Emission Inventory Methodology
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Guidance was sourced from the United States Environmental Protection Agency (US EPA) on pollutants to include in the inventory. The US EPA has compiled a list of "criteria pollutants" upon which it has focused attention and established national ambient air quality standards. The criteria pollutants are particulate matter (PM10 and PM2.5), lead, carbon monoxide (CO), oxides of nitrogen (NOx), sulphur dioxide (SO2) and ozone1. A volatile organic compound or VOC (which excludes certain non-reactive organic compounds) inventory is required in order to predict ozone levels. Ozone is not emitted in any great quantity as a pollutant but can be photochemically produced when VOC's are reacted with NOx in the presence of sunlight. Of the six criteria pollutants, lead was excluded from the emission inventory due to a lack of emission factors. Pollutants therefore included in the emission inventory are PM; CO; NOx; SO2 and total organic compounds (TOC).

The geographical region covered in the emission inventory was the DMA. Average annual emissions were estimated in the inventory. Emissions were represented spatially on the Durban Metro Geographical Information System (GIS). Emissions from each facility have been assigned to the area occupied by the facility. Likewise emissions from mobile and area sources have been assigned to the area in which they occur.

Emissions from three broad categories of sources were covered in the inventory; namely point sources, area sources and mobile sources. For the purpose of this study, emissions from stationary sources were, wherever possible, treated as point sources. Thus a facility where emissions occur was treated as one or more point sources. The emissions from each stack were determined and allocated to the point at which they arise. Emissions from sugar cane burning, solvent evaporation and the distribution of petroleum products were treated as area sources. Mobile emissions estimated included emissions from vehicular traffic, ships, airplanes and trains.

The purpose of the point source emission inventory was to estimate air emissions arising from industrial activity in the DMA. The following steps were undertaken in order to accomplish this task:
  • A mailing list of approximately 600 industries in the DMA was compiled.
  • A questionnaire was posted to each industry. The questionnaire covered the following.
    • General company information.
    • Details of emission points including stack positions and height and the physical conditions of the exit gas.
    • Emissions from each stack if known.
    • The type and quantity of fuel burnt in each fuel - burning unit and the stack to which each unit is attached.
    • Production data.
    • Air cleaning equipment including the efficiency of each unit.
    • Emissions from solvent use.
    • Fugitive emissions.
  • Each industry on the mailing list was contacted telephonically by ECOSERV in order to ensure timeous completion of questionnaires and to assist industries with any queries they might have had. One of the primary functions of this task was to establish the nature of emissions, if any, from each facility to enable follow up work to be done on those facilities that did not return questionnaires. Completed questionnaires were checked and filed.
  • A data base was designed in which questionnaire responses were analysed and emissions estimated. Emissions were recorded where industries kept such data. Otherwise, emissions were calculated from process data using emission factors and methodologies given in the Compilation of Air Pollutant Emission Factors (AP 42, US EPA, Nov. 1996)2. One database entry was included for each emission point.
  • Emissions were represented spatially on the Durban Metro Geographical Information System (GIS).

Emissions from mobile sources were calculated and spatially represented on the GIS as follows:
  • Emissions from vehicular traffic were determined from fuel-based emission factors determined for the South African vehicle fleet and from petroleum and diesel sales data for the DMA. Traffic densities were obtained and plotted on the GIS. Emissions were then apportioned across the DMA according to a model developed from traffic densities.
  • Emissions from trains were determined from fuel-based emission factors and fuel usages obtained from Spoornet. Rail emissions were allocated spatially using Durban Metro's GIS data on railway lines and by assuming a constant emission rate per length of track.
  • Emission factors have been determined by the United States Environmental Protection Agency (US EPA) for most aircraft engines running at power settings typically used by aircraft during landing and taking off. These emission factors and airport activity data were used to estimate aircraft emissions at Durban International Airport. These emissions were spatially allocated to the area occupied by Durban International Airport.
  • The results of a study of emissions from US harbours was used to determine emission factors for ships. Emissions from Durban harbour were then determined using these emission factors and harbour activity data obtained from Portnet. These emissions were spatially allocated to the area occupied by Durban harbour.

Emissions from area sources were calculated and spatially represented as follows:
  • ECOSERV adopted the approach used by the US EPA for determining TOC emissions from solvent evaporation: namely to treat large individual solvent evaporation sources as point sources and to account for the balance of emissions from solvent evaporation as area sources. Area source emissions from solvent evaporation were determined using US EPA population-based emission factors (which were scaled down for the purpose of the emission inventory). These emissions were spatially allocated using the Durban Metro GIS data on population demographics.
  • US EPA emission factors and petroleum and diesel sales data were used to estimate emissions from the marketing and distribution of petroleum products. Emissions that were estimated were loading losses at depots (excluding losses from storage tanks), transit losses and losses from service stations.
  • Emission factors published by the US EPA for estimating emissions from cane burning were used in conjunction with data provided by the S. A. Sugar Association Experiment Station. Emissions were then calculated from the area under cane and spatially allocated to the cane growing areas. 


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Last update: October 1999