Chapter 7: Precursors and Indirect Emissions

Although they are not included in global warming potential-weighted greenhouse gas emission totals, emissions of carbon monoxide (CO), oxides of nitrogen (NOx), non-methane volatile organic compounds (NMVOCs), and sulphur dioxide (SO2) are reported in greenhouse gas inventories. Carbon monoxide (CO), Nitrogen oxides (NOx) and NMVOC in the presence of sunlight contribute to the formation of the greenhouse gas ozone (O3) in the troposphere and are therefore often called ‘ozone precursors’. Furthermore, NOx emission plays an important role in the earth’s nitrogen cycle. Sulphur Dioxide emissions lead to formation of sulphate particles, which also play a role in climate change. Ammonia (NH3) is an aerosol precursor, but is less important for aerosol formation than SO2. Section 7.2 addresses the estimation and reporting of the precursors for national inventories. The methodologies for ambient air quality emission inventories have been elaborated in detail in the EMEP1 /CORINAIR Emission Inventory Guidebook (Guidebook), and these methodologies for CO, NOx, NMVOCs, and SO2 emissions are referenced in this chapter rather than to be included in the 2006 IPCC Guidelines for National Greenhouse Gas Inventories (2006 Guidelines). Exceptions are for sources not well-covered by the Guidebook.

Section 7.3 addresses nitrous oxide (N2O) emissions that result from the deposition of the nitrogen emitted as NOx and NH3. Nitrous oxide is produced in soils through the biological processes of nitrification and denitrification. Simply defined, nitrification is the aerobic microbial oxidation of ammonium to nitrate and denitrification is the anaerobic microbial reduction of nitrate to nitrogen gas (N2). Nitrous oxide is a gaseous intermediate in the reaction sequence of denitrification and a by-product of nitrification that leaks from microbial cells into the soil atmosphere. One of the main controlling factors in this reaction is the availability of inorganic nitrogen in the soil and therefore deposition of nitrogen resulting from NOx and ammonia (NH3) will enhance emissions. N2O emissions will also be enhanced if nitrogen is deposited in the ocean or in lakes. For this reason the 2006 Guidelines include guidance for estimating N2O emissions resulting from nitrogen deposition of all anthropogenic sources of NOx and NH3. Only agricultural sources of nitrogen were considered in the Revised 1996 Guidelines (IPCC, 1997).

Guidance is provided in Section 7.3 on estimating N2O emissions from atmospheric deposition resulting from all categories except agricultural soil management and manure management. Section 7.3 provides information on NOx emissions. Countries may use national methodologies to estimate emissions of NH3 not originating from agriculture. NH3 emissions are also covered in the EMEP/CORINAIR Emission Inventory Guidebook.