The concept of a “source strength” is much more difficult to define for aerosols than for most greenhouse gases. First, many aerosol species (e.g., sulphates, secondary organics) are not directly emitted, but are formed in the atmosphere from gaseous precursors. Second, some aerosol types (e.g., dust, sea salt) consist of particles whose physical properties, such as size and refractive index, have wide ranges. Since the atmospheric lifetimes and radiative effect of particles strongly depend on these properties, it makes little sense to provide a single value for the source strength of such aerosols. Third, aerosol species often combine to form mixed particles with optical properties and atmospheric lifetimes different from those of their components. Finally, clouds affect aerosols in a very complex way by scavenging aerosols, by adding mass through liquid phase chemistry, and through the formation of new aerosol particles in and near clouds. With regard to aerosol sources, we can report substantial progress over the previous IPCC assessment:

1. There are now better inventories of aerosol precursor emissions for many species (e.g., dimethylsulphide (DMS) and SO₂), including estimates of source fields for future scenarios. The present-day estimates on which this report is based are summarised in Table 5.2, see also Figure 5.2.
2. Emphasis is now on spatiotemporally resolved source and distribution fields.
3. There is now a better understanding of the conversion mechanisms that transform precursors into aerosol particles.
4. There is substantial progress towards the explicit representation of number/size and mass/size distributions and the specification of optical and hydration properties in models.

Table 5.2: Annual source strength for present day emissions of aerosol precursors (Tg N, S or C /year). The reference year is indicated in parentheses behind individual sources, where applicable.
	Northern Hemisphere	Southern Hemisphere	Globala	Range	Source
NOx (as TgN/yr)	32	9	41		(see also Chapter 4).
Fossil fuel (1985)	20	1.1	21		Benkovitz et al. (1996)
Aircraft (1992)	0.54	0.04	0.58	0.4-0.9	Penner et al. (1999b); Daggett et al. (1999)
Biomass burning (ca. 1990)	3.3	3.1	6.4	2-12	Liousse et al. (1996); Atherton (1996)
Soils (ca. 1990)	3.5	2.0	5.5	3-12	Yienger and Levy (1995)
Agricultural soils			2.2	0-4	Yienger and Levy (1995)
Natural soils			3.2	3-8	Yienger and Levy (1995)
Lightning	4.4	2.6	7.0	2-12	Price et al. (1997); Lawrence et al. (1995)
NH3 (as TgN/yr)	41	13	54	40-70	Bouwman et al. (1997)
Domestic animals (1990)	18	4.1	21.6	10-30	Bouwman et al. (1997)
Agriculture (1990)	12	1.1	12.6	6-18	Bouwman et al. (1997)
Human (1990)	2.3	0.3	2.6	1.3-3.9	Bouwman et al. (1997)
Biomass burning (1990)	3.5	2.2	5.7	3-8	Bouwman et al. (1997)
Fossil fuel and industry (1990)	0.29	0.01	0.3	0.1-0.5	Bouwman et al. (1997)
Natural soils (1990)	1.4	1.1	2.4	1-10	Bouwman et al. (1997)
Wild animals (1990)	0.10	0.02	0.1	0-1	Bouwman et al. (1997)
Oceans	3.6	4.5	8.2	3-16	Bouwman et al. (1997)
SO2 (as TgS/yr)	76	12	88	67-130
Fossil fuel and industry (1985)	68	8	76	60-100	Benkovitz et al. (1996)
Aircraft (1992)	0.06	0.004	0.06	0.03-1.0	Penner et al. (1998a); Penner et al. (1999b); Fahey et al. (1999)
Biomass burning (ca. 1990)	1.2	1.0	2.2	1-6	Spiro et al. (1992)
Volcanoes	6.3	3.0	9.3	6-20	Andres and Kasgnoc (1998) (incl. H2S)
DMS or H2S (as TgS/yr)	11.6	13.4	25.0	12-42
Oceans	11	13	24	13-36	Kettle and Andreae (2000)
Land biota and soils	0.6	0.4	1.0	0.4-5.6	Bates et al. (1992); Andreae and Jaeschke (1992)
Volatile organic emissions (as TgC/yr)	171	65	236	100-560
Anthropogenic (1985)	104	5	109	60-160	Piccot et al. (1992)
Terpenes (1990)	67	60	127	40-400	Guenther et al. (1995)
a The global figure may not equal the sum of the N. hemisphere and S. Hemisphere totals due to rounding.

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