Emission Factors for Selected Semivolatile Organic Chemicals from

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Emission Factors for Selected Semivolatile Organic Chemicals from Burning of Tropical Biomass Fuels and Estimation of Annual Australian Emissions Xianyu Wang, Carl P. Meyer, Fabienne Reisen, Melita Keywood, Phong K. Thai, Darryl W. Hawker, Jennifer Claire Powell, and Jochen F. Mueller Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01392 • Publication Date (Web): 07 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017

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Environmental Science & Technology

Emission Factors for Selected Semivolatile Organic Chemicals from Burning of Tropical Biomass Fuels and Estimation of Annual Australian Emissions Xianyu Wang,a,* Carl P. Meyer,b Fabienne Reisen,b Melita Keywood,b Phong K. Thai,a,c Darryl W. Hawker,d Jennifer Powell,b and Jochen F. Muellera

a

Queensland Alliance for Environmental Health Sciences, The University of Queensland, 39

Kessels Road, Coopers Plains, Queensland 4108, Australia b

CSIRO Oceans and Atmosphere Flagship, Aspendale Laboratories, 107-121 Station Street,

Aspendale, Victoria 3195, Australia c

International Laboratory for Air Quality and Health, Queensland University of Technology,

2 George St, Brisbane City, Queensland 4000, Australia d

Griffith School of Environment, Griffith University, 170 Kessels Road, Nathan, Queensland

4111, Australia

ACS Paragon Plus Environment


1

ABSTRACT

2

This study reveals that open-field biomass burning can be an important source of various

3

semivolatile organic chemicals (SVOCs) to the atmosphere including polycyclic aromatic

4

hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers

5

(PBDEs) and a range of pesticides. Emission factors (EFs) for 39 individual SVOCs are

6

determined from burning of various fuel types that are common in tropical Australia.

7

Emissions of PAHs are found to be sensitive to differences in combustion efficiencies rather

8

than fuel types reflecting a formation mechanism. In contrast, revolatilisation may be

9

important for other SVOCs such as PCBs. Based on the EFs determined in this work,

10

estimates of the annual emissions of these SVOCs from Australian bushfires/wildfires are

11

achieved, including for example ∑ PAHs (160 (min) – 1,100 (max) Mg), ∑ PCBs (14 – 300

12

kg), ∑ PBDEs (8.8 – 590 kg), α-endosulfan (6.5 – 200 kg) and chlorpyrifos (up to 1,400 kg),

13

as well as dioxin toxic equivalents (TEQ) of ∑ dl-PCBs (0.018 – 1.4 g). Emissions of SVOCs

14

that are predominantly revolatilised appear to be related to their use history, with higher

15

emissions estimated for chemicals that had a greater historical usage and were banned only

16

recently or are still in use.


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17


TABLE OF CONTENTS GRAPHIC

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19



20

INTRODUCTION

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Open-field biomass burning including agricultural waste burning, peat fires and

22

forest/savannah fires is an important source of emissions for a wide range of organic

23

pollutants including semivolatile organic chemicals (SVOCs).1-4 Many SVOCs, including

24

various important air pollutants such as polycyclic aromatic hydrocarbons (PAHs) and

25

halogenated organic compounds, are hazardous to humans.5 Release of these compounds

26

from biomass combustion events involves processes of de novo formation (i.e. compounds

27

newly formed from precursors and dependent on combustion conditions) or revolatilisation

28

(i.e. thermally stable chemicals remobilised untransformed due to increased temperatures).

29

Amongst types of open-field biomass burning, forest/savannah fires are dominant on a global

30

basis, accounting for 95% of total carbon emissions from this source.6 Globally, tropical

31

regions comprise most of the open-field biomass burning area, with the largest contributions

32

being from (central and southern) Africa and (central and northern) Australia.7, 8 Satellite-

33

derived data suggest that, from 1996 – 2012, the annual mean area burned across Australia

34

was the highest of any individual country, accounting for 15% of the global burned area.7, 9

35

Most of these fire-affected areas are in Australia’s northern tropical savannah woodlands and

36

central and northern arid rangelands.10, 11 As such, the contribution of these fires in tropical

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and arid Australia to the emission of harmful/toxic SVOCs is potentially significant. An

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estimate of these emissions is essential to understand the contribution from open-field

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biomass burning to the environmental burden of these chemicals.

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In order to achieve the above estimate for relevant SVOCs, it is first necessary to

41

measure/determine their emission factors (EFs), which are defined as mass of the compound

42

released to the atmosphere per unit mass of fuel consumed by combustion. This parameter is

43

important for quantitatively estimating the emissions of given chemicals from a regional or

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global perspective, in combination with knowledge of the mass of relevant vegetation

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combusted.6 It is also a key requirement in the construction of models of atmospheric

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transport and chemistry, necessary for evaluating the atmospheric impact of biomass

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burning.3 The typical approach to measure/determine EFs is through sampling the fire smoke

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emissions from burning of a known amount of biomass. Measurement of SVOC EFs from

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tropical biomass has been carried out to some extent for polychlorinated dibenzo-p-dioxins

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and dibenzofurans (PCDD/Fs) and dioxin-like (dl) polychlorinated biphenyls (PCBs).12-15 It

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has also been recognised that during biomass burning many other SVOCs such as PAHs and

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pesticides can be released.16-19 However, relevant EF data for PAHs are mostly limited to


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extratropical fuels while data for tropical biomass fuels are scarce.1, 20 There are essentially

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no relevant data for other SVOCs such as pesticides and polybrominated diphenyl ethers

55

(PBDEs).

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The aim of this study was to determine the EFs for a wide range of SVOCs from burning

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various fuel types that are common in tropical Australia. With these data, the present study

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also provides a first estimate of the annual emissions of many SVOCs from tropical

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Australian bushfires/wildfires.

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METHODOLOGY

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Sample collection. The study was conducted at the Mornington Sanctuary, a 3,500 km2

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nature reserve in the Kimberly region of Western Australia (17°31′44″ S, 126°6′12″ E).21 The

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region is typical of Australia’s open savannah woodlands and receives between 600 and

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1,000 mm annual rainfall.21 There is a lack of current local anthropogenic sources for SVOCs

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in the area contributing to the biomass-loading concentrations of these chemicals. Therefore

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we expect the sampling site (and emissions of SVOCs of interest from combustion of fuels

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naturally growing in the vicinity) to be representative of Australia’s most fire-prone areas, i.e.

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relatively unpopulated northern and central Australia. The vegetation comprises sparsely

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distributed trees less than 10 m in height (various Eucalyptus spp. and Corymbia spp.) with

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an understorey of hummock grasses (spinifex, Triodia spp.) and annual and perennial tussock

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grasses. Hummock grasses dominate the less fertile areas while tussock grasses tend to occur

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mainly on the richer volcanic and alluvial soils. These biomass types are among the most

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dominant in Australia in terms of areas, occupying at least 18%, 12% and 7% respectively of

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the whole continental land area.22 Further, the fact that they are distributed mostly in northern

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and central Australia means even higher proportions in these fire-affected areas. Test burns

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were conducted at a location close to the sources of the fuels whose emissions we aimed to

78

investigate. The biomass fuels used in this study comprised eucalypt leaf litter, eucalypt

79

coarse woody debris, spinifex and tussock grasses.

80

Measurements were conducted in August 2013, using a high volume smoke sampler with a

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sampling rate of approximately 1 m3 min-1. Details of the sampler design have been published

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elsewhere14, 15 and a schematic diagram is provided as Figure S1 in the Supporting

83

Information (SI). To minimise dilution from background/ambient air, smoke samples were

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collected directly above the fire plume (see Figure S2 in the SI as an example). Total



85

suspended particles (TSP) and particle-associated chemicals were collected on a quartz fibre

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filter (QFF, 203 × 254 mm) and gaseous chemicals separately collected on two subsequent

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130 mm diameter polyurethane foam (PUF) plugs (51 and 25 mm thicknesses for the front

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and back PUFs, respectively). A small bypass airflow was drawn into the associated carbon

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monoxide (CO) and carbon dioxide (CO2) analyser (Gascard II, Edinburgh Instruments,

90

Edinburgh, UK) to determine their concentrations (Figure S1).

91

The fuels of interest were collected from the surrounding undisturbed vegetation class

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immediately prior to each test and burned on an open hearth (within an area of 2 m2 and a

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height of 0.2 m) in beds constructed to approximate their undisturbed state and density.

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Smoke samples were taken from above the fire using the high volume sampler. The height of

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the sampling hood was adjusted throughout each test to ensure that surface temperatures of

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the hood and the line were less than 200 °C to minimise the risk of formation artefacts on the

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sampler surface, and that smoke levels (assessed by the CO2 and CO concentrations)

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remained within measurement range. In total, 11 smoke samples were collected with the

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sampling duration ranging from 18 to 80 min for each sample (Table S1). These experiments

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included fires with short (0.3 – 1.2 m) or long (1.5 – 2 m) flames and smoldering and full-

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course events in order to represent the range of complex burning conditions of actual

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bushfires. For the flaming samples, fuel was fed into the hearth progressively at the rate

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required to maintain the desired intensity of the flaming phase. Addition of the fuel was

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carefully conducted from the side of the burning pile. The smoldering phase was defined as

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the stage when little or no visible flame could be observed. For the full-course fires, sampling

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was from ignition until the fuels were burnt out and no additional fuel was loaded during the

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combustion. Collected QFF and PUF samples were stored at -25 °C until analysis.

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Chemical analysis. Details of chemical analysis are provided in Section 2 in the SI. Briefly,

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the mass of TSP within each sample was determined using a gravimetric method. The

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collected QFFs and PUFs were spiked with a solution containing 7 deuterated PAHs, 18 13C-

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PCB congeners, 7 13C-PBDE congeners and 14 13C-labelled pesticides at different levels as

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internal standards for quantification purposes (Table S2). QFF and PUF samples (both plugs

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combined) were then separately extracted in a Dionex ASE 350 Accelerated Solvent

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Extractor (Thermo Fisher Scientific) using n-hexane and acetone (1:1, v/v). Each extract was

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split 40%/40%/20% (v/v/v). The first aliquot (40%, F1) was cleaned up and analysed for non-

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acid resistant compounds (i.e., the analytes that would not survive the cleanup procedures

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involving concentrated sulfuric acid treatment) including 13 PAH compounds and 13


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pesticides. The second (40%, F2) was for acid resistant compounds targeting 18 PCB

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congeners, 14 polychlorinated naphthalene (PCN) congeners, 14 other pesticides and 7 PBDE

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congeners. The third (20%, F3) was analysed for the biomass burning tracer levoglucosan.

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(See details of sample cleanup in Section 2 of the SI. The full chemical list is provided in

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Table S2 of the SI.)

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Aliquots F1, F2 and F3 were analysed separately for the respective target compounds using a

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Thermo 1310 gas chromatograph coupled to a DFS Magnetic Sector high-resolution mass

125

spectrometer (GC-HRMS). The HRMS was operated in electron impact-multiple ion

126

detection (EI-MID) mode and resolution was set to ≥ 10,000 (10% valley definition).

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Quality assurance and quality control (QA/QC). Details on QA/QC are provided as

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Section 3 in the SI. Briefly, breakthrough effects were monitored for each sample. Solvent,

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matrix and field blank samples were integrated within sample batches and accounted for

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about 30% of the total sample numbers. Method detection limits (MDLs) were defined as the

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average field blank plus three times the standard deviation. If the relevant compounds could

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not be detected within the field blank samples, MDLs were determined based on half the

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instrument detection limits. MDLs were typically < 1 ng m-3 for PAH analytes and < 10 pg

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m-3 for other SVOCs as detailed in Table S3 for individual chemicals.

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Derivation of emission factors. EFs are derived using the carbon-balance model, which

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assumes that the total carbon in the fuels is a conserved quantity. Its principles are based on

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the work of Andreae and Merlet3 and Meyer et al.15

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The model or approach can be expressed by the following equation:

139

= ∆

∆

× =

×

(1)

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where is the emission factor (mass analyte kg-1 fuel) for a specific compound or

141

compound group , represents the fuel carbon content and and are the

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atmospheric concentrations (mass m-3) of the chemical or carbon under combustion

143

conditions and ambient (background) conditions respectively.

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Typically, the carbon content of dry biomass fuel is close to 50% and varies only within a

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limited range between different fuel types.3, 15 During the combustion process, more than

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85% of the carbon is emitted as CO2.15 Therefore for simplicity we approximated the mass of

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emitted carbon to be the mass of C in emitted CO2 (CO2-C). This will lead to a slight

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overestimation of the EF but is well within the typical uncertainty of SVOC analysis (relative



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standard deviation (RSD) of 20 – 50% for replicate QC samples fortified with analyte of

150

interest).23-26 The above equation is thus simplified to:

151

=

152

where '# and '# are concentrations of CO2-C (mass m-3) in the smoke and

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ambient air respectively.

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The Mornington Sanctuary sampling site is considered remote as mentioned, which means a

155

potentially low level of SVOCs in the ambient air. Taking some other remote sites in northern

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Australia as examples, PCB concentrations in air are typically reported as some hundred

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femtograms per cubic metre.27 Obtaining reliable results for ambient levels of these SVOCs

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at the Mornington Sanctuary sampling site would then be expected to require a sampling

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duration of approximately 12 to 24 hours. Several logistical challenges were recognised in the

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operation of the sampler for such an extended time at this site such as a lack of power supply,

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maintenance of the sampling equipment and ongoing sampling parameter monitoring and

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adjustment. We therefore decided not to collect background samples directly within this

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sampling campaign. Instead, background atmospheric concentration data for SVOCs and CO2

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refer to those from a study based on another remote site in the Northern Territory, Australia,

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namely the Australian Tropical Atmospheric Research Station (ATARS, 12°14'56.6"S,

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131°02'40.8"E) which provides better access to power supply and shelter for both personnel

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and equipment. These data were obtained in the year of 2014, using high volume air samplers

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(for SVOCs) and a high precision Fourier Transform Infrared trace gas and isotope analyser

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(for CO and CO2, Spectronus, Ecotech Pty. Ltd., Knoxfield, Australia). Samples identified as

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not being impacted by fire events were used. These were analysed in the same laboratory and

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using the same methods as those for the smoke plumes in the current work. (See details in

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Wang et al.1).

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RESULTS AND DISCUSSION

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Detection and concentrations of SVOCs in smoke samples. Overall, 47 out of the 79

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targeted chemicals were detected in over half of the samples, including all the PAH analytes,

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most PCB and PBDE congeners, some of the pesticides such as α-endosulfan, chlorpyrifos

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and hexachlorobenzene (HCB), and some PCN congeners. Concentrations of most of these

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SVOC analytes (39 out of 47), as well as TSP and the cellulose combustion product

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levoglucosan in the smoke samples were considerably higher (i.e. having a mean value at

180

least 10 times higher) than background levels (Table S4 in the SI) with the factor ranging

!"# !"#

× 0.5


(2)

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from 16 to 11,000 for different analytes. This suggested that combustion of these fuels

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represents an important source of these SVOCs to their ambient atmospheric environment.



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Table 1. Emission factors of TSP (g kg-1 fuel burnt), gaseous + particle-associated levoglucosan (g kg-1 fuel burnt) and selected target SVOCs

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(µg kg-1 fuel burnt) from burning of different fuels. For dioxin-like PCBs, the emission factor is expressed on the basis of dioxin toxic

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equivalents (TEQ) of ∑ dl-PCBs (pg kg-1 fuel burnt). Also shown is the modified combustion efficiency (MCE)

186 187 188 189 190

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Spinifex Tussock grasses Eucalypt leaf litter Eucalypt coarse woody debris Short Long Long flaming Short Long flaming Flaming + Full-course Flaming Smoldering Flaming Smoldering flaming flaming + smoldering flaming + smoldering smoldering TSP 8.6 24 11 2.7 7.4 7.2 4.2 24 12 3.2 31 Levoglucosan 0.082 0.21 0.090 0.0086 0.047 0.029 0.034 0.17 0.045 0.012 0.24 ∑ PAHs(a) 3,800 3,500 3,700 560 640 2,700 880 2,500 780 680 2,600 ∑ PCBs(b) 0.33 1.1 0.39 0.085 0.14 0.12 0.10 0.21 0.050 0.059 0.16 ∑ PCNs(c) 0.011 0.0088 0.0059 0.0047 0.0022 0.011 0.0012 0.0070 0.0011 0.00066 0.0025 ∑ PBDEs(d) 0.58 2.1 0.42 0.092 0.15 0.094 0.057 0.19 0.031 0.081 0.14 HCB 0.045 0.089 0.029 0.011 0.015 0.023 0.013 0.049 0.024 0.022 0.042 γ-HCH 0.040 0.10 0.093 0.014 0.013 0.012 0.024 0.015 0.0015 0.0084 0.016 p,p’-DDE 0.0067 0.021 0.0056 0.0016 0.0019 0.0038 0.00115 0.0021 0.00050 0.00068 0.00059 α-endosulfan 0.67 0.46 0.73 0.10 0.091 0.19 0.067 0.52 0.12 0.064 0.023 Chlorpyrifos NA 3.9 0.21 1.1 NA 0.34 NA 5.1 1.8 NA NA ∑ dl-PCBs TEQ(e) 1.2 5.0 1.6 0.18 0.33 0.16 0.15 0.28 0.068 0.064 0.22 MCE(f) 0.954 0.935 0.927 0.980 0.972 0.923 0.973 0.936 0.961 0.986 0.917 (a) Refers to sum of phenanthrene (Phe), anthracene (Ant), fluoranthene (Flu), pyrene (Pyr), benzo[a]anthrancene (BaA), chrysene (Chr), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), benzo[e]pyrene (BeP), benzo[a]pyrene (BaP), indeno[1,2,3-cd]pyrene (I123cdP), dibenzo[a,h]anthracene (DahA) and benzo[g,h,i]perylene (BghiP) data; (b) Refers to sum of data for congeners 28, 52, 101, 138, 153, 180, 77, 105, 114, 118, 156, 157 and 167; (c) Refers to sum of data for congeners 13, 27 and 28+36; (d) Refers to sum of data for congeners 28, 47, 99, 100 and 154; (e) Refers to sum of TEQ data for congeners 77, 105, 114, 118, 156, 157 and 167; (f) Expressed to three significant figures


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Group- and compound-specific emission factors for SVOCs. EF values were calculated

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for each chemical group and the aforementioned 39 individual chemicals that were detected

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in over half of the samples and had concentrations considerably higher than background

194

levels (Tables 1 and S5; values shown are gaseous (from PUFs) + particle-associated (from

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QFFs)). Also derived were the EF values for TSP and levoglucosan. The EF for TSP from

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this study ranged from 2.7 to 31 g kg-1, agreeing very well with the literature values from

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savannah, grassland and tropical forest (6.5 – 11 g kg-1).3 Levoglucosan EF (0.0086 – 0.24 g

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kg-1) was also within the range from the few relevant reports for tropical biomass

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combustions (up to 0.50 g kg-1).3, 28 As a group, PAHs had the highest emission factors,

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ranging from 560 to 3,800 µg kg-1 fuel burnt for ∑ PAHs depending on the fuel and

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combustion conditions. The individual compound with the highest EF was phenanthrene

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(Phe, 200 – 1,300 µg kg-1 (Table S5)). Other SVOCs/SVOC groups with relatively high EFs

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(µg kg-1 fuel burnt) were ∑ PCBs (0.050 – 1.1), ∑ PBDEs (0.031 – 2.1), and amongst

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pesticides, α-endosulfan (0.023 – 0.73) and chlorpyrifos (up to 5.1). Overall, the variation in

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EF data from all samples was less with PAHs than other SVOC groups. For example, the

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ratio of the highest to the lowest EF for ∑ PAHs is approximately 7, compared to the one of

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22 for ∑ PCBs.

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Variability in SVOC emissions can be discussed within two contexts: variability arising from

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combustion chemistry (i.e. inherent fuel chemical composition and the characteristics of the

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combustion event) and variability that arises from the revolatilisation of relatively stable

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SVOCs that have previously been deposited/uptaken on/by the fuel from other sources. With

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the former context, we expect to see variability between combustion types and fuel types

213

rather than within these types; with the latter we would expect to see variability within both

214

combustion and fuel types.

215

Emissions of PAHs from biomass burning are mainly through de novo formation processes

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(for example from aliphatic precursors such as propargyl moieties forming intermediate

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cyclopentadienyl radicals).16 Given there should be limited variation in the carbon content of

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the fuels of interest,3, 15 we should expect to see variation in PAH emissions associated with

219

combustion types rather than with fuel types. The emission profile of PAHs produced during

220

combustion is related to the relative completeness of the oxidation process, commonly

221

expressed as the modified combustion efficiency (MCE, shown for each sample in Table 1):29 ( =

)*+ , -*. + -*+ .



222

where -*. and -*+ . are the number of moles of each measured during the collection of a

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sample. Increased levels of the former are associated with reduced combustion efficiency.

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Both EF data for PAHs and levoglucosan are negatively correlated with MCE (Figure 1, p

Emission Factors for Selected Semivolatile Organic Chemicals from

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