The Role Of Water In Aerosol Climate Forcing

We’re familiar with the role greenhouse gases play as the driving force behind climate change, through their absorption of longwave radiation and heating of the lower atmosphere. A lesser-known participant in radiative forcing is atmospheric aerosol. Because of their short lifetime that is on the order of a week, the aerosol impact on climate is regional rather than the global impact of longer-lived species like CO₂ and CH₄. These tiny particles force climate by blocking short-wave visible radiation, thus creating a net cooling effect on the lower atmosphere, known as direct radiative forcing. Like CO_2, aerosols are products of fossil fuel and biomass burning but also form from soil and concrete dust, sea salt and organic biogenic emissions.

In addition, these tiny particles form the seed nuclei for cloud droplets. The aerosol number, size and chemistry influence cloud properties such as the cloud reflectivity or albedo, the cloud lifetime and precipitation probability. An injection of small pollution aerosol into a cloud can reduce the cloud droplet size, enhance the cloud albedo and also change the cloud lifetime.

Aerosols, water, clouds, and radiation form a closely coupled system with multiple feedbacks that perturb the climate by cooling or warming the atmosphere, altering the hydrological cycle and potentially affecting small-scale circulation. Predicting this forcing is a challenge not only from their complex interactions but also due to the high variability of these four parameters in space and time. A first step in understanding these interdependencies is to characterize the uptake of water onto aerosols.

In our Journal of Atmospheric Research paper we characterized changes in the aerosol water uptake related to the aerosol size and chemistry by studying changes in the aerosol optical properties: how aerosol scatter and absorb visible light. The data for this paper comes from a long-term study of aerosol hygroscopic growth at a Department of Energy Climate Research Facility in Oklahoma. The aerosol from this site is typical of a mid-latitude, agricultural region with seasonal growing cycles. Data and derived parameters from this study will serve as input to larger climate models or satellite data retrievals that help define the impact of aerosols on climate.

As aerosol grows in size from water uptake the aerosol scattering of light increases. The amount of scattered light scales with the aerosol surface area such that a 20% increase in a 1um diameter particle from water uptake translates roughly to a 44% increase in the light scattered from that particle. We use the strong relationship of aerosol size to the scattered light wavelength and angular dependence to gauge how particle size correlates to the aerosol’s hygroscopic growth potential. We then tied the aerosol chemistry to its size and hygroscopic growth with mass spectrometer measurements of the aerosol inorganic and organic components.

The results of this work show aerosol hygroscopic growth behavior varied with the aerosol size mode. Aerosol uptake of water with %RH in the smaller mode increased with the aerosol size. While water uptake with %RH in the larger mode decreased with particle size. The aerosol chemistry measurements helped explain this behavior by showing the organic composition to decrease and the inorganic salt composition to increase with particle size in the smaller mode. The lower growth with increasing size in the larger aerosol mode was likely due to a larger contribution from hygrophobic dust particles.

In a larger climate framework, the models can use this information to predict the aerosol influence on cloud and radiative forcing with emissions of organics, nitrates, sulfates, and dust. Changes in fire patterns, soil moisture, and burning of fossil fuels alter the aerosol size, chemistry, uptake of water and in turn the Earth’s energy balance and cloud properties.

This study, Seven years of aerosol scattering hygroscopic growth measurements from SGP: Factors influencing water uptake was recently published in the Journal of Geophysical Research: Atmospheres.