Global measurements and model calculations show that a simple formula can successfully capture the complex relationship between the chemistry and climate impact of aerosol particles.
The amount of water that aerosol particles can hold in the atmosphere determines how much of an impact they have on the climate. The capacity to hold water is referred to as hygroscopicity (K), which is determined by additional factors, most notably the particle size and chemical composition, which can be extremely variable and complex.
An international research team led by the Max Planck Institute for Chemistry (MPIC) and the Leibniz Institute for Tropospheric Research (TROPOS) was able to reduce the relationship between the chemical composition and the hygroscopicity of aerosol particles to a simple linear formula through extensive investigations. They demonstrated in a study published in the journal Nature Communications that hygroscopicity, when averaged globally, is largely determined by the proportion of organic and inorganic materials in the aerosol.
Our study was made possible by measurement campaigns with international partners in a wide range of locations around the world, as well as long-term observations at specific research stations, such as the ATTO observatory in the Brazilian rainforest.
Christopher Pöhlker
The hygroscopicity of aerosol particles is an important factor in the effect of aerosol particles on the climate and, as a result, in forecasting climate change using global climate models.”The capacity to hold water is determined by the composition of aerosol particles, which varies greatly in the atmosphere. However, we were able to demonstrate in our study that simplified assumptions can be made for the consideration of hygroscopicity in climate models,” Mira Pöhlker explains.
She is in charge of the “Atmospheric Microphysics” department at TROPOS and is a professor at the University of Leipzig. According to the aerosol and cloud researcher, this is the first study to use measurement results from across the world to show that a simple linear formula can be used without creating huge uncertainty in climate models.
Mira Pöhlker’s team evaluated data from 16 measurement campaigns between 2004 and 2020, during which hygroscopicity was determined using cloud condensation nuclei measurements and particle chemical composition was determined using aerosol mass spectrometry. The extensive data set covered a wide range of Earth’s regions and climate zones, from the Amazon’s tropical rainforest to Asian metropolitan areas with significant air pollution to the boreal pine forest of the Arctic Circle in Europe.
The evaluation of these data sets revealed: Effective aerosol hygroscopicity (κ) can be derived from the share of organic materials (ϵorg) and inorganic ions (ϵinorg) using a simple linear formula (κ = ϵorg ⋅ κorg + ϵinorg ⋅ κinorg). “Despite the chemical complexity of the organic matter, its hygroscopicity is successfully captured by the simple formula,” explains Christopher Pöhlker, Group Leader at the Max Planck Institute for Chemistry and co-author of the study. When averaged globally, he reports, hygroscopicity is κorg= 0.12 ± 0.02 for organic particle shares and κinorg = 0.63 ± 0.01 for inorganic ions.
Effect of the new formula on climate forecasts
The researchers used the global aerosol climate model ECHAM-HAM to put the new formula to the test. “In our study, we were able to use experiments to demonstrate that simplified assumptions in this area can be made without causing significant uncertainty in the model results.” This means that climate change investigations and forecasts are more reliable,” Mira Pöhlker summarizes. “Our study was made possible by measurement campaigns with international partners in a wide range of locations around the world, as well as long-term observations at specific research stations, such as the ATTO observatory in the Brazilian rainforest,” says Christopher Pöhlker of the Max Planck Institute for Chemistry in Mainz.
The interactions of atmospheric aerosols with solar radiation and clouds remain poorly understood and are among the greatest uncertainties in model description and forecasting of climate change. One reason for this is the numerous unanswered questions regarding the hygroscopicity of aerosol particles. Depending on their size and chemical composition, tiny aerosol particles can hold varying amounts of water. This is important for the scattering of solar radiation by aerosol particles as well as the formation of cloud droplets. Particles that contain more water scatter more sunlight back into the universe and can also have a cooling effect by forming more cloud droplets.
