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Environmental Processes
Evidence for the importance of semi-volatile organic ammonium salts in ambient particulate matter Ye Tao, and Jennifer Grace Murphy Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03800 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018
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Environmental Science & Technology
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Evidence for the importance of semi-volatile organic ammonium salts in ambient particulate matter
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Ye Tao1, Jennifer G. Murphy*2
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1Department
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2Department
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*Corresponding author:
[email protected] of Physical and Environmental Sciences, University of Toronto Scarborough, Toronto, ON, M1C 1A4, Canada of Chemistry, University of Toronto, Toronto, ON, M5S 3H6, Canada
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Abstract
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The gas/particle phase partitioning behavior of NH3/NH4+ and other semi-volatile constituents was
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measured by a custom-designed Denuder-MOUDI-Denuder integrated sampling system in Toronto,
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Canada. In this setup, upstream denuders were used to capture alkaline and acidic gaseous components,
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and particle phase components were captured by the filters on MOUDI stages. Downstream denuders
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captured any alkaline and acidic gases that exited the MOUDI apparatus, likely representing semi-volatile
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constituents. In the ambient gas phase HCOOH was the most abundant acidic gas, with an average mixing
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ratio ~2-3 times higher than that of SO2 and HNO3. It was found that the majority (49-96%) of filter-
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collected NH4+ volatilized during collection. NO3- volatilization could only explain 0.9-15% of NH4+ loss
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from the filters. Instead, a strong correlation and nearly 1:1 molar ratio between downstream HCOO- and
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NH4+ indicated that most of the semi-volatile NH4+ was originally balanced by organic acids in the ambient
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particle phase. The thermodynamic properties of HCOOH/HCOO- suggest that it should not have been
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present at high levels in the ambient particle phase, and we interpret its detection in the downstream
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denuder as evidence for larger organic acids that reacted to generate HCOOH prior to our offline
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measurement.
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Key words: ammonia, ammonium, phase partitioning, organic acids
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Environmental Science & Technology
1. Introduction
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Ammonia (NH3), the most abundant alkaline gas in the atmosphere, acts as one of the most important
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precursors to fine particulate matter1-3. By forming ammonium salts such as (NH4)2SO4 and NH4NO3, it
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can contribute significantly to regional haze formation4-6. One of the major challenges to accurately
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quantify the environmental impact of NH3 is to fully understand its gas/particle partitioning, which can
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alter its atmospheric lifetime and transport distance by orders of magnitude7, 8. By forming salts with
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sulfate, nitrate or organic acids, ammonium can also alter the ability of particles to act as cloud
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condensation nuclei9, 10, which has the potential to influence the Earth’s radiation budget and contributes
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significant uncertainty to Earth’s radiation forcing estimation11, 12. Thus, the phase partitioning behavior
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of ammonia is also very important for the study of the impact of anthropogenic activities on weather and
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climate.
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The formation of inorganic ammonium salts including (NH4)2SO4, (NH4)3H(SO4)2, NH4NO3 and
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NH4Cl have been well-documented8, 13-15. However, the interactions between organic compounds and NH3
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are not as well understood16, 17. The presence of NH3 can lower the vapor pressure of some organic acids
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by several orders of magnitude, enhancing their gas-to-particle conversion18. The contribution of organic
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acids to the enhancement of gas-to-particle conversion of NH3 has been tested by several laboratory
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studies but their exact role is still controversial in ambient atmosphere19, 20. Theoretically, organic acids
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can react with ammonia to form ammonium salts, and they have the potential to balance NH4+ in aqueous
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aerosol18, 20. In aerosol liquid water, the dissociation equilibrium of organic acids has the potential to alter
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aerosol acidity and impact ammonia’s phase partitioning behavior21. One of the major challenges to
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accurately measure the contribution from organic acids to the charge balance in aerosol is that they can
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decay into smaller carboxylic acids, or be oxidized into other compounds22, 23. Another important obstacle
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is that their reported or estimated thermodynamic parameters may be highly uncertain24, 25.
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A few field observations provide evidence for the existence of ammonium organic salts in ambient
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aerosol. Schlag, et al. 20 found that excess NH4+ in PM1 correlated well with (di-)carboxylic acids in the 3
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Netherlands atmosphere, and their lab experiments further proved that not only carboxylic acids, but even
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compounds with acidic -OH group (e.g. ascorbic acid) can promote the uptake of NH3 through acid-base
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reaction. A large amount of unexplained semi-volatile NH4+ was also found in the Great Smoky Mountains,
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which the authors proposed originating from ammonium organic salts26.
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In this study, we performed the measurement of ammonia/ammonium in both gas phase and particle
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phase through a custom-designed Denuder-MOUDI-Denuder (DMD) sampling system in the downtown
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Toronto area, an urban environment with annual PM2.5 mass loadings of less than 10 µg m-3
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(http://www.airqualityontario.com/press/publications.php), from August to October, 2017. The use of this
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DMD sampling system can account for the sampling losses of particle phase ammonium, nitrate and some
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organic acids due to dissociation or evaporation by collecting them downstream. Apart from common
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water-soluble inorganic components, several organic acids were also measured in both gas and particle
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phases to determine their phase partitioning behavior and contribution to balance ammonium in the
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particle phase.
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2. Method and instrument
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Sampling site. Ambient samples were collected over sampling periods of several hours between
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August and October, 2017. The sampling site was on the second floor of Lash Miller Chemistry Building
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located on the University of Toronto St. George campus (43o39’41” N, 79o23’56” W), which is close to
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the downtown area of Toronto, the largest city in Canada. The sampling inlet was about 6 meters above
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ground level. The air flow was drawn from the outside atmosphere with a high volume (1.4 m3 min-1)
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blower through a Teflon-coated 4-inch aluminum pipe. A branch flow of 30 L min-1 was taken from the
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main flow stream through a 1/2-inch wide, 33-cm long Teflon tube for aerosol and gaseous components
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sampling.
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Instrument description. Collection of gas phase and particle phase samples were performed using
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a custom-designed denuder-MOUDI-denuder (DMD) system as shown in Figure 1(a). Two annular
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denuders were attached in series to both the inlet and outlet of a size-segregated MOUDI (Micro-Orifice 4
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Uniform Deposit Impactor) cascade sampler (Model 110-NR, MSP Corp., USA) to efficiently capture
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alkaline and acidic gaseous components. Two upstream denuders were coated with H3PO3 (2% H3PO3,
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methanol: H2O=9:1) and Na2CO3 (1% Na2CO3, 1% glycerol, methanol: H2O=1:1) respectively for
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measurement of gas phase bases and acids. The collection efficiencies were tested by attaching two
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annular denuders in series to take ambient samples for 47-48 hours and were found to be above 99% for
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NH3 and about 95% for formic and acetic acids. Nearly complete removal for NH3 could be achieved and
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the breakthrough of upstream denuder contributed less than 5% of downstream concentrations for acidic
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components. The inlet flow then passed through the MOUDI sampler to collect total suspended particles
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(TSP) in different size ranges with the 50% cut-off sizes to be 0.056, 0.1, 0.18, 0.32, 0.56, 1.0, 1.8, 3.2,
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5.6, 10, 18 µm in aerodynamic diameters, respectively. The sample flow was controlled to be 30 l min-1
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to ensure the right cut-off sizes. No back-filters were applied to minimize the interference to the
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downstream denuders. Particles less than 56 nm were not collected and their deposition rate was too low
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to deposit in the downstream denuders27. Two downstream annular denuders were used to capture any
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semi-volatile material that volatilized following collection, a common sampling artifact in filter-based
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sampling methods28-30. The use of upstream denuders may enhance the sampling loss of semi-volatile
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components31, so in this study, we defined particle phase components to be the combination of those
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collected by MOUDI filters and those collected by the downstream denuders to have a more accurate
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measurement of ambient phase-partitioning. The residence time for ambient samples in the sampling
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apparatus was estimated
