Environmental Pollution 254 (2019) 112982
Contents lists available at ScienceDirect
Environmental Pollution
journal homepage: www.elsevier.com/locate/envpol
Characterization of M4 carbine rifle emissions with three ammunition
types*
Johanna Aurell a, Amara L. Holder b, Brian K. Gullett b, *, Kevin McNesby c,
Jason P. Weinstein b
a
University of Dayton Research Institute, 300 College Park, Dayton, OH 45469, USA
U.S. Environmental Protection Agency, Office of Research and Development, National Risk Management Research Laboratory (E343-04), Research Triangle
Park, NC 27711, USA
c
U.S. Army Research Laboratory, 2800 Powder Mill Rd, Adelphi, MD 20783, USA
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 March 2019
Received in revised form
2 July 2019
Accepted 29 July 2019
Available online 1 August 2019
Muzzle emissions from firing an M4 carbine rifle in a semi-enclosed chamber were characterized for an
array of compounds to provide quantitative data for future studies on potential inhalation exposure and
rangeland contamination. Air emissions were characterized for particulate matter (PM) size distribution,
composition, and morphology; carbon monoxide (CO); carbon dioxide (CO2); energetics; metals; polycyclic aromatic hydrocarbons; and methane. Three types of ammunition were used: a “Legacy” (Vietnamera) round, the common M855 round (no longer fielded), and its variant, an M855 round with added
potassium (K)-based salts to reduce muzzle flash. Average CO concentrations up to 1500 ppm significantly exceeded CO2 concentrations. Emitted particles were in the respirable size range with mass
median diameters between 0.33 and 0.58 mm. PM emissions were highest from the M855 salt-added
ammunition, likely due to incomplete secondary combustion in the muzzle blast caused by scavenging of combustion radicals by the K salt. Copper (Cu) had the highest emitted metal concentration for
all three round formulations, likely originating from the Cu jacket on the bullet. Based on a mass balance
analysis of each round's formulation, lead (Pb) was completely emitted for all three round types. This
work demonstrated methods for characterizing emissions from gun firing which can distinguish between round-specific effects and can be used to initiate studies of inhalation risk and environmental
deposition.
Published by Elsevier Ltd.
Keywords:
Emissions
Ammunition
Rifle
Carbon monoxide
Particulate matter
Metals
1. Introduction
Limited information is available on emissions from gun firing
and the resulting inhalation exposure and environmental deposition. This is likely due to the difficulties in quantitatively sampling
emissions from an unconfined source. Changes in ammunition
composition, such as the use of lead-free bullets, remain minimally
characterized. Recent work has begun to evaluate warfighter
exposure to particle inhalation due to repeated, enclosed range
firings (Grabinski et al., 2017; Voie et al., 2014; Sikkeland et al.,
2018; Borander et al., 2017), raising health concerns about prolonged exposure. The toxicity hazard from energetic material
*
This Paper has been recommended for acceptance by Bernd Nowack.
* Corresponding author.
E-mail address: gullett.brian@epa.gov (B.K. Gullett).
https://doi.org/10.1016/j.envpol.2019.112982
0269-7491/Published by Elsevier Ltd.
testing may result from skin contact with solid or liquid residues,
contact with contaminated water, contact with unconsumed energetic material, breathing aerosolized particulate matter, or
breathing toxic permanent gases produced during explosive energy
release.
Likewise, ground deposition of emissions, particularly on
training ranges, presents an unknown subsequent exposure and
environmental contamination. Some work has begun to characterize deposited residues from a variety of weapon systems (Brochu
et al., 2011; Walsh et al., 2012). Particles containing metals, organics, and salts are likely to deposit on the area around the range
or even be transported off-site. Depending on the chemical and the
properties of the soil, solubilization of contaminants, and subsequent ground water contamination may be a significant concern
(Fuller et al., 2014).
Emissions of interest from gun firings include metals and
organic combustion byproducts. Sources of metal in the
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J. Aurell et al. / Environmental Pollution 254 (2019) 112982
ammunition round include the projectile or projectile, the casing
material, the primer, and the propellant. Spalling of the gun barrel
lining itself is an additional metal source. Depending on the
composition of the ammunition, metals may include lead (Pb),
chromium (Cr), nickel (Ni), zinc (Zn) and copper (Cu) (Grabinski
et al., 2017), among others. The propellant ignitor (primer) can
contain metals such as Pb, barium (Ba), and antimony (Sb) compounds (Dalby et al., 2010) while other metallic compounds such as
bismuth (Bi) and antimony (Sb) are designed to mitigate Cu deposits in the rifled barrel. The double base (nitrocellulose, NC, and
nitroguanidine, NG) propellant may also contain Ba/K salts as a
flash suppressant. Other neat energetic materials containing
carbon-nitrogen (CeN) and nitrogen-hydrogen (NeH) bonds may
also produce toxic permanent gases (e.g., cyanide (CN), ammonia
(NH3)) which are hazardous to humans.
Sampling gun emissions presents significant measurement
challenges. The rapid dispersion of the emissions must be
accounted for to be quantitative in deriving total emissions per
firing. While the literature in this area is sparse, recent work by
Wingfors et al. (2014) constructed a polymethylmethacrylate
(PMMA) chamber into which multiple rounds of lead-free munitions were fired. Emission factors were calculated by multiplying
the time-averaged concentration (mass/sample volume) of the
compound sampled by the chamber volume, then dividing by the
number of firings (ten). The emissions were found to be rich in
particles at 29e30 mg/round. Metals and organics were also reported in relation to their reported composition in the ammunition.
More historic work with M16 rifle firings (Ase et al., 1985) has
identified relatively high levels of CO (over 30% by mass compared
to the propellant mass), hydrogen cyanide (HCN), carbon disulfide
(CS2), polycyclic aromatic hydrocarbons (PAHs), and metals, most
notably Ba, Cu, Pb, Sb, and Zn.
We build on previous small arms emissions measurements
(Brochu et al., 2011; Wingfors et al., 2014; Ase et al., 1985) by
developing and testing methods to quantify and characterize a
range of pollutants to determine if varying ammunition formulations result in differences that may impact personal exposure and
environmental contamination. For this work, we used the common
M4 carbine rifle with three versions of the M855 ammunition.
Targeted pollutants included CO, CO2, volatile organic compounds
(VOCs), particulate matter < 2.5 mm (PM2.5), PM < 10 mm (PM10), PM
elemental composition, particle size distribution, PAHs, and nitroaromatics. Results are expressed in terms of emission factors, or the
mass of emissions per gun firing and mass of emissions per mass of
fuel (propellant plus primer), the latter to more easily relate to
different propellant formulations. These emission factors are the
critical values necessary for informed prediction of risk for human
exposure and range contamination.
2. Materials and methods
2.1. Ammunition and rifle
Gun firings were conducted at the U.S. Army Research Laboratory (ARL), Aberdeen Proving Ground, MD (U.S.A.). An M4 carbine
rifle was selected for study due to its prevalence with U.S. military
and North American Treaty Organization (NATO) forces for the last
20 years. The barrel length is 36.8 cm plus the length of “A2”
muzzle, or “birdcage” device which is used to block/suppress light
emission from incandescing gases/particles immediately following
bullet exit. The M4 weapon was tested in operational configuration
and was not modified for the testing. It was capable of being
switched between semiautomatic and 3-shot burst mode but was
operated in single-shot mode. M855 ammunition was selected
because it is available in multiple compositions with the same base
propellant formulations (nitrocellulose ((C6H7(NO2)3O5)n)/nitroglycerin (C3H5N3O9) “double-base”) and contains Pb in the projectile, affording a chance to study the fate of metals. Three types of
M855 ammunition were tested, each of which used a M41 primer
containing Pb, aluminum (Al), Ba, Sb and a Cu jacketed lead bullet,
5.56 mm in diameter. The ammunition consisted of a 1990s era
M855 round, its “salted” counterpart which contains K for flash
suppression, and an older M855 Vietnam-era (1960s) “legacy”
round. While the M855 is in common sport use, this is not the same
formulation as in the current military-fielded M855A1 “green”
lead-free ammunition. A total of 20 shots were analyzed for the
three ammunition types. The composition of the ammunition used
here is described more fully in Supplemental Information (SI)
Tables SIe1. The metal and carbon fraction for these rounds are
found in Tables SIe2.
2.2. Test chamber and test setup
A PMMA rectangular enclosure (132 cm across, 76 cm high, and
66 cm in depth for a total volume of 0.3 m3) surrounded the M4
carbine muzzle to contain the rifle firing exhaust (Fig. 1). A 5 cm
diameter hole, later enlarged to 15 cm to mitigate the shock wave
stress on the enclosure, allowed for exit of the bullet. The clear side
of the box facilitated imaging measurements using high speed
cameras and spectrographs. The enclosure had a hinged side wall
that could be opened for insertion of samplers and cleaning of the
wall surfaces (Windex®) between firings (Fig. 1). The time between
each round fired was approximately 20 min with 1 h between
different ammunition types for cleaning of test chamber and
resetting of instruments. Multiple sampler inlets were positioned
approximately 15 cm below the center of the muzzle. Bulk PM2.5,
PM10, and PAH sampler inlets were inside the enclosure. All other
samples were extracted from the enclosure through stainless steel
sample lines.
2.3. Target analytes and sampling methods
The target analytes, their sampling techniques/instruments, and
methods are listed in Table 1. The CO2 (LICOR-820, Lincoln, NE,
U.S.A.) and the CO (E2vEC4-500-CO, SGX Sensortech, United
Kingdom) sensors were calibrated in accordance with U.S. EPA
Method 3A (U.S. EPA Method 3A, 2017), undergoing a daily three
point, zero, and calibration drift test. A precision dilution calibrator,
Serinus Cal 2000 (American ECOTECH L.L.C., Warren, RI, U.S.A.), was
used to dilute the high-level span gases to appropriate levels for the
CO2 and CO calibration curves. The E2v CO sensor has a CO detection range of 1e500 ppm with resolution of 1 ppm. The LICOR-820
has an analytical range of 0e20,000 ppm with an accuracy specification of less than 3% of reading. The LICOR-820 calibration range
for CO2 was set to 0e3,000 ppm.
PM2.5 and PM10 were sampled with SKC impactors on 47 mm
Teflon filters with a pore size of 2.0 mm using a Leland Legacy
sample pump (SKC Inc., Eighty Four, PA, U.S.A.) with a constant
airflow of 10 L/min. The Leland Legacy sample pump was calibrated
with a Gilibrator Air Flow Calibration System 610 (Sensidyne LP, St.
Petersburg, FL, U.S.A.). The sampling time for the PM impactors
ranged from 2 to 9 min, representing composite samples from
multiple shots and uncertainty over how much sample was
sufficient.
The elemental composition of PM was measured by energy
dispersive x-ray fluorescence spectrometry (ED-XRF) following U.S.
EPA Compendium Method IO-3.3 (U.S. EPA Compendium, 1999a).
Measurement of 13 standard reference materials resulted in recoveries (experimental value divided by given value) between 92
and 109%. The PM elemental carbon (EC) and organic carbon (OC)
J. Aurell et al. / Environmental Pollution 254 (2019) 112982
3
Fig. 1. Schematic of test chamber and test setup superimposed on shadowgraph image (not to scale).
Table 1
Target analytes, instrumentation, and methods.
Analyte
Technique/Instrument
CO2
CO
PM2.5
LICOR-820, NDIRa
Continuous 1 Hz
e2VEC4-500-CO, Electrochemical cellb
Continuous 1 Hz
c
SKC Impactor , 47 mm Teflon filter 2 mm pore size Batch e 10 L/min constant
flowd
SKC Impactorc, 47 mm Teflon filter 2 mm pore size Batch e 10 L/min constant
flowd
47 mm Quartz fiber filter
Batch e 10 L/min constant
flowd
47 mm Quartz fiber filter
Batch e 10 L/min constant
flowd
Batch e 5 L/min constant
Quartz fiber filter, PUFe, XAD-2
flowd
Teflon filter from PM2.5 batch filter, XRFf
Batch e 10 L/min constant
flowd
Quartz filter
Batch - 10 L/min constant
flow
6 L SUMMA Canister
Batch e 0.5 L/min
6 L SUMMA Canister
Batch e 0.5 L/min
Electrical Low Pressure Impactorg
Continuous & Batch
U.S. EPA Method 3A (U.S. EPA Method 3A, 2017)
U.S. EPA Method 3A (U.S. EPA Method 3A, 2017)
40 CFR 50, Appendix L (40 CFR Part 50Appendix, 1987)
Scanning Electron Microscope Energy Dispersive Batch
XRayh
Casuccio et al. (2004)
PM10
Nitrocellulose
Nitroaromatics
PAHs
Elements
Elemental, Organic Carbon
VOCs
CO, CO2, CH4
PN, Size Distribution,
Composition
PM Morphology
a
b
c
d
e
f
g
h
Frequency/Sampling rate
Method reference
40 CFR 50, Appendix J (40 CRF Part 50Appendix, 1987)
U.S. EPA Method 353.2 (U.S. EPA, 1993)
U.S. EPA Method 8330b (U.S. EPA Method 8330B, 2006)
Modified U.S. EPA Method TO-9A (U.S. EPA Compendium,
1999b)
EPA Compendium Method IO-3.3 (U.S. EPA Compendium,
1999a)
Panteliadis et al. (2015)
U.S. EPA Method TO-15 (U.S. EPA, 1999)
Modified U.S. EPA Method 25C (U.S. EPA, 2017)
(Kero and Jorgensen, 2016) and (Sowards et al., 2008)
LI-COR Biosciences, USA. NDIR - Non-dispersive infrared.
SGX Sensortech, United Kingdom.
SKC Inc., USA.
Leland Legacy sample pump, SKC Inc., USA.
PUF - Polyurethane foam.
XRF e X-ray fluorescence.
Dekati, Finland.
MIRA3 Tescan, Czech Republic.
were measured following the NIOSH 930 protocol cited in
Panteliadis et al. (2015) using a Sunset Laboratory Carbon Aerosol
Analyzer.
Gas phase and particle-bound PAHs were collected using polyurethane foam, XAD-2, and a quartz filter, extracted together with
toluene, concentrated, and then analyzed using a Modified U.S. EPA
Method 8270D (U.S. EPA, 1998) on a Thermo GC Trace 1310/ISQ
(ThermoScientific, Inc., Milan, IT/Austin, TX, U.S.A.) using a DB-5
chromatographic column (Agilent technologies, Santa Clara, CA,
U.S.A.). The method modifications included addition of pre-
4
J. Aurell et al. / Environmental Pollution 254 (2019) 112982
sampling, pre-extraction, and recovery standards with quantification via isotope dilution as per U.S. EPA Method 23 (U.S. EPA, 1991).
PAH analyses were conducted by a gas chromatograph-low resolution mass spectrometer (GC/LRMS) in selective ion monitoring
mode, targeting the 16 PAH priorities of U.S. EPA (Keith, 2015). PAH
emission factors were also evaluated using toxic equivalent factors
(TEFs) relative to benzo[a]-pyrene toxicity equivalent (B[a]]P-TEQs)
(Larsen and Larsen, 1998).
Energetics, including nitroaromatics, nitrocellulose, and their
byproducts, were sampled using a Leland Legacy pump (SKC, Inc.)
at a constant flow rate of 10 L/min onto a PM2.5 impactor with a
quartz fiber filter. Filter analyses followed U.S. EPA Method 8330b
(U.S. EPA Method 8330B, 2006) for nitroaromatics and possible
degradation products and U.S. EPA Method 353.2 (U.S. EPA, 1993)
for nitrocellulose by a nitrate-nitrite colorimetric method. The filters were analyzed by APPL Inc. (Clovis, CA, USA).
VOCs were sampled using laboratory-supplied 6L SUMMA
canisters via U.S. EPA Method TO-15 (U.S. EPA, 1999) at approximately 0.5 L/min. Analyses were done in accordance with U.S. EPA
Method TO-15 (U.S. EPA, 1999) using full scan mode GC/LRMS by
ALS Environmental (Simi Valley, California, USA). The SUMMA
canisters were also analyzed for CO2, CO, and CH4 by a GC/flame
ionization detector according to modified U.S. EPA Method 25C (U.S.
EPA, 2017).
Emissions were diluted with nitrogen using a porous tube
€nen et al., 2004) and an eductor (DI-1000,
dilution probe (Lyyra
Dekati Ltd., Kangasala, Finland) to reduce concentrations for
continuous PM measurements. The dilution ratio was monitored
continuously by measuring CO and CO2 concentrations in the
diluted sample. Continuous particle number concentration (PN)
and size distributions were measured after the gun firing with an
Electrical Low Pressure Impactor (ELPI, Dekati Ltd.). The ELPI
continuously measures particle mass and number concentrations
on a series of impactor plates supporting greased aluminum foils
(10 bins from 7 nme10 mm). The dilution was adjusted so that the
current observed on any single stage was below the maximum
current allowed by the instrument to ensure that all measurements
were within range. The mass median distribution was determined
using a density calculated from the elemental composition analysis
of the bulk PM10 aerosol. The resulting particle densities were 4.1,
3.0, and 4.2 g/cm3 for the M855, M855 Salted, and Legacy ammunition, respectively. The online ELPI sampled for approximately
2 ± 0.7 min for each shot.
For some gun firings lacey carbon transmission electron microscope (TEM) grids were placed on the ELPI impactors to collect
samples for morphological analysis. For other gun firings greased
polycarbonate filters were placed on the ELPI impactors to collect
PM for gravimetric and chemical analysis. Size resolved particle
elemental composition was measured by SEM-EDX according to
U.S. EPA Compendium Method IO-3.3 (U.S. EPA Compendium,
1999a). Select, individual particles of interest were imaged using
Scanning Electron Microscopy (SEM, Tescan MIRA3, Brno, Czech
Rep.) with the elemental composition measured by a SEM-EDX
(Bruker AXS, Inc., Madison, WI, U.S.A.). Particles were collected
with a lacey carbon-coated copper TEM grid affixed to an
aluminum SEM stub with carbon tape, allowing half of the TEM grid
to be suspended in the open space over the SEM stub. A background
analysis was conducted off-particle and the beam was placed in the
carbon matrix holes.
PM concentrations, CO2, and CO chamber air background samples were measured before each shot was fired. One VOC chamber
air background sample was collected after testing and one PAH trip
blank was collected.
2.4. Calculations
Emission factors were calculated using the carbon balance
method which takes the mass of the sampled target compound
divided by the mass of the sampled carbon. With knowledge of the
carbon composition of the source (propellant and primer), the total
mass of the target compound per bullet firing can be calculated
regardless of an unknown dilution amount. Emission factors thus
can be expressed as mass of target compound per mass of carbon or
mass of target compound per firing. Use of the carbon balance
method allows one to sample a subset of the carbine box gases
rather than the whole box atmosphere as the emission factor relies
only on the ratio of the pollutant to carbon which is preserved
regardless of sample volume. Emission factors calculated by this
method are expressed here as mass of pollutant per single round
firing, mass of pollutant per mass of “fuel”, where fuel is the mass of
the propellant and primer, and mass of pollutant per mass of
element present in the original propellant and primer (for example,
g Pb emitted/g Pb in the original propellant and primer). Additional
calculations allow determinations of the modified combustion efficiency (MCE) which is defined as the ratio of the CO2 increase
(above background) to the total amount of carbon emitted as
CO2 þ CO. Trace carbon species, such as non-methane organic
carbon and particulate carbon are commonly neglected in the total
amount of carbon emitted as their inclusion has been reported to
result in errors of less than a few percent (Sinha et al., 2003), well
within the error range of the measurements.
Standard deviations (SD) and relative standard deviations (RSD)
showed dispersion of three or more data values while relative
percent difference (RPD) was used as a quality indicator when only
two data values were obtained. One-way analysis of variance
(ANOVA) was used to determine significant differences between
ammunition formulations (p < 0.05 and F ¼ F/Fcrit > 1.0).
3. Results and discussion
3.1. CO2, CO, and CH4
Emission factors for major gaseous carbon species measured
from SUMMA canister samples are listed in Table 2 for each of the
ammunition types. Significant levels of incomplete CO oxidation
resulted in MCE values less than or equal to 0.337, higher than those
(0.165) from Wingfors et al. (2014). CO concentrations as high as
Table 2
CO2, CO, and CH4 emission factors with modified combustion efficiency.
Bullet type
855 no salt
855 Salt
Legacy
a
Unit
nc
Mean
Stand. Dev.
RSDd
RPDe
nc
Mean
Stand. Dev.
RSDd
nc
Mean
Stand. Dev.
RSDd
g/kg fuel
g/kg fuel
%
%
g/kg fuel
g/kg fuel
%
g/kg fuel
g/kg fuel
%
CO2
CO
CH4a
MCEb
4
339
73
22
N/A
6
330
66
20
9
414
84
20
4
517
47
9
N/A
6
510
42
8
9
460
54
12
2
3.4
N/A
N/A
1.15
3
3.6
0.1
3.9
1
4.1
N/A
N/A
4
0.297
0.064
22
N/A
6
0.288
0.056
20
9
0.345
0.080
23
Calculated from SUMMA Canister samples.
MCE ¼ modified combustion efficiency (DCO2 ppm/(DCO2 ppm þDCO ppm)),
unitless fraction.
c
n ¼ Number of samples collected and number of shots (equivalent).
d
RSD ¼ relative standard deviation, calculated when n ¼ 3 or more.
e
RPD ¼ relative percent difference, calculated when n ¼ 2. N/A e not applicable.
b
J. Aurell et al. / Environmental Pollution 254 (2019) 112982
2,000 ppm were observed in SUMMA samples. Shot to shot variation was reflected by RSD and RPD values between 13% and 24%.
The amount of CH4 to CO2 plus CO (DCH4/(DCO2þDCO)) was less
than 1%, implying that the CH4 has minimal impact on the MCE
value. No literature values for CH4 measurements from gun firing
could be located. Continuous CO and CO2 measurements indicated
that the combined M855 salted and M855 unsalted rounds (n ¼ 9)
exhibited less complete CO oxidation than the Legacy rounds
(n ¼ 8) although the result is not statistically distinct (p > 0.26,
F < 0.30). CO emission factors, 464e517 g/kg fuel, are higher than
those of Ase et al. (1985), 337 g/kg fuel, for an M16 firing.
3.2. Particulate matter
The PM2.5 and PM10 emission factor results for M855 and Legacy
ammunition were nearly the same, which indicates that the majority of the emitted particles are of diameter less than or equal to
2.5 mm and is confirmed by the measured size distributions (Table 3
and Fig. 2). However, the M855-salted ammunition PM10 emission
factor was nearly 50% larger than the PM2.5 emission factor. The
RSD(10e15%) and RPD(6e38%) indicate the level of consistency for
these rounds in the emissions as well as the testing and sampling
method. The emission factors for PM10 (Table 3) are consistent with
those measured by Wingfors et al. (2014) for total suspended particles (0.029e0.030 g/round), albeit using different methods and
different ammunition types. Total PM values for an M16 rifle were
found to be 4 mg/kg fuel (Ase et al., 1985) versus 11e34 g/kg fuel
reported in this current work for the M4 rifle.
The M855-Salted ammunition had the largest PM2.5/10 emission
factors, likely due to K-based particles (Table 3). The PM emission
factors for the two different ammunition compositions (M855Salted versus M855-Unsalted and Legacy) are statistically distinct,
(p<0.004, F>3.8). The Legacy ammunition resulted in emissions
within 50% of its more recently manufactured M855 counterpart
but these comparisons are limited by only two values for each
5
ammunition type.
PN emission factors showed no statistically significant differences between the different ammunition types (Table 3). To our
knowledge there are no other PN emission factors from gun firings
to compare with; however, these factors are an order of magnitude
larger than what has been measured from other combustion
sources. For example, PN emission factors from heavy duty diesel
trucks were ~2 1016 #/kg (Ban-Weiss et al., 2009), from ships
were 1.5 1016 #/kg (Beecken et al., 2015), and from woodstoves
were in the range of 2e7 1015 #/kg (Wardoyo et al., 2006). The
higher PN emission factors from gun firing as compared to other
combustion sources may be due to the additional particles formed
from non-combustion sources such as from the bullet fragments or
from the barrel.
The mass (Fig. 2A) and particle number distributions (not
shown) of the M855 and legacy ammunition types immediately
after firing are dominated by the smallest particles (<30 nm),
similar to distributions measured at firing ranges, where the count
median diameters ranged from 30 to 80 nm (Grabinski et al., 2017).
The size distribution evolved rapidly over the first few seconds
(Fig. 2B) as the particles aggregated due to the very high concentrations inside the enclosure. Initially, the mass distribution for the
M855 ammunition is entirely less than 1 mm and bimodal, with a
peak at 21 nm and one at approximately 100 nm. Over the first
2 min the particles mass median diameter shifts to around 770 nm
and approximately 30% of the mass is contained in particles larger
than 1 mm. This aggregation process explains the slightly larger
PM10 emission factors versus PM2.5 emission factors despite the
initial particle distributions all being significantly smaller than
10 mm in diameter. This shift in sizes is consistent with results reported by Wingfors et al. (2014), where the median diameter
increased from 0.2 mm at 2 min after firing to 1 mm after 12 min.
The initial size distribution was different for all three types of
ammunition, with the M855 and the Legacy exhibiting a peak in the
smallest size bin (<28 nm) and a second larger mode in the
Table 3
Particulate matter and element emission factors.
855 Salt (n ¼ 3, s ¼ 6)a
g/kg fuel
Particulates
PM2.5
PM10
PN (#/kg fuel)
Elements in the
Al
Ba
Bi
Cd
Co
Cr
Cu
Fe
K
Mg
Mn
Mo
Ni
Pb
S
Sb
V
W
Zn
a
23 ± 2.3
34 ± 5.3
6.5 ± 2.0 1016
PM2.5 fraction
0.072 ± 0.028
0.31 ± 0.039
2.91 ± 0.60
ND (0.0018)
ND (0.0013)
ND (0.0009)
16.4 ± 1.0
0.17 ± 0.021
9.07 ± 0.69
0.28 ± 0.02
0.003 ± 0.0001
0.001 ± 0.0004
ND (0.0014)
5.77 ± 0.42
0.76 ± 0.20
1.56 ± 0.22
0.017 ± 0.002
0.055 ± 0.004
2.12 ± 0.14
855 (n ¼ 2, s ¼ 4)b
Legacy (n ¼ 2, s ¼ 10)b
mg/round
g/kg element
g/kg fuel
mg/round
g/kg element
g/kg fuel
mg/round
g/kg element
40 ± 4.0
59 ± 9.1
NA
NA
15 ± 1.1
17 ± 0.5
1.0 ± 0.4 1017
25 ± 2.0
29 ± 0.8
NA
NA
10 ± 1.9
11 ± 1.2
7.5 ± 2.3 1016
17 ± 3.2
20 ± 2.1
NA
NA
0.12 ± 0.048
0.53 ± 0.07
5.02 ± 1.04
ND (0.0031)
ND (0.0022)
ND (0.0016)
28.3 ± 1.8
0.30 ± 0.04
15.7 ± 1.2
0.49 ± 0.03
0.0057 ± 0.0002
0.0019 ± 0.0007
ND (0.0024)
9.96 ± 0.73
1.31 ± 0.34
2.69 ± 0.37
0.029 ± 0.03
0.096 ± 0.006
3.66 ± 0.25
51 ± 20
90 ± 12
14804 ± 307
NA
NA
NA
NA
NA
65 ± 7.7
NA
NA
NA
NA
1672 ± 123
884 ± 232
720 ± 99
NA
NA
NA
0.086 ± 0.0004
0.33 ± 0.008
2.85 ± 0.47
0.003
ND (0.0012)
ND (0.0008)
15.6 ± 0.89
0.15 ± 0.003
0.57 ± 0.29
0.22 ± 0.005
ND (0.0015)
0.0009 ± 0.0003
ND (0.0013)
6.07 ± 0.22
0.0005
1.55 ± 0.42
0.09 ± 0.0004
0.064 ± 0.004
2.04 ± 0.11
0.15 ± 0.0007
0.57 ± 0.01
4.98 ± 0.82
0.0048
ND (0.0021)
ND (0.0014)
27.3 ± 1.5
0.26 ± 0.006
1.00 ± 0.51
0.39 ± 0.009
ND (0.0026)
0.0015 ± 0.0006
ND (0.0023)
10.60 ± 0.39
0.0008
2.70 ± 0.07
0.15 ± 0.0007
0.11 ± 0.006
3.55 ± 0.19
85 ± 0.4
441 ± 10
1440 ± 238
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
1765 ± 64
0.55
330 ± 9.0
NA
NA
NA
0.077 ± 0.013
0.28 ± 0.04
0.07 ± 0.04
0.002 ± 0.001
ND (0.0009)
ND (0.0005)
12.1 ± 1.7
0.50 ± 0.09
0.10 ± 0.001
0.17 ± 0.03
0.004 ± 0.001
0.002 ± 0.0005
ND (0.0009)
4.95 ± 0.5
ND
1.30 ± 0.2
0.01 ± 0.001
0.01
1.76 ± 0.26
0.13 ± 0.02
0.48 ± 0.07
0.13 ± 0.08
0.0042 ± 0.002
ND (0.00016)
ND (0.0009)
21.1 ± 2.9
0.87 ± 0.16
0.17 ± 0.002
0.29 ± 0.05
0.007 ± 0.003
0.004 ± 0.001
ND (0.002)
8.58 ± 0.87
ND
2.25 ± 0.35
0.02 ± 0.002
0.015
3.05 ± 0.45
76 ± 13
372 ± 52
38 ± 22
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
1438 ± 146
NA
280 ± 43
NA
NA
NA
Range of data equals ±1 standard deviation, n ¼ number of samples, s ¼ number of shots.
Range of data equals mean absolute deviation, n ¼ number of samples, s ¼ number of shots. NA ¼ not applicable, element not in the propellant or in the primer. ND ¼ not
detected, uncertainty level within parentheses.
b
6
J. Aurell et al. / Environmental Pollution 254 (2019) 112982
Fig. 2. A) Representative initial mass normalized mass weighted PM size distributions from M4 carbine for different ammunition types. B) Mass normalized mass weighted PM size
distributions from M4 carbine for M855 ammunition over the first 2 min after firing.
200e500 range. The M855 salted lacked the small particle mode,
and had a single mode at a mass median diameter of 0.570 mm. This
may be due to the K-salt suppressing the nucleation of the smallest
particles, however the mechanism by how this occurs is not known,
because the composition of the nuclei mode is presently unknown.
After several seconds the distributions were similar for all types,
with mass median diameters of 0.389 ± 0.109 mm for the M855
Salted, 0.330 ± 0.124 mm for the M855, and 0.575 ± 0.130 mm for the
Legacy ammunitions. PN emission factors calculated from the initial
particle concentrations (before significant aggregation occurred)
showed no statistically significant differences between the
different ammunition types (Table 3) elements.
Elemental emissions from analysis of the sample filters are
shown in Table 3, for the Salted, M855, and Legacy ammunition
types, respectively. Data are presented in terms of g/kg fuel, mg/
round and g/kg element, the latter being a measure of the amount
of the original metal that ends up in the emissions. The results
show relatively similar metal emission factors (Fig. 3) with the
exception of higher K from the salted (KNO3) ammunition. The
relative standard deviations for the element concentrations are
quite low, averaging 12%, despite only three replicates, indicating
the robustness of the combined sampling and analytical methods.
Cu has the highest metal emission factor (Fig. 3A) despite the
lack of Cu in the propellants and primers of the three tested bullets.
Rather, the bullets (the projectile) are encased in a Cu jacket which
is swaged by the rifled barrel to enable the bullet to spin, aiding in
accuracy. While the barrel etches the copper jacket, Cu is deposited
on the barrel interior and, as these results show, emitted from the
barrel. In addition to Cu, Pb and Sb are added to the propellant to
keep the rifle barrel clean. After Cu, Pb has the next highest emission factor for the three bullet types (other than K for the salted
ammunition) at around 5 g/kg (Fig. 3A). The abundance of Cu and
Pb is consistent with findings of Brochu et al. (2011). Elements not
reported in the primer and propellant formulation but observed in
the particle analyses include Na, Fe, Zn, and K (in the non-salted
rounds). This may be due to trace amounts of these elements
alloyed in the bullet (projectile) casing.
Emission factors presented in proportion to their reported
amounts in the primer plus propellant (g/g element) are shown in
Fig. 3B. Pb is consistently over 1,000 g Pb/kgPb for all three bullet
types suggesting a discrepancy with the composition in the manufacturer's Safety Data Sheet. While the projectile is a possible
source of measured Pb, images of launch survival (not covered in
this paper) saw no evidence of bullet breakup at uncorking. Bi also
exceeds the known amount except for the Legacy ammunition
despite its being reported as having the same amount of Bi in the
propellant (Tables SIe1) as the other two bullet formulations. S and
Sb both appear to near 100% emission but only for the Salted
ammunition. The excess reported for Pb and Bi may come from
barrel sloughing as the age of the barrel and its previous use are
unknown. As well, since we don't have unreported trace elements
from the primer and propellent, these may also provide a source.
Some trace elements are detected such as Cd, Co, Cr, Fe, Mn, Mo, Ni,
V, and W that were not reported in product formulations provided
by the manufacturer. Additionally, there may be other sources of
elements in stead of the primer and propellant, such as the jacket
material, projectile, muzzle, cleaning residues.
Wipes of the sampling chamber were taken after each round
type was finished and analyzed by ICP for Pb, the most likely
element to be observed. Between 1.3 and 1.9 mg of Pb were observed
per shot for the three ammunition round types (the blank wipe was
below detection limits at 0.4 mg) amounting to less than 3.1% of the
total air emission of Pb.
The element emission factors for PM2.5 and PM10 (Fig. 3) are
quite similar, indicating a similar mechanism of particle formation
despite their size difference. The Legacy ammunition alone showed
trace detectable levels of Cl contrary to its reported composition.
The M855 unsalted ammunition showed detectable levels of S,
unlike the Legacy ammunition.
The PM2.5 element emission factors by ammunition type are
compared with those reported for the respirable fraction (<4 mm
D50) from Wingfors et al. (2014) in Tables SIe3. There is general
agreement on the magnitude of the emission factors with the
exception of their low Pb values which are expected due to their
testing of Pb-free ammunition. Elemental emission data from Ase
et al. (1985) with the M16 rifle are not compared due to the substantially different propellant and primer composition.
3.3. PM composition by size
The smaller particles, which can penetrate deep into the lung,
were enriched with Pb. Particles less than 1 mm were composed of
approximately 14% Pb, while larger particles were only 6% Pb
(Table 4). Similar trends were observed for K, Sb, and Bi, which may
have been derived from combustion products. In contrast, Zn was
distributed uniformly across all size ranges. A sizable fraction of the
PM was not detected by XRF and was likely carbonaceous soot
formed from combustion. Almost all of the M855 PM10 mass not
J. Aurell et al. / Environmental Pollution 254 (2019) 112982
7
Fig. 3. Bullet element emission factors in A) g/kg fuel, where the denominator “fuel” accounts for the weight of the propellant and primer, and B) g/g element, where the denominator accounts for the weight of the element in the original propellant and primer composition; both PM2.5 and PM10 data shown.
Table 4
PM percent elemental mass composition by particle size determined using XRF.
Bullet type
PM Size Range (mm)
Cu
Pb
K
Sb
Zn
Bi
M855
0.14e0.27
0.27e0.37
0.37e0.63
0.63e0.94
0.94e1.55
1.55e2.4
PM2.5
PM10
0.27e0.37
0.37e0.63a
0.63e0.94
0.94e1.55
1.55e2.4
PM2.5
PM10
PM2.5
PM10
16.8
20.5
33.4
36.8
38.8
42.4
34.5
36.7
20.8
12.8
12.9
14.3
11.9
8.5
6.8
14.8
12.7
14.8
1.5
0.8
0.8
0.7
0.5
0.5
0.7
0.5
14.7
5.7
4.9
4.9
3.7
2.5
1.3
3.5
3.1
4.8
3.3
2.5
3.1
3.1
3.1
3.3
4.5
4.3
2.4
7.1
5.0
5.1
4.4
4.0
4.6
5.6
5.5
5.1
27.0
30.6
25.5
22.5
23.5
38.5
37.0
9.9
8.9
6.3
8.7
7.5
15.8
12.7
5.7
4.9
3.1
12.1
8.7
0.3
0.3
2.2
1.7
1.3
1.9
1.7
4.1
3.4
2.3
2.6
2.1
2.9
2.7
5.6
4.9
2.9
2.7
2.4
3.3
3.2
0.3
0.2
M855 Salted
Legacy
OC
11.7
10.0
EC
2.4
1.0
Oestb
Trace elements
Balancec
12.1
10.5
14.5
14.2
14.0
15.0
14.5
14.2
16.8
7.3
3.5
4.0
3.1
3.2
4.3
3.9
3.1
8.6
33.4
39.3
19.9
22.1
25.5
21.9
16.5
5.8
11.9
12.3
13.3
12.4
11.7
10.6
17.1
15.3
3.4
4.6
6.8
2.4
1.8
6.5
5.1
34.4
30.7
40.0
34.6
40.3
11.9
10.3
a
The gravimetric mass was less than the sum of the elements detected by XRF, which may have occurred due to contamination deposited on the filter after gravimetric
determination but before XRF analysis.
b
Oxygen estimated by assuming most common metal oxide form as described in Reff et al., 2009) (Reff et al., 2009).
c
Mass unable to be analyzed by XRF comprised of atomic number less than 12.
detected by XRF was carbonaceous, and most was in the form of
organic carbon (11.7%) as opposed to elemental carbon (2.4%). The
carbon content was not measured for the M855 salted ammunition
but is likely to be a major constituent of the particle mass not
accounted for by XRF. This is due to the function of the K-based salt
acting as a muzzle flash suppressant by inhibiting combustion of
unburned gases (the secondary muzzle flash) after bullet exit (K
acts as an H, O, OH scavenger). The unburned gases then contribute
to soot formation, resulting in a relative increase of carbon for the
salted rounds.
3.4. PM morphology
The particle morphology was mostly spherical, although some
particles appeared to be aggregates of smaller spherical particles.
Individual particles were composed of primarily Cu, and some
8
J. Aurell et al. / Environmental Pollution 254 (2019) 112982
containing Pb and lesser amounts of other elements (e.g., Bi, O, C,
Mg, Al) (Figure SI-2). These results suggest the formation of a metal
alloy upon condensation, which may have restructured into a larger
spherical particle in the cooling exhaust. Microscopy samples were
only obtained of the larger particles (>1 mm) but considering the
similarity of composition across all size ranges, the morphology and
the individual particle composition may be similar for the smaller
particles.
Similar aggregates of much smaller spherical particles (diameter < 100 nm) were observed at a firing range where M4 rifles
were being used with lead free ammunition (Grabinski et al., 2017).
These particles emitted from lead free ammunition were also
composed of copper and various other elements (Bi, Ni, Zn, Na, C),
but no lead was detected. The smaller particle size may have been
due to the higher ventilation rates at the firing range that suppressed particle aggregation. Additionally, in the present study the
presence of Pb in the emissions can form alloys with other metals,
reducing the melting point, which may promote the formation of
larger spheres upon aggregation.
M855 Salt. Naphthalene, pyrene, acenaphthylene, and phenanthrene are the most predominant PAHs for all three bullet types.
The most toxic PAH, common to all three bullet types, is benzo(a)
pyrene, which accounts for over 55% of the 16-PAH toxic equivalency value (Larsen and Larsen, 1998). These tests sampled for and
analyzed both gas phase and particle bound PAHs, finding values
that are often three orders of magnitude higher than particle phase
(only) data from Wingfors et al. (2014) indicating the predominantly volatile nature of the PAHs. For instance, the average emission factor for pyrene was 1.2 mg/kg or 2200 ng/round in this study
which is three times higher than derived by Wingfors et al. (2014)
(620 ng/round) and a hundred times higher than from open
burning of propellant (Aurell et al., 2015) (0.012 mg/kg fuel).
PAH emission factors are compared to those reported elsewhere
(Wingfors et al., 2014) in Tables SIe6. Reasonable agreement is
noted given the limited number of trials, two each for the three
ammunition types in this work and five for Wingfors et al. (2014)
However, our data were gas and particle phase whereas those of
Wingfors et al. (2014) were particle phase only.
3.5. Volatile organic compounds
3.7. Energetics
The VOC emission factors were background corrected and data
from the three bullet types are shown in Tables SIe4 in both mg/kg
fuel and mg/round. For most compounds, the variation of the individual emission factors was low, averaging 34%, despite the limited
number (n ¼ 1 to 3) of trials. For comparison with published
emission factors, the sole overlap with data from Wingfors et al.
(2014) is acrolein for which they obtained 8 ± 1 mg/round as
compared to this work at a 6-firing average of 3.8 ± 1.6 mg/round.
The benzene emission factor levels, 93.8e120 mg/kg are in same
range as benzene emissions from open detonation of munitions
(89e264 mg/kg (Aurell et al., 2011; Aurell et al., 2015)), although
higher than from open burning of propellant (2.5e4.7 g/kg (Aurell
et al., 2011; Aurell et al., 2015)).
Energetics were sampled by collecting three samples of the
Salted round, consisting of 2, 2, and 3 cumulative shots, two samples of the 855 round, consisting of 4 and 2 cumulative shots, and
two samples of the Legacy round, each sample consisting of four
shots each. None of the 17 nitroaromatics and energetic byproducts
examined in these seven samples resulted in levels above the
detection limit (Tables SIe7). The analysis method (U.S. EPA
Method 8330 B (U.S. EPA Method 8330B, 2006)) in our work had a
sample detection limit of 2 mg/sample which amounts to an emission factor of <0.039 mg/round for all three bullet types. However,
Wingfors et al. (2014) found that nitrogen-containing heterocyclic
compounds and aromatic nitrogen compounds accounted for a
relatively large proportion of the particulate-bound organics and
environmental sampling. Similarly, open range sampling by Brochu
et al. (2011) found a nitroglycerin emission mass of less than 0.9
mg/round for a number of ammunition and gun types.
3.6. Polycyclic aromatic hydrocarbons
PAH emission factors from the combined gas phase and particlebound phase are shown in Table 5 (the same table, expressed in
units of mg/kg of fuel is in Tables SIe5). PAHs were about 50%
higher in the Legacy round emissions than that of the M855 and
4. Conclusion
This
work
demonstrated
methods
to
comprehensively
Table 5
PAH emission factors from firing of M4 carbine.
Targets
Naphthalene
Acenaphthylene
Acenaphthene
Fluorene
Phenanthrene
Anthracene
Fluoranthene
Pyrene
Benzo(a)anthracene
Chrysene
Benzo(b)fluoranthene
Benzo(k)fluoranthene
Benzo(a)pyrene
Indeno(1,2,3-cd)pyrene
Dibenz(a,h)anthracene
Benzo(ghi)perylene
SUM 16-EPA PAHs
855-Salt, n ¼ 2
855, n ¼ 1
Average
Average
RPD
mg/round
mg B[a]P TEQ/round
%
1.59E-02
1.61E-03
ND
ND
1.56E-03
2.55E-04
7.08E-04
1.84E-03
1.22E-04
2.08E-04
3.31E-04
1.45E-04
2.35E-04
3.46E-04
4.70E-05
1.08E-03
2.44E-02
N/A
N/A
N/A
N/A
7.78E-07
1.27E-07
3.54E-05
1.84E-06
6.09E-07
6.24E-06
3.31E-05
7.27E-06
2.35E-04
3.46E-05
5.17E-05
2.17E-05
4.29E-04
25
25
N/A
N/A
4
37
7
4
6
4
5
37
6
11
88
0.04
18
mg/round
1.56E-02
1.44E-03
ND
ND
1.48E-03
2.33E-04
6.43E-04
1.67E-03
1.11E-04
2.02E-04
2.76E-04
1.41E-04
2.23E-04
2.70E-04
ND
8.88E-04
2.32E-02
Legacy, n ¼ 2
mg B[a]P TEQ/round
N/A
N/A
N/A
N/A
7.40E-07
1.17E-07
3.22E-05
1.67E-06
5.54E-07
6.05E-06
2.76E-05
7.05E-06
2.23E-04
2.70E-05
ND
1.78E-05
3.44E-04
Average
Average
RPD
mg/round
mg B[a]P TEQ/round
%
2.75E-02
2.27E-03
ND
ND
2.18E-03
3.05E-04
8.82E-04
2.81E-03
1.10E-04
2.64E-04
3.62E-04
1.61E-04
2.33E-04
3.16E-04
ND
1.89E-03
3.93E-02
N/A
N/A
N/A
N/A
1.09E-06
1.52E-07
4.41E-05
2.81E-06
5.49E-07
7.92E-06
3.62E-05
8.04E-06
2.33E-04
3.16E-05
ND
3.79E-05
4.03E-04
42
49
N/A
N/A
49
34
62
99
57
66
62
91
64
76
N/A
114
51
J. Aurell et al. / Environmental Pollution 254 (2019) 112982
characterize air emissions from gun firing, providing emission
factor data that can be used for environmental and health assessments. Distinctions between bullet/primer formulations could be
discerned, potentially allowing for optimization of propellant,
primer, and casing formulations to reduce risk of environmental
contamination and human exposure. PM emissions were highest
from the M855 salt-added ammunition, likely due to incomplete
secondary combustion in the muzzle blast caused by scavenging of
combustion radicals by the K salt. CO levels higher than CO2
resulted in MCE values lower than 0.35, indicating incomplete
carbon oxidation. Some metals, such as Pb, were completely
released in the emissions. The majority of particles were in the
respirable range, < 1 mm, and were enriched in Pb, among other
metals. Toxics including acrolein and benzo(a)pyrene were among
the volatiles and PAHs sampled. The emission of metals and organics could provide an environmental contamination concern for
outdoor firing ranges and an inhalation risk for indoor ranges
without sufficient ventilation. These data can be used in deposition
and exposure models to estimate potential environmental
contamination and health toxicity assessments.
Funding
This work was funded by the Strategic Environmental Research
and Development Program Exploratory Development (SERDP
SEED) WP-2611, and the U.S. Environmental Protection Agency’s
Office of Research and Development.
Disclaimer
The views expressed in this article are those of the authors and
do not necessarily represent the views or policies of the U.S.
Environmental Protection Agency (EPA). The publication of this
report does not indicate endorsement by the Department of Defense (DoD), nor should the contents be construed as reflecting the
official policy or position of the DoD. Reference herein to any specific commercial product, process, or service by trade name,
trademark, manufacturer, or otherwise, does not necessarily
constitute or imply its endorsement, recommendation, or favoring
by the DoD or EPA.
Declaration of interests
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
The authors declare the following financial interests/personal
relationships which may be considered as potential competing
interests: None.
Acknowledgments
Analytical and laboratory test-set up expertise was provided by
Mr. Dennis Tabor (U.S. EPA/ORD), Mr. Gene Summers (U.S. Army
Research laboratory), and Messrs. Will Sickels, Ray Sparks, and
Ronnie Thompson (Bowhead Total Enterprise Solutions, LLC).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.envpol.2019.112982.
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