Effects of Relative Humidity on Aerosols—Implications for Climate

Lacis, A. A., NASA - Goddard Institute for Space Studies

Aerosol Properties

Aerosol

"Refractive Indices of Three Hygroscopic Aerosols and their Dependence on Relative Humidity," October 2001.

Sponsored by the DOE Atmospheric Radiation Measurement (ARM) Program, science collaborators at the National Aeronautics and Space Administration (NASA) Goddard Institute for Space Studies (GISS) have created a new parameterization of hygroscopic aerosol properties for use in climate models. They have applied this parameterization in the GISS climate model to study the effects of aerosol on climate and are making it available to the broader scientific community. The parameterization begins with a set of laboratory measurements and then provides routines to interpolate the measurements across the full range of atmospheric conditions.

Many atmospheric aerosols—airborne particles that reflect the sun's energy away from earth and into space—are very sensitive to relative humidity (RH) changes. For example, when a hygroscopic aerosol—an aerosol that absorbs moisture from the air—begins to dissolve, its properties change as it absorbs water and the aerosol gets larger. Most hygroscopic aerosols stay in a dry, crystalline form in the atmosphere if the local RH is below some critical value around 65%. If the RH rises above that level, they absorb water and dissolve. As the RH increases, the particles continue to grow in size by absorbing more water from the air. If the RH level falls below the critical value, the aerosol begins to lose water and returns to its original dry state.

Larger particles are more effective at intercepting solar energy and therefore reflect more energy back to space. By determining how RH affects the size of the aerosol, scientists can calculate the effects aerosols have on solar and infrared radiation and subsequent impacts on climate change.

Using their parameterization in the NASA GISS general circulation model, the scientists included RH effects on aerosol size, real and imaginary refractive indices, and on scattering radiative parameters over the full range of solar and thermal wavelengths. Hygroscopic aerosols included in the parameterization are ammonium sulfate, sea salt, ammonium nitrate, and organic carbon. The scientists found that, compared to dry aerosols, the effects of RH increased the radiative forcing (which is a decrease in solar energy input to the earth system) as a result of sea salt by a factor of 2; of sulfate aerosols by about 50%; and of nitrate aerosols by about 25%.

This research is critical to understanding the effects that RH has on the size of aerosol properties, and the effect of aerosols on climate. Further treatment of aerosols is needed to help compare and corroborate general circulation model aerosol fields with ground-based and satellite measurements.