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Atmospheric Sciences & Global Change
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January 2011

Aerosols Become More Clear

Researchers examine the uncertainty of organic aerosols on climate

Solar Energy
Change in the effect of clouds on the global incoming and outgoing of solar and terrestrial energy to the Earth by tiny man-made particles is found to vary by 30% due to the uncertainties in our current understanding of how much water is absorbed by the tiny organic particles. Enlarge Image

Results:  Change in the effect of clouds on the global incoming and outgoing of solar and terrestrial energy to the Earth by tiny man-made particles—which are major contributors to smog and haze—is found to vary by a whopping 30%. According to a team of scientists, including Dr. Xiaohong Liu of Pacific Northwest National Laboratory, this large variation is caused by the different impacts of how much water is absorbed by the tiny organic particles, or organic hygroscopicity, on cloud droplet numbers in the present-day and in the time before the Industrial Revolution. This work presents the first study examining the uncertainty in organic aerosol hygroscopicity on climate.

Why it matters: Organic aerosols are currently the major topic in the aerosol research. Organics are among the most abundant aerosol components in the atmosphere, and the least understood. The organic aerosols may have significant impacts on cloud formation and climate change.

Methods: This study compared results in the present-day, in which human-emitted aerosols dominate the water uptake of particles and their cloud droplet formation, with pre-industrial results, before the Industrial Revolution when natural aerosol particles dominated the cloud droplet formation.

A Modal Aerosol Module in the National Center for Atmospheric Research's Community Atmospheric Model was used to examine sensitivities of aerosol indirect forcing, human impact on the cloud and precipitation formation, to hygroscopicity (represented by a single parameter "k") of primary organic aerosols, also known as POA, and secondary organic aerosols, SOA. The "k" value of POA is varied from 0 to 0.1, while the "k" value of SOA is varied by ±50% (from 0.14 to 0.07 and 0.21). These changes in "k" value of POA and SOA were well within the range of the current understanding of organic hygroscopicity. Pairs of present-day and pre-industrial simulations were performed to calculate the aerosol indirect forcing.

As organic hygroscopicity varies, the change in cloud droplet number concentration is less pronounced in present-day than that in pre-industrial, as the particle hygroscopicity at present-day is dominated by highly hygroscopic sulfate produced from anthropogenic (originating from industrial, domestic, and agriculture activities) emissions.

What's next: This work highlights the need for improved understanding of organic hygroscopicity and its representation in global models. The treatment of organic aerosol in the Modal Aerosol Module will be improved to better represent the different sources of primary organic aerosols and the mechanisms of secondary organic aerosols formation.

Acknowledgments: This study was supported by the DOE's Office of Science, DOE Atmospheric System Research, and the DOE Scientific Discovery through Advanced Computing program.

Research Team: Xiaohong Liu of Pacific Northwest National Laboratory and Jian Wang of Brookhaven National Laboratory.

Reference: Liu X and J Wang. 2010. "How Important Is Organic Aerosol Hygroscopicity to Aerosol Indirect Forcing?" Environmental Research Letters 5(4): Art. No. 044010. doi: 10.1088/1748-9326/5/4/044010.

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