rainwater acidic,’
Greg says.
Shortly afterwards, Ayers and Gillett headed south to make similar measurements
in Melbourne. They found little evidence of acidity. ‘It didn't surprise
us greatly. Melbourne is well ventilated. The strong prevailing westerly
winds on rainy days clear the air. In Sydney, the pollution often accumulates
for longer.’
Deposition, not rain
Greg points out that rainwater is just one way in which
acidic pollutants reach the ground. Atmospheric scientists believe that
just as much acid-causing material is directly deposited from the air.
They prefer the term ‘acid deposition’ to ‘acid rain’.
One-quarter of the sulfur in the atmosphere is natural, the rest is caused
by human activity. Nature releases sulfur through decomposing marine algae
and erupting volcanoes. Industry releases sulfur dioxide when fossil fuels
are burnt and sulfide ores smelted.
Sulfur dioxide readily forms sulfuric acid in the air or when it reaches
the ground. In the year 1900, global sulfur dioxide emissions were approximately
15 million tonnes. Annual emissions are now close to six times that amount.
Nitrogen oxides are generated by lightning and microbes, and by burning
of fossil fuel and biomass. In the atmosphere the oxides are often transformed
into nitric acid.
Throughout the late 1980s and the early `90s, Greg Ayers and his team
completed studies in three of Australia's largest power generating regions:
Victoria’s Latrobe Valley, and the Hunter Valley and Western Coalfields
of New South Wales.
Greg’s team needed some way of reliably monitoring and collecting
daily rainfall at each site. After some thought, they came up with an
automatic sampler containing eight polyethylene bottles mounted on a carousel.
Each morning the carousel automatically rotated to position the next bottle
beneath the funnel. The sampler logged rainfall to within 0.2 millimetres
as well as measuring the rate of rainfall in each shower.
So successful were the samplers that they are now being manufactured
and sold internationally under licence by Ecotech Ltd, a Melbourne-based
environmental equipment company.
Soils ain't soils
Environmental response to pollutants depends on many factors.
Some regions cope with acidification better than others, having larger
‘critical loads’. Critical load refers to the greatest assault
that an ecological system can withstand before showing measurable degradation.
Scientists determine critical load by examining rock and soil type, land
use and rainfall.
If soil is fertile with a pH greater than 4.5, and rainfall is relatively
low, the critical load will be high. The terrain can withstand moderately
large additions of acidity without undue suffering.
Conversely, in low pH soils, acidification mobilises toxic aluminium
ions. If coniferous forests predominate, or if land is devoted to rough
grazing, the result is a low critical load. Even minor acid deposition
is undesirable.
In the Hunter Valley, Greg concludes, the greatest determinant of critical
load is land use. The combined effect of farming and industry may be a
problem in some areas.
Acidification in Asia
In 1991, the Australian International Development Assistance
Bureau (now AusAID) financed the first ever survey of acid deposition
in Indonesia. Rob Gillett and Paul Selleck, a technical officer in Gregs’
team, measured rainwater chemistry for a year at four locations on the
main island, Java.
‘The levels of acid deposition we found in Indonesia are comparable
with those occurring in the worst affected parts of Europe and North America,’
Greg says.
Since the Indonesian work, the CSIRO team has travelled extensively,
initiating studies and providing advice to local authorities. They've
worked in Malaysia, New Guinea, Fiji and Brazil. The Melbourne laboratories
have seen a steady stream of overseas scientists completing training programs.
Future plans ...
‘We're aiming to establish a network of tropical sites
for rainwater and atmospheric measurements,’ Greg says. ‘We
will include five “hot spots” in Indonesia and Malaysia. The
baseline station at Charles Point in Darwin will give us relatively unpolluted
rainwater and air samples for comparison.’
He is also keen to build up the complete picture of Australian sources
and sinks of atmospheric sulfur, showing how the chemical gets into the
atmosphere and where it is consumed.
Unpolluted rainwater is not neutral. It is slightly
acidic, with a pH between five and six. (On the pH scale, seven is neutral,
anything less is considered acidic.) The atmosphere is full of acidic
particles released from natural processes. These acids dissolve in the
tiny cloud droplets that ultimately fall to earth as rain.
Article by Paul Holper |
