RCEI: Reactive Chlorine Emission Inventory

This activity was championed by Professor Bill Keene of the University of Virginia, who led the programme in the 1990s. There is now a group of over 18 scientists working in Canada, Germany, Russia, UK and USA. Their work has been published in eight papers which make up a special issue of the Journal of Geophysical Research.^1-8

The aims of the programme were to provide information on global atmospheric cycles of a number of chlorine compounds and so enable atmospheric scientists to calculate the extent to which the chemistry of processes in the air is influenced by chlorine atoms. These are powerful oxidising agents but are much less abundant than the hydroxyl radicals which sustain most of the processes that remove atmospheric pollutants.

The inventory gives values for the natural and man-made fluxes into the atmosphere during 1990, of the major chemical compounds that can react in the lower atmosphere to affect the reactive chlorine balance. The compounds enumerated were hydrogen chloride (HCl), nitryl chloride (ClNO₂), chloromethane (CH₃Cl), chloroform (CHCl₃), methyl chloroform (1,1,1-trichloroethane, CH₃CCl₃), tetrachloroethene (C₂Cl₄), trichloroethene (C₂HCl₃), dichloromethane (CH₂Cl₂), HCFC-22 (chlorodifluoromethane, CHClF₂) and particulate material containing chlorine. The sources are oceanic and terrestrial (fungi, soils and volcanoes), which are natural; biomass burning, which is mainly anthropogenic; and fossil fuel combustion, waste incineration and industrial processes, which are wholly anthropogenic. Nitryl chloride fluxes result from reactions of sea salt aerosol in the atmosphere and were calculated in this work.

The global fluxes estimated from these sources are shown in Table 1. Industrial processes contribute about eight times less than combustion processes (which are mainly anthropogenic). Only methyl chloroform and HCFC-22 have no natural sources. Significant fractions of dichloromethane (25%), trichloroethene (10%) and tetrachloroethene (5%) appear to be emitted from the oceans, although the numbers are rather uncertain, and mankind's activities account for about half of the methyl chloride in the atmosphere (in this case from biomass burning) but less than 20% of the chloroform.

Table 1. Annual Global fluxes for Major Sources of Reactive Chlorine into the Atmosphere.

Units are Tg Cl yr^-1 (10¹² grams of reactive chlorine per year)

Species	Ocean		Sea-salt	Terrestrial Ecosystems		Volca-noes	Biomass burning	Fossil Fuel burning	Waste incine-ration	Industrial processes
Species	Open	Coastal	Sea-salt	Fungi	Soils	Volca-noes	Biomass burning	Fossil Fuel burning	Waste incine-ration	Industrial processes
Particulate Cl	-	-	1785	-	15	-	-	-	-	-
HCl*	-	-	7.6	-	-	2	-	4.6	2	-
ClNO₂*	-	-	0.06	-	-	-	-	-	-	-
Tot. inorg. Cl	-	-	1785	-	15	2	6.3	4.6	2	-
CH₃Cl	0.46	0.0001	-	0.11	-	-	0.64	0.075	0.032	0.007
CHCl₃	0.32	0.0002	-	-	0.18	-	0.002	-	-	0.062
CH₃CCl₃	-	- -	-	-	-	-	0.013	-	-	0.572
C₂Cl₄	0.016	- -	-	-	-	-	-	0.002	-	0.313
C₂HCl₃	0.020	- -	- -	-	-	-	-	0.003	-	0.195
CH₂Cl₂	0.16	0.0003	- -	-	-	-	0.049	-	-	0.487
CHClF₂	-	- -	--	-	-	-	-	-	-	0.080

* HCl and ClNO₂ arise from sea-salt dechlorination in the atmosphere and are included in total inorganic chlorine

To be of value in modelling chemical processes going on in the atmosphere, the emissions need to be resolved on the same sort of geographical scale as those processes. Thus anthropogenic emissions, including those from biomass burning, were estimated for each of the 64,800 "gridboxes" bound by one degree of latitude and longitude. For man-made emissions in general the emission from each gridbox was calculated from the global fluxes by distribution using national economic statistics and population densities. Where potential point sources were known, such as chloromethane from coal combustion in power generation, subsidiary grids of these activities were developed. Natural sources, particularly those from oceans, are less clearly defined; consequently the best that could be achieved was latitudinal distribution in six bands, each of 30^o.

The investigators were able to conclude that most of the volatile chlorine in the troposphere is produced in the northern hemisphere in the winter. This is due to seasonally large amounts of sea salt aerosol and high levels of nitric acid, sulphur dioxide and dinitrogen pentoxide (which acidify the aerosol, releasing hydrogen chloride and nitryl chloride). Anthropogenic components appear to exert little influence on the extent or location of chlorine production.

Uncertainties in the distributions of emissions of individual compounds could be reduced with further work and it would be instructive to assess the extent to which the anthropogenic emissions are changing in time. RCEI is therefore looked on as a continuing process and it is anticipated that the gridded inventories will be updated early in the 21st century. A description of the programme, together with the complete texts of all papers and downloadable versions of the gridded inventories are on the RCEI website.

RCEI is a subgroup of the Global Emissions Inventory Activity (GEIA), itself part of the International Global Atmospheric Chemistry (IGAC) core project of the International Geosphere-Biosphere Program (IGBP). Financial support to RCEI was provided by the Chemical Manufacturers' Association through the Chlorine Chemistry Council and by Cefic (the European Council of Chemical Industry Federations) through Euro Chlor.

References:

1. Graedel T.E. and W.C. Keene, Overview: Reactive Chlorine Emissions Inventory, J. Geophys. Res., 104, 8331-8333, 1999.

2. Khalil M.A.K., R.M. Moore, D.B. Harper, J.M. Lobert, D.J. Erickson, V. Koropalov, W.T. Sturges and W.C. Keene, Natural emissions of chlorine containing gases: Reactive Chlorine Emissions Inventory, J. Geophys. Res., 104, 8333-8346, 1999.

3. Erickson D.J., C. Seuzaret, W.C. Keene and S.L. Gong, A general circulation model based calculation of HCl and ClNO2 production from sea salt dechlorination: Reactive Chlorine Emissions Inventory, J. Geophys. Res., 104, 8347-8372, 1999.

4. Lobert J., W.C. Keene, J.A. Logan and R. Yevich, Global chlorine emissions from biomass burning: Reactive Chlorine Emissions Inventory, J. Geophys. Res., 104, 8373-8390, 1999.

5. McCulloch A., M.L. Aucott, C.M. Benkovitz, T.E. Graedel, G. Kleiman, P.M. Midgley and Y.-F. Li, Global emissions of hydrogen chloride and chloromethane from coal combustion, incineration and industrial activities: Reactive Chlorine Emissions Inventory, J. Geophys. Res., 104, 8391-8403, 1999.

6. Aucott M.L., A. McCulloch, T.E. Graedel, G. Kleiman, P. Midgley and Y.-F. Li, Anthropogenic emissions of trichloromethane (chloroform, CHCl₃) and chlorodifluoromethane (HCFC-22): Reactive Chlorine Emissions Inventory, J. Geophys. Res., 104, 8405-8415, 1999.

7. McCulloch A., M.L. Aucott, T.E. Graedel, G. Kleiman, P.M. Midgley and Y.-F. Li, Industrial emissions of trichloroethene, tetrachloroethene and dichloromethane: Reactive Chlorine Emissions Inventory, J. Geophys. Res., 104, 8417-8427, 1999.

8. Keene W.C., M.A.K. Khalil, D.J. Erickson, A. McCulloch, T.E. Graedel, J.M. Lobert, M.L. Aucott, S.-L. Gong, D.B. Harper, G. Kleiman, P. Midgley, R.M. Moore, C. Seuzaret, W.T. Sturges, C.M. Benkovitz, V. Koropalov, L.A. Barrie and Y.-F. Li, Composite global emissions of reactive chlorine from anthropogenic and natural sources: Reactive Chlorine Emissions Inventory, J. Geophys. Res., 104, 8429-8440.