Heterogeneous Reactions on Salts

Spontaneous Molecular Bromine Production in Sea-Salt Aerosols

Bromine chemistry is responsible for the catalytic ozone destruction in the atmosphere. The heterogeneous reactions of sea-salt aerosols are the main abiotic sources of reactive bromine in the atmosphere. Here, we present a novel mechanism for the activation of bromide ions (Br-) by O2 and H2O in the absence of additional oxidants. The laboratory and theoretical calculation results demonstrated that under dark conditions, Br-, O2 and H3O+ could spontaneously generate Br and HO2 radicals through a proton-electron transfer process at the air-water interface and in the liquid phase. Our results also showed that light and acidity could significantly promote the activation of Br- and the production of Br2. The estimated gaseous Br2 production rate was up to 1.55×1010 molecules cm-2 ⋅ s-1 under light and acidic conditions; these results showed a significant contribution to the atmospheric reactive bromine budget. The reactive oxygen species (ROS) generated during Br- activation could promote the multiphase oxidation of SO2 to produce sulfuric acid, while the increase in acidity had a positive feedback effect on Br- activation. Our findings highlight the crucial role of the proton-electron transfer process in Br2 production; here, H3O+ facilitates the activation of Br- by O2, serves as a significant source of atmospheric reactive bromine and exerts a profound impact on the atmospheric oxidation capacity.

Mechanistic Insights into N2O5-Halide Ions Chemistry at the Air-Water Interface

The activation of halogens (X = Cl, Br, I) by N2O5 is linked to NOx sources, ozone concentrations, NO3 reactivity, and the chemistry of halide-containing aerosol particles. However, a detailed chemical mechanism is still lacking. Herein, we explored the chemistry of the N2O5···X- systems at the air-water interface. Two different reaction pathways were identified for the reaction of N2O5 with X- at the air-water interface: the formation of XNO2 or XONO, along with NO3-. In the case of the Cl- system, the ClNO2 generation pathway is more favorable, while for the Br- and I- systems, the formation of BrONO and IONO is barrierless, making them the predominant products. Furthermore, the mechanisms of formation of X2 from XNO2 and XONO were also investigated. The high energy barriers of reactions and the high free energies of the products compared to those of the reactants indicate that ClNO2 is stable at the air-water interface. Contrary to the widely held belief regarding X2 producing from the reaction of XNO2 with X-, our calculations demonstrate that BrONO and IONO initially form stable BrONO···Br- and IONO···I- complexes, which then subsequently react with Br- and I- to form Br3- and I3-, respectively. Finally, Br3- and I3- decompose to form Br2 and I2. These findings have significant implications for experimental interpretation and offer new insights into halogen cycling in the atmosphere.

Revealing the Contribution of Interfacial Processes to Atmospheric Oxidizing Capacity in Haze Chemistry

The atmospheric oxidizing capacity is the most important driving force for the chemical transformation of pollutants in the atmosphere. Traditionally, the atmospheric oxidizing capacity mainly depends on the concentration of O3 and other gaseous oxidants. However, the atmospheric oxidizing capacity based on gas-phase oxidation cannot accurately describe the explosive growth of secondary particulate matter under complex air pollution. From the chemical perspective, the atmospheric oxidizing capacity mainly comes from the activation of O2, which can be achieved in both gas-phase and interfacial processes. In the heterogeneous or multiphase formation pathways of secondary particulate matter, the enhancement of oxidizing capacity ascribed to the O2/H2O-involved interfacial oxidation and hydrolysis processes is an unrecognized source of atmospheric oxidizing capacity. Revealing the enhanced oxidizing capacity due to interfacial processes in high-concentration particulate matter environments and its contribution to the formation of secondary pollution are critical in understanding haze chemistry. The accurate evaluation of atmospheric oxidizing capacity ascribed to interfacial processes is also an important scientific basis for the implementation of PM2.5 and O3 collaborative control in China and around the world.

Magic cluster sizes of cationic and anionic sodium chloride clusters explained by statistical modeling of the complete phase space

As one of the main components of sea salt aerosols, sodium chloride is involved in numerous atmospheric processes. Gas-phase clusters are ideal models to study fundamental physical and chemical properties of sodium chloride, which are significantly affected by the cluster size. Of particular interest are magic cluster sizes, which exhibit high intensities in mass spectra. In order to understand the origin of these magic cluster sizes, quantum chemical calculations at the CCSD(T)//DFT level are performed, yielding structures and binding energies of neutral (NaCl)x, anionic (NaCl)xCl- and cationic (NaCl)xNa+ clusters up to x = 8. Our calculations show that the clusters can easily isomerize, enabling dissociation into the lowest-energy isomers of the fragments. Energetics can explain the special stability of (NaCl)4Cl-, but (NaCl)4Na+ actually offers low-lying dissociation channels, despite being a magic cluster size. Collision-induced dissociation experiments reveal that the loss of neutral clusters (NaCl)x, x = 2, 4, is in most cases more favorable than the loss of NaCl or the atomic ion, i.e. sodium chloride clusters actually fragment via the cleavage of the entire cluster, not by evaporating small cluster building blocks. This is rationalized by the calculated high stability of even-numbered neutral clusters (NaCl)x, especially x = 2, 4. Analysis of the density of states and rate constants calculated with a modified Rice-Ramsperger-Kassel-Marcus (RRKM) equation called AWATAR - considering all energetically accessible isomers of reactants and fragments - shows that entropic effects are responsible for the magic cluster character of (NaCl)4Na+. In particular, low-lying vibrational modes provide a high density of states of the near-planar cluster. Together with the small contribution of an atomic ion to the sum of states in a loose transition state for dissociation, this leads to a very small unimolecular rate constant for dissociation into (NaCl)4 and Na+, which is the lowest energy fragmentation pathway. Thus, entropic effects may override energetics for certain magic cluster sizes.

Comparison of the heterogeneous reaction of NO2 on the surface of clay minerals and desert dust particles

Mineral particles in air could provide atmospheric chemical reaction interface for gaseous substances and participate in atmospheric chemical reaction process, and affecting the status and levels of gaseous pollutants in air. However, differences of the heterogenous reaction on the surface minerals particles are not very clear. Considering main mineral composition of ambient particles was from dust emission, therefore, typical clay minerals (chlorite, illite) and desert particles (Taklimakan Desert) were selected to analysize chemical reaction of NO2, one of major gaseous pollutants, on mineral particles by using of In-situ DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) under different condition. And In situ near-ambient pressure X-ray photoelectron spectroscopy (In situ NAP-XPS) was employed to investigate iron (one of the major metals) species variation on the surface of mineral dust particles during the heterogeneous reactions. Our data show that humidity controlled by deuterium oxide (D2O) has a greater effect on chemical reactions compared to light and temperature. Under dry conditions, the amount of heterogeneous reaction products of NO2 on the particles shows Xiaotang dust > chlorite > illite > Tazhong dust regardless of dark or light conditions. In contrast, under humidity conditions, the order of nitrate product quantity under moderate conditions was chlorite > illite > Xiaotang dust > Tazhong dust. In situ NAP-XPS results demonstrate that specie variation of the Fe could promote the heterogenous reactions. These data could provide useful information for understanding the formation mechanism of nitrate aerosols and removal of nitrogen oxides in the atmosphere.

Effects of SO2 on NH4NO3 Photolysis: The Role of Reducibility and Acidic Products

Nitrate photolysis is a vital process in secondary NOx release into the atmosphere. The heterogeneous oxidation of SO₂ due to nitrate photolysis has been widely reported, while the influence of SO₂ on nitrate photolysis has rarely been investigated. In this study, the photolysis of nitrate on different substrates was investigated in the absence and presence of SO₂. In the photolysis of NH₄NO₃ on the membrane without mineral oxides, NO, NO₂, HONO, and NH₃ decreased by 17.1, 6.0, 12.6, and 57.1% due to the presence of SO₂, respectively. In the photolysis of NH₄NO₃ on the surface of mineral oxides, SO₂ also exhibited an inhibitory effect on the production of NOx, HONO, and NH₃ due to its reducibility and acidic products, while the increase in surface acidity due to the accumulation of abundant sulfate on TiO₂ and MgO promoted the release of HONO. On the photoactive oxide TiO₂, HSO₃^-, generated by the uptake of SO₂, could compete for holes with nitrate to block nitrate photolysis. This study highlights the interaction between the heterogeneous oxidation of SO₂ and nitrate photolysis and provides a new perspective on how SO₂ affects the photolysis of nitrate absorbed on the photoactive oxides.

Emerging investigator series: ozone uptake by urban road dust and first evidence for chlorine activation during ozone uptake by agro-based anti-icer: implications for wintertime air quality in high-latitude urban environments

High-latitude urban regions provide a unique and complex range of environmental surfaces for uptake of trace pollutant gases, including winter road maintenance materials (e.g., gravel, rock salts, and anti-icer, a saline solution applied to roads during winter). In an effort to reduce the negative environmental and economic impacts of road salts, many municipalities have turned to agro-based anti-icing materials that are rich in organic material. To date, the reactivity of both anti-icer and saline road dust with pollutant gases remain unexplored, which limits our ability to assess the potential impacts of these materials on air quality in high-latitude regions. Here, we used a coated-wall flow tube to investigate the uptake of ozone, an important air pollutant, by road dust collected in Edmonton, Canada. At 25% relative humidity (RH) and 50 ppb ozone, γ_BET for ozone uptake by this sample is (8.0 ± 0.7) × 10^-8 under dark conditions and (2.1 ± 0.1) × 10^-7 under illuminated conditions. These values are 2-4× higher than those previously obtained by our group for natural mineral dusts, but are not large enough for suspended road dust to influence local ozone mixing ratios. In a separate set of experiments, we also investigated the uptake of ozone by calcium chloride (i.e., road salt) and commercial anti-icer solution. Although ozone uptake by pure calcium chloride was negligible, ozone uptake by anti-icer was significant, which implies that the reactivity of anti-icer is conferred by its organic content. Importantly, ozone uptake by anti-icer-and, to a lesser extent, road dust doped with anti-icer-leads to the release of inorganic chlorine gas, which we collected using inline reductive trapping and quantified using ion chromatography. To explain these results, we propose a novel pathway for chlorine activation: here, ozone oxidation of the anti-icer organic fraction (in this case, molasses) yields reactive OH radicals that can oxidize chloride. In summary, this study demonstrates the ability of road dust and anti-icer to influence atmospheric oxidant mixing ratios in cold-climate urban areas, and highlights previously unidentified air quality impacts of winter road maintenance decisions.

Seasonal source analysis of nitrogen and carbon aerosols of PM2.5 in typical cities of Zhejiang, China

Fine particulate matter (PM_2.5) significantly impacts global air quality and human health due to its smaller particle size and larger specific surface area. Nitrogen and carbon aerosols, as the main components of PM_2.5, play key roles in air pollution. This study identified the sources and seasonal variation of nitrogen and carbon aerosols in PM_2.5 in typical cities of Zhejiang. The annual average PM_2.5 concentrations of Hangzhou (HZ), Ningbo (NB), and Huzhou (HUZ) were 39.8 ± 19.1 μg m^-‍3, 40.0 ± 21.5 μg m^-3, and 50.1 ± 22.6 μg m^-3, respectively, which exceeded the Chinese air quality limit of 35.0 μg m^-3. The results showed that the concentrations of nitrogen aerosols (NO₃^- and NH₄⁺) in water-soluble inorganic ions were higher at 9.6 ± 4.6 μg m^-3, 9.0 ± 4.5 μg m^-3 and 11.5 ± 5.4 μg m^-3 in HZ, NB and HUZ, respectively, especially in winter, accounting for over 60% of the total. The annual average δ¹⁵N values of PM_2.5 were 6.2 ± 1.9‰, 6.4 ± 2.2‰ and 6.7 ± 1.9‰ in HZ, NB and HZ, respectively; the δ¹⁵N values in winter were relatively low. A Bayesian isotopic mixing model was employed to analyse the sources of nitrogen aerosols in winter; the results showed that nitrogen concentration was mainly affected by NH₃ and NO_X emitted by motor vehicle exhaust, coal combustion, biomass combustion, biogenic soil emissions, animal wastes and ocean evaporation (NB). In addition, the carbon component analysis of PM_2.5 showed that the annual average mass concentration of TC accounted for 18.7%, 16.4% and 20.1% of PM_2.5 in HZ, HUZ and NB, respectively. The same isotope model was used to analyse the sources of carbon aerosols; the results showed that carbon aerosols were mainly affected by the sources of motor vehicle exhaust, coal combustion, biomass combustion and dust. In the PM_2.5 in Zhejiang, the most contributory sources of nitrogenous aerosols and carbon aerosols were motor vehicle exhaust sources.

Physical and chemical properties of sea salt deliquescent brines as a function of temperature and relative humidity

Thermodynamic modeling has been used to predict chemical compositions of brines formed by the deliquescence of sea salt aerosols. Representative brines have been mixed, and physical and chemical properties have been measured over a range of temperatures. Brine properties are discussed in terms of atmospheric corrosion of austenitic stainless steel, using spent nuclear fuel dry storage canisters as an example. After initial loading with spent fuel, during dry storage, the canisters cool over time, leading to increased surface relative humidities and evolving brine chemistries and properties. These parameters affect corrosion kinetics and damage distributions, and may offer important constraints on the expected timing, rate, and long-term impacts of canister corrosion.

Comprehensive Study about the Photolysis of Nitrates on Mineral Oxides

Nitrates formed on mineral dust through heterogeneous reactions in high NO_x areas can undergo photolysis to regenerate NO_x and potentially interfere in the photochemistry in the downwind low NO_x areas. However, little is known about such renoxification processes. In this study, photolysis of various nitrates on different mineral oxides was comprehensively investigated in a flow reactor and in situ diffuse reflectance Fourier-transform infrared spectroscopy (in situ DRIFTS). TiO₂ was found much more reactive than Al₂O₃ and SiO₂ with both NO₂ and HONO as the two major photolysis products. The yields of NO₂ and HONO depend on the cation basicity of the nitrate salts or the acidity of particles. As such, NH₄NO₃ is much more productive than other nitrates like Fe(NO₃)₃, Ca(NO₃)₂, and KNO₃. SO₂ and water vapor promote the photodegradation by increasing the surface acidity due to the photoinduced formation of H₂SO₄/sulfate and H⁺, respectively. O₂ enables the photo-oxidation of NO_x to regenerate nitrate and thus inhibits the NO_x yield. Overall, our results demonstrated that the photolysis of nitrate can be accelerated under complex air pollution conditions, which are helpful for understanding the transformation of nitrate and the nitrogen cycle in the atmosphere.

Direct field evidence of autocatalytic iodine release from atmospheric aerosol

Recycling of reactive iodine from heterogeneous processes on sea-salt aerosol was hypothesized over two decades ago to play an important role in the atmospheric cleansing capacity. However, the understanding of this mechanism has been limited to laboratory studies and has not been confirmed in the atmosphere until now. We present atmospheric measurement of gas-phase iodine interhalogen species and show that their production via heterogeneous processing on marine aerosols is remarkably fast. These observations reveal that the atmospheric recycling of atomic iodine through photolysis of iodine interhalogen species is more efficient than previously thought, which is ultimately expected to lead to higher ozone loss and faster new particle formation in the marine environment.

Reactive iodine plays a key role in determining the oxidation capacity, or cleansing capacity, of the atmosphere in addition to being implicated in the formation of new particles in the marine boundary layer. The postulation that heterogeneous cycling of reactive iodine on aerosols may significantly influence the lifetime of ozone in the troposphere not only remains poorly understood but also heretofore has never been observed or quantified in the field. Here, we report direct ambient observations of hypoiodous acid (HOI) and heterogeneous recycling of interhalogen product species (i.e., iodine monochloride [ICl] and iodine monobromide [IBr]) in a midlatitude coastal environment. Significant levels of ICl and IBr with mean daily maxima of 4.3 and 3.0 parts per trillion by volume (1-min average), respectively, have been observed throughout the campaign. We show that the heterogeneous reaction of HOI on marine aerosol and subsequent production of iodine interhalogens are much faster than previously thought. These results indicate that the fast formation of iodine interhalogens, together with their rapid photolysis, results in more efficient recycling of atomic iodine than currently considered in models. Photolysis of the observed ICl and IBr leads to a 32% increase in the daytime average of atomic iodine production rate, thereby enhancing the average daytime iodine-catalyzed ozone loss rate by 10 to 20%. Our findings provide direct field evidence that the autocatalytic mechanism of iodine release from marine aerosol is important in the atmosphere and can have significant impacts on atmospheric oxidation capacity.

Water Adsorption vs Phase Transition of Aerosols Monitored by a Quartz Crystal Microbalance

A quartz crystal microbalance (QCM) with a high sensitivity of 0.1 ng was applied to monitor the oscillation frequency variation (Δf) of standard single species, two-component systems with typical ambient aerosol compositions, and ambient aerosol filter samples as a function of relative humidity (RH) and determine their deliquescence RH (DRH) and phase transition. Δf is associated with the adsorption or desorption process of water molecules for solid samples and physical properties of the sample film during solid-to-aqueous phase transition (deliquescence). During the pre-deliquescence stage, the water adsorption process led to the increased mass with decreasing Δf, especially for the hydrates such as MgCl₂ and Ca(NO₃)₂, which have more than 20% and 40% increased mass, respectively. The water adsorption process might cause a mass deviation of ambient particulate matter measurement using similar instrument principles. During the deliquescence stage, the observed rapid increasing Δf with RH was caused by a significant change in the physical properties (such as density and viscosity) of the sample film. The determined DRH for a given single-component system is consistent with the results estimated from the thermodynamic models. For a complex system, the QCM can determine the DRH^1st well as the eutonic point and track the possible variation of the physical properties of inorganic or with organic acid mixture systems. During the post-deliquescence stage, the gradual increasing trend of Δf with RH for Ca(NO₃)₂ and an external mixture of NaCl-Ca(NO₃)₂ was mainly contributed by a stronger RH dependent of physical properties for Ca(NO₃)₂(aq). Overall, this study provides the possible physical properties variation of common aerosol composition as a function of RH, which was consistent with the results calculated from the thermodynamic models. The stronger water adsorption for MgCl₂ and Ca(NO₃)₂ with solid-like viscosity at RH < DRH might lead to different chemical reactivities in the atmospheric chemistry in addition to the radiative forcing of aerosols caused by the hysteresis.

The role of anthropogenic chlorine emission in surface ozone formation during different seasons over eastern China

Anthropogenic chlorine emission is an important source of Cl radicals, which plays an important role in the oxidative chemistry of the troposphere. However, its seasonal impacts on surface ozone levels in China have yet been comprehensively explored. In this study, we conducted numerical simulations for January, April, July and October 2015 by using the Community Multiscale Air Quality (CMAQ) modeling system with updated heterogeneous reactions of nitrogen oxides with particulate chlorine and updated Anthropogenic Chlorine Emission Inventory for China (ACEIC). Two experiments with and without ACEIC in the model were established, and their results were compared with each other. The model can faithfully reproduce the magnitudes and variations of meteorological parameters and air pollutant concentrations. Cl radicals were generated by the photolysis of ClNO₂, ClNO and Cl₂, HCl oxidation by OH radicals, and the heterogeneous reactions of NO₃ with particulate Cl^-. ClNO₂ and ClNO were mainly produced from the heterogeneous reactions of N₂O₅ and NO₂ with particulate Cl^-, respectively. The spatial and seasonal variations ofz these chlorinated species and their responses to the implementation of ACEIC were revealed in this study. Our results suggested that besides N₂O₅, the heterogeneous reactions of NO₂ and NO₃ with particulate Cl^- could be important sources of Cl radicals. Anthropogenic chlorine emission increased the Cl radical concentration through enhancing the photolysis of ClNO, Cl₂, and ClNO₂. The implementation of ACEIC in the model increased the degradation of Volatile Organic Compounds (VOCs) not only by Cl radicals but also by OH radicals. Although the seasonal variation of AECIE was insignificant, the larger formation of Cl radicals caused by higher levels of NO_x in January was counteracted by the larger loss of them due to more VOCs degradations, resulting in a lower increase in Cl radicals due to the implementation of ACEIC compared with other months. The anthropogenic chlorine emissions increased the monthly mean maximum daily 8-hour average (MDA8) O₃ mixing ratio by up to 4.9 ppbv, and increased the 1-hour O₃ mixing ratio by up to 34.3 ppbv. The impact of ACEIC was the most significant in January and the least in July due to the high emissions of NO_x and VOCs and adverse meteorological conditions in winter. It indicated that although the ozone concentration was low, the anthropogenic chlorine emission significantly contributed to the atmospheric oxidation capacity and increase ozone concentrations in winter.

Identification of most preferable reaction pathways for chloride depletion from size segregated sea-salt aerosols: A study over high altitude Himalaya, tropical urban metropolis and tropical coastal mangrove forest in eastern India

Depletion of chloride from sea-salt aerosols affects their hygroscopicity, cloud condensation nuclei activity as well as microphysical and chemical properties of aerosols and clouds modifying earth-atmosphere radiative balance. Here, we proposed five possible reaction pathways through which the inorganic acids (H₂SO₄ and HNO₃) could deplete chloride from sea-salt aerosols. We determined "maximum potential contribution" (MPC) of each acid and compared the MPC with actual chloride depletion. This step-by-step approach enables us to identify the most preferable reaction pathway(s) for coarse, superfine, accumulation and ultrafine aerosols over a Himalayan station (Darjeeling), a tropical urban station (Kolkata) and a tropical mangrove forest at the north-east coast of Bay of Bengal (Sundarban) in India. Over Kolkata and Darjeeling, locally generated acids reacted with transported sea-salts. Over Sundarban, the locally generated sea-salts from the Bay of Bengal reacted with the acids of biomass burning plume transported from Eastern Ghat and continental haze transported from upper Indo-Gangetic Plain. The average chloride depletion in PM10 ranged between 70 and 74% over Sundarban and 31-34% over Kolkata and Darjeeling. We observed that HNO₃(g) depleted the larger (>1 μm) chlorides whereas H₂SO₄(g) depleted the smaller (<1 μm) chlorides over Kolkata and Darjeeling. However, in addition to H₂SO₄(g) and HNO₃(g), some other species could be involved in chloride depletion over Sundarban mainly during winter. The study reveals that Sundarban acts as the major sink of the inorganic acids transported from Eastern Ghat biomass burning plume inhibiting their further advection towards inland regions.

Nitrite-Mediated Photooxidation of Vanillin in the Atmospheric Aqueous Phase

Nitrite (NO₂^-) and its conjugate acid, nitrous acid (HNO₂), have long been recognized as a ubiquitous atmospheric pollutant as well as an important photochemical source of hydroxyl radicals (·OH) and reactive nitrogen species (·NO, ·NO₂, ·N₂O₃, etc.) in both the gas phase and aqueous phase. Although NO₂^-/HNO₂ plays an important role in atmospheric chemistry, our understanding on its role in the chemical evolution of organic components in atmospheric waters is rather incomplete and is still in dispute. In this study, the nitrite-mediated photooxidation of vanillin (VL), a phenolic compound abundant in biomass burning emissions, was investigated under pH conditions relevant for atmospheric waters. The influence of solution pH, dissolved oxygen, and ·OH scavengers on the nitrite-mediated photooxidation of VL was discussed in detail. Our study reveals that the molecular composition of the products is dependent on the molar ratio of NO₂^-/VL in the solution and that nitrophenols are the major reaction products. We also found that the light absorbance of the oxidative products increases with increasing pH in the visible region, which can be attributed to the deprotonation of the nitrophenols formed. These results contribute to a better understanding of methoxyphenol photooxidation mediated by nitrite as a source of toxic nitrophenols and climatically important brown carbon in atmospheric waters.

Significant impact of heterogeneous reactions of reactive chlorine species on summertime atmospheric ozone and free-radical formation in north China

Heterogeneous reactions of N₂O₅, O₃, OH, ClONO₂, HOCl, ClNO₂, and NO₂, with chlorine-containing particles are incorporated in the Community Multiscale Air Quality (CMAQ) model to evaluate the impact of heterogeneous reactions of reactive chlorine species on ozone and free radicals. Changes of summertime ozone and free radical concentrations due to the additional heterogeneous reactions in north China were quantified. These heterogeneous reactions increased the O₃, OH, HO₂ and RO₂ concentrations by up to 20%, 28%, 36% and 48% for some regions in the Beijing-Tianjin-Hebei (BTH) area. These areas typically have a larger amount of NOx emissions and a lower VOC/NOx ratio. The zero-out method evaluates that the photolysis of ClNO₂ and Cl₂ are the major contributors (42.4% and 57.6%, respectively) to atmospheric Cl in the early morning hours but the photolysis of Cl₂ is the only significant contributor after 10:00 am. The results highlight that heterogeneous reactions of reactive chlorine species are important to atmospheric ozone and free-radical formation. Our study also suggests that the on-going NOx emission controls in the NCP region with a goal to reduce both O₃ and secondary nitrate can also have the co-benefit of reducing the formation Cl from ClNO₂ and Cl₂, which may also lead to lower secondary organic aerosol formation and thus the control of summertime PM_2.5 in the region.

Spatial differences in ambient coarse and fine particles in the Monterrey metropolitan area, Mexico: Implications for source contribution

The ambient air of the Monterrey Metropolitan Area (MMA) in Mexico frequently exhibits high levels of PM₁₀ and PM_2.5. However, no information exists on the chemical composition of coarse particles (PM_c = PM₁₀ - PM_2.5). A monitoring campaign was conducted during the summer of 2015, during which 24-hr average PM₁₀ and PM_2.5 samples were collected using high-volume filter-based instruments to chemically characterize the fine and coarse fractions of the PM. The collected samples were analyzed for anions (Cl^-, NO₃^-, SO₄^2-), cations (Na⁺, NH₄⁺, K⁺), organic carbon (OC), elemental carbon (EC), and 35 trace elements (Al to Pb). During the campaign, the average PM_2.5 concentrations did not showed significance differences among sampling sites, whereas the average PM_c concentrations did. In addition, the PM_c accounted for 75% to 90% of the PM₁₀ across the MMA. The average contribution of the main chemical species to the total mass indicated that geological material including Ca, Fe, Si, and Al (45%) and sulfates (11%) were the principal components of PM_c, whereas sulfates (54%) and organic matter (30%) were the principal components of PM_2.5. The OC-to-EC ratio for PM_c ranged from 4.4 to 13, whereas that for PM_2.5 ranged from 3.97 to 6.08. The estimated contribution of Secondary Organic Aerosol (SOA) to the total mass of organic aerosol in PM_2.5 was estimated to be around 70-80%; for PM_c, the contribution was lower (20-50%). The enrichment factors (EF) for most of the trace elements exhibited high values for PM_2.5 (EF: 10-1000) and low values for PM_c (EF: 1-10). Given the high contribution of crustal elements and the high values of EFs, PM_c is heavily influenced by soil resuspension and PM_2.5 by anthropogenic sources. Finally, the airborne particles found in the eastern region of the MMA were chemically distinguishable from those in its western region. Implications: Concentration and chemical composition patterns of fine and coarse particles can vary significantly across the MMA. Public policy solutions have to be built based on these observations. There is clear evidence that the spatial variations in the MMA's coarse fractions are influenced by clearly recognizable primary emission sources, while fine particles exhibit a homogeneous concentration field and a clear spatial pattern of increasing secondary contributions. Important reductions in the coarse fraction can come from primary particles' emission controls; for fine particles, control of gaseous precursors-particularly sulfur-containing species and organic compounds-should be considered.

In Situ Chemical Analysis of the Gas-Aerosol Particle Interface

The gas-aerosol particle interface is believed to contribute to the growth of secondary organic aerosols in the atmosphere. Despite its importance, the chemical composition of the interface has not been probed directly because of a lack of suitable interface-specific analytical techniques. The preliminary result in our early work has demonstrated direct observations of molecules at the gas-aerosol particle interface with the development of second harmonic scattering (SHS). However, the SHS technique is far away from being an analytical tool of chemical compositions at the gas-aerosol particle interface. In this work, we continued to develop the interface-specific SHS for in situ chemical analysis of molecules at the gas-aerosol particle interface. As an example, we demonstrated coherent SHS signal of a new SHS probe, crystal violet (CV), from interfaces of aerosol particles. The development of the SHS technique includes: (1) Optimization for a more efficient femtosecond laser system in the generation of SHS from aerosol particles. A near 5 MHz repetition rate of a femtosecond laser was found to be optimal for the generation of SHS; (2) exploration of a more effective detector for SHS of aerosol particles. We found that both a CCD detector and a single-photon counter produce similar signal-to-noise ratios of the interfacial SHS signals from aerosol particles. The CCD detector is a more effective option for the detection of SHS and could greatly reduce sampling time of the interfacial responses; (3) combination of the optimal laser system with the CCD detector, which has greatly improved the detection sensitivity of interfacial molecules by more than 2 orders of magnitude and could potentially detect interfacial SHS from a single aerosol particle. These experimental results not only provided a thorough analysis of the SHS technique but also built a solid foundation for further development of a new vibrational sum frequency scattering (SFS) technique for chemical structures at the gas-aerosol particle interface.

Heterogeneous Reaction of HCOOH on NaCl Particles at Different Relative Humidities

The contribution of volatile organic acids to chloride depletion still remains unclear under ambient conditions in the coast and inland. In this work, the heterogeneous reaction of HCOOH on the NaCl surface at a series of relative humidities (RHs) was investigated using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The formate was found to be formed on NaCl surface under dry and wet conditions, accompanied by the corresponding chloride depletion. The adsorbed HCOOH and the formation of formate on NaCl surface decreased with increasing RH below 30% RH. The adsorbed HCOOH decreased, while the formation of formate increased with enhanced RH at 45-70% RH. The variation in the formation of formate with RH suggests that chloride depletion may undergo similar changes. Additionally, the mechanism and kinetics for uptake of HCOOH on NaCl surface at various RHs were discussed and analyzed. Our results highlight the role of heterogeneous chemistry of volatile organic acid in the chloride depletion of NaCl in the coast and inland.

Sea spray aerosol chemical composition: elemental and molecular mimics for laboratory studies of heterogeneous and multiphase reactions

Schematic representation of the reactive uptake of N₂O₅to a sea spray aerosol particle containing a thick organic film.

Airborne Observations of Reactive Inorganic Chlorine and Bromine Species in the Exhaust of Coal-Fired Power Plants

We present airborne observations of gaseous reactive halogen species (HCl, Cl₂, ClNO₂, Br₂,BrNO₂, and BrCl), sulfur dioxide (SO₂), and nonrefractory fine particulate chloride (pCl) and sulfate(pSO₄) in power plant exhaust. Measurements were conducted during the Wintertime INvestigation of Transport, Emissions, and Reactivity campaign in February-March of 2015 aboard the NCAR-NSF C-130 aircraft. Fifty air mass encounters were identified in which SO₂ levels were elevated ~5 ppb above ambient background levels and in proximity to operational power plants. Each encounter was attributed to one or more potential emission sources using a simple wind trajectory analysis. In case studies, we compare measured emission ratios to those reported in the 2011 National Emissions Inventory and present evidence of the conversion of HCl emitted from power plants to ClNO₂. Taking into account possible chemical conversion downwind, there was general agreement between the observed and reported HCl: SO₂ emission ratios. Reactive bromine species (Br₂, BrNO₂, and/or BrCl) were detected in the exhaust of some coal-fired power plants, likely related to the absence of wet flue gas desulfurization emission control technology. Levels of bromine species enhanced in some encounters exceeded those expected assuming all of the native bromide in coal was released to the atmosphere, though there was no reported use of bromide salts (as a way to reduce mercury emissions) during Wintertime INvestigation of Transport, Emissions, and Reactivity observations. These measurements represent the first ever in-flight observations of reactive gaseous chlorine and bromine containing compounds present in coal-fired power plant exhaust.

Participation of the Halogens in Photochemical Reactions in Natural and Treated Waters

Halide ions are ubiquitous in natural waters and wastewaters. Halogens play an important and complex role in environmental photochemical processes and in reactions taking place during photochemical water treatment. While inert to solar wavelengths, halides can be converted into radical and non-radical reactive halogen species (RHS) by sensitized photolysis and by reactions with secondary reactive oxygen species (ROS) produced through sunlight-initiated reactions in water and atmospheric aerosols, such as hydroxyl radical, ozone, and nitrate radical. In photochemical advanced oxidation processes for water treatment, RHS can be generated by UV photolysis and by reactions of halides with hydroxyl radicals, sulfate radicals, ozone, and other ROS. RHS are reactive toward organic compounds, and some reactions lead to incorporation of halogen into byproducts. Recent studies indicate that halides, or the RHS derived from them, affect the concentrations of photogenerated reactive oxygen species (ROS) and other reactive species; influence the photobleaching of dissolved natural organic matter (DOM); alter the rates and products of pollutant transformations; lead to covalent incorporation of halogen into small natural molecules, DOM, and pollutants; and give rise to certain halogen oxides of concern as water contaminants. The complex and colorful chemistry of halogen in waters will be summarized in detail and the implications of this chemistry for global biogeochemical cycling of halogen, contaminant fate in natural waters, and water purification technologies will be discussed.

Inland Sea Spray Aerosol Transport and Incomplete Chloride Depletion: Varying Degrees of Reactive Processing Observed during SOAS

Multiphase reactions involving sea spray aerosol (SSA) impact trace gas budgets in coastal regions by acting as a reservoir for oxidized nitrogen and sulfur species, as well as being a source of halogen gases (HCl, ClNO₂, etc.). Whereas most studies of multiphase reactions on SSA have focused on marine environments, far less is known about SSA transported inland. Herein, single-particle measurements of SSA are reported at a site >320 km from the Gulf of Mexico, with transport times of 7-68 h. Samples were collected during the Southern Oxidant and Aerosol Study (SOAS) in June-July 2013 near Centreville, Alabama. SSA was observed in 93% of 42 time periods analyzed. During two marine air mass periods, SSA represented significant number fractions of particles in the accumulation (0.2-1.0 μm, 11%) and coarse (1.0-10.0 μm, 35%) modes. Chloride content of SSA particles ranged from full to partial depletion, with 24% of SSA particles containing chloride (mole fraction of Cl/Na ≥ 0.1, 90% chloride depletion). Both the frequent observation of SSA at an inland site and the range of chloride depletion observed suggest that SSA may represent an underappreciated inland sink for NO_x/SO₂ oxidation products and a source of halogen gases.

The effects of coexisting Na2SO4 on heterogeneous uptake of NO2 on CaCO3 particles at various RHs

Atmospheric particles can undergo nucleation, coagulation, chemical-aging, dissolution-precipitation or other atmospheric processes, resulting in complex multicomponent aerosols. The coexisting species have potentially important consequences in the heterogeneous reactions of multicomponent aerosol particles with polluted gases, which are still poorly understood. The effect of coexisting Na₂SO₄ on heterogeneous uptake of NO₂ on CaCO₃ particles is investigated in a broad RH range. The combination of DRIFTS, Raman, SEM and IC provides qualitative and quantitative information about the formation of nitrate and other surface species. Ca(NO₃)₂ and NaNO₃ are generated on mixed CaCO₃-Na₂SO₄ particles under dry condition. Both the amount of NO₃^- formed and the NO₃^- formation rates for the mixtures can be predicted based on the linear addition of those for pure CaCO₃ and Na₂SO₄ particles under dry condition. The further reaction of Ca(NO₃)₂ with Na₂SO₄ could lead to the formation of crystal NaNO₃ and CaSO₄·0.5H₂O at 30% RH. Coagulation between Ca²⁺ and SO₄^2- in surface adsorbed water is observed after part conversion of CaCO₃ to Ca(NO₃)₂, resulting in the formation of CaSO₄·2H₂O at 80% RH. The amount of NO₃^- formed on the mixtures is dramatically enhanced relative to the predictions at 30% and 80% RH. The findings presented here highlight the role of coexisting species in the heterogeneous reactions of trace gases with multicomponent aerosols due to the complexity of atmospheric particles.

Marine Primary Productivity as a Potential Indirect Source of Selenium and Other Trace Elements in Atmospheric Deposition

Atmospheric processes play an important role in the supply of the trace element selenium (Se) as well as other essential trace elements to terrestrial environments, mainly via wet deposition. Here we investigate whether the marine biosphere can be identified as a source of Se and of other trace elements in precipitation samples. We used artificial neural network (ANN) modeling and other statistical methods to analyze relationships between a high-resolution atmospheric deposition chemistry time series (March 2007-January 2009) from Plynlimon (UK) and exposure of air masses to marine chlorophyll a and to other source proxies. Using ANN sensitivity analyses, we found that higher air mass exposure to marine productivity leads to higher concentrations of dissolved organic carbon (DOC) in rainfall. Furthermore, marine productivity was found to be an important but indirect factor in controlling Se as well as vanadium (V), cobalt (Co), nickel (Ni), zinc (Zn), and aluminum (Al) concentrations in atmospheric deposition, likely via scavenging by organic compounds derived from marine organisms. Marine organisms may thus play an indirect but important role in the delivery of trace elements to terrestrial environments and food chains.

Nanoscopic characterization of the water vapor-salt interfacial layer reveals a unique biphasic adsorption process

Our quantitative understanding of water adsorption onto salt surfaces under ambient conditions is presently quite poor owing to the difficulties in directly characterizing this interfacial layer under these conditions. Here we determine the thickness of the interfacial layer on NaCl at different relative humidities (RH) based on a novel application of atomic force spectroscopy and capillary condensation theory. In particular, we take advantage of the microsecond-timescale of the capillary condensation process to directly resolve the magnitude of its contribution in the tip-sample interaction, from which the interfacial water thickness is determined. Further, to correlate this thickness with salt dissolution, we also measure surface conductance under similar conditions. We find that below 30% RH, there is essentially only the deposition of water molecules onto this surface, typical of conventional adsorption onto solid surfaces. However, above 30% RH, adsorption is simultaneous with the dissolution of ions, unlike conventional adsorption, leading to a rapid increase of surface conductance. Thus, water adsorption on NaCl is an unconventional biphasic process in which the interfacial layer not only exhibits quantitative differences in thickness but also qualitative differences in composition.

Heterogeneous Chemistry of Lipopolysaccharides with Gas-Phase Nitric Acid: Reactive Sites and Reaction Pathways

Recent studies have shown that sea spray aerosol (SSA) has a size-dependent, complex composition consisting of biomolecules and biologically derived organic compounds in addition to salts. This additional chemical complexity most likely influences the heterogeneous reactivity of SSA, as these other components will have different reactive sites and reaction pathways. In this study, we focus on the reactivity of a class of particles derived from some of the biological components of sea spray aerosol including lipopolysaccharides (LPS) that undergo heterogeneous chemistry within the reactive sites of the biological molecule. Examples of these reactions and the relevant reactive sites are proposed as follows: R-COONa(s) + HNO3(g) → NaNO3 + R-COOH and R-HPO4Na(s) + HNO3(g) → NaNO3 + R-H2PO4. These reactions may be a heterogeneous pathway not only for sea spray aerosol but also for a variety of other types of atmospheric aerosol as well.

Atmospheric chemistry of bioaerosols: heterogeneous and multiphase reactions with atmospheric oxidants and other trace gases

Once airborne, biologically-derived aerosol particles are prone to reaction with various atmospheric oxidants such as OH, NO₃, and O₃.

Advances in analytical techniques and instrumentation have now established methods for detecting, quantifying, and identifying the chemical and microbial constituents of particulate matter in the atmosphere. For example, recent cryo-TEM studies of sea spray have identified whole bacteria and viruses ejected from ocean seawater into air. A focal point of this perspective is directed towards the reactivity of aerosol particles of biological origin with oxidants (OH, NO₃, and O₃) present in the atmosphere. Complementary information on the reactivity of aerosol particles is obtained from field investigations and laboratory studies. Laboratory studies of different types of biologically-derived particles offer important information related to their impacts on the local and global environment. These studies can also unravel a range of different chemistries and reactivity afforded by the complexity and diversity of the chemical make-up of these particles. Laboratory experiments as the ones reviewed herein can elucidate the chemistry of biological aerosols.

Hygroscopic growth of water-soluble matter extracted from remote marine aerosols over the western North Pacific: Influence of pollutants transported from East Asia

We examined the hygroscopic properties of water-soluble matter (WSM) nebulized from water extracts of total suspended particles (TSP) collected at Chichijima Island in the western North Pacific during January to September 2003. The hygroscopic growth factor g(RH) of the aerosol particles was measured using a hygroscopic tandem differential mobility analyzer (HTDMA) with an initial dry particle diameter of 100nm and relative humidity (RH) of 5-95%. The measured growth factor at 90% RH, g(90%), ranged from 1.51 to 2.14 (mean: 1.76±0.15), significantly lower than that of sea salts (2.1), probably owing to the heterogeneous reactions associated with chloride depletion in sea-salt particles and water-soluble organic matter (WSOM). The g(90%) maximized in summer and minimized in spring. The decrease in spring was most likely explained by the formation of less hygroscopic salts or particles via organometallic reactions during the long-range transport of Asian dust. Cl(-) and Na(+) dominate the mass fractions of WSM, followed by nss-SO4(2-) and WSOM. Based on regression analysis, we confirmed that g(90%) at Chichijima Island largely increased due to the dominant sea spray; however, atmospheric processes associated with chloride depletion in sea salts and WSOM often suppressed g(90%). Furthermore, we explored the deviation (average: 18%) between the measured and predicted g(90%) by comparing measured and model growth factors. The present study demonstrates that long-range atmospheric transport of anthropogenic pollutants (SO2, NOx, organics, etc.) and the interactions with sea-salt particles often suppress the hygroscopic growth of marine aerosols over the western North Pacific, affecting the remote background conditions. The present study also suggests that the HCl liberation leads to the formation of less hygroscopic aerosols over the western North Pacific during long-range transport.

Development of a gas phase source for perfluoroalkyl acids to examine atmospheric sampling methods

A continuous perfluoroalkyl acid gas phase source was constructed to improve current atmospheric sampling techniques.

Adsorption and transformation mechanism of NO2 on NaCl(100) surface: A density functional theory study

To understand the heterogeneous reactions between NO2 and sea salt particles in the atmosphere of coastal areas, the absorption of an NO2 molecule on the NaCl(100) surface, the dimerization of NO2 molecules and the hydrolysis of N2O4 isomers at the (100) surface of NaCl are investigated by density functional theory. Calculated results show that the most favorable adsorption geometry of isolated NO2 molecule is found to reside at the bridge site (II-1) with the adsorption energy of -14.85 kcal/mol. At the surface of NaCl(100), three closed-shell dimers can be identified as sym-O2N-NO2, cis-ONO-NO2 and trans-ONO-NO2. The reactions of H2O with sym-O2N-NO2 on the (100) surface of NaCl are difficult to occur because of the high barrier (33.79 kcal/mol), whereas, the reactions of H2O with cis-ONONO2 and trans-ONONO2 play the key role in the hydrolysis process. The product, HONO, is one of the main atmospheric sources of OH radicals which drive the chemistry of the troposphere.

Anaerobic microbial transformation of halogenated aromatics and fate prediction using electron density modeling

Halogenated homo- and heterocyclic aromatics including disinfectants, pesticides and pharmaceuticals raise concern as persistent and toxic contaminants with often unknown fate. Remediation strategies and natural attenuation in anaerobic environments often build on microbial reductive dehalogenation. Here we describe the transformation of halogenated anilines, benzonitriles, phenols, methoxylated, or hydroxylated benzoic acids, pyridines, thiophenes, furoic acids, and benzenes by Dehalococcoides mccartyi strain CBDB1 and environmental fate modeling of the dehalogenation pathways. The compounds were chosen based on structural considerations to investigate the influence of functional groups present in a multitude of commercially used halogenated aromatics. Experimentally obtained growth yields were 0.1 to 5 × 10(14) cells mol(-1) of halogen released (corresponding to 0.3-15.3 g protein mol(-1) halogen), and specific enzyme activities ranged from 4.5 to 87.4 nkat mg(-1) protein. Chlorinated electron-poor pyridines were not dechlorinated in contrast to electron-rich thiophenes. Three different partial charge models demonstrated that the regioselective removal of halogens is governed by the least negative partial charge of the halogen. Microbial reaction pathways combined with computational chemistry and pertinent literature findings on Co(I) chemistry suggest that halide expulsion during reductive dehalogenation is initiated through single electron transfer from B12Co(I) to the apical halogen site.

The effect of cations on NO2 production from the photolysis of aqueous thin water films of nitrate salts

Cations are shown to enhance nitrate photochemistry by changing the concentrations of nitrate ions in the interface region.

Influence of stearic acid coating of the NaCl surface on the reactivity with NO2 under humidity

In the atmosphere, sea salt aerosols, containing mainly NaCl, can accumulate fatty acids on surface and undergo spatially heterogeneous chemistry with atmospheric nitrogen oxides.

An unconventional bilayer ice structure on a NaCl(001) film

Water-solid interactions are of broad importance both in nature and technology. The hexagonal bilayer model based on the Bernal-Fowler-Pauling ice rules has been widely adopted to describe water structuring at interfaces. Using a cryogenic scanning tunnelling microscope, here we report a new type of two-dimensional ice-like bilayer structure built from cyclic water tetramers on an insulating NaCl(001) film, which is completely beyond this conventional bilayer picture. A novel bridging mechanism allows the interconnection of water tetramers to form chains, flakes and eventually a two-dimensional extended ice bilayer containing a regular array of Bjerrum D-type defects. Ab initio density functional theory calculations substantiate this bridging growth mode and reveal a striking proton-disordered ice structure. The formation of the periodic Bjerrum defects with unusually high density may have a crucial role as H donor sites in directing multilayer ice growth and in catalysing heterogeneous chemical reactions on water-coated salt surfaces.

Production of gas phase NO2 and halogens from the photolysis of thin water films containing nitrate, chloride and bromide ions at room temperature

Nitrate and halide ions coexist in particles generated in marine regions, around alkaline dry lakes, and in the Arctic snowpack. Although the photochemistry of nitrate ions in bulk aqueous solution is well known, there is recent evidence that it may be more efficient at liquid-gas interfaces, and that the presence of other ions in solution may enhance interfacial reactivity. This study examines the 311 nm photolysis of thin aqueous films of ternary halide-nitrate salt mixtures (NaCl-NaBr-NaNO3) deposited on the walls of a Teflon chamber at 298 K. The films were generated by nebulizing aqueous 0.25 M NaNO3 solutions which had NaCl and NaBr added to vary the mole fraction of halide ions. Molar ratios of chloride to bromide ions were chosen to be 0.25, 1.0, or 4.0. The subsequent generation of gas phase NO2 and reactive halogen gases (Br2, BrCl and Cl2) were monitored with time. The rate of gas phase NO2 formation was shown to be enhanced by the addition of the halide ions to thin films containing only aqueous NaNO3. At [Cl(-)]/[Br(-)] ≤ 1.0, the NO2 enhancement was similar to that observed for binary NaBr-NaNO3 mixtures, while with excess chloride NO2 enhancement was similar to that observed for binary NaCl-NaNO3 mixtures. Molecular dynamics simulations predict that the halide ions draw nitrate ions closer to the interface where a less complete solvent shell allows more efficient escape of NO2 to the gas phase, and that bromide ions are more effective in bringing nitrate ions closer to the surface. The combination of theory and experiments suggests that under atmospheric conditions where nitrate ion photochemistry plays a role, the impact of other species such as halide ions should be taken into account in predicting the impacts of nitrate ion photochemistry.

Characteristics of water-soluble inorganic ions in PM2.5 and PM 2.5-10 in the coastal urban agglomeration along the Western Taiwan Strait Region, China

PM2.5 and PM2.5-10 aerosol samples were collected in four seasons during November 2010, January, April, and August 2011 at 13 urban/suburban sites and one background site in Western Taiwan Straits Region (WTSR), which is the coastal area with rapid urbanization, high population density, and deteriorating air quality. The 10 days average PM2.5 concentrations were 92.92, 51.96, 74.48, and 89.69 μg/m(3) in spring, summer, autumn, and winter, respectively, exceeding the Chinese ambient air quality standard for annual average value of PM2.5 (grade II, 35 μg/m(3)). Temporal distribution of water-soluble inorganic ions (WSIIs) in PM2.5 was coincident with PM2.5 mass concentrations, showing highest in spring, lowest in summer, and middle in autumn and winter. WSIIs took considerable proportion (42.2 ∼ 50.1 %) in PM2.5 and PM2.5-10. Generally, urban/suburban sites had obviously suffered severer pollution of fine particles compared with the background site. The WSIIs concentrations and characteristics were closely related to the local anthropogenic activities and natural environment, urban sites in cities with higher urbanization level, or sites with weaker diffuse condition suffered severer WSIIs pollution. Fossil fuel combustion, traffic emissions, crustal/soil dust, municipal constructions, and sea salt and biomass burnings were the major potential sources of WSIIs in PM2.5 in WTSR according to the result of principal component analysis.

Heterogeneous uptake and adsorption of gas-phase formic acid on oxide and clay particle surfaces: the roles of surface hydroxyl groups and adsorbed water in formic acid adsorption and the impact of formic acid adsorption on water uptake

Organic acids in the atmosphere are ubiquitous and are often correlated with mineral dust aerosol. Heterogeneous chemistry and the uptake of organic acids on mineral dust particles can potentially alter the properties of the particle. In this study, heterogeneous uptake and reaction of formic acid, HCOOH, the most abundant carboxylic acid present in the atmosphere, on oxide and clays of the most abundant elements, Si and Al, present in the Earth's crust are investigated under dry and humid conditions. In particular, quantitative adsorption measurements using a Quartz Crystal Microbalance (QCM) coupled with spectroscopic studies using Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopy are combined to allow for both quantification of the amount of uptake and identification of distinct adsorbed species formed on silica, alumina, and kaolinite particle surfaces at 298 K. These oxides and clay particles show significant differences in the extent and speciation of adsorbed HCOOH due to inherent differences in surface -OH group reactivity. Adsorbed water, controlled by relative humidity, can increase the irreversible uptake of formic acid. Interestingly, the resulting layer of adsorbed formate on the particle surface decreases the particle hydrophilicity thereby decreasing the amount of water taken up by the surface as measured by QCM. Atmospheric implications of this study are discussed.

Laboratory study on the hygroscopic behavior of external and internal C2-C4 dicarboxylic acid-NaCl mixtures

Atmospheric aerosol is usually found to be a mixture of various inorganic and organic components in field measurements, whereas the effect of this mixing state on the hygroscopicity of aerosol particles has remained unknown. In this study, the hygroscopic behavior of mixtures of C2-C4 dicarboxylic acids and NaCl was investigated. For both externally and internally mixed malonic acid-NaCl and succinic acid-NaCl particles, correlation between water content and chemical composition was observed and the water content of these mixtures at relative humidity (RH) above 80% can be well predicted by the Zdanovskii-Stokes-Robinson (ZSR) method. In contrast, a nonlinear relation between the total water content of the mixtures and the water content of each chemical composition separately was found for oxalic acid-NaCl mixtures. Compared to the values predicted by the ZSR method, the dissolution of oxalic acid in external mixtures resulted in an increase in the total water content, whereas the formation of less hygroscopic disodium oxalate in internal mixtures led to a significant decrease in the total water content. Furthermore, we found that the hygroscopicity of the sodium dicarboxylate plays a critical role in determining the aqueous chemistry of dicarboxylic acid-NaCl mixtures during the humidifying and dehumidifying process. It was also found that the hydration of oxalic acid and the deliquescence of NaCl did not change in external oxalic acid-NaCl mixtures. The deliquescence relative humidity (DRHs) for both malonic acid and NaCl decreased in both external and internal mixtures. These results could help in understanding the conversion processes of dicarboxylic acids to dicarboxylate salts, as well as the substitution of Cl by oxalate in the atmosphere. It was demonstrated that the effect of coexisting components on the hygroscopic behavior of mixed aerosols should not be neglected.

Hydroxyl radical oxidation of phospholipid-coated NaCl particles

Biological organic compounds mixed with NaCl and other inorganic compounds in sea-salt aerosol particles react in air with oxidants such as ozone and hydroxyl (OH) radicals. Laboratory studies of model systems can provide insight into these reactions. We report here studies of the kinetics and mechanism of oxidation of unsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) on NaCl by gas phase OH in air at room temperature and 1 atm pressure using diffuse reflection infrared Fourier transform spectrometry (DRIFTS) and matrix-assisted laser desorption/ionization-time-of-flight-mass spectrometry (MALDI-TOF-MS) to identify possible structures of surface-bound reaction products. For comparison, some studies were also carried out on the saturated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) on NaCl. The calculated concentration of hydroxyl radicals, generated by photolysis of isopropyl nitrite, was (1.6-6.4) × 10(8) radicals cm(-3). Surface-bound aldehydes, ketones, organic nitrates and nitrate ions were identified as products of these reactions and structures of potential products were proposed based on mechanistic considerations combined with the MALDI-TOF-MS and DRIFTS spectra. The loss rate of vinyl hydrogen, =C-H, at 3008 cm(-1) was used to obtain a lower limit for the rate constant (k1) for addition of OH to the double bond, k1 > (3 ± 1) × 10(-13) cm(3) molecule(-1) s(-1) (1s), corresponding to a reaction probability of γ(add) > (4 ± 1) × 10(-3) (1s). Assuming that abstraction from -CH2- groups in POPC is the same as for DPPC, the percentage of the reaction that occurs by addition is ~80%. This is similar to the percent addition predicted using structure-reactivity relationships for gas-phase reactions. Decreasing the amount of POPC relative to NaCl resulted in more nitrate ion formation and less relative loss of POPC, and increasing the OH concentration resulted in a more rapid loss of POPC and faster product formation. These studies suggest that under atmospheric conditions with an OH concentration of 5 × 10(6) radicals cm(-3), the lifetime of POPC with respect to OH is <6 days.