10.1021/jp072005p.s001
Y. Liu
Y.
Liu
J. P. Cain
J. P.
Cain
H. Wang
H.
Wang
A. Laskin
A.
Laskin
Kinetic Study of Heterogeneous Reaction of Deliquesced NaCl Particles with Gaseous HNO<sub>3</sub>
Using Particle-on-Substrate Stagnation Flow Reactor Approach
American Chemical Society
2007
uptake coefficient decreases monotonically
uptake coefficient declines
HNO 3 uptake
particle size range
sea salt particles
sea salt types
HNO 3 concentration
sea salt
RH
Deliquesced NaCl Particles
electron microscopy grid substrate
0.82 μ m
scanning electron microscopy
particle size
particle loading
sea salt aerosols
Results show behavior
Gaseous HNO 3
CCSEM
reaction kinetics
reaction uptake coefficient
HNO 3
reactant diffusion
2007-10-11 00:00:00
Journal contribution
https://acs.figshare.com/articles/journal_contribution/Kinetic_Study_of_Heterogeneous_Reaction_of_Deliquesced_NaCl_Particles_with_Gaseous_HNO_sub_3_sub_Using_Particle_on_Substrate_Stagnation_Flow_Reactor_Approach/2981281
Heterogeneous reaction kinetics of gaseous nitric acid with deliquesced sodium chloride particles NaCl(aq)
+ HNO<sub>3</sub>(g) → NaNO<sub>3</sub>(aq) + HCl(g) were investigated with a novel particle-on-substrate stagnation flow
reactor (PS-SFR) approach under conditions, including particle size, relative humidity, and reaction time,
directly relevant to the atmospheric chemistry of sea salt particles. Particles deposited onto an electron
microscopy grid substrate were exposed to the reacting gas at atmospheric pressure and room temperature by
impingement via a stagnation flow inside the reactor. The reactor design and choice of flow parameters were
guided by computational fluid dynamics to ensure uniformity of the diffusion flux to all particles undergoing
reaction. The reaction kinetics was followed by observing chloride depletion in the particles by computer-controlled scanning electron microscopy with energy-dispersive X-ray analysis (CCSEM/EDX). The validity
of the current approach was examined first by conducting experiments with median dry particle diameter <i>D̄</i><sub>p</sub>
= 0.82 μm, 80% relative humidity, particle loading densities 4 × 10<sup>4</sup> ≤ <i>N</i><sub>s</sub> ≤ 7 × 10<sup>6</sup> cm<sup>-2</sup> and free stream
HNO<sub>3</sub> concentrations 2, 7, and 22 ppb. Upon deliquescence the droplet diameter <i>D̄</i><sub>d</sub> approximately doubles.
The apparent, pseudo-first-order rate constant determined in these experiments varied with particle loading
and HNO<sub>3</sub> concentration in a manner consistent with a diffusion-kinetic analysis reported earlier (Laskin, A.;
Wang, H.; Robertson, W. H.; Cowin, J. P.; Ezell, M. J.; Finlayson-Pitts, B. J. <i>J. Phys. Chem. A </i><b>2006</b>, <i>110</i>,
10619)<i>.</i> The intrinsic, second-order rate constant was obtained as <i>k</i><sub>II</sub> = 5.7 × 10<sup>-15</sup> cm<sup>3</sup> molecule<sup>-1</sup> s<sup>-1</sup> in the
limit of zero particle loading and by assuming that the substrate is inert to HNO<sub>3</sub>. Under this loading condition
the experimental, net reaction uptake coefficient was found to be γ<sub>net</sub> = 0.11 with an uncertainty factor of 3.
Additional experiments examined the variations of HNO<sub>3</sub> uptake on pure NaCl, a sea salt-like mixture of
NaCl and MgCl<sub>2</sub> (Mg-to-Cl molar ratio of 0.114) and real sea salt particles as a function of relative humidity.
Results show behavior of the uptake coefficient to be similar for all three types of salt particles with <i>D̄</i><sub>p</sub> ∼
0.9 μm over the relative humidity range 20−80%. Gaseous HNO<sub>3</sub> uptake coefficient peaks around a relative
humidity of 55%, with γ<sub>net</sub> well over 0.2 for sea salt. Below the efflorescence relative humidity the uptake
coefficient declines with decreasing RH for all three sea salt types, and it does so without exhibiting a sudden
shutoff of reactivity. The uptake of HNO<sub>3</sub> on sea salt particles was more rapid than that on the mixture of
NaCl and MgCl<sub>2</sub>, and uptake on both sea salt and sea salt-like mixture was faster than on pure NaCl. The
uptake of HNO<sub>3</sub> on deliquesced, pure NaCl particles was also examined over the particle size range of 0.57
≤ <i>D̄</i><sub>p</sub> ≤ 1.7 μm (1.1 ≤ <i>D̄</i><sub>d</sub> ≤ 3.4 μm) under a constant relative humidity of 80%. The uptake coefficient
decreases monotonically with an increase in particle size. Application of a resistance model of reaction kinetics
and reactant diffusion over a single particle suggests that, over the range of particle size studied, the uptake
is largely controlled by gaseous reactant diffusion from the free stream to the particle surface. In addition, a
combined consideration of uptake coefficients obtained in the present study and those previously reported for
substantially smaller droplets (<i>D̄</i><sub>d</sub> ∼ 0.1 μm) (Saul, T. D.; Tolocka, M. P.; Johnston, M. V. <i>J. Phys. Chem.
A</i> <b>2006</b>, <i>110</i>, 7614) suggests that the peak reactivity occurs at a droplet diameter of ∼0.7 μm, which is
immediately below the size at which sea salt aerosols begin to notably contribute to light scattering.