Kinetic Study of Heterogeneous Reaction of Deliquesced NaCl Particles with Gaseous HNO3 Using Particle-on-Substrate Stagnation Flow Reactor Approach

Heterogeneous reaction kinetics of gaseous nitric acid with deliquesced sodium chloride particles NaCl(aq) + HNO3(g) f NaNO3(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 computercontrolled 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 D hp ) 0.82 µm, 80% relative humidity, particle loading densities 4 × 104 e Ns e 7 × 106 cm-2 and free stream HNO3 concentrations 2, 7, and 22 ppb. Upon deliquescence the droplet diameter D h d approximately doubles. The apparent, pseudo-first-order rate constant determined in these experiments varied with particle loading and HNO3 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. J. Phys. Chem. A 2006, 110, 10619). The intrinsic, second-order rate constant was obtained as kII ) 5.7 × 10-15 cm3 molecule-1 s-1 in the limit of zero particle loading and by assuming that the substrate is inert to HNO3. Under this loading condition the experimental, net reaction uptake coefficient was found to be γnet ) 0.11 with an uncertainty factor of 3. Additional experiments examined the variations of HNO3 uptake on pure NaCl, a sea salt-like mixture of NaCl and MgCl2 (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 D hp ∼ 0.9 µm over the relative humidity range 20-80%. Gaseous HNO3 uptake coefficient peaks around a relative humidity of 55%, with γnet 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 HNO3 on sea salt particles was more rapid than that on the mixture of NaCl and MgCl2, and uptake on both sea salt and sea salt-like mixture was faster than on pure NaCl. The uptake of HNO3 on deliquesced, pure NaCl particles was also examined over the particle size range of 0.57 eD h p e 1.7 µm (1.1 e D h d e 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 (D h d ∼ 0.1 µm) (Saul, T. D.; Tolocka, M. P.; Johnston, M. V. J. Phys. Chem. A 2006, 110, 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.