Azadeh Tabazadeh

Distribution and fate of selected oxygenated organic species in the troposphere and lower stratosphere over the Atlantic

A large number of oxygenated organic chemicals (peroxyacyl nitrates, alkyl nitrates, acetone, formaldehyde, methanol, methylhydroperoxide, acetic acid and formic acid) were measured during the 1997 Subsonic Assessment (SASS) Ozone and Nitrogen Oxide Experiment (SONEX) airborne field campaign over the Atlantic. In this paper, we present a first picture of the distribution of these oxygenated organic chemicals (Ox-organic) in the troposphere and the lower stratosphere, and assess their source and sink relationships. In both the troposphere and the lower stratosphere, the total atmospheric abundance of these oxygenated species (ΣOx-organic) nearly equals that of total nonmethane hydrocarbons (ΣNMHC), which have been traditionally measured. A sizable fraction of the reactive nitrogen (10–30%) is present in its oxygenated organic form. The organic reactive nitrogen fraction is dominated by peroxyacetyl nitrate (PAN), with alkyl nitrates and peroxypropionyl nitrate (PPN) accounting for <5% of total NOy. Comparison of observations with the predictions of the Harvard three-dimensional global model suggests that in many key areas (e.g., formaldehyde and peroxides) substantial differences between measurements and theory are present and must be resolved. In the case of CH3OH, there appears to be a large mismatch between atmospheric concentrations and estimated sources, indicating the presence of major unknown removal processes. Instrument intercomparisons as well as disagreements between observations and model predictions are used to identify needed improvements in key areas. The atmospheric chemistry and sources of this group of chemicals is poorly understood even though their fate is intricately linked with upper tropospheric NOx and HOx cycles.

Simulating equilibrium within aerosols and nonequilibrium between gases and aerosols

A numerical method to solve chemical equilibrium equations and a method of coupling the equilibrium calculations to nonequilibrium growth and evaporation are discussed. The equilibrium program solves any number of equations for gas, aqueous, ionic, and solid equilibrium concentrations over large spatial grids and particle size grids. It also simultaneously computes electrolyte mean mixed activity coefficients and aerosol liquid water content. Mean mixed activity coefficient calculations require mean binary activity coefficient information. Temperature-dependent mean binary activity coefficient polynomials were constructed using mean binary activity coefficient data at 298 K, apparent molal enthalpy data, and apparent molal heat capacity data. The equilibrium solver is mole and charge conserving, requires iteration, but always converges. Solutions to the equilibrium equations are used for two purposes. The first is to estimate surface vapor pressures over particles containing a solution and/or a solid phase. Such vapor pressures are then applied in gas-aerosol transfer equations. The second is to estimate intraparticle composition and size immediately after gas-aerosol transfer.

Ice nucleation processes in upper tropospheric wave‐clouds observed during SUCCESS

We have compared in situ measurements near the leading-edges of wave-clouds observed during the SUCCESS experiment with numerical simulations. Observations of high supersaturations with respect to ice (>50%) near the leading edge of a very cold wave cloud (T <−60°C) are approximately consistent with recent theoretical and laboratory studies suggesting that large supersaturations are required to homogeneously freeze sulfate aerosols. Also, the peak ice crystal number densities observed in this cloud (about 4 cm−3) are consistent with the number densities calculated in our model. In the warmer wave-cloud (T ≃−37°C) relatively large ice number densities were observed (20–40 cm−3). Our model calculations suggest that these large number densities are probably caused by activation of sulfate aerosols into liquid droplets followed by subsequent homogeneous freezing. If moderate numbers of effective heterogeneous freezing nuclei (0.5–1 cm−3) had been present in either of these clouds, then the number densities of ice crystals and the peak relative humidities should have been lower than the observed values.

A conceptual model of the dehydration of air due to freeze-drying by optically thin, laminar cirrus rising slowly across the tropical tropopause

In this study, we use a cloud model to simulate dehydration which occurs due to formation of optically thin, laminar cirrus as air rises slowly across the tropopause. The slow ascent and adiabatic cooling, which balances the radiative heating near the tropopause, drives nucleation of a very small number of ice crystals (<1 L−1). These crystals grow rapidly and sediment out within a few hours. The clouds never become optically thick enough to be visible from the ground. The ice crystal nucleation and growth prevents the relative humidity with respect to ice (RHI) from rising more than a few percent above the threshold for ice nucleation (RHInuc ≃ 110–160%, depending upon the aerosol composition); hence, laminar cirrus can limit the mixing ratio of water vapor entering the stratosphere. However, the ice number densities are too low and their sedimentation is too rapid to allow dehydration of the air from RHInuc down to saturation (RHI = 100%). The net result is that air crosses the tropopause with water vapor mixing ratios about 1.1 to 1.6 times the ice saturation mixing ratio corresponding to the tropopause temperature, depending on the threshold of ice nucleation on aerosols in the tropopause region. If the cross-tropopause ascent rate is larger than that calculated to balance radiative heating (0.2 cm s−1), then larger ice crystal number densities are generated, and more effective dehydration is possible (assuming a fixed temperature). The water vapor mixing ratio entering the stratosphere decreases with increasing ascent rate (approaching the tropopause ice saturation mixing ratio) until the vertical wind speed exceeds the ice crystal terminal velocity (about 10 cm s−1). More effective dehydration can also be provided by temperature oscillations associated with wave motions. The water vapor mixing ratio entering the stratosphere is essentially controlled by the tropopause temperature at the coldest point in the wave. Hence, the efficiency of dehydration at the tropopause depends upon both the effectiveness of upper tropospheric aerosols as ice nuclei and the occurrence of wave motions in the tropopause region. In situ humidity observations from tropical aircraft campaigns and balloon launches over the past several years have provided a few examples of ice-supersaturated air near the tropopause. However, given the scarcity of data and the uncertainties in water vapor measurements, we lack definitive evidence that air entering the stratosphere is supersaturated with respect to ice.

A new parameterization of H2SO4/H2O aerosol composition: Atmospheric implications

Recent results from a thermodynamic model of aqueous sulfuric acid are used to derive a new parameterization for the variation of sulfuric acid aerosol composition with temperature and relative humidity. This formulation is valid for relative humidities above 1% in the temperature range of 185 to 260 K. An expression for calculating the vapor pressure of supercooled liquid water, consistent with the sulfuric acid model, is also presented. We show that the Steele and Hamill [1981] formulation underestimates the water partial pressure over aqueous H2SO4 solutions by up to 12% at low temperatures. This difference results in a corresponding underestimate of the H2SO4 concentration in the aerosol by about 6% of the weight percent at approximately 190 K. In addition, the relation commonly used for estimating the vapor pressure of H2O over supercooled liquid water differs by up to 10% from our derived expression. The combined error can result in a 20% underestimation of water activity over a H2SO4 solution droplet in the stratosphere, which has implications for the parameterization of heterogeneous reaction rates in stratospheric sulfuric acid aerosols. The influence of aerosol composition on the rate of homogeneous ice nucleation from a H2SO4 solution droplet is also discussed. This parameterization can also be used for homogeneous gas phase nucleation calculations of H2SO4 solution droplets under various environmental conditions such as in aircraft exhaust or in volcanic plumes.

Nitric acid scavenging by mineral and biomass burning aerosols

The abundance of gas phase nitric acid in the upper troposphere is overestimated by global chemistry-transport models, especially during the spring and summer seasons. Recent aircraft data obtained over the central US show that mineral aerosols were abundant in the upper troposphere during spring. Chemical reactions on mineral dust may provide an important sink for nitric acid. In regions where the mineral dust abundance is low in the upper troposphere similar HNO3 removal processes may occur on biomass burning aerosols. We propose that mineral and biomass burning aerosols may provide an important global sink for gas phase nitric acid, particularly during spring and summer when aerosol composition in the upper troposphere may be greatly affected by dust storms from east Asia or tropical biomass burning plumes.

Analysis of lidar observations of Arctic polar stratospheric clouds during January 1989

We present analyses of lidar backscatter and depolarization ratios for polar stratospheric clouds (PSCs) observed during the 1989 Airborne Arctic Stratospheric Experiment. The backscatter and depolarization ratios are available at one visible and one infrared wavelength. Water ice PSCs were identified at low ambient temperatures based upon their relatively large back-scattering and depolarization ratios. The remaining clouds fall into four major categories. First, we observe a class of clouds that are not depolarizing at either of the two wavelengths. These clouds are identified as Type Ib PSCs, which are assumed to be composed of ternary solutions of H2SO4/HNO3/H2O. Type Ib clouds were never dominant, though on some dates they accounted for 25 to 40% of the observations. We find from the wavelength dependence of the backscattering by these clouds that their size distributions must be very narrow. Other optical observations of these clouds should consider the possible impact of these narrow size distributions on their data analysis. These clouds have a relatively large total particulate mass that is comparable to the known gas phase reservoir of nitric acid. The number density of Type Ib particles is similar to the concentrations of the ambient sulfate aerosols. The second category of clouds is highly depolarizing at both lidar wavelengths, but has relatively low backscattering ratios. We identify these clouds as Type 1a PSCs (assumed to be nitric acid tri- or dihydrate) which form only on a small subset by number, approximately 1% or less, of the ambient sulfate aerosols. These clouds were the first to be observed, and were especially common on the first few flights. They accounted for more than 70% of the observations on three flights. Type Ia particles are near 1 μm or larger in radius. The third type of cloud is depolarizing at visible wavelengths, but not at near infrared wavelengths. These clouds were seen during portions of nearly every flight and comprised 10 to 25% of all of the observations. These clouds, which we refer to as Type 1c, are composed of small, solid particles. These clouds contain a relatively large mass of material, comparable to the gas phase reservoir of nitric acid. Type 1c particles are a few tenths of a micrometer in radius and have a concentration that is similar to that of the ambient sulfate aerosols. The final class of clouds has no depolarization at the lidars visible wavelength but has significant depolarization at the infrared wavelength. Such particles were seen on many of the flights and sometimes accounted for as much as 30% of all the observations. We interpret these clouds as being mixtures of Type 1a and 1b PSCs. Although some polar stratospheric clouds have fairly homogeneous properties over very large spatial scales, many have variable properties at relatively small scales. Thus the various types of particles are often observed within a single cloud. Homogeneous clouds composed only of solid particles seem inconsistent with a wave cloud origin. However, clouds in which several types of particles occur together may represent situations in which mountain waves have triggered solid particle formation. Alternatively, clouds composed of different types of particles may be in the process of freezing. We observed in regions upwind of the coldest temperatures that the air often did not contain clouds but may have had large supersaturations with respect to NAT. In contrast, regions downwind of the coldest temperatures often contained large solid particles in air that may have been close to NAT supersaturation. The correlation between PSC types and their respective temperature histories suggest that, given enough time, liquid particles will convert to solids.

Oxygenated volatile organic chemicals in the oceans: Inferences and implications based on atmospheric observations and air-sea exchange models

(10 � 9 mol L � 1 ) and 2 nM and net fluxes of 1.1 � 10 � 12 g cm � 2 s � 1 and 0.4 � 10 � 12 gc m � 2 s � 1 are calculated for acetaldehyde and propanal, respectively. Large surface seawater concentrations are also estimated for methanol (100 nM) and acetone (10 nM) corresponding to an undersaturation of 6% and 14%, and a deposition velocity of 0.08 cm s � 1 and 0.10 cm s � 1 , respectively. These data imply a large oceanic source for acetaldehyde and propanal, and a modest sink for methanol and acetone. Assuming a 50–100 meter mixed layer, an extremely large oceanic reservoir of OVOC, exceeding the atmospheric reservoir by an order of magnitude, can be inferred to be present. Available seawater data are both preliminary and extremely limited but indicate rather low bulk OVOC concentrations and provide no support for the existence of a large oceanic reservoir. We speculate on the causes and implications of these findings. INDEX TERMS: 0312 Atmospheric Composition and Structure: Air/sea constituent fluxes (3339, 4504); 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry. Citation: Singh, H. B., A. Tabazadeh, M. J. Evans, B. D. Field, D. J. Jacob, G. Sachse, J. H. Crawford, R. Shetter, and W. H. Brune, Oxygenated volatile organic chemicals in the oceans: Inferences and implications based on atmospheric observations and air-sea exchange models, Geophys. Res. Lett., 30(16), 1862, doi:10.1029/ 2003GL017933, 2003.

The presence of metastable HNO3/H2O solid phases in the stratosphere inferred from ER 2 data

We present data to show that at least some of the observed polar stratospheric cloud (PSC) particles sampled by the ER 2 on January 20, 1989, during the Airborne Arctic Stratospheric Expedition (AASE 1) were composed of a water-rich HNO3/H2O solid phase. The PSC water content derived from the ER 2 data on this day is larger than that of nitric acid trihydrate (NAT), nitric acid dihydrate (NAD) or an aqueous ternary solution of H2SO4/HNO3/H2O. Here, we suggest that these particles were composed of a water-rich metastable HNO3/H2O solid phase and refer to such clouds as Type 1c PSCs, which are different from Type 1a (crystalline NAT or NAD particles) or Type 1b (aqueous ternary solution droplets) PSCs. Type 1c PSCs could either be crystalline (a higher hydrate of HNO3 and H2O) or amorphous, and the data analysis presented here cannot distinguish between these different solid phases. The observed PSC on this day could have been a mixture of water-rich HNO3-containing solid particles with liquid ternary droplets and/or NAT (NAD) aerosols. However, a mixture of cloud particles composed of ternary solution droplets and NAT (NAD) aerosols is inconsistent with the data. A surface adsorption/reaction process occurring on frozen sulfate surfaces is suggested as a formation mechanism for Type 1c PSCs in the stratosphere. A possible mechanism for the formation of large HNO3-containing aerosols (NAT or NAD) starting with Type 1c PSCs is discussed.

A model for heterogeneous chemical processes on the surfaces of ice and nitric acid trihydrate particles

A model is developed that incorporates the physics and physical chemistry of ice surfaces relevant to polar stratospheric clouds. The Langmuir and Brunauer, Emmett, and Teller (BET) adsorption isotherms are used to compute surface concentrations of H2O, HCl, HOCl, ClONO2 and N2O5 on ice and nitric acid trihydrate (NAT) crystals. Assuming pseudo-first-order kinetics with respect to adsorbed HOCl, ClONO2 and N2O5, surface reaction rates and reaction probabilities (sticking coefficients) are determined. The model parameters (surface morphology and energies) are extracted from measured uptake coefficients and reaction probabilities. For gas pressures of about 10−7 torr and temperatures in the range of 180–200 K, HCl completely coats ice and water-rich NAT surfaces, while HOCl, ClONO2 and N2O5 may cover 0.01–1% of these surfaces. The model is applied to analyze laboratory data, leading to estimates of adsorption free energies, enthalpies and entropies for HCl, HOCl, ClONO2 and N2O5 on ice and NAT surfaces, and activation energies for the heterogeneous reactions of HCl and H2O with HOCl, ClONO2 and N2O5 on these surfaces. The energy parameters are used to calculate surface parameters such as adsorption and desorption constants, surface coverages, reaction rate coefficients, surface diffusion coefficients and reaction probabilities for varies species and chemical interactions on ice and NAT surfaces. Implications for chemical processing on polar stratospheric clouds are discussed.