Aerosols Before Pollution

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Science  05 Jan 2007:
Vol. 315, Issue 5808, pp. 50-51
DOI: 10.1126/science.1136529

Atmospheric aerosols play a large role in human-induced climate change because of their effects on solar radiation transfer and cloud processes. To assess the impact of human perturbations on the atmosphere's aerosol content, we need to know the prehuman aerosol burden. This is especially important for understanding the cloud-mediated effects of aerosols on climate, because cloud properties respond to aerosols in a nonlinear way and are most sensitive to the addition of particles when the background concentration is very low (1). Because cloud droplets can nucleate only on particles above a certain size (typically about 60 to 90 nm), this subset of the aerosol population—called cloud condensation nuclei (CCN)—is of particular importance. In the following, I try to provide a rough estimate of what CCN concentrations might have been in the prehuman atmosphere.]

Information about atmospheric aerosol contents in the absence of human activity is very difficult to obtain. Human activities are causing the emission of huge amounts of aerosol particles and their gaseous precursors. Aerosol particles have typical atmospheric lifetimes of 3 to 10 days; on average, after three such lifetimes, about 5% of the initial burden remains in the atmosphere. Given that air masses can easily travel several thousand kilometers in 15 days, there are really no places where we can expect to find truly pristine conditions, especially in the Northern Hemisphere.

Aerosol concentrations approaching pristine conditions are mostly found over the oceans, especially in the Southern Hemisphere, where large expanses of open ocean and a low density of population and industry contribute to keeping the human impact at a minimum. The natural aerosol over these remote ocean regions consists mainly of a mixture of sea salt particles, organics, and sulfates from the oxidation of biogenic dimethylsulfide; some mineral dust and smoke from wildfires may also be present (see the figure). In biologically productive ocean regions, typical concentrations of CCN are in the low hundreds per cm3. Much lower concentrations of a few tens of CCN per cm3 are found over the mid-latitude oceans in wintertime, when biological and photochemical activity are low.

The determination of pristine CCN concentrations over continental regions presents a much more difficult problem. Measurements at sites away from obvious sources of pollution are very few, and even among these data, it is usually difficult to assess how much of the observed aerosol results from pollution. Aerosol compositions at remote sites in the Northern Hemisphere suggest that the continental “background” aerosol nowadays consists mostly of pollution aerosols at varying levels of dilution: The concentration of black carbon, a unique indicator of combustion and pollution, is strongly correlated to that of the dominant sulfate and organic aerosols (2). Even in the Southern Hemisphere, pollution aerosols, especially from biomass burning, dominate in most continental areas, with CCN concentrations typically in the upper hundreds to thousands per cm3.

Sources of aerosol particles to the natural atmosphere.

Primary particles—such as sea spray, soil dust, smoke from wildfires, and biological particles including pollen, microbes, and plant debris—are emitted directly into the atmosphere. Secondary particles are formed in the atmosphere from gaseous precursors; for example, sulfates form from biogenic dimethyl sulfide and volcanic sulfur dioxide (SO2), and secondary organic aerosol from biogenic volatile organic compounds.

Over remote continental regions, the cleanest conditions prevail when unpolluted air masses of marine origin flow over nearly uninhabited lands. For example, measurements have been made in the center of the Amazon Basin during the rainy season, when clean air masses from the Atlantic Ocean are transported for several days over the Amazon forest. CCN concentrations were in the low hundreds per cm3, more or less identical to the concentrations over the tropical oceans (3). Similar concentrations have been reported from other remote continental sites, such as southeast Australia, the western United States and Alaska, and northern Finland (47). Clearly, all these measurements represent upper limits to the natural CCN populations, because even these locations are influenced to varying degrees by the long-range transport of pollution.

An alternative way of assessing the pristine continental CCN background is by estimating the number of new particles in the CCN size range produced from biogenic precursors at remote sites. During summer, bursts of particle production occur in such places about twice a week, but this mechanism cannot sustain a substantial CCN population on a continuous basis. To get a more representative perspective on aerosol particle formation, Tunved et al. (8) determined the increase in the number of particles as air masses traveled from the Atlantic over land to research sites in Finland. Particle numbers increased with travel time and the rate of terpene emission from plants. At typical terpene emission rates, total particle concentrations of ∼1000 to 2000 per cm3 were reached, of which ∼100 to 300 were larger than 90 nm and therefore potential CCN.

These data are from a region where nucleation is favored because of trace amounts of anthropogenic SO2, and they only apply to the spring and summer seasons. Thus, they probably still represent an upper limit to natural CCN production at mid-latitudes. Overall, natural production of CCN-active particles over biologically active regions on the continents probably cannot account for more than 100 to 300 per cm3, not much greater than the levels found over the oceans. During the cold seasons, much lower particle production must be expected.

In recent years, modelers have tried to reproduce pristine aerosol conditions by running their global chemistry/transport/climate models with industrial or anthropogenic sources turned off (9). Unfortunately, the production rates and mechanisms for primary biogenic aerosols (plant particles, spores, microbes, etc.) and secondary organic aerosols (from natural hydrocarbons) are still very poorly understood. These two components may be responsible for a large fraction of the natural continental aerosol, and current model results can therefore only be considered rough estimates of preindustrial aerosol abundance over the continents. This applies especially to number concentrations and size distributions, which are our primary concern here.

I am not aware of any modeling studies that have attempted to look at the atmosphere before the advent of humans. Instead, the models use as a reference state either the preindustrial period or the present-day atmosphere with anthropogenic sources turned off. All models agree that anthropogenic emissions have caused large enhancement of aerosol loads even over remote parts of the continents, with typical enhancements by 50 to 300% over remote regions of Asia, North America, and South America. From these studies, we can estimate preindustrial CCN concentrations over the continents of 50 to 200 per cm3, similar to the values over the remote oceans in the same models. Higher aerosol concentrations are predicted over the tropical continents, because of biomass burning by preindustrial human populations.

Thus, prehuman aerosol levels may have been very similar over continents and oceans, ranging from a few tens per cm3 in biogenically inactive regions or seasons to a few hundreds per cm3 under biologically active conditions. This conclusion renders invalid the conventional classification of air masses into maritime and continental according to their aerosol content. It also implies that, before the onset of human-induced pollution, cloud microphysical properties over the continents resembled those over the oceans, whereas nowadays, cloud processes over most of the continents are shaped by the effects of human perturbation.


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