Temperature dependence of the uptake coefficients of nitric acid, hydrochloric acid and nitrogen oxide (N2O5) by water droplets

Molecular Insights into Chemical Reactions at Aqueous Aerosol Interfaces

Atmospheric aerosols facilitate reactions between ambient gases and dissolved species. Here, we review our efforts to interrogate the uptake of these gases and the mechanisms of their reactions both theoretically and experimentally. We highlight the fascinating behavior of N2O5 in solutions ranging from pure water to complex mixtures, chosen because its aerosol-mediated reactions significantly impact global ozone, hydroxyl, and methane concentrations. As a hydrophobic, weakly soluble, and highly reactive species, N2O5 is a sensitive probe of the chemical and physical properties of aerosol interfaces. We employ contemporary theory to disentangle the fate of N2O5 as it approaches pure and salty water, starting with adsorption and ending with hydrolysis to HNO3, chlorination to ClNO2, or evaporation. Flow reactor and gas-liquid scattering experiments probe even greater complexity as added ions, organic molecules, and surfactants alter the interfacial composition and reaction rates. Together, we reveal a new perspective on multiphase chemistry in the atmosphere.

Influence of plasma-treated air on surface microbial communities on freshly harvested lettuce

Plant-based foods like lettuce are an important part of the human diet and worldwide industry. On a global scale, the number of food-associated illnesses increased in the last decades. Conventional lettuce sanitation methods include cleaning either with tap or chloritized water. Beside these water-consuming strategies, physical plasma is an innovative and effective possibility for food sanitation. Recent studies with plasma-treated water showed an effective reduction of the microbial load. Plasma-processed air (PPA) is another great opportunity to reduce the microbial load and save water. To test the efficiency of PPA, the surface microbiome of treated lettuce was analyzed via proliferation assays with special agars, live/dead assays and tests for respiratory activity of the microorganisms. PPA showed a reduction of the colony forming units (CFU/mL) on all tested microbial groups (Gram-negative and Gram-positive bacteria, yeasts and molds). These results were supported by the live/dead assay. For further insights, the PPA-ingredients were detected with Fourier Transformation Infrared Spectroscopy (FTIR), which revealed NO2, NO and N2O5 as the main reactive species in the PPA. In the future, PPA could be an outstanding, on-demand sanitation step for higher food safety standards, especially in situations where humidity and high temperature should be avoided.

Concurrent measurements of nitrate at urban and suburban sites identify local nitrate formation as a driver for urban episodic PM2.5 pollution

Nitrate (NO3-) is often among the leading components of urban particulate matter (PM) during PM pollution episodes. However, the factors controlling its prevalence remain inadequately understood. In this work, we analyzed concurrent hourly monitoring data of NO3- in PM2.5 at a pair of urban and suburban locations (28 km apart) in Hong Kong for a period of two months. The concentration gradient in PM2.5 NO3- was 3.0 ± 2.9 (urban) vs. 1.3 ± 0.9 μg m-3 (suburban) while that for its precursors nitrogen oxides (NOx) was 38.1 vs 4.1 ppb. NO3- accounted for 45 % of the difference in PM2.5 between the sites. Both sites were characterized to have more available NH3 than HNO3. Urban nitrate episodes, defined as periods of urban-suburban NO3- difference exceeding 2 μg m-3, constituted 21 % of the total measurement hours, with an hourly NO3- average gradient of 4.2 and a peak value of 23.6 μg m-3. Our comparative analysis, together with 3-D air quality model simulations, indicates that the high NOx levels largely explain the excessive NO3- concentrations in our urban site, with the gas phase HNO3 formation reaction contributing significantly during the daytime and the N2O5 hydrolysis pathway playing a prominent role during nighttime. This study presents a first quantitative analysis that unambiguously shows local formation of NO3- in urban environments as a driver for urban episodic PM2.5 pollution, suggesting effective benefits of lowering urban NOx.

SN2 Reactions of N2O5 with Ions in Water: Microscopic Mechanisms, Intermediates, and Products

Reactions of dinitrogen pentoxide (N₂O₅) greatly affect the concentrations of NO₃, ozone, OH radicals, methane, and more. In this work, we employ ab initio molecular dynamics and other tools of computational chemistry to explore reactions of N₂O₅ with anions hydrated by 12 water molecules to shed light on this important class of reactions. The ions investigated are Cl^-, SO₄^2-, ClO₄^-, and RCOO^- (R = H, CH₃, C₂H₅). The following main results are obtained: (i) all the reactions take place by an S_N2-type mechanism, with a transition state that involves a contact ion pair (NO₂⁺NO₃^-) that interacts strongly with water molecules. (ii) Reactions of a solvent-separated nitronium ion (NO₂⁺) are not observed in any of the cases. (iii) An explanation is provided for the suppression of ClNO₂ formation from N₂O₅ reacting with salty water when sulfate or acetate ions are present, as found in recent experiments. (iv) Formation of novel intermediate species, such as (SO₄NO₂^-) and RCOONO₂, in these reactions is predicted. The results suggest atomistic-level mechanisms for the reactions studied and may be useful for the development of improved modeling of reaction kinetics in aerosol particles.

Anions dramatically enhance proton transfer through aqueous interfaces

Proton transfer (PT) through and across aqueous interfaces is a fundamental process in chemistry and biology. Notwithstanding its importance, it is not generally realized that interfacial PT is quite different from conventional PT in bulk water. Here we show that, in contrast with the behavior of strong nitric acid in aqueous solution, gas-phase HNO(3) does not dissociate upon collision with the surface of water unless a few ions (> 1 per 10(6) H(2)O) are present. By applying online electrospray ionization mass spectrometry to monitor in situ the surface of aqueous jets exposed to HNO(3(g)) beams we found that NO(3)(-) production increases dramatically on > 30-μM inert electrolyte solutions. We also performed quantum mechanical calculations confirming that the sizable barrier hindering HNO(3) dissociation on the surface of small water clusters is drastically lowered in the presence of anions. Anions electrostatically assist in drawing the proton away from NO(3)(-) lingering outside the cluster, whose incorporation is hampered by the energetic cost of opening a cavity therein. Present results provide both direct experimental evidence and mechanistic insights on the counterintuitive slowness of PT at water-hydrophobe boundaries and its remarkable sensitivity to electrostatic effects.

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) --> 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 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 D(p) = 0.82 microm, 80% relative humidity, particle loading densities 4 x 10(4) <or= N(s) <or= 7 x 10(6) cm(-2) and free stream HNO3 concentrations 2, 7, and 22 ppb. Upon deliquescence the droplet diameter D(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 x 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 gamma(net) = 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(p) approximately 0.9 miccrom over the relative humidity range 20-80%. Gaseous HNO3 uptake coefficient peaks around a relative humidity of 55%, with gamma(net) 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 <or= D(p) <or= 1.7 microm (1.1 <or= D(d) <or= 3.4 microm) 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(d) approximately 0.1 microm) (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 approximately 0.7 microm, which is immediately below the size at which sea salt aerosols begin to notably contribute to light scattering.

On the Reactive Uptake of Gaseous Compounds by Organic-Coated Aqueous Aerosols: Theoretical Analysis and Application to the Heterogeneous Hydrolysis of N2O5

The presence of organic coatings on aerosols may have important consequences to the atmospheric chemistry, in particular to the N2O5 heterogeneous hydrolysis. This is demonstrated by recent experiments which show that the uptake of N2O5 by aqueous aerosols is slowed considerably when an organic coating consisting of monoterpene oxidation products is added on the particles. To treat the mechanisms behind the suppression, an extension of the resistor model, which has been widely applied in investigation of the heterogeneous uptake by aerosols, was derived. The extension accounts for dissolution, diffusion, and chemical reactions in a multilayered organic coating, and it provides a parametrization for the heterogeneous uptake by organic-coated aerosols that can be applied in large-scale models. Moreover, the framework was applied to interpret the findings regarding the decreased uptake of N2O5 by the organic-coated aerosols. Performed calculations suggested that the reaction rate constant of N2O5 in the coating is decreased by 3-5 orders of magnitude, in addition to which the product of the solubility of N2O5 and its diffusion coefficient in the coating is reduced more than an order of magnitude compared to the corresponding value for the aqueous phase. The results suggest also that the accommodation coefficient of N2O5 to such coatings is no more than a factor of 2 smaller than that to pure water surfaces. Finally, the relevance of the results to the atmospheric N2O5 heterogeneous hydrolysis is discussed and implications to planning further laboratory studies focusing on secondary organic aerosol formation are pointed out.

Reactive Uptake of Nitric Acid onto Sodium Chloride Aerosols Across a Wide Range of Relative Humidities

Reactive uptake coefficients for nitric acid onto size-selected (d(ve) = 102 and 233 nm) sodium chloride aerosols are determined for relative humidities (RH) between 85% and 10%. Both pure sodium chloride and sodium chloride mixed with magnesium chloride (X(Mg/Na) = 0.114, typical of sea salt) are studied. The aerosol is equilibrated with a carrier gas stream at the desired RH and then mixed with nitric acid vapor at a concentration of 60 ppb in a laminar flow tube reactor. At the end of the reactor, the particle composition is determined in real time with a laser ablation single particle mass spectrometer. For relative humidities above the efflorescence relative humidity (ERH), the particles exist as liquid droplets and the uptake coefficient ranges from 0.05 at 85% RH to >0.1 near the ERH. The droplet sizes, relative humidity and composition dependencies, are readily predicted by thermodynamics. For relative humidities below the ERH, the particles are nominally "solid" and uptake depends on the amount of surface adsorbed water (SAW). The addition of magnesium chloride to the particle phase (0.114 mole ratio of magnesium to sodium) facilitates uptake by increasing the amount of SAW. In the presence of magnesium chloride, the uptake coefficient remains high (>0.1) down to 10% RH, suggesting that the displacement of chloride by nitrate in fine sea salt particles is efficient over the entire range of conditions in the ambient marine environment. In the marine boundary layer, displacement of chloride by nitrate in fine sea salt particles should be nearly complete within a few hours (faster in polluted areas)-a time scale much shorter than the particle residence time in the atmosphere.