Air quality expert group - Fine particulate matter (PM2.5) in ... - Defra

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4.3.1 Primary versus secondary PM PM2.5 emissions and receptor modelling 49. Not all of the particulate matter found in the atmosphere has been directly emitted into the atmosphere by primary sources. There are significant sources of both primary PM and secondary PM, the latter being formed in the atmosphere by chemical reactions involving primary emitted precursor species. Each secondary PM component would thus have its own primary PM precursor or precursors. 50. PM sulphate, for example, is an important component of secondary PM. It is formed by homogeneous gas phase oxidation of SO2 by hydroxyl (OH) radicals and by cloud phase oxidation of SO2 by hydrogen peroxide and ozone. These chemical reactions lead to the formation of particles of sulphuric acid which may take up ammonia from the atmosphere, leading to partial and ultimately complete neutralisation through the formation of ammonium sulphate. Particulate sulphate is a mixture of sulphuric acid and ammonium sulphate dissolved in the water associated with the atmospheric aerosol. Particulate sulphate is thus a secondary PM component with SO2 and NH3 as its primary pollutant precursors. Because of the exceedingly low volatility of sulphuric acid and ammonium sulphate, particulate sulphate is stable in the atmosphere and, once formed irreversibly, will not decompose back to ammonia and sulphuric acid vapours under normal atmospheric conditions. 51. There is an important class of secondary PM components whose atmospheric formation is reversible. The most important example of which is ammonium nitrate, formed by the chemical reaction of gaseous ammonia with gaseous nitric acid on pre-existing particles. Ammonium nitrate formation is thus associated with an increase in the PM mass rather than an increase in the PM number density. Ammonium nitrate is thermally unstable and may revert to gaseous ammonia and nitric acid with a time constant of minutes to hours depending on atmospheric conditions. 52. The atmospheric oxidation of certain volatile organic compounds (VOCs) can lead to the formation of low volatility, multi-functional organic compounds. If the volatilities of these oxidation products are sufficiently low, they can absorb onto pre-existing particles, passing into the particulate phase and increasing the PM mass but not the PM number density. Since the absorption is reversible, this component of organic PM may pass back into the atmosphere with a time constant of minutes to hours, again depending on atmospheric conditions. Once back in the atmosphere, these semi-volatile organic compounds may be further oxidised to oxidation products of even lower volatility, which may again absorb onto pre-existing particles, further increasing the PM mass, or they may be oxidised through to CO and water, depending on their chemical structures. 53. A wide range of VOCs are able to contribute to the formation of secondary organic aerosol. Laboratory studies show that precursors can include both anthropogenic and natural, biogenic compounds. Studies of airborne particles using carbon-14 as a tracer of contemporary (as opposed to fossil) carbon (Heal et al., 2011) suggest that biogenic precursors play a substantial role (Section 4.6.3). Naturally-emitted VOCs from vegetation, termed biogenic VOCs, are an important contributor to the formation of secondary organic particles. Current knowledge of emissions within the UK is inadequate and AQEG recommends development of a natural speciated inventory for biogenic VOC. 97
PM2.5 in the UK 98 4.4 A critical assessment of emission inventories for modelling of PM2.5 concentrations 54. Modelling concentrations of PM2.5 from emissions data is complicated by the fact that it requires inventories for a range of pollutants, including direct emissions of PM2.5 itself as well as its precursor gases SO2, NOx, NH3 and NMVOCs. These pollutants are emitted in varying amounts from different sources and exhibit different spatial and temporal behaviour. With contributions from four different precursor gases and direct emissions to concentrations of ambient PM2.5, there are therefore more emission sources to include in the inventories used in modelling PM2.5 concentrations than are required to model other pollutant concentrations. Many of these, especially non-combustion and diffuse sources, are very difficult to quantify. Many of the major sources of PM2.5 in the atmosphere, both primary and secondary, are also difficult to regulate and control. Because of the wide variety of pollutants and sources that contribute to direct PM2.5 emissions and to its formation in the atmosphere, the uncertainties in the emission inventories for each of these pollutants and sources are compounded in the models used to estimate concentrations. 55. Modellers turn to emission inventories as the primary source of emissions data. For anthropogenic emissions these are developed mainly by national inventory agencies. There is a well-established mechanism for reporting national emission inventories driven by the requirements of international bodies, such as the UN and EU, under various protocols. The emphasis is on providing inventories using common methodologies so that inventories provided by different countries are comparable and are also consistent with other national statistics, for example on energy consumption. This leads to a method of accounting for emissions which is ideal for policy-makers, who can identify which are the major sources and assess the costs and benefits of alternative control strategies. This approach works particularly well for greenhouse gases where the long-lived nature of these emitted gases means they can be treated almost as a commodity that can be traded; this is evident in the concept of “emission trading schemes”. Using common inventory approaches that define what sources are and are not included in national inventories is also ideal for tracking progress against national emission reduction targets. For this, most national inventories are fit for purpose. They may also be adequate for air quality modelling of primary pollutants. 56. However, national inventories may fall short of what is required by models for PM2.5 concentrations. Simply knowing the annual rate of emissions for a given pollutant and source sector is not sufficient, nor is it sufficient to consider only anthropogenic sources. The spatial and temporal variation in emissions is important and this is far less well-understood than the annual rate of emissions at a national level which is usually derived from national statistical datasets. The question is how do emissions vary with time of day and day of the week and season, where exactly do they occur and what are the influences of meteorological factors? Whilst some of these aspects can be assessed, e.g. for combustion sources, many cannot, especially for fugitive sources of dust and many agricultural processes.
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AIR QUALITY EXPERT GROUP Fine Parti
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This is a report from the Air Quali
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II Membership Chair Professor Paul
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IV Acknowledgements The Air Quality
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VI 2.5 Summary 39 2.5.1 What does t
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VIII 5.4 Components of PM2.5 and so
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PM2.5 in the UK 2 Air Quality Direc
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PM2.5 in the UK 4 I.2.1 Concentrati
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PM2.5 in the UK 6 • PM2.5 emissio
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PM2.5 in the UK 8 I.4.1 Modelling r
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PM2.5 in the UK 10 4. In broad term
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PM2.5 in the UK Table 1.2: National
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PM2.5 in the UK 14 17. Dry depositi
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PM2.5 in the UK 16 in ozone in the
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PM2.5 in the UK 18
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PM2.5 in the UK 20 5. The exact for
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PM2.5 in the UK 22 12. These diffic
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PM2.5 in the UK 24 2.3 Methods used
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PM2.5 in the UK 26 (e) During the
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PM2.5 in the UK 28 2.3.3 The Partis
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PM2.5 in the UK 30 35. For non-auto
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PM2.5 in the UK 32 43. Ratification
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PM2.5 in the UK 34 49. Many of the
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PM2.5 in the UK 36 57. Black carbon
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PM2.5 in the UK 38 2.4.4 PM2.5 elem
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PM2.5 in the UK 40 average of 93.6%
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PM2.5 in the UK Annex 1 - PM2.5 mon
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PM2.5 in the UK 44
Page 57 and 58: PM2.5 in the UK 46 3.2.1 Diurnal va
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Page 61 and 62: PM2.5 in the UK 50 3.2.4 Seasonal v
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Page 65 and 66: PM2.5 in the UK 54 3.4.2 Relationsh
Page 67 and 68: PM2.5 in the UK 56 Correlation 0.8
Page 69 and 70: PM2.5 in the UK Figure 3.11: Polar
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Page 75 and 76: PM2.5 in the UK 64 secondary ammoni
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Page 79 and 80: PM2.5 in the UK 68 3.9 Composition
Page 81 and 82: PM2.5 in the UK 70 43. The UK has i
Page 83 and 84: PM2.5 in the UK 72 NO 3 6 4 2 2000
Page 85 and 86: PM2.5 in the UK 74 54. PM ammonium
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Page 89 and 90: PM2.5 in the UK 78 (k) Road traffic
Page 91 and 92: PM2.5 in the UK 80 provided by spec
Page 93 and 94: PM2.5 in the UK 82 14. The London L
Page 95 and 96: PM2.5 in the UK Table 4.1: UK emiss
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Page 99 and 100: PM2.5 in the UK 88 Figure 4.5: Spat
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Page 105 and 106: PM2.5 in the UK Emissions (ktonnes)
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Page 115 and 116: PM2.5 in the UK 104 4.5.2 Quantifyi
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Page 119 and 120: PM2.5 in the UK 108 4.6.1 Receptor
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Page 123 and 124: PM2.5 in the UK 112 PM2.5 contribut
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Page 129 and 130: PM2.5 in the UK 118 100% 90% 80% 70
Page 131 and 132: PM2.5 in the UK 120 105. Receptor m
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Page 135 and 136: PM2.5 in the UK 124 5. At the regio
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Page 139 and 140: PM2.5 in the UK 128 Figure 5.2: Tot
Page 141 and 142: PM2.5 in the UK 130 µgm -3 160 140
Page 143 and 144: PM2.5 in the UK 132 PM component (
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Page 147 and 148: PM2.5 in the UK 136 Figure 5.8: UKI
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Page 153 and 154: PM2.5 in the UK 142 the North Sea (
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PM2.5 in the UK 148 2. Figure A2.1.
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PM2.5 in the UK 150 6. The non-line
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PM2.5 in the UK 152 Model 50 45 40
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PM2.5 in the UK 154 (a) (b) modPM 2
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PM2.5 in the UK 156 NO3 - concentra
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PM2.5 in the UK 158 26. This was in
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PM2.5 in the UK 160 SIA components
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PM2.5 in the UK 162 x 50 km grid fo
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PM2.5 in the UK 164 A2.6: Photochem
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PM2.5 in the UK 166 PM2.5 Emission
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PM2.5 in the UK 168 A2.8: PCM PM mo
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PM2.5 in the UK 170 illustrated by
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PM2.5 in the UK 172 trajectory mode
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PM2.5 in the UK 174
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PM2.5 in the UK 176 4. The delivery
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PM2.5 in the UK 178 12. With respec
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PM2.5 in the UK 180
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PM2.5 in the UK Chapter 2: Measurin
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PM2.5 in the UK RoTAP (2012). Revie
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PM2.5 in the UK Harrison R.M. and Y
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PM2.5 in the UK Chapter 5 Modelling
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PM2.5 in the UK Jones A.M., Harriso
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PB13837 Nobel House 17 Smith Square
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