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Uptake of gaseous alkylamides by suspended sulfuric acid particles: Formation of ammonium/aminium salts Hangfei Chen, MingYi Wang, Lei Yao, Jianmin Chen, and Lin Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03175 • Publication Date (Web): 14 Sep 2017 Downloaded from http://pubs.acs.org on September 15, 2017
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Uptake of gaseous alkylamides by suspended sulfuric acid particles: Formation of ammonium/aminium salts
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Hangfei Chen1, Mingyi Wang1,a, Lei Yao1, Jianmin Chen1,2, Lin Wang1,2 *
1 2
5 6 7 8 9 10 11
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ABSTRACT Amides represent an important class of nitrogen-containing compounds
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in the atmosphere that can in theory interact with atmospheric acidic particles and
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contribute to secondary aerosol formation. In this study, uptake coefficients (γ) of six
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alkylamides (C1 to C3) by suspended sulfuric acid particles were measured using an
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aerosol flow tube coupled to a High Resolution Time-of-Flight Chemical Ionization
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Mass Spectrometer (HRToF-CIMS). At 293 K and < 3% relative humidity (RH), the
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measured uptake coefficients for six alkylamides were in the range of (4.8-23) × 10-2.
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A negative dependence upon RH was observed for both N-methylformamide and
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N,N-dimethylformamide, likely due to decreased mass accommodation coefficients (α)
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at lower acidities. A negative temperature dependence was observed for
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N,N-dimethylformamide under < 3% RH, also consistent with the mass
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accommodation-controlled uptake processes. Chemical analysis of reacted sulfuric
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acid particles indicates that alkylamides hydrolyzed in the presence of water
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molecules to form ammonium or aminium. Our results suggest that multiphase uptake
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of amides will contribute to growth of atmospheric acidic particles and alter their
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chemical composition.
Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3), Department of Environmental Science & Engineering, Fudan University, Shanghai 200433, China 2 Institute of Atmospheric Sciences, Fudan University, Shanghai 200433, China a now at: Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, PA 15213, USA
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1. Introduction
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Amides refer to a category of nitrogen-containing organic compounds with a general
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formula of R1C(O)NR2R3 (R1, R2, and R3 = a hydrogen atom or an alkyl group). They
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are directly emitted from various sources, including biomass burning1, cooking1,
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combustion2, animal husbandry3, sewage treatment4, and industrial manufacture
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processes of lubricants, inks, and many other products5. In addition to the primary
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emission sources, amides can be generated in situ in the atmosphere as well. For
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example, amides are products formed from the gas-phase reactions of amines with
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OH and NO3 radicals and ozone6-9, and from OH radical-initiated reactions of
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2-aminoethanol10, 11. Amides can also be formed via atmospheric accretion reactions
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of organic acids with amines or ammonia12. In addition, reactions of particulate
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amines with stabilized Criegee intermediates or secondary ozonides will lead to the
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formation of amides13.
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Gaseous amides have been detected close to their sources. Leach et al. measured
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over 4000 pptv (parts per trillion by volume) of dimethylformamide (DMF) near
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waste and sewage operations4. Large emissions (4.21 g kg-1) of acetamide (AA) were
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detected from peat fires burning14. Formamide (FA) was detected as a degradation
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product of 2-aminoethanol emitted from an industrial carbon capture facility15. In
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addition to measurements of amides near the emission sources, Yao et al. recently
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reported the detection of C1- to C6-amides in the urban atmosphere of Shanghai using
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a High Resolution Time-of-Flight Chemical Ionization Mass Spectrometer
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(HRToF-CIMS)16: up to hundreds of pptv of amides, and even peak concentrations of
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8700 pptv for C3-amides, were observed in urban Shanghai.
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The detection of high concentrations of amides in the ambient air urges a complete
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understanding on their atmospheric transformation. Previous studies reported that
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amides can undergo reactions with atmospheric oxidants including OH and NO3
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radicals, O3, and Cl atoms5, 17-20, leading to nitrogen-containing products that can
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contribute to the formation of secondary aerosols20-22. On the other hand, acetamide
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was found to only have a very weak enhancement capability on sulfuric acid
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nucleation23. Nevertheless, amides are expected to interact with acids since they are
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prone to accept protons. Indeed, previous studies suggest that amides would
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hydrolyze via the AOT2 (acid-catalyzed, oxygen-protonated, tetrahedral intermediate
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and bimolecular) mechanism in acidic media in the presence of water molecules,
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giving rise to the formation of ammonium or aminium24, 25. Therefore, amides likely
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interact with atmospheric acidic particles, especially newly-formed sulfuric acid
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particles with a high acidity, and alter their physicochemical properties.
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In this study, we investigated uptake of six alkylamides by suspended sulfuric acid
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particles in an aerosol flow tube coupled to a HRToF-CIMS using protonated ethanol
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reagent ions. The influences of temperature (293-355 K) and relative humidity (
196 kcal mol-1 for alkylamides31).
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Alkylamides were detected through the following ion-molecule reaction: (C2H5OH)n·H+ + R1C(O)NR2R3 →
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(C2H5OH)j·R1C(O)NR2R3·H+ + (n-j) C2H5OH
(R1)
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where
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(C2H5OH)j·R1C(O)NR2R3·H+ donates the protonated alkylamides and their clusters
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with ethanol. Concentrations of alkylamides were in the range of hundreds of pptv to
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a few ppbv, estimated using the calibration coefficients from a previous study16. Note
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that the measured signal intensities for protonated alkylamides were then corrected by
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subtracting the instrumental background, which was generally less than 7% of the
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initial signal intensities of the alkylamides in all of our uptake experiments.
n
=
1,
2,
3; j=0,
1;
R1C(O)NR2R3
donates
alkylamides; and
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The chemical composition of sulfuric acid particles after exposure to FA or DMF
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was analyzed by HRToF-CIMS that was interfaced to a Filter Inlet for Gases and
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AEROsols (FIGAERO). Briefly, FIGAERO-HRToF-CIMS provides unperturbed
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measurements of gaseous samples while collecting particles on a PTFE filter via an
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entirely separate sampling port32. Analysis of collected particles was conducted via
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evaporation from the filter using temperature-programmed thermal desorption by
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heated UHP N2. Note that, before filter collection, we specifically directed the aerosol
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sample flow through a cylindrical diffusion scrubber filled with activated charcoal to
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remove the redundant gaseous alkylamides. Particle loss in the scrubber was
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negligible. The typical collection time for the aerosol flow was 10 min.
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Uptake of a gas-phase species by a liquid surface can be treated by a diffusional
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and reactional equation33. The observed first-order rate coefficient ݇௦ is derived
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from the observed decay in the absence and presence of sulfuric acid aerosols, as
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given in eq 1:
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௨
ூ
݇௦ = ln ቀ ூబቁ
(eq 1)
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where ݑis the carrier gas flow velocity (cm s-1), ܮis the contact distance between
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the gaseous alkylamide and suspended sulfuric acid particles, ܫ is the initial signal
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intensity of the alkylamide and ܫ௧ is the signal intensity of the alkylamide after a
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reaction time of t (corresponding to a contact distance of )ܮ.The uptake coefficient γ
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is then determined using eq 2: ସ
γ = ௌ·ఠ
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ೌ
(eq 2)
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where ݇ = ݇௦ − ݇௪ is the first-order rate coefficient for loss of alkylamides onto
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the suspended aerosol particles, S is the total aerosol surface area-to-volume ratio of
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the aerosol flow obtained with the SMPS, ߱ௗ is the average molecular speed of
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the alkylamide in the gas phase, and ݇௪ is the first-order rate coefficient for
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alkylamides’ loss to the flow tube wall that is determined in the same manner as ݇௦
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but in the absence of particles. ݇௪ is generally larger at higher RH, and a ݇௪ of
