Air pollution emissions

Anthropogenic and Natural Emissions

Most pollutants are emitted both by natural as well as by anthropogenic sources. Natural sources are not influenced by humans or by human-induced activities. Volcanoes are a good example of this type of source. Many emissions are biogenic, i.e., produced by living organisms, but these emissions are very often influenced by human’s activities. Nitrous oxide (N₂O) is a greenhouse gas that is for a large part emitted during nitrification and de-nitrification processes (the conversion of ammonium to nitrate and of nitrate to N₂O and ammonium respectively), which take place in the soil. The largest N₂O emissions are observed where nitrogen-containing fertilizer is applied in agriculture.

The ratio between anthropogenic and natural emissions is very important, as only the anthropogenic portion can be influenced, e.g., by abatement measures. A good example is provided by abatement measures for photochemical smog, with ozone and peroxyacyl nitrates (PAN) as important secondary products. Photochemical smog is caused by nitrogen oxide (NO_x) and volatile organic compound (VOC) emissions. Therefore, the U.S. Environmental Protection Agency (EPA) decided, some 20 years ago, to combat photochemical smog in the U.S. by stringent reduction of VOC emissions and to a lesser extent, reduction of NO_x; however, this policy did not produce the desired results. Natural emissions, especially of terpenes and isoprenes, proved to be so large that sufficient VOC concentrations were available in the atmosphere for oxidant production, despite efforts to limit anthropogenic sources.

Emissions Acid Deposition and Oxidant Formation

Table 1. Global, natural and anthropogenic emissions of SO2 and NOx
Emissions in Tg (1012)/year	SO2/IPCC	NOx/Graedel	Volatile Organic comp.
Biomass burning	2.2	6	45
Volcanoes	9.3	-	-
Lightning	-	5	-
Biogenic emissions from land areas	1.0	15	350 (isoprenes) + 480 (terpenes)
Biogenic emissions from oceans	24	-	27
Industrial and utility activities	76	22	45
Solvents	-	-	15
Total natural emissions	34	21	855
Total anthropogenic emissions	78	27	105
Total emissions	113	48	960

The anthropogenic sources of sulfur dioxide(SO₂), (see Table 1) are much larger compared to natural sources; in fact, they exceeded natural sources as early as 1950. The natural emissions of NO_x are, according to the estimate compiled by Graedel and Crutzen, in the same order of magnitude as anthropogenic emissions. Some NO_xsources, especially natural ones, are quite uncertain, as is the contribution of natural sources to ambient VOC concentrations.

Table 2. Overview of emissions of sulfur, nitrogen and organic compounds (Source: IPCC, 2005)

(It should be noted that the emissions due to biomass burning and from land areas are partly natural and partly anthropogenic. "IPCC" indicates data provided by Intergovernmental Panel on Climate Change. "Graedel" indicates data provided by Graedel and Crutzen).

Natural VOC emissions are predominant on a global scale, but near industrialized areas anthropogenic sources are the most important contributors to ambient concentrations.

A very recent overview of emissions of sulfur and nitrogen compounds is presented in Table 2. In this table the differences in emissions between Northern and Southern Hemisphere are highlighted, which makes clear that, in general, the North's contribution to emissions is much larger.

Sulfur dioxide emissions are declining in Europe and the U.S., while they remain constant or are increasing slightly in developing countries. Nitrous oxide emissions have stabilized or declined in developed countries, but are significantly increasing in developing countries. These trends suggest that acid deposition (a consequence of sulfur dioxide emissions) is being exchanged with exposure to increased oxidant concentrations.

Emissions: Global Problems

Figure 1. Ratio of historical/present concentrations of greenhouse gases

An extreme case is reactive chlorine (the main compound responsible for destroying stratospheric ozone, see Antarctic ozone hole), where at least 90% of the relevant emissions are anthropogenic. Chlorofluorocarbons (CFCs) were widely applied, beginning in 1945, until their use was banned through enactment of the Montreal Protocol in 1998. No natural sources of these compounds are known.

Table 3. Overview of the emissions of greenhouse gases

In Figure 1 the atmospheric concentrations of the most important greenhouse gases (CO₂, CH₄,CFCs and N₂O) have been plotted from 1850 until present as a fraction of the present global ambient concentration. If it is assumed that ambient concentrations vary linearly with emissions (which is not wholly accurate), and that only natural emissions are responsible for concentrations before the year 1800, we can see that the ratio of natural versus anthropogenic sources varies significantly for the different greenhouse gases; the ratio is highest for N₂O and lowest for CFCs.

A simplified overview of the emissions of greenhouse gases is given in Table 3. Natural sources for CO₂ are not mentioned as the exchange between atmosphere and ecosystems, consists actually of largecarbon cycles (e.g., decay of biomass in the Fall and uptake during leaf formation and growth in the Spring) and these cycles of uptake and emissions are in equilibrium, as explained in the article on the greenhouse effect.

Aerosols

Table 4. Natural and human-made primary emissions of aerosols

Aerosols, defined as solid or liquid particles which have a certain life time suspended in air (particle size varying from a few nanometers to several hundred micrometers, as in the case of dust particles), are either emitted as such (primary aerosol), or are formed by way of the transformations ofpollutants such as sulfur dioxide, nitrogen oxides and ammonia into sulfates,nitrates and ammonium respectively (secondary aerosol). Many volatile organic compounds are converted to oxidized organic species with low volatility, thus becoming a component of ambient aerosol.

Ambient aerosol concentrations are partly of primary and partly of secondary nature. Sea salt particles caused by waves and dust swept up by wind are examples of primary aerosol. Primary aerosol emissions, especially dust and seasalt particles, constitute the largest contribution to total aerosol mass. An overview of primary emissions, both natural and human-made is given in Table 4.

Table 5. Secondary emissions of aerosols

A large part of secondary aerosols, however, consists of sulfates and, depending on the specific conditions,nitrate and organic compounds, which are formed by atmospheric conversion with sulfur dioxide, nitrogen oxides and VOCs as precursors. An overview of the amount of secondary aerosol is found in Table 5. Near areas of high emissions (e.g., industrialized regions of Europe, the U.S. and Asia) the anthropogenic (human-made) emissions are more important than the natural.

Sulfate still represents the largest fraction of anthropogenic contributions to ambient aerosols, but the present composition of aerosol reflects a shift in emissions, especially in developed countries in the last 10 years. For example, emissions of sulfur oxides have gone down due to measures to reduce acid deposition. Emissions from the transportation sector have been stabilized or even increased, notwithstanding the introduction of exhaust catalysts in cars, because some countries, e.g., the U.S. and Europe have seen an increase in the number of cars on the road as well as the total driven distances per car. As a result, there has been a shift from sulfate to nitrate in ambient aerosol.

Aerosols have a large impact on local pollution problems, but also contribute to regional environmental issues (e.g., acidification) and global environmental issues (e.g., the Antarctic ozone hole and climatic change).