The structure of atmospheric aerosol particles affects their reactivity and growth rates. Measurements of aerosol properties over the Amazon rainforest indicate that organic particles above tropical rainforests are simple liquid drops.
Most of us think of a plasticizer as an industrial solvent that is added to plastics or other materials to make them more flexible, workable or durable. But substances that act like plasticizers also occur in nature, and their effects can be profound. For the diverse population of microscopic aerosol particles suspended in air, water is a plasticizer. Whether a particle is liquid, solid or semi-solid affects its chemical reactivity, growth and physical properties. These properties influence its direct and indirect radiative effects as well as air quality. Writing in Nature Geoscience, Bateman and colleagues1 reveal that the plasticizing effect of water makes organic aerosols liquid over the humid Amazon.
Organic aerosols are composed of primary and secondary particulate organic matter. The primary material is emitted directly into the atmosphere, mostly from fossil fuel combustion and biomass burning, and consists mostly of hydrocarbons. The secondary material is formed in the atmosphere when volatile organic compounds emitted from these same sources, as well as from vegetation, are oxidized to low volatility products that can condense into particulate matter2. Over the past few decades, numerous studies have investigated the chemical and physical processes by which secondary organic aerosols are formed. A key step in this process is the partitioning of oxidized organic compounds between the gas and particle phases. Field and laboratory studies3 and thermodynamic analysis4 indicate that a significant fraction of particulate organic matter is likely to exist in a single liquid phase. This greatly simplifies the information needed to model the partitioning process and consequently, models of secondary organic aerosol formation have also assumed that particulate organic matter exists as a single liquid phase3.
It came as a great surprise to many, then, when in 20105 measurements from a boreal forest in Finland demonstrated that organic aerosol particles were not liquid, but solid or semi-solid. Since then, researchers have sought to better understand how factors such as chemical composition, humidity and temperature influence particle phase, and atmospheric models have been developed that can begin to account for the effects of phase on particle formation, growth and chemistry6.
Now, Bateman et al.1 show that organic particles are often liquid, at least in the Amazon, and that the observed differences in phase are primarily due to the effects of humidity.
Bateman and colleagues present measurements of these tiny aerosol particles sampled in the Amazon forest during the wet and dry seasons. They added laboratory studies in which particles were formed by oxidizing isoprene and α-pinene, the predominant volatile organic compounds emitted in the forest, under simulated atmospheric conditions. Measuring particles roughly one-hundredth the width of a human hair in a remote tropical forest is no small feat. Particles were identified as solid or liquid by using a home-built apparatus to determine whether a particle stuck or bounced when it impacted a surface, since only solid particles bounce. When the relative humidity was above 80%, essentially all particles stuck to the impaction surface — the particles were liquid.
Because even in the dry season the relative humidity in the Amazon is almost always higher than 80%, these observations suggest that aerosol particles there are almost always liquid. Since relative humidity levels are similar in most other tropical rainforests, atmospheric models evaluating the effects of particulate matter on air quality and climate can simply assume that particles are liquid over these regions. The study of Bateman et al. represents a wonderful complement to the earlier study in Finland5 that came to the opposite conclusion regarding the phase of aerosol particles in the much drier boreal forests of Eurasia and North America.
There are still a number of uncertainties regarding the phase state of organic aerosols. Future studies will need to fill in the geographical gap between the tropics and high latitudes. Although Bateman et al. suggest that biogenic organic aerosols with a range of compositions change from solid to liquid when approximately one-quarter of the particle volume is water, this relationship will need to be verified for aerosols formed from other organic precursors under a greater variety of conditions before it is adopted in models. Furthermore, although the study sampled some air affected by urban emissions, the effect on organic phase was inconclusive. The emphasis of the study was on clean conditions in which particles were entirely secondary organic matter. Since aerosols influenced by urban, marine, soil and biomass-burning emissions can contain inorganic salts as well as soot and soil dust, studies of the effects of these components on particle phase are necessary. And even when a particle is liquid, it may be composed of separate organic and aqueous phases with very different chemical and physical properties that can affect its growth, chemistry and radiative effects.
Bateman et al.1 highlight the biome-dependency and importance of relative humidity in determining the physical state of organic aerosols. Improving our understanding of the geographically variable and complex environmental controls over aerosol phase state will be essential for predicting how secondary organic aerosols form and for evaluating their impacts on climate.
About this article
Nature Communications (2017)