Deliquescent Particles Research Articles

Kinetics of nitrosamine and amine reactions with NO3 radical and ozone related to aqueous particle and cloud droplet chemistry

Aqueous phase reactivity experiments with the amines dimethylamine (DMA), diethanolamine (DEA) and pyrrolidine (PYL) and their corresponding nitrosamines nitrosodimethylamine (NDMA), nitrosodiethanolamine (NDEA) and nitrosopyrrolidine (NPYL) have been performed. NO3 radical reaction rate coefficients for DMA, DEA and PYL were measured for the first time and are 3.7×105, 8.2×105 and 8.7×105M−1s−1, respectively. Rate coefficients for NO3+NDMA, NDEA and NPYL are 1.2×108, 2.3×108 and 2.4×108M−1s−1. Compared to OH radical rate coefficients for reactions with amines, the NO3 radical will most likely not be an important oxidant but it is a potential nighttime oxidant for nitrosamines in cloud droplets or deliquescent particles. Ozone is unreactive towards amines and nitrosamines and upper limits of rate coefficients suggest that aqueous ozone reactions are not important in atmospheric waters.

Phototransformation of 4-phenoxyphenol sensitised by 4-carboxybenzophenone: Evidence of new photochemical pathways in the bulk aqueous phase and on the surface of aerosol deliquescent particles

In addition to direct photolysis, degradation of organic compounds by solar light can also occur by indirect photolysis or photo-sensitised processes. These reactions are important because they are involved in, among others, direct and indirect climate changes, adverse health effects from inhaled particles, effects on cloud chemistry and ozone production. In this work, the importance of atmospheric photo-sensitisation is evaluated in bulk aqueous solution and on the surface of aerosol deliquescent particles. Irradiation experiments in aqueous solution indicate that 4-carboxybenzophenone (CBP) is able to photosensitise the degradation of 4-phenoxyphenol (4 PP). The process takes place via the CBP triplet state (3CBP*), which has an oxidising nature. 4 PP is fluorescent, unlike the photosensitiser CBP, with two emission bands at ∼320 and ∼380 nm. However, addition of CBP to a 4 PP solution considerably decreases the intensity of 4 PP fluorescence bands and causes a very intense new band to appear at ∼420 nm. This behaviour suggests a possible interaction between CBP and 4 PP in solution, which could favour further light-induced processes. Moreover, the new band overlaps with the fluorescence spectrum of atmospheric HULIS (HUmic-LIke Substances), suggesting that supramolecular photosensitiser–substrate interactions may have a role in HULIS fluorescence properties. The interaction between CBP and 4 PP coated on silica particles (gas–solid system) was also investigated under simulated sunlight, and in the presence of variable relative humidity. The water molecules inhibit the degradation of 4 PP, induced by 3CBP* on the surface of aerosol particles, indicating that the process could be even faster on particles than in solution. We demonstrate that phenol substances adsorbed on aerosol surfaces and in bulk solution are substantially altered upon photosensitised processes.

Modelling multiphase chemistry in deliquescent aerosols and clouds using CAPRAM3.0i

Modelling studies were performed with the multiphase mechanism RACM-MIM2ext/CAPRAM 3.0i to investigate the tropospheric multiphase chemistry in deliquesced particles and non-precipitating clouds using the SPACCIM model framework. Simulations using a non-permanent cloud scenario were carried out for two different environmental conditions focusing on the multiphase chemistry of oxidants and other linked chemical subsystems. Model results were analysed by time-resolved reaction flux analyses allowing advanced interpretations. The model shows significant effects of multiphase chemical interactions on the tropospheric budget of gas-phase oxidants and organic compounds. In-cloud gas-phase OH radical concentration reductions of about 90 % and 75 % were modelled for urban and remote conditions, respectively. The reduced in-cloud gas-phase oxidation budget increases the tropospheric residence time of organic trace gases by up to about 30 %. Aqueous-phase oxidations of methylglyoxal and 1,4-butenedial were identified as important OH radical sinks under polluted conditions. The model revealed that the organic C3 and C4 chemistry contributes with about 38 %/48 % and 8 %/9 % considerably to the urban and remote cloud / aqueous particle OH sinks. Furthermore, the simulations clearly implicate the potential role of deliquescent particles to operate as a reactive chemical medium due to an efficient TMI/HOx,y chemical processing including e.g. an effective in-situ formation of OH radicals. Considerable chemical differences between deliquescent particles and cloud droplets, e.g. a circa 2 times more efficient daytime iron processing in the urban deliquescent particles, were identified. The in-cloud oxidation of methylglyoxal and its oxidation products is identified as efficient sink for NO3 radicals in the aqueous phase.

Mechanism development and modelling of tropospheric multiphase halogen chemistry: The CAPRAM Halogen Module 2.0 (HM2)

A new detailed multiphase halogen mechanism, the CAPRAM Halogen Module 2.0 (HM2), has been developed and coupled to the multiphase chemistry mechanism RACM-MIM2ext/CAPRAM 3.0n. The overall mechanism comprises 1,705 reactions including 595 reactions of the HM2. Halogen chemistry box model studies have been, for the first time, performed with a non-permanent cloud scenario for pristine open ocean regions in mid-latitudes. Moreover, detailed time-resolved reaction flux analysis has been used to investigate the multiphase halogen reaction cycles in more detail. Clouds significantly change the multiphase halogen chemical system and new reaction cycles are proposed for in-cloud conditions. While most gas phase concentrations are decreased for chlorine and iodine species, they are increased for bromine. Flux analyses determined the relative contributions of the methylene dihalides CH2IX (X = Cl, Br, I) as the main I atom source with a contribution of about 80 % to the total iodocarbon sources. Furthermore, HOI was confirmed to be important for chlorine activation. It is shown that 25 % of the ozone loss can be attributed to halogens. VOC oxidation by halogens is important as halogens account for about 20 % of the methane oxidation and up to 80 % of the oxidation of other VOCs. In other cases, enhanced VOC and VOC oxidation product concentration levels were found. For example, 15 % of the methyl peroxyl radicals are formed after the reaction of chlorine atoms with methane or methyl hydroperoxide. In the aqueous phase, changes in the oxidation of organics do only occur for highly oxidised organics without a C-H bond. For example, over 80 % of oxalic acid are oxidised by electron transfer with Cl2 − in deliquescent particles during non-cloud periods.

Spectroscopic Evidence of Keto−Enol Tautomerism in Deliquesced Malonic Acid Particles

Scanning transmission X-ray microscopy combined with near-edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS) and optical microscopy coupled with Fourier transform infrared spectroscopy (micro-FTIR) have been applied to observe hygroscopic growth and chemical changes in malonic acid particles deposited on substrates. The extent of the hygroscopic growth of particles has been quantified in terms of the corresponding water-to-solute ratios (WSR) based on STXM/NEXAFS and micro-FTIR data sets. WSR values derived separately from two applied methods displayed a remarkable agreement with previous data reported in the literature. Comparison of NEXAFS and FTIR spectra acquired at different relative humidity (RH) shows efficient keto-enol tautomerization of malonic acid, with the enol form dominating at higher RH. The keto-enol equilibrium constants were calculated using relevant peak intensities in the carbon and oxygen K-edge NEXAFS spectra as a function of RH. We report strong dependence of the equilibrium constant on RH, with measured values of 0.18 ± 0.03, 1.11 ± 0.14, and 2.33 ± 0.37 corresponding to 2, 50, and 90% RH, respectively. Enols are important intermediates in aldol condensation reactions pertaining to formation and atmospheric aging of secondary organic aerosol (SOA). The present knowledge assumes that constituents of atmospheric deliquesced particles undergo aqueous chemistry with kinetic and equilibrium constants analogous to reactions in bulk solutions, which would estimate absolute dominance of the keto form of carboxylic acids. For instance, the keto-enol equilibrium constant of malonic acid in diluted aqueous solution is <10(-4). Our results suggest that in deliquesced micrometer-size particles, carboxylic acids may exist in predominantly enol forms that need to be explicitly considered in atmospheric aerosol chemistry.

Internal structure, hygroscopic and reactive properties of mixed sodium methanesulfonate-sodium chloride particles

Internal structures, hygroscopic properties and heterogeneous reactivity of mixed CH(3)SO(3)Na/NaCl particles were investigated using a combination of computer modeling and experimental approaches. Surfactant properties of CH(3)SO(3)(-) ions and their surface accumulation in wet, deliquesced particles were assessed using molecular dynamics (MD) simulations and surface tension measurements. Internal structures of dry CH(3)SO(3)Na/NaCl particles were investigated using scanning electron microscopy (SEM) assisted with X-ray microanalysis mapping, and time-of-flight secondary ion mass spectrometry (TOF-SIMS). The combination of these techniques shows that dry CH(3)SO(3)Na/NaCl particles are composed of a NaCl core surrounded by a CH(3)SO(3)Na shell. Hygroscopic growth, deliquescence and efflorescence phase transitions of mixed CH(3)SO(3)Na/NaCl particles were determined and compared to those of pure NaCl particles. These results indicate that particles undergo a two step deliquescence transition: first at ∼69% relative humidity (RH) the CH(3)SO(3)Na shell takes up water, and then at ∼75% RH the NaCl core deliquesces. Reactive uptake coefficients for the particle-HNO(3) heterogeneous reaction were determined at different CH(3)SO(3)Na/NaCl mixing ratios and RH. The net reaction probability decreased notably with increasing CH(3)SO(3)Na and at lower RH.

Radical-driven carbonyl-to-acid conversion and acid degradation in tropospheric aqueous systems studied by CAPRAM

Abstract Model studies on the aqueous phase radical-driven processing of carbonyl compounds and acids in clouds and deliquescent particles were performed. The model exposed that aqueous radical conversions of carbonyl compounds and its oxidation products can contribute potentially to the formation of functionalised organic acids. The main identified C 2 –C 4 organic gas phase precursors are ethylene glycol, glycolaldehyde, glyoxal, methylglyoxal and 1,4-butenedial. The aqueous phase is shown to contribute significantly with about 93%/63%, 47%/8%, 31%/4%, 7%/4%, 36%/8% to the multiphase oxidative fate of these compounds under remote/urban conditions. Interestingly, the studies revealed that aqueous chemical processing is not only limited to in-cloud conditions but also proceeds in deliquescent particle phase with significant fluxes. Oxalic acid is shown to be formed preferably in deliquescent particles subsequent to the in-cloud oxidations. Mean aqueous phase oxalate formation fluxes of about 12, 42 and 0.4 ng m −3 h −1 in the remote, urban and maritime scenario, respectively. Additionally, the turnovers of the oxidation of organics such as methylglyoxal by NO 3 radical reactions are identified to be competitive to their OH pendants. At the current state of CAPRAM, mean C 2 –C 4 in-cloud oxidation fluxes of about 0.12 and 0.5 μg m −3 h −1 are modelled under the idealised remote and urban cloud conditions. Finally, turnovers from radical oxidations were compared with those of thermal reactions. It is demonstrated that, based on the sparse kinetic data available organic accretion reaction might be of interest in just a few cases for cloud droplets and aqueous particles but generally do not reach the oxidative conversion rates of the main radical oxidants OH and NO 3 . Interestingly, oxidation reactions of H 2 O 2 are shown to be competitive to the OH radical conversions in cases when H 2 O 2 is not readily used up by the S(IV) oxidation.

Atmospheric Stability of Levoglucosan: A Detailed Laboratory and Modeling Study

Levoglucosan, an important molecular marker for biomass burning, represents an important fraction of the water-soluble organic carbon in atmospheric particles influenced by residential wood burning and wildfires. However, particle phase oxidation processes of levoglucosan by free radicals are not well-known. Hence, detailed kinetic studies on the reactivity of levoglucosan with OH, NO(3), and SO(4)(-) radicals in aqueous solutions were performed to better understand the levoglucosan oxidation in the deliquescent particles. The data obtained were implemented into a parcel model with detailed microphysics and complex multiphase chemistry to investigate the degradation fluxes of levoglucosan in cloud droplets and in deliquescent particles. The model calculations show that levoglucosan can be oxidized readily by OH radicals during daytime with mean degradation fluxes of about 7.2 ng m(-3) h(-1) in summer and 4.7 ng m(-3) h(-1) in winter for a polluted continental plume. This indicates that the oxidation of levoglucosan in atmospheric deliquescent particles is at least as fast as that of other atmospherically relevant organic compounds and levoglucosan may not be as stable as previously thought in the atmosphere, especially under high relative humidity conditions.

Deliquescence and hygroscopic growth of succinic acid particles measured with LACIS

Succinic acid, a dicarboxylic acid which is only slightly soluble in water, was examined with regard to its deliquescence process and its hygroscopic growth in comparison with simple Köhler theory. Investigations were done at LACIS (Leipzig Aerosol Cloud Interaction Simulator), which can measure hygroscopic growth of aerosol particles at relative humidities up to values close to 100%. The deliquescence relative humidity of dry succinic acid particles with diameters of 200 nm was found at 99% (±0.2%). Deliquescence took place on a time scale faster than 0.3 seconds, for some measurements even faster than 0.15 seconds. Comparison of the measured equilibrium sizes of the succinic acid particles with modeled hygroscopic growth revealed evaporation of succinic acid from the deliquesced particles, an effect, which has been described in literature previously.

Inter-particle and gas-particle interactions in sampling artifacts of PM in filter-based samplers

Despite the recent rapid development of real-time measurements of chemical compositions in atmospheric aerosols, filter-based aerosol sampling for an extended period of time with post-sampling chemical analysis is still very common. The collected particles interact with both the in-coming gas and each other. These interactions lead to sampling artifacts. This paper investigates the role of aerosol ionic composition, inter-particle and gas-particle interactions in the sampling artifacts of PM 2.5 at high relative humidity. This is achieved by comparing PM 2.5 samples collected with and without a denuder/filter pack system at urban and suburban sites in Hong Kong. The samples collected with a denuder/filter pack were used to estimate the artifact-free ambient gas and particulate concentrations. Because of the prevailing high relative humidity (>70%RH) in HK, the collected particles were expected to have deliquesced and form an internal mixture, according to thermodynamic predictions. It was found that an ambient molar particulate [NH 4 + ]/[SO 4 2− ] ratio of 1.5 is a critical condition to the sampling artifact characteristics of PM 2.5 . The urban samples were all ammonium rich (AR; [NH 4 + ]/[SO 4 2− ] greater than 1.5) and were characterized by a high nitrate concentration and low strong-acidity. They had a significant strong acidity loss of 73%, roughly half of which was due to the neutralization of acidity by ammonia. The suburban samples were all ammonium poor (AP; [NH 4 + ]/[SO 4 2− ] less than or equal to 1.5) and were characterized by a low nitrate concentration and high strong-acidity. Sampling artifacts of the acidity, ammonium and sulfate were not important in the AP samples. In both the AR and AP samples, the individual evaporation of HNO 3 and HCl were the primary reactions for the artifacts of nitrate and chloride while the concomitant evaporation of HNO 3 and HCl with NH 3 was insignificant. In general, the extents of the sampling artifacts of nitrate and chloride increased with increasing gas-to-particulate concentration ratio of these two species. A methodology to estimate the artifact-free ambient concentrations of nitrate and chloride from measurements of filter pack samplers (without any denuders) for deliquesced particles is proposed.

Deliquescence of Deposited Atmospheric Particles on Leaf Surfaces

A large fraction of deposited aerosol particles on leafsurfaces represents hygroscopic material in a high humidity environment, likely to become deliquescent within the water vapour transpired by the leaf. Microscopic observations on leaf surfaces of beech, kohlrabi and elder leaves grown in a particle-free environment and/or treated with defined particle load are presented. Spreading of deliquescent particles, formation of salt crusts, and encoating of and by waxes was observed. Deliquescence of NaNO3 particles (deliquescence point 74% RH at 25 °C) was observed on and near the stomata at 35% relative humidity (RH) of the surrounding air, illustrating the influence of the boundary layer. Aerosols are difficult to identify on leaf surfaces, as similar patterns may be produced by deliquescent particles and processes like ‘salt creep’. These problems are especially important on leaves from the natural environment if only scanning electron microscopy (SEM) is used. These patterns could appear similar to the fused waxes described in forest decline research. Strong gradients are formed between deliquescent particles and the apoplastic solution within the leaf, promoting cuticular and stomatal uptake.

Initial Precipitation Formation in Warm Florida Cumulus

The microphysical processes that lead to the development of precipitation in small, warm cumulus are examined using data from the Small Cumulus Microphysics Study near Cape Canaveral, Florida. Aircraft measurements are used to determine the concentration and size distribution of giant and ultragiant nuclei in clear air as a function of relative humidity, altitude, wind speed, and wind direction. The clear-air particle distributions show that ultragiant particles (radii extending from 10 to 150 mm) exist from the surface to cloud base in concentrations that correspond to the concentrations of raindrops observed during drizzle to moderate rainfall events. A shift of the spectra toward larger size with increasing relative humidity was observed, suggesting that the spectra are composed of deliquesced particles growing by condensation. The small cumulus clouds are shown to contain cores where the observed liquid water content was nearly adiabatic. The observed evolution of the cloud droplet distribution within the near-adiabatic cores as a function of height showed an increase in the small droplet mode associated with condensation and an increase in the concentration of larger droplets associated with growth by accretion. Droplets with radii extending to nearly 100 mm were present just above cloud base. These measurements were consistent with the clear-air measurements and provided evidence that the ultragiant nuclei can immediately act as embryos for raindrop growth by accretion upon entering cloud base. Comparisons of reflectivity computed from the cumulus core composite droplet distributions with the radar-observed reflectivity data provided independent evidence that the composite spectra reasonably represented the evolving microstructure of the cores of small cumulus clouds as they grew vertically. The analyses provide strong evidence of an efficient process for the initial development of precipitation in small Florida cumuli. This process consists of raindrop embryo formation on ultragiant nuclei followed by growth by accretion as the newly formed drops proceed upward through the adiabatic cores of the cumulus clouds. These data support the conceptual model of raindrop formation in marine clouds first proposed by Woodcock a half century ago.

EMEP Eulerian model for atmospheric transport and deposition of nitrogen species over Europe

As a part of EMEP, MSC-W in Oslo has the task to compute atmospheric transport and deposition of sulphur and nitrogen species over Europe. To perform this task, a three-dimensional Eulerian multilayer model was developed at MSC-W. This model has a 50 km horizontal resolution with 20 vertical layers, of which 10 are below 2 km level. Meteorological input, from a dedicated weather prediction model, is updated at 6-hour intervals. Emission inventories for NO x (NO + NO 2), NH 3 (and SO 2), submitted officially by the individual countries, are used as input data. In the model, NO x may be converted to PAN by the CH 3COO 2 radical, or to nitrate (as HNO 3 in the gas phase and ammonium nitrate as particulate matter). The oxidation of NO x is assumed to proceed either by the OH radical in the gas phase, or as a night-time reaction via N 2O 5 on deliquescent particles. Both OH and CH 3COO 2 are prescribed as a function of solar zenith angle and cloud cover. Furthermore, ammonium sulphate is formed by NH 3 and sulphate. Excess NH 3 may react with HNO 3, forming ammonium nitrate in an equilibrium reaction. Nitrogen is removed from the atmosphere by dry and wet deposition. A description of the model, as well as the model results, are presented and discussed. We also present a comparison of calculated concentrations and deposition fields for nitrogen with available measurements of concentrations and wet deposition for 1992.

The Production of Moisture on the Pulmonary Alveolar Wall

Deliquescent particles elaborated by the granular pneumonocytes may play a role in the production of alveolar mucoid fluid.