Impact of atmospheric particles on climate underestimated

Levels of global secondary organic aerosol (SOA) in the Earth's atmosphere have increased by 60 per cent since pre-industrial times, according to new research, which suggests that the effects of SOA have been previously underestimated.

This new information could help lead to better estimates of future climate change. SOA is made up of fine particles and droplets suspended in the atmosphere and is the product of many complex photo-chemical processes. It affects the climate by increasing the reflection of the sun's rays and so cools the Earth's surface. It also contributes to atmospheric haze, as well as having an impact on human health.

There has been a considerable change to the composition and magnitude of emissions from human activities since pre-industrial times. This study, partly conducted under the EU-funded EUCAARI project1, investigates changes affecting the distribution and global burden of organic aerosols since 1750.

The increase is believed to be due to rises in industrial or fossil fuel emissions rather than increases in biomass burning.

The researchers processed meteorological data from 2004 using a computer model. They modelled SOA formation from the by-products of mainly biogenically emitted (produced by plants and animals) substances such as monoterpenes, isoprene, benzene, toluene, xylene and other volatile organic compounds. The experiments also compared data from 1750 and 2004 to assess the effects of increases in ammonium sulphate aerosol.

The present day global warming effects of SOA were calculated including the radiative effects of aerosols, clouds, light scattering and absorption by gases, at 40 levels of the lower to middle atmosphere. Among the results were:

* The production of SOA increased from about 43 teragrams (1 Tg = 1012 grams) a year to 69 Tg a year since pre-industrial times, leading to an increase in the global annual mean SOA burden from 0.44 Tg to 0.70 Tg, or about 60 per cent.

* Emissions from fossil and biofuel burning contribute twice as much to the SOA increase as biomass burning emissions. * The increases are greatest over industrialised areas, as well as over regions with high emissions of precursor gases that will cause the formation of SOA.

* The increase is mainly caused by emissions of primary organic aerosols (POA) from fossil fuel and biofuel burning.

* The largest distribution increases in SOA at surface levels are in the biomass burning regions of South America, Southern Africa and South East Asia, as well as industrialised areas such as Europe and the East Coast of America.

* Increases higher in the atmosphere were seen in the northern Hemisphere, where significant increases were found at polar latitudes, while very small increases were found at high southern latitudes.

* Radiative forcing was much stronger over industrialised areas, in eastern Europe and on the east coast of the US.

Radiative forcing (measured in watts per square meter) provides a simplified means of comparing the various factors that are believed to influence climate change.

As yet, very few radiative forcing estimates of SOA exist and no radiative forcing estimates were provided for SOA in the latest IPCC report. The authors believe that the radiative forcing of SOA was previously underestimated and these results may improve estimates of future climate change.

1. EUCAARI (European Integrated project on Aerosol Cloud Climate and Air Quality Interactions) is supported by the European Commission under the Sixth Framework Programme. See: www.atm.helsinki.fi/eucaari/

Source: Hoyle, C.R., Myhre, G. , Berntsen, T.K. and Isaksen, I.S.A. (2008). Anthropogenic influence on SOA and the resulting radiative forcing. Atmospheric Chemistry and Physics Discussions. 8: 18911-18936. Contact: [email protected]