Reacto-Diffusive Length of N2O5 in Aqueous Sulfate- and Chloride

Feb 2, 2016 - A fundamental question about this mechanism is what the resultant length scale of N2O5 chemistry is in systems relevant to atmospheric a...
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Reacto-Diffusive Length of N2O5 in Aqueous Sulfate- and ChlorideContaining Aerosol Particles Cassandra J. Gaston† and Joel A. Thornton* Department of Atmospheric Sciences, University of Washington, Seattle, Washington 98195, United States ABSTRACT: Heterogeneous reactions of dinitrogen pentoxide (N2O5) on aerosol particles impact air quality and climate, yet aspects of the relevant physical chemistry remain unresolved. One important consideration is the competing effects of diffusion and the rate of chemical reaction within the particle, which determines the length that N2O5 travels within a particle before reacting, referred to as the reactodiffusive length (l). Large values of l imply a dependence of the reactive uptake efficiency of N2O5, i.e., γ(N2O5), on particle size. We present measurements of the size dependence of γ(N2O5) on aqueous sodium chloride, ammonium sulfate, and ammonium bisulfate particles. γ(N2O5) on ammonium sulfate and ammonium bisulfate particles ranged from 0.016 ± 0.005 to 0.036 ± 0.001 as the surface-areaweighted particle radius increased from 39 to 127 nm, resulting in an estimated l of 32 ± 6 nm. In contrast, γ(N2O5) on sodium chloride particles was independent of particle size, suggesting a near-surface reaction dominated the uptake of N2O5. Differences in the reactivity of the N2O5 intermediate, NO2+, with water and chloride can explain the observed dependencies. These results allow for parameterizations in atmospheric models to determine a more robust population mean value of γ(N2O5) that accounts for the distribution of particle sizes. α

1. INTRODUCTION

N2O5(g) ↔ N2O5(aq)

Heterogeneous reactions between atmospheric trace gases and aerosol particles significantly impact air quality and global climate through effects on budgets and lifetimes of greenhouse gases and on the size and composition of aerosols.1−5 An important example is the reaction of dinitrogen pentoxide (N2O5) on aqueous aerosol particles. N2O5 is formed at night from the reaction of nitrogen oxide radicals, NOx (NOx = NO + NO2), with ozone. NOx radicals, primarily emitted by anthropogenic activities, catalyze the production of ozone, a criteria air pollutant, a greenhouse gas, and the primary source of the hydroxyl radical, which is the atmosphere’s primary initiator of free radical oxidation.6,7 As such, understanding processes that lead to NOx removal are important for constraining air quality and coupled-chemistry-climate models. N2O5 undergoes reactive uptake onto particles with an efficiency denoted as γ(N2O5) and defined as the probability that gas−particle collisions result in irreversible reaction on or in the aerosol particle.2,8,9 On deliquesced inorganic salt solution particles, γ(N2O5) has been shown to range from 0.015 to 0.28−15 depending upon particle composition, which affects the reactions N2O5 undergoes in the condensed phase, and on temperature, which presumably affects accommodation and/or solubility.1,16−21 Key compositional factors affecting γ(N2O5) include particulate concentrations of water and chloride, which generally enhance the reactive uptake of N2O5, and nitrate, which inhibits uptake.10,16,17,19,22,23 The reactive uptake of N2O5 has been proposed to occur in the following steps: © 2016 American Chemical Society

(R1)

N2O5(aq) + H 2O(l) ↔ H 2ONO2+(aq) + NO3−(aq) (R2) k

II H2O

H 2ONO2+(aq) + H 2O(l) ⎯⎯⎯⎯⎯→ HNO3(aq) + H3O+(aq) (R3) k IICl

H 2ONO2+(aq) + Cl−(aq) ⎯⎯⎯→ ClNO2 (aq) + H 2O(l) (R4)

The first step represents mass accommodation and dissolution of N2O5 into a thin layer adjacent to the particle surface (reaction R1). Dissolved N2O5 then diffuses through the particle bulk depending on its condensed-phase diffusivity (denoted Daq).1,23 Dissolved N2O5 is proposed to react in a reversible process to form a hydrated intermediate, H2ONO2+(aq),22 and nitrate anion (reaction R2). Further reaction of the H2ONO2+(aq) intermediate with water results in the formation of nitric acid (HNO3) (reaction R3), which equilibrates between the gas and particle depending on the pH of the particle solution. HNO3 represents a terminal product for reactive nitrogen in the atmosphere. Therefore, the net hydrolysis of N2O5 (reactions R1−R3) represents a terminal sink of two NOx. Alternatively, the hydrated N2O5 intermediate can react with other components of the solution, such as halide Received: December 5, 2015 Revised: January 26, 2016 Published: February 2, 2016 1039

DOI: 10.1021/acs.jpca.5b11914 J. Phys. Chem. A 2016, 120, 1039−1045

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The Journal of Physical Chemistry A

Table 1. Composition and Weight Percent of Solution Used To Generate Aerosol, Total Particle Surface Area (SA) of Aerosol Generated, Surface-Area-Weighted Geometric Mean Radius of Aerosol (Rp), γ(N2O5), and Yield of ClNO2 (Y(ClNO2))a composition

wt %

SA (cm2/cm3)

ammonium bisulfate

0.004 0.02 0.03 0.04 0.05 0.1 0.3 0.0065 0.011 0.03 0.05 0.3 0.0075 0.01 0.03 0.085 0.15 0.3

7.19 1.57 1.93 2.99 1.39 1.08 1.48 6.57 8.47 1.13 1.01 1.20 7.90 1.06 1.17 1.27 1.63 1.57

ammonium sulfate

sodium chloride

a

× × × × × × × × × × × × × × × × × ×

Rp (nm)

10−05 10−04 10−04 10−04 10−04 10−04 10−04 10−05 10−05 10−04 10−04 10−04 10−05 10−04 10−04 10−04 10−04 10−04

39 57 70 72 79 95 127 44 50 64 73 125 39 48 61 69 81 93

γ(N2O5) 0.016 0.020 0.022 0.027 0.032 0.033 0.032 0.018 0.023 0.031 0.036 0.032 0.036 0.035 0.030 0.033 0.034 0.029

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.005 0.002 0.003 0.001 0.002 0.008 0.001 0.004 0.006 0.004 0.003 0.004 0.002 0.003 0.005 0.003 0.006 0.002

Y(ClNO2) N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A 0.79 ± 0.75 ± 0.88 ± 1.69 ± 1.18 ± 1.19 ±

0.29 0.26 0.26 0.67 0.29 0.29

Stated uncertainties are 95% confidence intervals.

ions, the most commonly studied being Cl− (reaction R4).10,24,25 Reaction of N2O5 with chloride produces one HNO3 and one nitryl chloride (ClNO2) per N2O5 reacted (reaction R4).10,24−28 ClNO2 readily volatilizes from particles,10,24,29 and undergoes photolysis regenerating one NOx and producing one Cl-atom. Cl-atoms are highly reactive toward hydrocarbons, directly impacting the lifetime of the greenhouse gas methane in the atmosphere.24,27,30,31 The rate constant for reaction of the H2ONO2+(aq) with Cl−(aq) has been estimated to be a factor of 500−800 larger than the corresponding reaction with H2O, and thus even trace concentrations of Cl−(aq) in particles influence the rate and product yield.10,16 A fundamental question about this mechanism is what the resultant length scale of N2O5 chemistry is in systems relevant to atmospheric aqueous aerosol particles. Do the reactions proceed essentially at the particle surface,9 limited only by surface accommodation, or do the reactions proceed as dissolved N2O5 diffuses throughout the particle bulk?16,22 Parameterizations based on either of these two extreme cases have largely been able to explain the reactivity of N2O5 across a number of aqueous solution particle systems.16,19,32,33 The reacto-diffusive length (l) is a measure of the competing effects of chemical reaction and diffusion of N2O5 upon collision with the particle surface. Physically, it is a measure of the average distance from the particle surface an accommodated N2O5 (or other reactant) will travel through the particle due to molecular diffusion before reacting. For short values of l, reaction is faster than diffusion, and N2O5 on average reacts near the particle surface; large values of l indicate that N2O5 will mix throughout the volume of the particle before reacting.22,23,34 Such a distinction has several implications. Reaction throughout the particle bulk implies that the γ(N2O5) scales with the volumeto-surface ratio of the particle population (i.e., Rp/3), decreasing with smaller particles and following a threshold behavior if the reaction on large particles is accommodation limited.34 Reaction probabilities are often determined on polydisperse particle populations, which might not overlap with atmospheric particle composition-size distributions, and

the latter vary through time and space.16,34−36 If, however, N2O5 reacts at the particle surface, γ(N2O5) would show no particle size dependence for the same composition, but knowledge of particle surface composition and surface area would be necessary for accurate parameterizations of γ(N2O5). Previous work has suggested a size dependence of γ(N2O5) on aqueous particles composed of organic acids;22 however, a robust assessment of the size dependence of γ(N2O5) has been lacking, particularly in the presence of different nucleophiles. Here we present measurements of γ(N2O5) as a function of particle size for particles of differing inorganic compositions. Specifically, by contrasting the size dependence of γ(N2O5) on aqueous particles with or without Cl−(aq), we in theory change the lifetime of the proposed H2ONO2+(aq) intermediate by a factor of 500 whereas the diffusivity of N2O5 in the aqueous solutions remains relatively constant. The size dependence is then an independent metric of the change in reactivity relative to dilution and can be used to extract absolute values of reactivity assuming a value for aqueous-phase diffusion coefficients. We end with a discussion of the implications for accurate parameterization of the reactive uptake of N2O5 on tropospheric aerosol particles.

2. EXPERIMENTAL SECTION Particle Generation. Measurements of the reactive uptake of N2O5 were made using an entrained aerosol flow tube coupled to an iodide-adduct chemical ionization mass spectrometer (CIMS).37 Aerosols were generated with a constant output atomizer (TSI Inc., Model 3076) from dilute solutions (0.004−0.3 wt %) of ammonium bisulfate (SigmaAldrich, 99.99% purity), ammonium sulfate (Sigma-Aldrich, ≥99% purity), and sodium chloride (Sigma-Aldrich, 99.99% purity). The atomizer output (flow ranging from 1400 to 2500 sccm) was diluted and conditioned to 50% RH by mixing 5 standard liters per minute (slpm) of humidified ultrahigh purity (UHP) N2 with the atomizer output. The humidity of the aerosol flow was measured with a Vaisala humidity probe (accuracy ±2%) just upstream of the flow reactor.12,16,22,38,39 Different weight percent solutions were used in the atomizer to 1040

DOI: 10.1021/acs.jpca.5b11914 J. Phys. Chem. A 2016, 120, 1039−1045

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introduced axially down the center of the flow reactor in a 0.1 slpm UHP N2 carrier flow through a Teflon-lined movable stainless steel injector.16 N2O5 mixing ratios of up to 8 ppbv were used for the work presented here. N2O5 was detected as a cluster with Iodide ions, IN2O5−, using Iodide adduct CIMS.37 Determination of γ(N2O5). N2O5 and the conditioned aerosol interacted within a pyrex, halocarbon wax coated flow tube with an inner diameter (ID) of 6 cm and a length of 90 cm. Measurements were limited to the central 60 cm to maintain well-mixed, laminar flow conditions. Moving the injector to the top and bottom of the flow tube altered the interaction time between N2O5 and the generated particles (see Figure 2 for a representative time trace of N2O5 uptake using

generate particle distributions having similar particle surface area concentrations but different mean geometric diameters. Table 1 shows the different surface-area-weighted geometric mean particle radius (which we define as Rp in this work) for each solution used; Rp tends to decrease as the weight percent of the solution decreases.22 Particle size distributions and total surface area concentrations (SA) at the flow tube exit were measured using a scanning mobility particle sizer (SMPS) consisting of a differential mobility analyzer (DMA) and condensation particle counter (Grimm Technologies). SA typically ranged from (0.66−2.9) × 10−4 cm2/cm3. To ensure that the measured SA was representative of the SA in the flow tube, the DMA sheath flow was conditioned to 50% RH by sampling from the flow reactor for ∼1 h prior to the start of the experiment. The SA at the flow tube entrance was within 10% of that measured at the flow tube exit. We assume all particles remain deliquesced, supersaturated solutions in the flow reactor as at no point during delivery and transit through the tube do the particles experience RH < 50%. The distributions of particle surface areas for each particle type and weight percent solution are shown in Figure 1. The surface-area-weighted geometric mean

Figure 2. Representative N2O5 time trace during an uptake experiment onto particles using the pseudoparticle modulation technique. Minima in N2O5 signal indicate periods when the injector was at the top of the flow tube, allowing maximum exposure time between N2O5 and aerosol in the flow tube. N2O5 was taken up onto NaCl particles in this case resulting in the production of ClNO2.

this method). This method has been shown to accurately measure the reactive uptake for N2O5 compared to a standard decay method while providing more robust statistics of γ(N2O5).16,40,42 The first-order rate loss (khet) was determined using a pseudoparticle modulation technique:12,16,43 ⎛ 1 ⎞ ⎛ [N2O5]top ⎞ ⎟ k het = −⎜ ⎟ ln⎜ ⎝ tres ⎠ ⎝ [N2O5]bottom ⎠

(1)

tres is the resulting interaction time between the gases and particles, which was 40 ± 4 s, set by the flow drawn through a critical orifice on the CIMS entrance upstream of a scroll pump and the SMPS sample flow. The total flow through the reactor was 2.1 slpm. Wall losses of N2O5 to the flow tube (kwall) were determined for each experiment in the same manner as khet except in the absence of particles. γ(N2O5) was then determined from the equation:

Figure 1. Normalized surface area distributions for ammonium bisulfate (ABS), ammonium sulfate (AS), and sodium chloride (NaCl) particles generated from solutions of different weight percents.

γ(N2O5) ≈

4(k het − k wall) ωSA

(2)

ω represents the mean molecular velocity of N2O5. We neglect gas-phase diffusion limitations to reactive uptake, which are small ( ∼90 nm in ABS and AS particles, which is not predicted by a single value of l used in eq 3 (Figure 3). It is unclear why this relatively sharp transition is observed when the theory developed by Hanson et al.34 predicts a much gentler approach to the maximum reaction probability for the l we derive from the observed size dependence. We speculate that the equilibrium described by (reaction R2) above leads to an ultimate limitation in the reactive flux of N2O5 not accounted for in the current resistor-model framework. More detailed microphysical models of the coupled reaction and diffusion in the near surface region are clearly warranted. We also note that, although l ∼ 2−5 nm on aqueous NaCl particles certainly implies that γmeas is reflecting a near surface phenomenon, the resistor-model framework we employ does not explicitly resolve the surface region. Surface saturation is possible for ABS and AS particles, in which the dominant reactant, H2O, reacts slower with NO2+ than chloride in NaCl particles. The concentrations of N2O5 used in our experiments are similar to those in polluted regions (approximately a few ppbv) whereas the available particle surface area per volume of air is orders of magnitude higher than in the atmosphere. Therefore, surface saturation effects are likely no more important in our experiments than in the atmosphere.

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AUTHOR INFORMATION

Corresponding Author

*J. A. Thornton. E-mail: [email protected]. Ph: 206543-4010. Present Address †

Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149, United States

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

■

REFERENCES

F. D. Lopez-Hilfiker is acknowledged for assistance with calibrating the CIMS for N2O5 and ClNO2. Funding for this work was provided by the National Science Foundation through award ECS-623046.

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4. CONCLUSIONS Results from this work suggest that γ(N2O5) will depend on particle size for dilute, nonacidic, non-chloride-containing aerosol particles, representative of a significant fraction of tropospheric aerosol particles. In contrast, particles containing significant concentrations of chloride had no dependence on particle size down to Rp ∼ 50 nm. The difference in size dependence is explained with a mechanism that incorporates differential reactivity of a hydrated NO2+ intermediate with water and Cl−. Indeed, the kinetics inferred from the size dependencies alone are in good agreement with those inferred from varying the particle concentrations of Cl−(aq) relative to water. The dependence of γ(N2O5) on size is likely most significant for aerosol particles in polluted regions, which due to the presence of fresh emissions can have a larger fraction of the surface area distribution contained in particles less than 150 nm in size. The results we present here allow for measurements of γ(N2O5) made on larger particles to be adjusted in a physically accurate way for incorporation into models of atmospheric chemistry. The low value of l observed for NaCl particles 1044

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