Air quality assessment of benzo(a)pyrene from asphalt plant operation

Abstract

A study has been carried out to assess the contribution of Polycyclic Aromatic Hydrocarbons (PAHs) from asphalt plant operation, utilising Benzo(a)pyrene (BaP) as a marker for PAHs, to the background air concentration around asphalt plants in the UK. The purpose behind this assessment was to determine whether the use of published BaP emission factors based on the US Environmental Protection Agency (EPA) methodology is appropriate in the context of the UK, especially as the EPA methodology does not give BaP emission factors for all activities. The study also aimed to improve the overall understanding of BaP emissions from asphalt plants in the UK, and determine whether site location and operation is likely to influence the contribution of PAHs to ambient air quality. In order to establish whether the use of US EPA emissions factors is appropriate, the study has compared the BaP emissions measured and calculated emissions rates from two UK sites with those estimated using US EPA emission factors. A dispersion modelling exercise was carried out to show the BaP contribution to ambient air around each site. This study showed that, as the US EPA methodology does not provide factors for all emission sources on asphalt plants, their use may give rise to over- or under-estimations, particularly where sources of BaP are temperature dependent. However, the contribution of both the estimated and measured BaP concentrations to environmental concentration were low, averaging about 0.05 ng m⁻³ at the boundary of the sites, which is well below the UK BaP assessment threshold of 0.25 ng m⁻³. Therefore, BaP concentrations, and hence PAH concentrations, from similar asphalt plant operations are unlikely to contribute negatively to ambient air quality.

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Impact statement

This study was carried out to assess the contribution of Polycyclic Aromatic Hydrocarbons (PAHs) from UK asphalt plant and to gain a better understanding of Benzo(a)pyrene (BaP) emissions in this context. The assessment determines whether the use of published BaP emission factors from the US Environmental Protection Agency (EPA) methodology is appropriate in the UK context to estimate BAP/PAH contribution to background air concentration. It also determines whether site location and operation is likely to influence the contribution. The study has shown that use of the US EPA methodology may give rise to over- and/or under-estimations, particularly where sources are temperature dependent. However, it also shows that the contribution of both estimated and measured BaP concentrations to ambient air were low.

Introduction

A study has been carried out in order to assess whether the use of published benzo(a)pyrene (BaP) emissions factors based on the US Environmental Protection Agency (EPA) AP-42 methodology is appropriate in the UK context to use when assessing the contribution of Polycyclic Aromatic Hydrocarbons (PAHs), utilising BaP as a marker, from asphalt plant operation to the background air concentration. Although the USEPA AP-42 data set is reasonably comprehensive, emission factors for BaP are not provided for all process activities.

The study also aimed to improve overall understanding of BaP emissions from asphalt plants in the UK and determine whether plant design or siting is likely to impact on the contribution of PAHs to ambient air quality.

Background

Polycyclic aromatic hydrocarbons

Polycyclic Aromatic Hydrocarbons (PAHs) are a large group of organic compounds whose molecular structure comprises of carbon and hydrogen atoms only, within two or more fused aromatic rings. Sources of PAHs emit varying proportions and amounts of individual PAH compounds. PAH compounds that require assessment under the Fourth Daughter Directive of the European Parliament and of the Council (which relates to polycyclic aromatic hydrocarbons in ambient air) include benzo(a)pyrene, benzo(a)anthracene, benzo(b)fluoranthene, benzo(j)fluoranthene, benzo(k)fluoranthene, indeno(1,2,3-cd)pyrene, dibenz(a,h)anthracene and fluoranthene.

The chemical and physical properties of different PAHs vary, as do their carcinogenicity. Human exposure to PAH occurs through a number of environmental pathways and includes internal absorption (through eating and drinking) and inhalation of air both indoors and outdoors. A number of PAH have been classified by the International Agency for Research on Cancer (IARC) as either carcinogenic to humans (Group 1), a probable carcinogen (Group 2A) or a possible carcinogen (Group 2B) human carcinogens, with benzo(a)pyrene (BaP) being the only PAH to fall into the Group 1 category. It is usual for the carcinogenic potencies of PAH to be published relative to BaP.

The EU Working Group on Polycyclic Aromatic Hydrocarbons has proposed using BaP as a marker for PAHs. For this reason, as well as because of the harmful effect on human health, BaP has been utilised as a marker for PAHs during this air quality assessment study.

Benzo(a)pyrene

Article 3 and Annex 1 of the Fourth Daughter Directive proposes that a target value of 1 ng m⁻³ should not be exceeded for BaP in ambient air†, when assessed by the methodology suggested in the directive over a calendar year.

A lower assessment threshold for BaP of 0.4 ng m⁻³ and an upper assessment threshold of 0.6 ng m⁻³ are also set by the Fourth Daughter Directive primarily as a measure to set appropriate future assessment methods required to monitor the concentration of PAH. Below the upper assessment threshold the Directive allows for assessment by a combination of measurement and modelling. Below the lower threshold the use of modelling or objective estimation can be used.

The UK Expert Panel on Air Quality (EPAQS) air quality standard for BaP is 0.25¹ ng m⁻³. This concentration has been adopted as a provisional national air quality strategy objective for England, Wales and Scotland, to be achieved by 31 December 2010.

The Air Quality Strategy for England, Scotland, Wales and Northern Ireland projects that there will be exceedances of the 2010 objective for PAH in some major urban areas and alongside busy roads.²

The asphalt manufacturing process

Asphalt production during roadstone coating involves combining bitumen with mineral aggregate to produce asphalt (see Fig. 1). This takes place at a hot mix asphalt plant. Such plants fall into three general categories: drum (continuous) plants, asphalt plants (semi-continuous) and batch plants. In all plants mineral aggregates are heated and dried before being mixed with bitumen in a mixing unit. The resultant asphalt mix is either transferred directly to a transport truck, which takes it to a paving site, or stored in silos prior to transport.

Fig. 1 Schematic overview of the asphalt process.

In an asphalt or batch plant, mineral aggregates are heated in a rotary drum dryer to a temperature between 135 °C and 180 °C. Heated and dried mineral aggregate is placed into a pug mill mixer, where it is coated with bitumen and mineral filler. Bitumen is pumped into a pug mill mixer in discrete portions from bitumen storage tanks. Drum plants differ from batch plants in that the heating and drying of the mineral aggregate as well as the addition of bitumen and filler take place in the rotary drum.

Emissions of BaP will arise from incomplete combustion of fuel in the drier as well as from bitumen delivery and storage, the coating process, asphalt storage, loading and from vehicles before sheeting.

Assessing the impact of BaP

When planning and locating a new asphalt plant, the contribution of pollutants to the existing air quality background should be assessed in accordance with UK Government's Planning Policy Statement 23³ based on the annual rate of production and published emissions; for example EPA factors and emission limit values in the Process Guidance Note (PGN) 3/15a (04).⁴

The use of published emissions factors is a recognised method for estimating the emissions from plants that are still in the development phase. The information that underpins the emission factor development tends to be the culmination of a significant number of separate studies that may not necessarily have been carried out for establishing detailed emission factors, especially across the full range of hazardous air pollutants. This is particularly true for the EPA emissions factors, as published in their AP-42 emission factor handbook.⁵ This handbook provides an interpretation of asphalt plant emissions based on the US design and operation of asphalt plants and may not necessarily transpose directly to the UK situation. As discussed, the purpose of this study has been to assess the suitability of the AP-42 data for the purpose of determining the impact of BaP in the vicinity of two asphalt plants in the UK and, through this determination, to identify whether any improvements can be made to the understanding of the significance of BaP from the various stages of the asphalt process and how this may impact on the siting of asphalt plants in future.

Methodology

Study sites

During this study the PAH emissions from two asphalt manufacturing plant sites were measured. The sites were constructed in 1995 and 2002 and thus the information gathered for the sites will broadly reflect current operating practices. The characteristics of the two sites are summarised in Table 1. The main difference between the two sites is that on site 1 the emissions from the pug mixer were discharged into the discharge flue from the drier, while on site 2 these emissions were introduced into the drier as part of the combustion support air. Likewise, site 2 processes a larger fraction of reclaimed asphalt, while site 1 makes use of recycled foundry sand.

Table 1 Characteristics of the study sites used in this study

Study site 1
Annual plant through put	ca. 108 000 tonnes asphalt per year
Operating hours	24 h per day, with peak collection period between 0600–1000 and 1200–1400
Recycled materials used	Foundry sand which is dried alongside roadstone
Main stack	26 m serving emissions from drier and pug mixer
Maximum operating temperature	186 °C
Bitumen storage	Four grades used and blended on site
Bitumen delivery	Daily delivery of approximately 27 tonnes, 5 days per week
Study site 2
Annual plant throughput	ca. 240 000 tonnes asphalt per year
Operating hours	Permitted hours between 0530–2200 (Monday–Friday); 0600–1400 (Saturday)
Recycled materials used	Various percentages of Reclaimed Asphalt Product (RAP) added which is dried alongside roadstone
Main stack	30 m serving emissions from drier
Operating temperature	160 °C
Bitumen storage	4 × 100 tonne tanks
Bitumen delivery	1–2 daily deliveries of approximately 27 tonnes each, 5 days per week

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The BaP emissions measured on each of these two sites were compared with the estimated emissions calculated using US EPA emission factors. This established whether the use of US EPA emissions factors in the context of a typical UK roadstone coating plant is an appropriate method for estimating emissions.

Following on from this, a dispersion modelling exercise, using ADMS v3.3, was carried out around both sites to show the BaP contribution to ambient air. This allowed an improved understanding of how site design contributes to BaP concentrations in ambient air, and may impact on the design and siting of asphalt plants in future.

Monitoring methods

The PAH monitoring involved collecting samples from the main stack, storage bins and the bitumen tanks of both sites. Samples were also collected from beneath vehicle sheeting at site 2 to assess ‘yard’ emission. The following monitoring methods were used for the various locations:

Main stack. BaP was sampled at the main stacks in accordance with BS ISO 11338:2003 (Part 1: Determination of PAH in stack), with a small number of deviations:

The resin traps were not supplied at a temperature below −5 °C or kept in the dark.

The sampling volumetric flow rate was in the order of 0.9 m³ h⁻¹, which was below the recommended rate of 1 m³ h⁻¹.

Due to safety issues a sample was collected from only one port on the stack.

Gas velocities and temperatures were measured across the two sampling axes to enable the determination of isokinetic flow rates. Exhaust gas flow rates were measured using pitot probes at the stack, sampled gas volumes were recorded using calibrated gas meters.

Silo and loadout sources. BaP was sampled using a methodology based upon NIOSH 5506 “polynuclear aromatic hydrocarbons by HPLC” at all other locations, although the sample flow rate was restricted to c. 1.5 l min⁻¹ compared to the target rate of c. 2 l min⁻¹ due to “back pressure” within the sample train. The analysis of BaP was undertaken using Gas chromatography with mass spectroscopy (GC-MS) and results were determined for combined particulate and vapour phase samples.

Yard emissions. BaP was sampled from vehicles by extracting air, through a lagged stainless steel probe, from underneath the tarpaulin sheeting of a vehicle loaded with asphalt. The air extraction rate was about 65 litres per minute. Samples were sub-sampled from this gas stream in accordance with NIOSH Method 5506. As with the silo and loadout samples, the analysis of BaP was undertaken using GC-MS.

In order to determine emissions rates, a number of other factors, including gas flow rates, measurement/sampling periods and production rates, were also measured where possible.

Results

The measured concentrations, averages and standard deviations for each process on site 1 and site 2 are shown in Tables 2 and 3 respectively.

Table 2 Site 1 measured concentrations, averages and standard deviations

	BaP concentration (μg m^−3)	Total PAH concentration (μg m^−3)^a	BaP as % of Total PAH	BaP Standard Deviation
a PAH emissions measured on both sites included naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benzo(a)anthracene, chrysene, benzo(b/j)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, indeno(123-cd)pyrene, dibenzo(ah)anthracene, benzo(ghi)perylene.
Stack Total	0.001	61.9	0.002	—
Mixer 1–3	0.006	6.1	0.098
Mixer 2–3	0.001	7.0	0.014
Mixer 5–6	0.010	9.7	0.103
Mixer Average	0.006	7.6	0.075	0.0045
Unload 1	0.008	376.0	0.002
Unload 2	0.007	327.5	0.002
Bitumen Unload Average (Delivery)	0.008	351.8	0.002	0.0007
Blow-out 1	0.245	413.0	0.059
Blow-out 2	0.108	380.8	0.028
Blow-out average	0.177	396.9	0.044	0.0969

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Table 3 Site 2 measured concentrations, averages and standard deviations

	BaP concentration (μg/m^3)	Total PAH concentration (μg m^−3)^a	BaP as % of Total PAH	BaP standard deviation
a See footnote on Table 2. b The proportion of naphthalene determined in the bin samples was lower than for the other samples resulting in a comparatively high BaP as % of Total PAH.
Stack Vapour	0.005	63.2	0.008
Stack PM	0.047	273.1	0.017
Stack Total	0.052	336.3	0.015	—
Bin 6/7	0.084	30.0	0.280^b
Bin 4/5	0.026	9.0	0.289^b
Bin Average	0.055	19.5	0.282^b	0.0410
Bitumen Unload (Delivery)	0.092	240.6	0.038	—
Blow-out	No data	No data	No data	—
Vehicle test 1	1.2	596.5	0.201
Vehicle test 2	0.5	710.6	0.070
Vehicle test 3	0.3	662.8	0.045
Vehicle test 4	0.2	781.1	0.026
Vehicle test 5	0.4	952.4	0.042
Vehicle average	0.5	740.7	0.070	0.396

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Following the completion of PAH monitoring for both sites, the measured BaP concentrations from each individual process on site were converted to emission factor estimates, in grams per tonne of asphalt (Table 5). The method used to carry out this conversion is discussed below. BaP contributions were also predicted using the US EPA AP-42 methodology, and results also displayed in grams per tonnes. This will allow a direct comparison between the two sets of results.

Measured BaP concentrations

The measured BaP concentrations from both sites were converted to emissions factor estimates by the calculations outlined below.

Stack contribution point source emissions (stack, bitumen tank etc).

Unit Emission (g/tonne) = Total Emission (g)/total production during the sample period (tonnes)

where

Total Emission (μg) = stack concentration (μg/m³) × gas flow rate (N m³/s) × sample period (seconds)

and

Stack Concentration (μg m⁻³) = Stack vapour phase contribution (μg m⁻³) + PM phase contribution (μg m⁻³)

Emission from load out and silo and yard activities

Emission from load out, silo and yard Activities have been estimated using the USEPA organic PM emission estimate in conjunction with USEPA PAH fraction and measured BaP fraction determined from measurements. The calculations used were:

Unit Emission (g/tonne) = Total Emission (g/yr)/production per year (tonnes/yr)

where

Total Emission (g/yr) = USEPA organic PM emission estimate (kg/yr) × USEPA PAH % of total × % of BaP in average emission from bin/mixer

Emissions from bitumen delivery.

Unit Emission (g/tonne) = Total Emission During Monitoring Period (g)/Production During Monitoring Period (tonnes)

where

Total Emission During Monitoring Period (g) = Unload Emission (g) + Blow-out Emission (g)

and

Unload/Blow-out Emission (μg) = Concentration (μg/m³) × Flow Rate (m³ s⁻¹) × Unload Time (seconds)

While there may be some small breathing loss associated with the storage of bitumen in the tanks on site, this is considered to be negligible.

The figures used for the calculation and resulting emission estimates are show in Table 4.

Table 4 Results of sampling exercise carried out on the yard activities (Site 2 only)

	Vehicle test 1	Vehicle test 2	Vehicle test 3	Vehicle test 4	Vehicle test 5
Measured BaP (μg m^−3)	1.2	0.5	0.3	0.2	0.4
Volume sampled (m^3)	1.617	1.617	1.557	1.111	0.515
Estimated tonnes per load	19.5	19.5	19.5	19.5	19.5
Number of loads per test	3	3	3	2	1
Total tonnes per test	58.5	58.5	58.5	39	19.5
BaP emissions for measurement period (μg/tonne)	0.03317	0.01382	0.00798	0.00570	0.01056
Total sample time per test (min)	80	80	75	40	35
Unsheeted time per test (min)	24	24	24	16	8
% time unsheeted	30.0%	30.0%	32.0%	40.0%	22.9%
BaP emissions for unsheeted period (μg/tonne)	0.00995	0.00415	0.00256	0.00228	0.00241
Average (μg/tonne)	4.27 × 10^−3
Emission rate (g/tonne)	4.27 × 10^−9

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Emission from yard activities at site 2.

Unit Emission (g/tonne) = Average[Unit Emission (g/tonne) of vehicle loads]

where

Unit Emission (g/tonne) of vehicle loads = Total Emission During Exposed Period (g)/Total Vehicle Load per Test (tonnes)

and

Total Emission During Exposed Period (g) = BaP emission for measurement period (g/tonne) × % load unsheeted time

and

BaP emission for measurement period (g/tonne) = measured BaP (g m⁻³) × volume sampled (m³)/load weight (tonnes)

US EPA AP42 predictions

Table 6 summarises the emission factor calculations as published by the US EPA, as well as the emissions estimated for the two asphalt plant using these calculations.

Table 5 BaP emission factor estimates

Emission calculation	Site 1	Site 2
a The yard contributions for site 1 were incomplete. b At site 2, the level of detail around the bitumen unloading activity was lower, as the blow off activity, where bitumen is cleared from the transfer pipe, was carried out too quickly for any samples to be collected. c See Table 4.
Main stack
Total Stack Contribution (μg m^−3)	0.001	0.052
Gas Flow Rate (N m^3 s^−1)	8.2	3.39
Sample period (seconds)	14 400	14 400
Total Emission (g)	1.18 × 10^−04	0.0025
Sample Period Production (tonnes)	179	152
Emission (g/tonne)	6.6 × 10^−07	1.67 × 10^−05
Silo emission
USEPA organic PM emission estimate (kg/yr)	39.15	22.06
USEPA PAH % of Organic PM	11.40	11.40
% of BaP in average emission from bin/mixer	0.075	0.28
Total Annual Emission (kg/yr)	0.0033	0.0071
Production (tonnes/yr)	108 000	240 000
Emission (g/tonne)	3.08 × 10^−05	2.96 × 10^−05
Load out
USEPA organic PM emission estimate (kg/yr)	52.57	29.62
USEPA PAH % of Organic PM	5.93	5.93
% of BaP in average emission from bin/mixer	0.075	0.28
Total Annual Emission (kg/yr)	0.0023	0.0050
Production (tonnes/yr)	108 000	240 000
Emission (g/tonne)	2.15 × 10^−05	2.06 × 10^−05
Bitumen Delivery
Unloading Concentration (μg m^−3)	0.0075	0.0920
Unloading Flow Rate (m^3 s^−1)	0.0157	0.0118
Unload Time (seconds)	2,400	3120
Total Unloading Emission (μg)	0.2827	3.3816
Blow-out Concentration (μg m^−3)	0.1765	Not determined/measured^b
Blow-out Flow Rate (m^3 s^−1)	0.0471
Blow-out Time (seconds)	120
Total Blow-out Emission (μg)	0.9981
Total Emission During Monitoring Period (g)	1.28 × 10^−06
Production During Monitoring Period	415	758
Emission (g/tonne)	3.09 × 10^−09	>4.46 × 10^−09
Yard Emissions
Emission (g/tonne)^c	Not determined^a (assumed to be 1.27 × 10^−05 g/t using the USEPA methodology)	4.27 × 10^−09
Total Emission Factor (g/tonne)	6.57 × 10 ^−05	>6.69 × 10 ^−05

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Table 6 Predicted BaP contributions based on the US EPA AP42 methodology^5,a,b,c,d

Emission Rate Calculation	Site 1	Site 2
a V is the asphalt volatility as determined by ASTM Method D2872-88, set at default value of −0.5. b T is operating temperature in °F. c EF is the organic particulate matter emission factor (g or kg per tonne asphalt). d AF is the proportion of the organic particulate matter fraction that is BaP (in %).
T/°C	186	160
Temperature (°F)	366.8	320
Main Stack
ERBaP (g/t)	1.55 × 10^−07	1.55 × 10^−07
Silo Emission
EFPM = 0.00105(−V)e^((0.0251)(T + 460)−20.43) (kg/t)	0.00036	0.00011
AFBaP (%)	No data	No data
EFBaP (g/t) = EFPM × AFBaP	No value	No value
Load Out
EFPM = 0.0141(−V)e^((0.0251)(T + 460)−20.43) (kg/t)	0.00049	0.00015
AFBaP (%)	0.0023%	0.0023%
EFBaP (g/t) = EFPM × AFBaP	1.12 × 10^−05	3.46 × 10^−06
Bitumen Storage /Delivery
EFPM determined using US EPA ‘TANKS’ programme (kg/t)	Not possible for bitumen PM	Not possible for bitumen PM
AFBaP (%)	No data	No data
EFBaP (g/t) = EFPM × AFBaP	No value	No value
Yard Emissions
EFPM (kg/t)	0.00055	0.00055
AFBaP (%)	0.0023%	0.0023%
EFBaP (g/t) = EFPM × AFBaP	1.27 × 10^−05	1.27 × 10^−05
Total Emission Factor (g/tonne)	>2.40 × 10 ^−05	>1.63 × 10 ^−05

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Impact on ambient air

To assess the impact of the contributions from the individual BaP sources on the ambient BaP concentrations in the vicinity of each of the sites, a dispersion modelling exercise has been carried out using the dispersion model ADMS. Table 7 shows the source characteristics that have been used as part of this modelling exercise.

Table 7 Source characteristics for dispersion modelling

Parameter - stack parameters	Site 1	Site 2
Assumed operation hours	24 per day	24 per day
Efflux velocity	14.9 m s^−1	11.9 m s^−1
Exit temperature	111 °C	103 °C
Exit diameter	1.02 m	1.18 m
Stack height	26 m	30 m
Flow rate (actual) (m^3 h^−1)	43,920	47,700
Parameters - Other sources	Site 1	Site 2
Discharge Heights
Load out	1 m	1 m
Silo filling	5 m	5 m
Yard emissions	1 m	1 m
Bitumen tanks	0.5 m	0.5 m
Discharge velocity
Load out
Silo filling	0 m s^−1	0 m s^−1
Yard emissions
Bitumen tanks

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Fig. 2 and 3 show the BaP contribution superimposed on the site plans. In each case the emission rates applied were those calculated from the measured BaP concentrations, as shown in Table 2. In each case the dispersion modelling shows that the contribution of BaP to the ambient concentration is focused around the main processing building, where both the silos and load out activity are located. In addition to this, in Fig. 2 (site 1), a small area where the where the BaP contribution may exceeds 0.25 ng m⁻³, i.e. within the 0.1 ng m⁻³ contour, lies very close to the eastern side of the plant boundary. This feature coincides with the yard activities, which were assigned the predicted US EPA emission factor, which was much higher than the emission factor determined at site 2. It is important to note that the findings of this dispersion modelling exercise are site specific. Other building/discharge configurations may result in significantly different impact profiles.

Fig. 2 Annual mean BaP concentration (ng m⁻³) around site 1. Contours shown are 0.25, 0.05 and 0.01 ng m⁻³.

Fig. 3 Annual mean BaP concentration (ng m⁻³) around site 2. Contours shown are 0.25, 0.05 and 0.01 ng m⁻³.

Discussion

BaP concentrations

The US EPA methodology does not provide factors for all emission sources on such plants and the yard activity emission factors that are provided are independent of temperature‡, which means that their use could give rise to over-estimated emissions. In addition, the measured stack contribution to emissions on both sites was higher than that predicted by the US EPA methodology.

However, notwithstanding the omissions in the US EPA methodology, this study has shown that the estimated BaP emissions measured on the two sites (6.57 × 10⁻⁰⁵ g/t and 6.69 × 10⁻⁰⁵ g/t) is broadly similar to that determined using the US EPA method: >2.40 × 10⁻⁰⁵ g/t and >1.63 × 10⁻⁰⁵ g/t for sites 1 and 2, respectively.

Impact on ambient air

In terms of actual impact of BaP concentrations from the asphalt plants to ambient air quality, it can be seen that, on balance, the process contribution to the actual environmental concentration is low, at about 0.05 ng m⁻³ at each of the site boundaries. These predictions are below the EPAQS air quality standard of 0.25 ng m⁻³. Clearly, the consequence of any activity located close to the boundary fence may cause elevated but localised BaP. The location of these activities should therefore be taken into consideration in site design. Fig. 2 and 3 also show that based on this size of asphalt plant the contribution of BaP to the ambient concentration rapidly falls with distance from the activity.

The annual average BaP concentration in large urban centres such as London and Glasgow in 2005 was found to be 0.08 ng m⁻³ and 0.10 ng m⁻³ respectively⁶ and in industrial areas, that contain major BaP emission sources, the BaP concentrations are likely to be higher. Based on the findings of this study, it is highly unlikely that the BaP assessment threshold of 0.25 ng m⁻³ will be exceeded when siting asphalt plants. In locations where there is already an existing and significant source of BaP the implications of additional activities that might generate further emissions should be considered.

Conclusions

Overall, it can be concluded that the use of published BaP emission factors based on the US EPA methodology needs to be applied carefully, in particular where sources of BaP are dependent on temperature. While the estimated BaP emissions measured on the study sites were broadly similar to those estimated using the US EPA methodology, omissions in the US EPA methodology mean that emissions can be over or under estimated. However, on balance, the contribution of the estimated emissions from these asphalt plants to environmental concentration is low, averaging about 0.05 ng m⁻³ at the site boundary - well below the BaP assessment threshold of 0.25 ng m⁻³. Therefore, BaP concentrations, and hence PAH concentrations, from similar asphalt plant operations are unlikely to significantly affect local air quality. This outcome can be ensured by:

•Careful control of the process temperatures to minimise the generation of organic particulate matter.

•Containment of fugitive emissions, especially close to the site boundary.

•Ensuring that any new plant is not sited in close proximity to potentially sensitive receptors who may be affected over the exposure period of a year.

•Ensuring that BaP producing activities, where located near to site boundary, are located on sides away from potentially sensitive receptors.

Notes and references

1. Expert Panel on Air Quality Standards Polycyclic Aromatic hydrocarbons, 1999, published by The Stationery Office.
2. AEA Group (on behalf of Defra) Annual Report for 2008 on the UK PAH Monitoring and Analysis Network, 2009http://www.airquality.co.uk/reports/.
3. Planning Policy Statement 23: Planning and Pollution Control, 2004, published by The Stationery Office.
4. Defra, Process Guidance Note 3/15a (04), Secretary of State's Guidance for Roadstone Coating Processes, 2004, http://archive.defra.gov.uk/environment/quality/pollution/ppc/localauth/pubs/guidance/notes/pgnotes/documents/minpg3-15a.pdf .
5. US Environmental Protection Agency; AP-42 emission factor handbook, updated 2004, http://www.epa.gov/ttn/chief/ap42/ch11/final/c11s01.pdf.
6. UK Air Quality Archive, 2008, annual data to 2006, www.airquality.co.uk/archive/data/pah/Annual_to_2006v1_2sf.xls.

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Footnotes

† 1 ng m⁻³ is equal to 1 × 10⁻⁹ g m⁻³.

‡ The consequence of operating temperature on organic particulate matter can be seen from the US EPA equations reproduced in Table 6 and the estimates for organic PM emission in Table 5. It can be seen that the generation of organic particulate matter is a temperature dependent function. For example, comparing the BaP contributions at 160 °C and 180 °C against 140 °C will result in a 3 and 9 fold increase in BaP emission. It is therefore critical for this type of operation to maintain the drier temperature as low as practicable whilst maintaining the required asphalt temperature at the point of use. Clearly, the same effects will be found for the silo and probably the yard emissions.

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