IGACtivities No. 22
December 2000

Emission inventories of
natural sources

Contributed by
Jos Olivier, National Institute of Public Health and the Environment (RIVM),
The Netherlands.

Introduction

The Global Emissions Inventory Activity (GEIA) aims at providing global gridded emissions inventories to science and policy communities for all trace gases, in particular those that are relevant for global atmospheric chemistry. These emissions may stem from anthropogenic or natural sources. Natural sources may relate to processes in the soil (e.g., bacteria), in vegetation (plants), in the oceans (emission from dissolved amounts or produced by algae, or acting as a sink by absorption and dissolution in the water), in the Earth's crust (volcanoes or gas/oil seepage), or in the atmosphere (lightning). These emissions often depend strongly on climate, soil or water characteristics and thereby show a strong temporal variation in seasonality, diurnal cycle or both. Thus, natural sources often have a distinctly different character than have anthropogenic sources, which are comparably constant with respect to seasonality. In addition, the source strength as well as the spatial distribution of natural sources may differ substantially from year to year. An extreme example is volcanic emissions. Besides natural sources, natural sinks may occur locally, e.g., in some ocean areas where compounds are absorbed from the atmosphere.

Even if one is primarily interested in the effect of man-made emissions, the background of natural emissions (or sinks) of the same compounds and of related species must also be considered. For example, large datasets concerning temperature, precipitation, soil characteristics and oceanic parameters must be taken into account. Many of the natural emissions inventories are thus the result of modeling of the underlying processes using these global datasets. The resulting emission inventories either relate to a multi-year average showing the characteristic spatial and seasonal distribution of the natural emission sources, or present episodic inventories for specified years based on geo and climate data for these specific years.

In this paper we will review the GEIA emissions inventories of natural sources and also discuss others published in the literature, comparing their global source strengths with anthropogenic sources.

GEIA inventories

Species presently covered by GEIA are related to acidification, ozone depletion, tropospheric ozone formation, climate change, aerosol formation and pollutants such as heavy metals and persistent organic pollutants (POPs) that are poisoning people and ecosystems. The extent to which N–, S–, Cl–, and C–containing compounds and other species are emitted by natural sources differs from compound to compound. For example, in Figure 1 total global emissions from natural and anthropogenic sources of NOX, NH3 and N2O are compared. It clearly shows that the share of man-made emissions in the total source strength can differ considerably but also that uncertainties can be quite large.

Figure 1. Contribution and uncertainty of global anthropogenic and natural N emissions in 1990 [from Olivier et al. (1999)].

Emission inventories are available for the following species:

  • acidification: NOX from soils and lightning; NH3 from natural soils, oceans and wild animals
  • aerosol formation: SO2 from volcanoes; DMS from oceans
  • climate change: CH4 from wetlands, termites, oceans/hydrates; N2O from natural soils and oceans
  • tropospheric ozone: CO from vegetation and oceans; CO soil sink; NMVOC from vegetation
  • major reactive chlorine compounds: CH3Cl, CHCl3, CH2Cl2, C2HCl3, and C2Cl4 from oceans; CH3Cl and CHCl3 from land-based sources; HCl and ClNO2 from sea salt dechlorination.

For the following sources inventories are in progress:

  • radionuclides
  • emissions from natural biomass burning (wildfires in temperate regions) (for various species).

Natural CH4 inventories do not reside at the GEIA website, but are available at http://www.giss.nasa.gov/data/ch4fung. Inventories for DMS from oceans are published in the literature, but not yet available at the GEIA site. Natural CO emissions from vegetation and oceans are discussed in the literature and used by many atmospheric modellers, but to date no comprehensive emission inventory at 1º x 1º has been compiled. Within the EU-funded project 'POET' which studies the Precursors CO, NOx, CH4 and NMVOC of Ozone and their Effect on the Troposphere [Granier, 1999], the gridded inventory for CO from vegetation will be based on the GEIA inventory for NMVOC from vegetation by Yienger and Levy [1995] and the small ocean source of 13 Tg CO will be based on Bates et al. [1995]. For the soil sink of CO the project will not provide one dataset, since the models calculate this sink from the calculated surface CO concentration and the assumed deposition velocity. Resulting gridded emission inventories will be made available for the IGAC modeling community at large.

N– and S–inventories

For the natural sources of the nitrogen compounds NOx, N2O, NH3, mainly soils under natural vegetation and oceans, the reader is referred to Bouwman et al. [1995; 1997] for detailed descriptions of inventories of N2O and NH3, to Yienger and Levy [1995] for NOx from soils and vegetation and to Price et al. [1997a,b] for NOx from lightning. Davidson and Kingerlee [1997] present an alternative estimate of global NOX emissions from soils, but they do not provide gridded emissions. In Nevisson et al. [1995] a more detailed description is provided of N2O emissions (and regional sinks) from oceans. Furthermore, Seitzinger and Kroeze [1998] have developed a global inventory of N2O emissions from freshwater and coastal marine ecosystems, originating from N inputs from mainly anthropogenic sources (so-called indirect N2O emissions, mainly from agriculture). The vertical profile of lightning emissions may be taken from Pickering et al. [1998], who also provide an uncertainty range of 2-10 Tg N, whereas Price et al. [1997b] conclude from a constraint analysis that the range would be 5-25 Tg N. As is the case for soils, alternative datasets also exist for lightning in the literature, but these are not always available on a 1° x 1° grid. As illustrated in Figure 1, total natural emissions of nitrogen gases have a share of 40%, 20% and 80% in the global total emission of NOx, NH3 and N2O, respectively.

Table 1. Natural emissions of nitrogen and sulfur compounds.

On the sulfur emissions from volcanoes we refer to the separate paper by Robert Andres [this issue].

CO, CH4 and NMVOC inventories

Interestingly, for CO at present no inventories on a 1º x 1º basis are reported for the natural sources from vegetation and oceans. Müller [1992] presented a 10° x 10° inventory for oceanic emissions of 162 Tg based on Erickson [1989], but has revised his estimate to 20 Tg, and Bates et al. [1995] estimate the ocean source at 13 Tg CO. However, various estimates are reported in the literature and in the 1997 GIM/IGAC model inter-comparison of 3D tropospheric CO distributions a range of source strengths was used [Kanakidou et al., 1999]. For vegetation/soil the average was 160 Tg within a range of 100-280 Tg; for oceans these values were 50 (13-162) Tg. Table 2 provides an overview of the various estimates found in the literature, including the 'best estimate' provided by Khalil [2000] in the introduction of the special issue on CO of Chemosphere: Global Change Science, No. 1 of 1999. For comparison, we note that the second IPCC Assessment Report of 1996 estimated vegetation and ocean emissions to be in the range of 60-160 Tg and 20-200 Tg, respectively. Within the framework of EDGAR 3.0, an estimate is made of CO emissions from vegetation fires in temperate regions, which is 35 Tg in 1995 [Olivier et al., 2000]. In addition, the global budget includes a soil sink, of which the total strength and spatial distribution are generally calculated from the surface CO concentration and the assumed deposition velocity.

Table 2. Natural emissions of CO, CH4 and NMVOC.

Matthews and Fung [1987] presented a gridded inventory of CH4 from natural wetlands based on an extensive analysis and arrived at a source strength of 110 Tg. Matthews [2000] reviews the literature up to 1997 and concludes that estimates of wetland emissions are converging to a level around 100 Tg, however with an uncertainty still of about 50%. Recently, Darras et al. [1999] have reported on an IGBP-DIS Wetland Data Initiative to determine the global extent of wetlands. In this paper the Matthews and Fung dataset was compared to three other wetland datasets (ISLSCP, DISCover and Ramsar). Fung et al. [1991] and Gornitz and Fung [1994] provide gridded inventories for CH4 from termites and hydrate/clathrate in the Soviet Arctic and between 76º-85º N of 20 and 10 Tg CH4, respectively. These inventories are, however, highly uncertain. An alternative inventory of termites' emissions, with the same global source strength of 20 Tg is presented in Sanderson [1996]. All of these inventories, except for the termite CH4 of Sanderson, are available at http://www.giss. nasa.gov/data ch4fung.

Finally, within the framework of the POET project [Granier, 1999], a CH4 and NMVOC inventory for emissions from oil and gas seepage from continental shelves and from land will be developed. This will be based on Hovland et al. [1993] for CH4 from gas seepage through the seabed, presenting a global total estimate of 8-65 Tg of CH4, of which a fraction will pass the water column and into the atmosphere. This paper builds on a study by Wilson et al. [1994], which provides an estimate of natural marine oil seepage of 0.2-6 Tg oil discharged into the water with a best estimate of 0.6 Tg. A part of the oil will dissolve and emit into the air. Lambert and Schmidt [1993] argue that the oceanic source of methane is likely to be 50-60 Tg, of which 3.5 (3-4) Tg is emitted from open ocean. Clark et al. [2000] measured that gas bubbles at the ocean surface above seeps contain about 60% CH4 and about 10% NMVOC. The seepage inventories are on the continental shelf in addition to the hydrate/clathrate source estimated by Fung et al. [1991].

The major natural source of NMVOCs is vegetation, although the global source strength is rather uncertain [Guenter et al., 1995]. The main compound groups emitted by plants and trees are isoprene and monoterpenes. Also, there are a few small oceanic sources (see Table 2). In a separate paper in this issue the natural NMVOC inventory is discussed in more detail by Alex Guenther.

Reactive chlorine emission inventories (RCEI)

Recently a comprehensive set of gridded inventories of reactive chlorine compounds has been published in a series of eight consecutive papers in Journal of Geophysical Research - Atmospheres, 104 (D7), 8331–8440, April 1999. These include estimates of natural sources, of which the global source strength is summarized in Table 3. Oceans appear to account for about 12% of the global annual emissions of methyl chloride, much lower than often has been assumed. For chloroform, both oceans and land appear to be major sources.

Table 3: Natural emission of reactive chlorine species.

Acknowledgement

I thank Jean-Francois Müller for the informative discussions on the natural source inventories to be used in the POET project.

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