Soluble Trace Gases Research Articles

Wind tunnel experiments on the retention of trace gases during riming: nitric acid, hydrochloric acid, and hydrogen peroxide

Abstract. Laboratory experiments were carried out in a vertical wind tunnel to study the retention of different atmospheric trace gases during riming. In the experiments, the rimed ice particles floated in a laminar air stream carrying a cloud of supercooled droplets with radii between 10 and 20 μm. Ice particles, dendritic ice crystals, and snow flakes with diameters between 6 mm and 1.5 cm were allowed to rime at temperatures between −5 and −12 °C where riming mainly proceeds in the atmosphere and with cloud liquid water contents between 1 and 1.5 g m−3 which are values typically found in atmospheric mixed-phase clouds. Three trace species were investigated, nitric and hydrochloric acid, and hydrogen peroxide. They were present in the supercooled liquid droplets in concentrations from 1 to 120 ppmv, i.e. similar to the concentrations measured in cloud drops. The chemical analyses of the rimed ice particles allow one to determine the trace species concentration in the ice phase. Together with the known liquid phase concentration the retention coefficients were calculated in terms of the amount of the species which remained in the ice phase after freezing. It was found that the highly soluble trace gases, nitric and hydrochloric acid, were retained nearly completely (98.6±8% and 99.7±9%, respectively) while for hydrogen peroxide a retention of 64.3±11% was determined. No influence of the riming temperature on the retention was found which can be explained by the fact that in the observed range of temperature and liquid water content, riming proceeded in the dry growth regime.

Uptake of soluble gaseous pollutants by rain droplets in the atmosphere with nocturnal temperature profile

We analyze the uptake of gaseous pollutants by the rain droplets falling in the atmosphere with nocturnal temperature inversion. The rate of uptake of soluble trace gases by falling rain droplets is determined by solving energy and mass conservation equations. In the analysis we accounted for the accumulation of the soluble gas and energy in the bulk of the falling rain droplet. The problem is solved in the approximation of a thin concentration and temperature boundary layers in the vicinity of the droplet surface. It is assumed that the bulk of a droplet, beyond the diffusion boundary layer, is completely mixed and distributions of concentration of the absorbate and temperature are homogeneous and time-dependent in the bulk. The problem is reduced to a system of linear-convolution Volterra integral equations of the second kind which is solved numerically. Calculations are performed using available experimental data on nocturnal temperature profiles in the atmosphere. It is shown than if the concentration of gaseous pollutants in the atmosphere is homogeneous and the absolute temperature in the atmosphere increases with altitude, a droplet absorbs gas during all the period of its fall. In the case when the temperature–altitude curve comprises the nocturnal inversion and temperature fall segments, gas absorption by a falling rain droplet can be replaced by desorption and vice versa. Neglecting temperature inhomogeneity in the atmosphere caused by nocturnal temperature profile leads to significant underestimation of the concentration of gaseous pollutants inside a droplet on the ground. The calculations performed using temperature profiles measured by Corsmeier et al. (1997) showed that the underestimation of the concentration of gaseous pollutants in rain droplets at the ground can exceed 20%.

Scavenging of soluble trace gases by falling rain droplets in inhomogeneous atmosphere

We analyze non-isothermal absorption of trace gases by the rain droplets with internal circulation which is caused by interfacial shear stresses. It is assumed that the concentration of soluble trace gases and temperature in the atmosphere varies in a vertical direction. The rate of scavenging of soluble trace gases by falling rain droplets is determined by solving heat and mass transfer equations. In the analysis we accounted for the accumulation of the absorbate in the bulk of the falling rain droplet. The problem is solved in the approximation of a thin concentration and temperature boundary layers in the droplet and in the surrounding air. We assumed that the bulk of a droplet, beyond the diffusion boundary layer, is completely mixed and concentration of the absorbate and temperature are homogeneous and time-dependent in the bulk. By combining the generalized similarity transformation method with Duhamel's theorem, the system of transient conjugate equations of convective diffusion and energy conservation for absorbate transport in liquid and gaseous phases with time-dependent boundary conditions is reduced to a system of linear convolution Volterra integral equations of the second kind which is solved numerically. Calculations are performed using available experimental data on concentration and temperature profiles in the atmosphere. It is shown than if concentration of a trace gas in the atmosphere is homogeneous and temperature in the atmosphere decreases with height, beginning from some altitude gas absorption is replaced by gas desorption. Neglecting temperature inhomogenity in the atmosphere described by adiabatic lapse rate leads to essential overestimation of the trace gas concentration in a droplet on the ground.

Effect of Altitude Concentration Gradient of Soluble Gaseous Pollutants on Their Scavenging by Falling Rain Droplets

Abstract This paper analyzes absorption of soluble atmospheric trace gases by falling rain droplets with internal circulation, which is caused by interfacial shear stresses. It is assumed that the concentration of soluble trace gases in the atmosphere varies in a vertical direction. In the analysis the accumulation of the absorbate in the bulk of the falling rain droplet was accounted for. The problem is solved in the approximation of a thin concentration boundary layer in the droplet and in the surrounding air. It was assumed that the bulk of a droplet, beyond the diffusion boundary layer, is completely mixed and that concentration of the absorbate is homogeneous and time dependent in the bulk. By combining the generalized similarity transformation method with Duhamel’s theorem, the system of transient conjugate equations of convective diffusion for absorbate transport in liquid and gaseous phases with time-dependent boundary conditions is reduced to a linear-convolution Volterra integral equation of the second kind, which is solved numerically. It is shown that the nonuniform vertical distribution of absorbate in a gaseous phase strongly affects mass transfer during gas absorption by a falling droplet.

Model sensitivity studies regarding the role of the retention coefficient for the scavenging and redistribution of highly soluble trace gases by deep convective cloud systems

Abstract. The role of the retention coefficient (i.e. the fraction of a dissolved trace gas which is retained in hydrometeors during freezing) for the scavenging and redistribution of highly soluble trace gases by deep convective cloud systems is investigated using a modified version of the Weather Research and Forecasting (WRF) model. Results from cloud system resolving model runs (in which deep convection is initiated by small random perturbations in association with so-called "large scale forcings (LSF)") for a tropical oceanic (TOGA COARE) and a mid-latitude continental case (ARM) are compared to two runs in which bubbles are used to initiate deep convection (STERAO, ARM). In the LSF runs, scavenging is found to almost entirely prevent a highly soluble tracer initially located in the lowest 1.5 km of the troposphere from reaching the upper troposphere, independent of the retention coefficient. The release of gases from freezing hydrometeors leads to mixing ratio increases in the upper troposphere comparable to those calculated for insoluble trace gases only in the two runs in which bubbles are used to initiate deep convection. A comparison of the two ARM runs indicates that using bubbles to initiate deep convection may result in an overestimate of the influence of the retention coefficient on the vertical transport of highly soluble tracers. It is, however, found that the retention coefficient plays an important role for the scavenging and redistribution of highly soluble trace gases with a (chemical) source in the free troposphere and also for trace gases for which even relatively inefficient transport may be important. The large difference between LSF and bubble runs is attributed to differences in dynamics and microphysics in the inflow regions of the storms. The dependence of the results on the model setup indicates the need for additional model studies with a more realistic initiation of deep convection, e.g., considering effects of orography in a nested model setup.

On the Assessment of Atmospheric Deposition of Sulfur and Nitrogen Species to the Surface of Large Inland Lakes—Lake Champlain

Early work indicated that wet deposition of radioactive fallout to the water surface of a lake greatly exceeded dry, when calculated as annual averages. To test whether this result also applies to the deposition rates of soluble trace gases from the lower atmosphere, data collected at land sites near Lake Champlain have been used to estimate deposition rates to the lake itself, using an analysis of the wind speed-up factor as an intermediate step. The contribution of dry deposition of the major nitrogen and sulfur chemical species is estimated to have been less than 20% of the total atmospheric deposition. However, this result must not be extrapolated to the watershed in which Lake Champlain resides, since evidence obtained elsewhere indicates that the dry deposition contribution over the entire watershed will likely be similar to the wet. The analysis indicates that for the period from 1992 to 1997 the annual total deposition rates of oxidized nitrogen and sulfur ranged from 300 to 500 tonnes per year and 600 to 1,100 tonnes per year, respectively.

Experimental Studies of Droplet Evaporation Kinetics: Validation of Models for Binary and Ternary Aqueous Solutions

Abstract Experiments were conducted with an electrodynamic levitation system to study the kinetics of droplet evaporation under chemically rich conditions. Single solution droplets of known composition (HNO3/H2O or H2SO4/HNO3/H2O) were introduced into an environmentally controlled cubic levitation cell. The gaseous environment was set intentionally out of equilibrium with the droplet properties, thus permitting the HNO3 mass accommodation coefficient to be determined. Measurements were performed at room temperature and various pressures (200–1000 hPa). Droplet sizes (initial radii in the range 12–26 μm) were measured versus time to high precision (±0.03 μm) via Mie scattering and compared with sizes computed by different models for mass and heat transfer in the transition regime. The best agreement between the theoretical calculations and experimental results was obtained for an HNO3 mass accommodation coefficient of 0.11 ± 0.03 at atmospheric pressure, 0.17 ± 0.05 at 500 hPa, and 0.33 ± 0.08 at 200 hPa. The determination of the mass accommodation coefficient was not sensitive to the transport model used. The results show that droplet evaporation is strongly limited by HNO3 and occurs in two stages, one characterized by rapid H2O mass transfer and the other by HNO3 mass transfer. The presence of a nonvolatile solute (SO2−4) affects the activities of the volatile components (HNO3 and H2O) and prevents complete evaporation of the solution droplets. These findings validate recent attempts to include the effects of soluble trace gases in cloud models, as long as suitable model parameters are used.

Soluble trace gas effect on cloud condensation nuclei activation: Influence of initial equilibration on cloud model results

Influence of initial equilibration of NH3 and HNO3 on number of droplets activated during a cloud model simulation is studied. This is done with a size‐resolved droplet box model including detailed microphysics and liquid phase thermodynamics. The results show that when both ammonia and nitric acid are present in the gas phase simultaneously, the number of activated droplets is dependent on the history of the air masses forming the cloud. Simulations show that in situations where the aerosol distribution is in equilibrium with condensable trace gases prior to cloud formation, the number of cloud droplets is lower than in the nonequilibrium situation.

Closure evaluation of size‐resolved aerosol pH in the New England coastal atmosphere during summer

During summer 2002, soluble trace gases, the ionic composition of size‐resolved aerosols, and meteorological conditions were measured from the National Oceanic and Atmospheric Administration ship Ronald H. Brown in coastal air along the eastern U.S. seaboard. Aerosol acidities were extrapolated from direct pH measurements in minimally diluted extracts and were also inferred from the measured phase partitioning and thermodynamic properties of HCl, HNO3, NH3, HCOOH, and CH3COOH. Median pHs for 0.75‐ through 25‐μm geometric mean diameter (GMD) size fractions based on direct measurements (2.6–3.9) were higher than those inferred from HCl partitioning (1.7–3.3). Phase disequilibria caused negative deviations in median pHs inferred from HNO3 partitioning with aerosol size fractions greater than 2.8‐μm GMD; pHs inferred for smaller size fractions (median values of 1.9–3.0) were similar to those based on HCl (median values of 1.5–3.0). The pHs inferred from NH3 partitioning were significantly lower than those estimated by other approaches; causes for this apparent bias are not known. The dominance of gas‐phase HCOOH and CH3COOH was generally consistent with predicted phase partitioning with acidic aerosols. Typically large diel excursions in most gases implied corresponding variability in aerosol acidity. The pHs inferred from maximum and minimum mixing ratios of gases over each aerosol sampling interval suggested median 12‐hour variations of ∼0.4–0.7 pH unit. Total acidity (Ht = H+ + undissociated acids) was greater than H+ by 1–2 orders of magnitude in all size fractions; most Ht was in the form of HSO4−.

Membrane-based parallel plate denuder for the collection and removal of soluble atmospheric gases.

A continuously wetted cellulose acetate membrane-based parallel plate diffusion denuder is described. This is the first membrane-based denuder that has a small enough internal liquid holdup volume to permit reasonably rapid response time (10 --> 90% rise time of approximately 1.2 min for a transient event at a liquid flow rate of 500 microL/min) while permitting quantitative removal of common soluble atmospheric trace gases at flow rates up to 1.7 L/min. The latter attribute permits the use of the device as the first element in a particle sampling and analysis system for the quantitative removal of potentially interfering soluble trace gases. Particle losses in the denuder range from 0.9 to 2.9% over an aerodynamic diameter range of 0.38-3.48 microm, averaging 1.8%. However, only approximately 0.5% of the particles actually appears in the denuder effluent liquid. The relatively compact (300 mm H x 57 mm W x 26 mm D) wet denuder should be attractive in a number of applications. We show excellent agreement for HONO measurements with a conventional larger parallel plate wetted denuder in field measurements.

Köhler theory for a polydisperse droplet population in the presence of a soluble trace gas, and an application to stratospheric STS droplet growth

Abstract. We consider the equilibrium behavior of a polydisperse aqueous droplet population as a function of relative humidity (RH) when a soluble trace gas, such as nitric acid, is present in the system. The droplet population experiences a splitting when the RH is increased sufficiently. This splitting is not related to the traditional Köhler activation of cloud droplets, as it may occur at relative humidities below 100%. Remarkably, the splitting always takes place in such a way that the largest size class of the (discretized) droplet population starts taking up the soluble trace gas efficiently, growing steeply as a function of RH, and forcing the smaller droplets to shrink. We consider this behavior in terms of open and closed system Köhler curves (open system referring to one in which the trace gas concentration remains constant and closed system to one in which the gas concentration decreases as a result of uptake of the trace gas). We show how the open and closed system Köhler curves are related, and that the splitting of the population can be explained in terms of closed system curves crossing the Köhler maxima of the open system curves. We then go on to consider time-dependent situations, and show that due to gas-phase mass transfer limitations, the splitting of the size distributions moves toward smaller sizes as the rate of RH increase becomes more rapid. Finally, we consider stratospheric supercooled ternary solution droplet populations, and show that the splitting described using the new theory may lead to formation of bimodal size distributions in the stratosphere.

Parametrization of the soluble trace gas effect on CCN activation

Parametrization of the soluble trace gas effect on CCN activation

The impact of cloud particle gravitational settling on soluble trace gas distributions

The effect of gravitational settling of cloud droplets and ice particles on the tropospheric distributions of soluble trace gases is examined, focusing in particular on nitric acid and hydrogen peroxide. For this study, we separate cloud condensate into two classes: large (rain, snow and graupel) and small (cloud water and cloud ice). The first class has typical fall speeds of several m/s and frequently reaches the ground. The redistribution and loss of soluble trace gases due to this type of precipitation is generally already included in global photochemical models (although the manner in which this is done varies widely between models). However, an additional redistribution can result from the gravitational settling of the second class, small cloud droplets or ice particles, whose mean fall speeds can reach 1 m/s. When we include this additional downward flux term in our model simulations, we find a significant impact on the distributions of the soluble trace gases HNO 3 and H 2 O 2 . In the upper troposphere, the settling of ice particles leads to a strong reduction in the mixing ratios of these gases, due to the relatively high terminal velocities of ice crystals (generally > 10 cm/s). The impact of cloud droplet settling in the lower troposphere is also found to be significant in some regions, though it is generally smaller due to the slower settling velocities of liquid droplets; in addition, the settling loss in the lower troposphere is computed to be frequently overwhelmed by the supply of HNO 3 and H 2 O 2 via sublimating hydrometeors settling in from above. Our findings may help explain the overestimate of free tropospheric HNO 3 found in several global photochemical modeling studies. The reductions computed for HNO 3 and H 2 O 2 also have an effect on the distributions of related trace gases, such as NO x and OH. Gravitational settling might also directly affect other soluble trace gases and aerosols. There are substantial uncertainties involved in computing the global impact of gravitational settling, such as the uptake of gases and aerosols onto and into ice particles and the high degree of variability and complexity of clouds. Results of sensitivity studies examining some of these uncertainties are presented. Even employing a low-end settling velocity for ice particles (5 cm/s) and a minimum ice uptake efficiency for HNO 3 based on laboratory data, we still compute a notable impact on the upper tropospheric HNO 3 and H 2 O 2 distributions. DOI: 10.1034/j.1600-0889.1998.t01-2-00005.x

Modification of the Köhler Equation to Include Soluble Trace Gases and Slightly Soluble Substances

Abstract A generalized reformulation of the Kohler theory to include the effect of soluble gases and slightly soluble aerosol substances is presented. A single equation is derived that takes into account 1) the Kelvin effect; 2) the Raoult effect caused by highly soluble aerosol material (salt); 3) increase in droplet radius due to an undissolved, insoluble, or slightly soluble core; 4) contribution of solute into the droplet by a slightly soluble substance; and 5) contribution of hygroscopic material into the droplet by a soluble trace gas allowed to deplete from the gas phase because of the condensational growth of the droplets (assuming a monodisperse size distribution). Model calculations are presented for a system in which the aerosol is composed of a slightly soluble CaSO4 core coated with ammonium sulfate, and the gas phase contains HNO3. It is shown that the resulting equilibrium curves allow the occurrence of stable, unactivated droplets with radii up to about 10 μm at realistic ambient condition...

The impact of cloud particle gravitational settling on soluble trace gas distributions

The effect of gravitational settling of cloud droplets and ice particles on the tropospheric distributions of soluble trace gases is examined, focusing in particular on nitric acid and hydrogen peroxide. For this study, we separate cloud condensate into two classes: large (rain, snow and graupel) and small (cloud water and cloud ice). The first class has typical fall speeds of several m/s and frequently reaches the ground. The redistribution and loss of soluble trace gases due to this type of precipitation is generally already included in global photochemical models (although the manner in which this is done varies widely between models). However, an additional redistribution can result from the gravitational settling of the second class, small cloud droplets or ice particles, whose mean fall speeds can reach 1 m/s. When we include this additional downward flux term in our model simulations, we find a significant impact on the distributions of the soluble trace gases HNO3 and H2O2. In the upper troposphere, the settling of ice particles leads to a strong reduction in the mixing ratios of these gases, due to the relatively high terminal velocities of ice crystals (generally > 10 cm/s). The impact of cloud droplet settling in the lower troposphere is also found to be significant in some regions, though it is generally smaller due to the slower settling velocities of liquid droplets; in addition, the settling loss in the lower troposphere is computed to be frequently overwhelmed by the supply of HNO3 and H2O2 via sublimating hydrometeors settling in from above. Our findings may help explain the overestimate of free tropospheric HNO3 found in several global photochemical modeling studies. The reductions computed for HNO3 and H2O2 also have an effect on the distributions of related trace gases, such as NOx and OH. Gravitational settling might also directly affect other soluble trace gases and aerosols. There are substantial uncertainties involved in computing the global impact of gravitational settling, such as the uptake of gases and aerosols onto and into ice particles and the high degree of variability and complexity of clouds. Results of sensitivity studies examining some of these uncertainties are presented. Even employing a low-end settling velocity for ice particles (5 cm/s) and a minimum ice uptake efficiency for HNO3 based on laboratory data, we still compute a notable impact on the upper tropospheric HNO3 and H2O2 distributions.

A generalized reformulation of the Köhler theory: effects of soluble trace gases and slightly soluble substances

A generalized reformulation of the Köhler theory: effects of soluble trace gases and slightly soluble substances

Deducing droplet concentration and supersaturation in marine boundary layer clouds from surface aerosol measurements

Air parcels in the marine boundary layer (MBL) are mixed up through nonprecipitating clouds at the top of the MBL many times (on average) before they can be removed by precipitation scavenging. The equivalent dry size of the particles (cloud condensation nuclei, CCN) upon which droplets are formed increases because of liquid phase oxidation of soluble trace gases during the cloud processing. The observed separation of the submicron size distribution into an interstitial mode and cloud droplet residue mode makes it possible to infer the effective MBL cloud supersaturation and cloud droplet concentrations from surface measurements of the aerosol size distribution during periods when nonprecipitating MBL clouds are present in the back trajectory and the MBL is well mixed. The effect of particle composition on the accuracy of the inferred cloud supersaturations is evaluated. A large database of hundreds of size distributions taken on an Atlantic and a Pacific cruise and an airship flight off the Oregon coast are used to calculate the range of effective MBL cloud supersaturations and droplet concentrations encountered during these expeditions. The inferred droplet concentrations on the Pacific cruise were mostly in the 25 to 150 cm−3 range, whereas they were mostly in the 50 to 500 cm−3 range for the Atlantic cruise. The inferred effective supersaturation in the tropical MBL clouds was typically in the 0.15% to 0.25% range. Recent work of Tang and Munkelwitz [1994] would indicate that particles consisting of mixtures of ammonium sulfate and sulfuric acid would not have recrystalized in the differential mobility analyzer (DMA) within the range of relative humidities (45% to 60%) at which the DMA was operated. At these humidities the hydrated size can be as much as 20% greater than the dry size. Corrections for the hydrated size within the DMA at the time of measurement are included and are also used to correct previous measurements of the relationship between dry size and critical supersaturation made using the Naval Research Laboratory (NRL) DMA and NRL thermal gradient CCN counters.

The Environmental Control of Individual Aqueous Particles in a Cubic Electrodynamic Levitation System

ABSTRACT The experimental investigation of aerosol particles often requires special apparatus to ensure that the walls of the system interfere minimally with the measurements. Electrodynamic levitation offers an attractive approach because the particle, when suitably charged, is held away from the system walls by time-dependent electric fields. A new electrodynamic levitation system has been developed expressly for investigating aqueous particles in controlled gaseous environments. A cubic cell was used in conjunction with the carefully controlled introduction of gas into the cell to form a wall-less flow reactor that is ideal for thermodynamic and kinetic studies involving reactive reagents. Unlike most previous cell configurations, the cubic geometry permits three-axis position control of the particle and an ability to measure the gravitational and drag forces simultaneously and independently when the gas flow is horizontally aligned. Preliminary results at room temperature of the mass gained by salt and sulfuric acid particles exposed to various humidities show the basic characteristics of the cell. New results with sulfuric acid particles exposed simultaneously to the vapors of water and nitric acid show that nonvolatile and volatile solutes cause aqueous particles to behave in qualitatively different ways. Such results confirm that cloud formation in the atmosphere should be enhanced by the presence of soluble trace gases under suitably low-temperature conditions.

A modeling study of the effects of ice-phase microphysical processes on trace chemical removal efficiencies

A simple cloud-chemistry model has been developed to study the effects of specific microphysical processes of precipitation formation and scavenging on the efficiency with which clouds remove trace chemicals form the atmosphere. The model is designed to let an isolated precipitation particle fall through the mixed-phase zone of a steady-state background cloud derived from the ascent of a Lagrangian parcel in which given mixing ratios of SO 2, CO 2 and NH 3 are in equilibrium with the aqueous phase. The precipitation particle is allowed to acquire water mass by both vapor deposition and accretion of supercooled cloud water and solute content by gaseous entrapment in rime ice, as parameterized from prior laboratory experiments. The concept of a relative removal efficiency has been employed to identify the important microphysical fractionation processes within the cloud. The model calculations show that the removal efficiencies of the chemical constituents in the cloud nuclei and soluble trace gases are controlled largely by the accretional growth process. Even with modest solubilities, a trace gas such as SO 2 is estimated to be removed much more efficiently than is water vapor via cold-cloud precipitation because of substantial entrapment of the gas during rime-ice formation.

Chemistry and physics of a winter stratus cloud layer: A case study

The chemical and physical properties of a supercooled stratus cloud layer and surrounding clear air covering southern Ontario on February 20, 1984, were studied with the objectives of identifying the processes responsible for the cloud water chemical composition. The cloud layer, which extended from circa 650 to 1050 m mean sea level, was bounded by a strong temperature inversion just at cloud top. The air below this inversion was well mixed vertically, as indicated by the near independence of concentrations of various trace gas species with altitude and by conservation of number concentration of particles from the surface to cloud top, with below‐cloud aerosol number concentrations (0.2 < d < 3 μm) approximately equal to the sum of the interstitial aerosol and cloud droplet (2 < d < 30 μm) number concentrations. Fractional incorporation of aerosol sulfate into cloud water, determined by comparison of interstitial and cloud water sulfate concentrations, was in the range of 85–90%, similar to the result inferred from comparison of interstitial aerosol and cloud droplet number concentrations. Gas phase equivalent concentrations of cloud water species were the same (within experimental error) as the concentrations of aerosol and soluble trace gas species in the below‐cloud air. These results suggest that the dominant processes determining cloud water composition in this cloud layer were nucleation scavenging of sulfate aerosol and scavenging of gaseous HNO3, with no significant contribution from reactive scavenging processes.