A life cycle methodology for mapping the flows of pollutants in the urban environment

Abstract

This paper presents an integrated life cycle methodology for mapping the flows of pollutants in the urban environment, following the pollutants from their sources through the environment to receptors. The sources of pollution that can be considered by this methodology include products, processes and human activities. Life cycle assessment (LCA), substance flow analysis (SFA), fate and transport modelling (F&TM) and geographical information systems (GIS) have been used as tools for these purposes. A mathematical framework has been developed to enable linking and integration of LCA and SFA. The main feature of the framework is a distinction between the foreground and background systems, where the foreground system includes pollution sources of primary interest in the urban environment and the background comprises all other supporting activities occurring elsewhere in the life cycle. Applying the foreground–background approach, SFA is used to track specific pollutants in the urban environment (foreground) from different sources. LCA is applied to quantify emissions of a number of different pollutants and their impacts in both the urban (foreground) and in the wider environment (background). Therefore, two “pollution vectors" are generated: one each by LCA and SFA. The former comprises all environmental burdens or impacts generated by a source of interest on a life cycle basis and the latter is defined by the flows of a particular burden (substance or pollutant) generated by different sources in the foreground. The vectors are related to the “unit of analysis" which represents a modified functional unit used in LCA and defines the level of activity of the pollution source of interest. A further methodological development has also included integration of LCA and SFA with F&TM and GIS. A four-step methodology is proposed to enable spatial mapping of pollution from sources through the environment to receptors. The approach involves the use of GIS to map sources of pollution, application of the LCA–SFA approach to define sources of interest and quantify environmental burdens and impacts on a life-cycle basis. This is followed by F&TM to track pollution through the environment and by the quantification of site-specific impacts on human health and the environment. The application of the integrated methodology and the mathematical framework is illustrated by a hypothetical example involving four pollution sources in a city: incineration of MSW, manufacture of PVC, car travel and truck freight.

Appendices

Appendix 1: Integrated LCA and F&T models

See Table 3

Table 3 Multimedia fate models used in conjunction with LCA (Hertwich et al. 2002)

Appendix 2: Definition of environmental impacts in the problem-oriented approach in LCA

(Adapted from Azapagic et al. 2004)

Non-renewable resource depletion includes depletion of fossil fuels, metals and minerals. The total impact is calculated as

$${E_{1} = {\sum\limits_{j = 1}^J {\frac{{B_{j}}}{{ec_{{1,j}}}}}} \quad(-)}$$

(A1)

where B _j is the quantity of a resource used per functional unit and ec _1,j represents the estimated total world reserves of that resource.

Global warming potential (GWP) is equal to the sum of emissions of the greenhouse gases multiplied by their respective GWP factors, e _2,j :

$${E_{2} = {\sum\limits_{j = 1}^J {ec_{{2,j}} B_{j}}} \quad (\rm kg)}$$

(A2)

where B _j represents the emission of greenhouse gas j. GWP factors ec _2,j for different greenhouse gases are expressed relative to the global warming potential of CO₂, which is therefore unity. The values of GWP depend on the time horizon over which the global warming effect is assessed. GWP factors for shorter times (20 and 50 years) provide an indication of the short-term effects of greenhouse gases on the climate, while GWP for longer periods (100 and 500 years) are used to predict the cumulative effects of these gases on the global climate.

Ozone depletion potential (ODP) indicates the potential of emissions of chlorofluorohydrocarbons (CFCs) and other halogenated HCs for depleting the ozone layer and is expressed as

$${E_{3} = {\sum\limits_{j = 1}^J {ec_{{3,j}} B_{j}}} \quad ({\rm kg})}$$

(A3)

where B _j is the emission of an ozone depleting gas j. The ODP factors ec _3,j are expressed relative to the ozone depletion potential of CFC-11.

Acidification potential (AP) is based on the contributions of SO₂, NOx, HCl, NH₃, and HF to the potential acid deposition, i.e. on their potential to form H+ ions. AP is calculated according to the formula:

$${E_{4} = {\sum\limits_{j = 1}^J {ec_{{4,j}} B_{j}}}\quad (\rm kg)}$$

(A4)

where ec _4,j represents the acidification potential of gas j expressed relative to the AP of SO₂,  and  B _j is its emission in kg per functional unit.

Eutrophication potential (EP) is defined as the potential of nutrients to cause over-fertilisation of water and soil, which can result in an increased growth of biomass. It is calculated as:

$${E_{5} = {\sum\limits_{j = 1}^J {ec_{{5,j}} B_{j}}} \quad (\rm kg)}$$

(A5)

where B _j is an emission of species such as N, NOx, NH ⁺₄ , PO ³⁻₄ , P, and COD and ec _5,j are their respective eutrophication potentials. EP is expressed relative to PO ³⁻₄ .

Photochemical oxidants creation potential (POCP) is related to the potential of VOCs and NOx to generate photochemical or summer smog. It is usually expressed relative to the POCP classification factors of ethylene and can be calculated as:

$${E_{6} = {\sum\limits_{j = 1}^J {ec_{{6,j}} B_{j}}}\quad (\rm kg)}$$

(A6)

where B _j are the emissions of the species participating in the formation of summer smog and ec _6,j are their classification factors for photochemical oxidation formation.

Human toxicity potential (HTP) is calculated by taking into account releases toxic to humans to three different media, i.e. air, water and soil:

$${{{E}}_{{{7}}} ={\sum\limits_{{{j = 1}}}^{{J}}{{{ec}}_{{{{7,jA}}}} {{B}}_{{{{jA}}}}}} +{\sum\limits_{{{j}} = {{1}}}^{{J}}{{{ec}}_{{{{7,jW}}}} {{B}}_{{{{jW}}}}}} +{\sum\limits_{{{j}} = {{1}}}^{{J}}{{{ec}}_{{{{7,jS}}}} {{B}}_{{{{jS}}}}}}\quad (\rm kg)}$$

(A7)

where ec _7,jA ,   ec _7,jW , and  ec _7,jS are human toxicological classification factors for substances emitted to air, water and soil, respectively, and B _jA ,   B _jW   and  B _jS represent the respective emissions of different toxic substances into the three environmental media. The toxicological factors are calculated relative to 1,4 dichlorobenzene (1,4-DB).

Aquatic toxicity potential (ATP) can be calculated as

$${{{E}}_{{{8}}} ={\sum\limits_{{{j = 1}}}^{{J}}{{{ec}}_{{{{8,jA}}}} {{B}}_{{{{jA}}}}}} +{\sum\limits_{{{j}} = {{1}}}^{{J}}{{{ec}}_{{{{8,jW}}}} {{B}}_{{{{jW}}}}}} +{\sum\limits_{{{j}} = {{1}}}^{{J}}{{{ec}}_{{{{8,jS}}}} {{B}}_{{{{jS}}}}}} \quad(m^3)}$$

(A8)

where ec _8,jA , ec _8,jW , and  ec _8,jS represent the toxicity classification factors of different substances emitted to air, water and land, respectively, that may reach waterways and affect aquatic organisms. B _jA ,  B _jW  and  B _jS represent the respective emissions of different toxic substances into the three environmental media. ATP is also expressed relative to 1,4 dichlorobenzene (1,4-DB).

Table 4 Relative contribution (e _k,j ) of selected burdens (B _j ) to impacts (E _k ), as defined by the problem-oriented approach in LCA [see Eq. 6 and (A1)–(A8)]