Aerosol Swelling — A Source of Error When Estimating the Aerosol First Indirect Effect

Scientists use data from four ARM sites to understand differences between observationally-based and model-based estimates of the aerosol effect on cloud properties.

The Science

Aerosol particles in the atmosphere can impact the Earth’s energy balance directly through scattering and absorbing solar energy and indirectly by impacting cloud properties. In the aerosol “first indirect effect,” an increase in aerosol number concentration while the liquid water amount in the cloud stays the same leads to more, but smaller cloud droplets and higher solar reflectivity. As cloud condensation nuclei (CCN; the number of aerosol particles that serve as nuclei for cloud droplets) is difficult to measure globally, scientists often use aerosol optical depth (which can be measured by visible satellite or ground-based radiometers) as a proxy for CCN when estimating the first indirect effect. Estimates for the first aerosol indirect effect from ground-based, aircraft, and satellite measurements vary widely and are often lower than the values produced by model simulations. For this reason, aerosol-cloud interactions are considered one of the largest current sources of uncertainty in climate model projections. In this study, scientists use detailed measurements from four ARM sites around the world to investigate reasons for the difference in observationally-based and model-based estimates of the aerosol indirect effect.

The Impact

The study finds that swelling of aerosol particles in high relative humidity conditions may be an important factor in differing estimates of the aerosol first indirect effect. The amount of swelling that an individual aerosol particle experiences is based on the chemical composition of the aerosol particle as well as the environmental conditions. As aerosol particles swell, their optical scattering increases, leading to a larger aerosol optical depth even though the concentration of aerosol particles is unchanged. Therefore, observationally-based estimates that use aerosol optical depth as a proxy for CCN may underestimate the first indirect effect. This finding helps explain the systematic difference between satellite-based estimates of the first indirect effect and those simulated by earth system models.


Scientists used extensive measurements of aerosol and cloud properties made at four Atmospheric Radiation Measurement sites around the world to identify and quantify the influence of aerosol hygroscopicity and aerosol swelling on the aerosol first indirect effect. These sites have distinct aerosol properties and experience different meteorological conditions. The magnitude of the first indirect effect for aerosol particles with stronger aerosol hygroscopicity is systematically larger than that for aerosol particles with weaker aerosol hygroscopicity. A one-unit enhancement in the aerosol scattering coefficient by the swelling effect leads to a systematic underestimation of the aerosol first indirect effect by about 23%. This can result in a significant underestimation of the indirect effect-related radiative forcing (by several W m-2 depending on aerosol properties and relative humidity). This likely contributes significantly to the systematic difference between observation-based, especially satellite-based, estimates of the first indirect effect and those simulated by general circulation models. It may also partially explain systematic variations in the first indirect effect with water vapor amount as noted by others.

Principal Investigator(s)

Zhanqing Li
University of Maryland


The study is supported by the following research grants: the National Science Foundation of U.S. (AGS1534670) and China (91544217) and the Key R&D Program (2017YFC1501702). Ground-based observations from the U.S. Southern Great Plains, Graciosa Island in the Azores, the Ganges Valley in India, and ShouXian, China, were obtained from the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program (Data can be accessed at http://www.archive.arm.gov/discovery/). The large-scale dynamic and thermodynamic data were obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) model runs for ARM analysis provided by the ECMWF.


Liu, J. and Z. Li. “Significant Underestimation in the Optically Based Estimation of the Aerosol First Indirect Effect Induced by the Aerosol Swelling Effect.” Geophysical Research Letters, 45(11), 5690-5699 (2018). [DOI: