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Measurements of Gas Phase Acids in Diesel Exhaust: A Relevant Source of HNCO? Jeremy Wentzell, John Liggio, Shao-Meng Li, Alexander Vlasenko, Ralf M. Staebler, Gang Lu, M.J. Poitras, Tak Chan, and Jeffrey Robert Brook Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es401127j • Publication Date (Web): 19 Jun 2013 Downloaded from http://pubs.acs.org on June 19, 2013
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Measurements of Gas Phase Acids in Diesel Exhaust: A Relevant Source of HNCO?
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Jeremy J.B. Wentzell1, John Liggio1*, Shao-Meng Li1, A. Vlasenko1, Ralf Staebler1, Gang
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Lu1, Marie-Josée Poitras2, Tak Chan2 and Jeffrey R. Brook1
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1. Air Quality Processes Research Section, Environment Canada, 4905 Dufferin Street,
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Toronto, Ontario, Canada M3H 5T4
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2. Emissions Research and Measurement Section, Environment Canada, 335 River Road,
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Ottawa, Ontario, Canada K1A 0H3
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* Corresponding author
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Abstract
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Gas-phase acids in light duty diesel (LDD) vehicle exhaust were measured using
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chemical ionization mass spectrometry (CIMS) and fuel based emission factors (EF) and
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NOx ratios for these species were determined under differing steady state engine
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operating conditions. The derived HONO and HNO3 EFs agree well with literature
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values, with HONO being the single most important acidic emission. Of particular
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importance is the quantification of the EF for the toxic species, isocyanic acid (HNCO).
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The emission factors for HNCO ranged from 0.69 – 3.96 mg kgfuel-1, and were
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significantly higher than previous biomass burning emission estimates. Further ambient
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urban measurements of HNCO demonstrated a clear relationship with the known traffic
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markers of benzene and toluene, demonstrating for the first time that urban commuter
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traffic is a source of HNCO. Estimates based upon the HNCO-benzene relationship
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indicate that upwards of 23 tonnes of HNCO are released annually from commuter traffic
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in the Greater Toronto Area, far exceeding the amount possible from LDD alone.
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Nationally, 250 – 770 tonnes of HNCO may be emitted annually from on-road vehicles,
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likely representing the dominant source of exposure in urban areas, and with emissions
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comparable to that of biomass burning..
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1.0 Introduction
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Acidic species contribute to environmental issues including air quality, acid rain,
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and secondary organic aerosol formation1. A better understanding of the sources of
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organic and inorganic acids is important for understanding these issues. These sources are
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numerous, including biogenic and anthropogenic emissions, in addition to
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transformations of precursors in aqueous, gas and particulate phases1. Direct
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anthropogenic emissions represent an important source of carboxylic (organic) acids in
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urban and industrial environments1, of which vehicle emissions (gasoline and diesel) are
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likely the most important primary source2, 3 despite generally being a product of many
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types of fuel combustion4-7. Automobile exhaust is also a known source of inorganic
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acids such as nitric (HNO3) and nitrous (HONO) acids8. HONO in particular has also
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been shown to be an important source of the OH radical, and once emitted can react with
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ambient amines leading to the production of carcinogenic nitrosamines9. Reported
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primary fuel based emission factors (EF’s) for gaseous acids are few, yet are required as
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inputs into air quality models to constrain model predictions of organic aerosol, organic
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aerosol O:C ratios and aqueous phase chemistry.
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From a health perspective, recent studies have implicated isocyanic acid as a
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highly toxic gaseous acid and a potential health concern due to its dissociation at
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physiological pH10. Isocyanic acid (HNCO) and its aqueous anion isocyanate (CNO-)
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have been linked to health issues such as atherosclerosis, cataracts and rheumatoid
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arthritis through carbamylation reactions10-13. Carbamylation is a chemical process
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whereby amine, hydroxyl and sulfhydryl groups in human proteins add across the N-C
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bond of the CNO group11, 12 impairing protein function in the body13. Roberts et al10
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estimated that inhalation of concentrations as low as 1 ppbv may be sufficient to
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commence carbamylation reactions in the human body. Despite its toxicity, HNCO-
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specific exposure or air quality standards do not exist, although limits on occupational
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exposure to methyl isocyanate (the simplest organic isocyanate) have been established in
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some Canadian provinces14 and US states15.
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Isocyanic acid has been shown to be a product of various forms of combustion
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such as biomass burning, cooking and cigarette smoking10. Despite its important health
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implications, the magnitudes of HNCO emissions from all primary sources have not been
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clearly quantified. Laboratory proxies for real world automobile catalysts have shown
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that HNCO could be a trace intermediate in automobile catalytic converters during the
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warm-up phase16, 17. It has also been suggested as a direct additive in selective catalytic
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reduction (SCR) catalyst systems as a method of limiting NOx emissions18 and has been
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quantified as a by-product in urea based SCR systems.19 While urea-type SCR system
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have been commercialised, they are so far only sparingly used on heavy-duty diesel
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(HDD) vehicles and light-duty trucks, however stricter NOx emissions requirements will
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likely increase their use20. While these are known or potential sources of HNCO, EFs for
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HNCO (and most other acids) from the transportation sector in particular, do not exist.
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These sources are important to focus on due to the close proximity between the areas of
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high emissions and populations. Consequently, EF estimates for HNCO are important for
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determining the potential risk of harmful population exposure.
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In the current study, measurements of several organic and inorganic acids from
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light duty diesel exhaust in an engine lab using acetate ion chemical ionisation mass
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spectrometry (Acid-CIMS) during the Diesel Engine Emission Research Experiment
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(DEERE 2012) are presented. Emission ratios (mg of compound/kg of fuel consumed)
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were calculated for several species, including HNCO. To further demonstrate the
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relevance of the engine lab study results, ambient measurements of HNCO and vehicle
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emission tracers, suggest that HNCO is present as a result of vehicle emissions in urban
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areas. The results imply that vehicle emissions are a dominant source of HNCO in urban
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areas, but cannot be neglected as a relevant source even in the presence of biomass
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burning.
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2.0 Experimental
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2.1 Operation of the Acid-CIMS during DEERE
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The Acid-CIMS is a differentially pumped mass spectrometer (THS Instruments
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Inc.) of similar design and operation to other CIMS instruments described previously21, 22;
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a brief description follows here with additional details in the Supporting Information. The
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Acid-CIMS sampled air through a 0.635 cm O.D. PFA Teflon (PFA) tube externally
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heated to 80 ºC and into the instrument through a critical orifice (0.067 cm). Acetate ions
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were generated by passing 10 mL min-1 of N2 over the headspace of a diffusion tube
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containing acetic anhydride, followed by dilution with N2 and ionization by Po210. In the
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flow tube the acetate ions undergo the following reaction22
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CH3COO- + HA→CH3COOH + A-
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(Eq 1)
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where HA is the acid of interest and A- is the respective anion. The Acid-CIMS was
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operated primarily in selected ion mode. A full list of the monitored ions is given in the
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supplemental information (SI).
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2.2 CIMS Calibration The Acid-CIMS was calibrated daily using a certified standard of HCl (Air-
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Liquide Canada) which was used as a relative reference compound. Before and after the
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study in conjunction with HCl, calibrations of other acids were performed using certified
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permeation tubes which were verified by ion chromatography. Calibrations of HONO
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were performed using a source similar to that of others22, 23 by flowing HCl over a bed of
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NaNO2 undergoing the following reaction
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HCl + NaNO2→HONO + NaCl
(Eq 2)
The output of the HONO source was verified by ion chromatography. Calibrations of HNCO were performed by heating solid cyanuric acid to 300C
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while passing nitrogen over the headspace to produce a stable source of HNCO22, 24.
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Concentrations of CNO- were verified by ion chromatography. Backgrounds were
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subtracted from raw signal prior to applying calibration factors. The instrument response
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was linear over the range of 10 ppbv for all species.
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2.3 Test Vehicle Operation and Dilution Experiments performed during DEERE 2012 were performed in a manner
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described previously25. A Turbo Diesel Injection (TDI) engine removed from a 2001
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Volkswagen Jetta, equipped with a diesel oxidation catalyst (DOC), and operating on
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ultra-low sulphur diesel was used during the study. A schematic of the experimental setup
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is shown in Figure S1 (SI). Measurements were performed from a Constant Volume
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Sampler (CVS) which diluted the raw exhaust with HEPA filtered room air, by factors
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ranging from ~16-77 depending on driving mode. Four different steady state driving
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modes were studied (Table 1), determined by a constant catalytic converter temperature.
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These steady state modes were derived from the average speed and engine torque during
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transient drive cycles including; (1) the US06 Supplemental Federal Test Procedure
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(US06) representative of aggressive driving, (2) the Highway Fuel Economy Driving
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Schedule (HWFET) representing normal highway driving conditions, (3) U.S. Federal
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Test Procedure 75 (FTP75), representing city driving and (4) the engine operating at idle.
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Some degree of uncertainty likely exists in translating steady state conditions to actual
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on-road driving, particularly for Mode 3 (city driving). Nonetheless, these conditions
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provide a reasonable estimate of emissions.
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In order to ensure pseudo first-order kinetics within the instrument the exhaust
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was further diluted at a ratio of ~17:1 with zero air. Background measurements were
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obtained every 10 minutes by sampling the HEPA filtered dilution air which was further
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diluted to 17:1 as above. high background signal prevented the measurements of certain
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species which are known to exist26, 27 such as m/z 45 (formic acid). Concentrations of
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CO, CO2 (non-dispersive infrared analyser), NOx (chemiluminescence) and total
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hydrocarbons (THC, flame ionisation detector) were measured in the CVS dilution tunnel
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without additional dilution (Table 1). These instruments were zeroed and spanned every
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60 minutes. The THC monitor was calibrated with propane. Approximately 60 hours of
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CIMS data, across the 4 driving modes was obtained.
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2.4 Ambient Measurements
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Ambient measurements of HNCO, HNO3 and HCOOH were performed in
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Toronto, Ontario, Canada between September 17 and October 2, 2012 at a suburban
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location approximately 100 m from a major roadway. Ambient air was sampled through
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a short heated PFA tube with a residence time of 0.03 s. The Acid-CIMS was background
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corrected 10 minutes every hour using a bicarbonate scrubber28 with calibrations
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performed as above, and minimal data loss (
