Total Aerosol Surface Area Research Articles

Simulating Southern Ocean Aerosol and Ice Nucleating Particles in the Community Earth System Model Version 2

AbstractSouthern Ocean (SO) low‐level mixed phase clouds have been a long‐standing challenge for Earth system models to accurately represent. While improvements to the Community Earth System Model version 2 (CESM2) resulted in increased supercooled liquid in SO clouds and improved model radiative biases, simulated SO clouds in CESM2 now contain too little ice. Previous observational studies have indicated that marine particles are major contributor to SO low‐level cloud heterogeneous ice nucleation, a process that initiates a number of cloud processes that govern cloud radiative properties. In this study, we utilize detailed aerosol and ice nucleating particle (INP) measurements from two recent measurement campaigns to assess simulated aerosol abundance, number size distributions, and composition and INP parameterizations for use in CESM2. Our results indicate that CESM2 has a positive bias in simulated surface‐level total aerosol surface area at latitudes north of 58°S. Measured INP populations were dominated by marine INPs and we present evidence of refractory INPs present over the SO assumed here to be mineral dust INPs. Results highlight a critical need to assess simulated mineral dust number and size distributions in CESM2 in order to adequately represent SO INP populations and their response to long‐term changes in atmospheric transport patterns and land use change. We also discuss important cautions and limitations in applying a commonly used mineral dust INP parameterization to remote regions like the pristine SO.

Mechanism of SOA formation determines magnitude of radiative effects

Secondary organic aerosol (SOA) nearly always exists as an internal mixture, and the distribution of this mixture depends on the formation mechanism of SOA. A model is developed to examine the influence of using an internal mixing state based on the mechanism of formation and to estimate the radiative forcing of SOA in the future. For the present day, 66% of SOA is internally mixed with sulfate, while 34% is internally mixed with primary soot. Compared with using an external mixture, the direct effect of SOA is decreased due to the decrease in total aerosol surface area and the increase of absorption efficiency. Aerosol number concentrations are sharply reduced, and this is responsible for a large decrease in the cloud albedo effect. Internal mixing decreases the radiative effect of SOA by a factor of >4 compared with treating SOA as an external mixture. The future SOA burden increases by 24% due to CO2 increases and climate change, leading to a total (direct plus cloud albedo) radiative forcing of -0.05 W m-2 When the combined effects of changes in climate, anthropogenic emissions, and land use are included, the SOA forcing is -0.07 W m-2, even though the SOA burden only increases by 6.8%. This is caused by the substantial increase of SOA associated with sulfate in the Aitken mode. The Aitken mode increase contributes to the enhancement of first indirect radiative forcing, which dominates the total radiative forcing.

Study of the unknown HONO daytime source at a European suburban site during the MEGAPOLI summer and winter field campaigns

Abstract. Nitrous acid measurements were carried out during the MEGAPOLI summer and winter field campaigns at SIRTA observatory in Paris surroundings. Highly variable HONO levels were observed during the campaigns, ranging from 10 ppt to 500 ppt in summer and from 10 ppt to 1.7 ppb in winter. Significant HONO mixing ratios have also been measured during daytime hours, comprised between some tenth of ppt and 200 ppt for the summer campaign and between few ppt and 1 ppb for the winter campaign. Ancillary measurements, such as NOx, O3, photolysis frequencies, meteorological parameters (pressure, temperature, relative humidity, wind speed and wind direction), black carbon concentration, total aerosol surface area, boundary layer height and soil moisture, were conducted during both campaigns. In addition, for the summer period, OH radical measurements were made with a CIMS (Chemical Ionisation Mass Spectrometer). This large dataset has been used to investigate the HONO budget in a suburban environment. To do so, calculations of HONO concentrations using PhotoStationary State (PSS) approach have been performed, for daytime hours. The comparison of these calculations with measured HONO concentrations revealed an underestimation of the calculations making evident a missing source term for both campaigns. This unknown HONO source exhibits a bell-shaped like average diurnal profile with a maximum around noon of approximately 0.7 ppb h−1 and 0.25 ppb h−1, during summer and winter respectively. This source is the main HONO source during daytime hours for both campaigns. In both cases, this source shows a slight positive correlation with J(NO2) and the product between J(NO2) and soil moisture. This original approach had, thus, indicated that this missing source is photolytic and might be heterogeneous occurring at ground surface and involving water content available on the ground.

Modeling the atmospheric transport and outflow of polycyclic aromatic hydrocarbons emitted from China

An Euler atmospheric transport model CanMETOP (Canadian Model for Environmental Transport of Organochlorine Pesticides) was applied to the atmospheric transport and outflow of polycyclic aromatic hydrocarbons (PAHs) in China in 2003 based on a square kilometer resolution emission inventory. The reaction with OH radical, gas/particle partition by considering the adsorption onto total aerosol surface area, and dynamic soil/ocean–air exchange of PAHs were also considered. The results show that the spatial distribution of PAH concentration levels in the atmosphere is greatly controlled by emission and meteorological conditions. Elevated concentration levels are predicted in Shanxi, Guizhou, North China Plain, Sichuan Basin and Chongqing metropolitan areas due to the high emission densities at those locations. High concentrations are also modeled in environments offshore of China and in the western Pacific Ocean. The model also predicts a slightly decreasing vertical profile in the planetary boundary layer (lower than ∼1 km), but concentration decreases ∼2 orders of magnitude in the free atmosphere. The Westerlies as well as the East Asian Monsoon and local topographical forcings are identified as key factors influencing the transport pattern of PAHs in China. In 2003, ∼3800°tons of the sixteen parent PAHs listed on USEPA priority control list were transported out of China with about 80% transported through the eastern boundary. The outflow concentrates near 30°N, signifying a slight discrepancy from the position of emission density peaks. The center of the outflow plume is located at a height of ∼1 km at 120°E, and climbs to 3.5 km and 5 km at 130°E and 140°E, respectively. A seasonal variation of 5–6 fold is also found for the outflow flux with greatly elevated transport flux in spring and winter. ► Use a state-of-the-art Euler numerical model to study the transport of PAHs. ► The total outflow flux of PAHs was estimated to be 3800 tons per year. ► The high spatial resolution revealed transport patterns at smaller scales.

Prediction of daytime variations of HO2 radical concentrations in the marine boundary layer using BP network

A Back-Propagation Neural Network (BPNN) was established to predict the daytime variations of HO2 radical concentration observed in the field campaign RISFEX 2003 (RIShiri Fall Experiment 2003) conducted in September 2003 at Rishiri Island (45.07 N, 141.12 E, and 35m asl) in the Sea of Japan. The initial weight matrices and bias vectors for the network were optimized by a bee evolutionary genetic algorithm (BEGA). It was found that the input variables sensitive to HO2 variation were photolysis frequency of O3 to O(1D) (J(O1D)), a composite parameter defined as the ratio of HC to NOx reactivity towards OH radicals (Φ), and the total aerosol surface area (A). The predicted results are closely correlated with the experimental data with the coefficient of determination (R2) close to 1. In addition, the means and ranges of the predicted HO2 concentration agree with the observed data with the correlation coefficient (R), the index of agreement (IA) and the fractional bias (FB) in the range of 0.84–0.93, 0.88–0.95 and −14%–7%, respectively. This study demonstrates that BPNN is a potential tool to predict the daytime variations of HO2 radical concentrations in the marine boundary layer (MBL).

Impact of the Knudsen number and mass-transfer expression on multi-phase kinetic modeling

Three different mass-transfer expressions are employed within the Model of Aerosol, Gas, and Interfacial Chemistry (MAGIC) to study gas-phase molecular chlorine and bromine production from NaCl and NaBr aerosols, respectively. Simulations of chamber experiments are performed in which NaCl aerosols react with gas-phase ozone in the presence of UV light, in order to identify the importance of the Knudsen number and mass-transfer expression in systems with varying contributions from gas-phase, aqueous-phase, and interfacial chemistry. In the case of NaBr aerosols, simulations are performed of both dark and photolytic conditions. A range of Knudsen numbers spanning the continuum, transition and free-molecular regimes is studied. Particle size is varied over three orders of magnitude, and particle concentration is changed to keep either (a) total aerosol volume or (b) total aerosol surface area constant. When total aerosol volume is constant, the total amount of surface area available for interfacial reaction increases linearly with Knudsen number. Consequently peak gas-phase Cl 2 and Br 2 concentrations increase by two orders of magnitude from the continuum regime to the free-molecular regime. When total aerosol surface area is constant, total aerosol volume is inversely proportional to Knudsen number, with lesser volume being available at higher Knudsen numbers. Consequently Cl − depletion in the kinetic regime leads to most gas-phase Cl 2 being produced in the transition regime. Gas-phase Br 2 concentration trends are determined by aqueous-phase reaction mechanisms, leading to a monotonic decrease in production with Knudsen number. At all Knudsen numbers, more gas-phase bromine is produced in the photolytic case than in the dark case, the difference being significant in the transition regime. Results of this study suggest that halogen production is insensitive to the mass-transfer expression used in the simulations.

Secondary aerosol formation in continental outflow conditions during ACE‐Asia

Ground‐based aerosol size distribution measurements at the Gosan, Korea, sampling site during ACE‐Asia showed occasionally elevated concentrations of nucleation mode aerosol particles, with subsequent growth to the Aitken mode. Similar results from aircraft and ship‐based measurements at around the same time indicated that at least one event exhibited a broad spatial extent. One of the most pronounced events, with total number concentrations of particles as high as 105 cm−3, occurred on 12 April at the same time as a large dust event, when the total aerosol surface area of particles with diameters less than 10 μm was increase by a factor of 1.7 compared to the average value for the study. Aircraft data showed layers of enlarged SO2 concentration coinciding with an increased aerosol total number concentration, with the highest detected SO2 concentration being 12 ppbv and aerosol number concentration reaching 8 × 104 cm−3 at 800 m altitude. Aircraft data showed that newly formed particles and dust were stratified in different layers in marine environment and transported to the surface close to the island. Atmospheric thermodynamic data suggested that particles were formed above the marine boundary layer and transported by vertical mixing toward the surface as a result of a 3 K north‐to‐south increase in the sea surface temperature. This temperature increase changed atmosphere stratification from stable to neutral/slightly unstable and deepened the internal mixed layer from below 300 m to above 800 m altitude. Measured hygroscopic properties of nucleation mode aerosol particles were consistent with those of ammonium sulfate particles. Here we reported a novel case study of tropospheric aerosol formation identifying particle chemical composition of nucleation mode aerosols in real time by measuring their water uptake properties. Then we demonstrated that particles formed in a layer aloft and were transported to the ground site by growth of the mixed boundary layer while nucleation was chemically induced and did not require turbulent mixing.

Aerosol size distributions measured at the South Pole during ISCAT

Abstract Aerosol physical size distributions were measured at the South Pole during December 1998 and December 2000 as part of the ISCAT program. The size ranges covered by these measurements were 3 to 250 nm in 1998 and 3 nm–2 μm in 2000. “Typical background aerosols” measured during both periods were similar. Total aerosol number concentrations ranged from 100 to 300 cm−3 with occasional spikes as high as 10,000 cm−3. We believe the spikes were due to local emissions. The number mean size of background aerosols ranged from 50 to 70 nm, and total aerosol surface area concentrations were 2.8±0.4 μm2 cm−3. Aerosols measured in December 2000 were cleanly separated into “low volume” and “high volume” periods. During the low-volume periods, volume concentrations were 0.07±0.01 μm3cm−3 with a volume mean diameter of 0.27±0.05 μm, and these volume concentrations were mostly within a factor two of values that would be expected based on reconstructed mass from particulate chemical composition. Volume concentrations during the “high volume” periods exceed levels that can be explained from aerosol chemistry and calculated light-scattering coefficients exceed values that have been recorded historically. We have been unable to identify why this might have occurred. We observed one 4-h event on December 15, 2000 during which nanoparticles grew slowly from ∼3.0 to 3.6 nm.We believe these particles had recently been formed by nucleation. Because this occurred during a period of stagnation, it is possible that this event was associated with local emissions.

Uptake of reactive nitrogen on cirrus cloud particles during INCA

The uptake of reactive nitrogen species on ice crystals in cirrus clouds was investigated by simultaneous aircraft‐based measurements of gas‐ and condensed‐phase NOy, ice particle size distribution and total aerosol surface area. The measurements were performed in 2000 during the INCA campaign at northern and southern midlatitudes at local autumn. Cirrus ice particles were frequently encountered during these flights in the upper troposphere. Concurrently with the occurrence of cirrus ice particles particulate NOy was observed. It was found that the coverage of the cirrus ice particles with particulate NOy strongly depends on the temperature and the gas‐phase NOy concentration. A substantial coverage of the ice particles with values larger than one percent of a monolayer was observed only for temperatures below about 217 K. On average only about one percent of the available gas‐phase NOy was found as particulate NOy.

Influence of the source formulation on modeling the atmospheric global distribution of sea salt aerosol

Three different sea salt generation functions are investigated for use in global three‐dimensional atmospheric models. Complementary observational data are used to validate an annual simulation of the whole size range (film, jet, and spume droplet derived particles). Aerosol concentrations are corrected for humidity growth and sampler inlet characteristics. Data from the North American deposition network are corrected for mineral dust to derive sea salt wet fluxes. We find that sea salt transport to inner continental areas requires substantial mass in the jet droplet range, which is best reproduced with the source of Monahan et al. [1986]. The results from this source formulation also shows the best agreement with aerosol concentration seasonality and sea salt size distributions below 4 μm dry radius. Measured wind speed dependence of coarse particle occurrence suggests that above 4 μ the source from Smith and Harrison [1998] is most appropriate. Such sea salt simulations are relevant for assessing heterogeneous chemistry and radiative effects. Sea salt aerosol provides on an annual average, in marine regions, an aggregate surface area equal to 1–10% of the area of the underlying Earth's surface. Together with mineral dust, sulfate, and carbonaceous aerosol the total aerosol surface area globally amounts to 13% of that of the Earth's surface. On the basis of atmospheric column burdens, sea salt represents 21% of the total global aerosol surface area. Equal partitioning of the aerosol surface area among the four components suggests that one has to consider all of them if the global aerosol impact is to be fully determined.

The Great Dun Fell Experiment 1995: an overview

During March and April of 1995 a major international field project was conducted at the UMIST field station site on Great Dun Fell in Cumbria, Northern England. The hill cap cloud which frequently envelopes this site was used as a natural flow through reactor to examine the sensitivity of the cloud microphysics to the aerosol entering the cloud and also to investigate the effects of the cloud in changing the aerosol size distribution, chemical composition and associated optical properties. To investigate these processes, detailed measurements of the cloud water chemistry (including the chemistry of sulphur compounds, organic and inorganic oxidised nitrogen and ammonia), cloud microphysics and properties of the aerosol and trace gas concentrations upwind and downwind of the cap cloud were undertaken. It was found that the cloud droplet number was generally strongly correlated to aerosol number concentration, with up to 2000 activated droplets cm −3 being observed in the most polluted conditions. In such conditions it was inferred that hygroscopic organic compounds were important in the activation process. Often, the size distribution of the aerosol was substantially modified by the cloud processing, largely due to the aqueous phase oxidation of S(IV) to sulphate by hydrogen peroxide, but also through the uptake and fixing of gas phase nitric acid as nitrate, increasing the calculated optical scattering of the aerosol substantially (by up to 24%). New particle formation was also observed in the ultrafine aerosol mode (at about 5 nm) downwind of the cap cloud, particularly in conditions of low total aerosol surface area and in the presence of ammonia and HCl gases. This was seen to occur at night as well as during the day via a mechanism which is not yet understood. The implications of these results for parameterising aerosol growth in Global Climate Models are explored.

Is aerosol production within the remote marine boundary layer sufficient to maintain observed concentrations?

To evaluate the impact of aerosols on climate we must consider the aerosol dynamics of the remote marine atmosphere. Marine aerosols are subject to losses due to precipitation, dry deposition, and coagulation; yet, observed remote marine aerosol concentrations and size distributions are relatively constant. This maintenance of the aerosol distribution requires a particle source. This work focuses on the potential of H2SO4 nucleation within the marine boundary layer (MBL) to supply these particles. Spatial and temporal variability in meteorology and species concentrations are considered in a mathematical model to evaluate the effect of natural deviations from average MBL conditions on the highly nonlinear aerosol system. A dynamic, vertically dimensioned, size‐resolved aerosol model is used with parameterized heterogeneous chemical processes. The results suggest that MBL nucleation may be an important source of new particle number in the remote MBL. However, though our model shows that typical remote MBL aerosol distributions can on average be maintained by MBL nucleation and sea‐salt emissions, large oscillations in particle number concentration occur. Because such oscillations are only occasionally reported in measurements, MBL nucleation may not be the dominant source of new particles in the remote MBL. The nucleation events, which cause these oscillations, are predicted to occur at the top of the MBL after rain and/or entrainment of clean free tropospheric air. Predictions are particularly sensitive to the H2SO4 accommodation coefficient, nucleation tuner, and washout efficiency. Reduction of the accommodation coefficient is shown to increase the predicted accumulation mode concentration because the nucleation rate is enhanced. Entrainment of clean free tropospheric air is shown to increase the frequency of nucleation events within the MBL and may help to explain the observed correlation between subsidence and MBL small particles. A small constant addition of particles from the free troposphere, ocean, etc. suppresses H2SO4 nucleation and can lead to a reduction in total predicted aerosol number. This is because nucleation events require low total aerosol surface area and the constant addition of particles reduces the severity of aerosol surface area minimums. Larger external aerosol sources can maintain the observed remote MBL aerosol distribution. However, the temporal and spatial variability of such sources could have a large impact on aerosol concentrations and requires further investigation.

Particle nucleation in the tropical boundary layer and its coupling to marine sulfur sources

New particle formation in a tropical marine boundary layer setting was characterized during NASA's Pacific Exploratory Mission-Tropics A program. It represents the clearest demonstration to date of aerosol nucleation and growth being linked to the natural marine sulfur cycle. This conclusion was based on real-time observations of dimethylsulfide, sulfur dioxide, sulfuric acid (gas), hydroxide, ozone, temperature, relative humidity, aerosol size and number distribution, and total aerosol surface area. Classic binary nucleation theory predicts no nucleation under the observed marine boundary layer conditions.

First gaseous ion composition measurements in the exhaust plume of a jet aircraft in flight: Implications for gaseous sulfuric acid, aerosols, and chemiions

Mass spectrometric composition measurements of gaseous negative ions have been made in the exhaust plume of a commercial jet aircraft (Airbus A310) in flight at altitudes around 10.4 km and at two plume ages around 3.0 and 3.6 s. Negative ions observed inside the exhaust plume are mostly NO3−(HNO3)m and HSO4−(HNO3)m with m ≤2. Outside the plume in the “background” atmosphere the same negative ion species with the same R = (HSO4−(HNO3)m)/(NO3−(HNO3)m) were observed. This indicates that the ions observed in the plume were entrained ambient atmospheric ions. By contrast no indications for negative chemiions (with masses ≤1100 amu) produced by the airbus engines were found in the plume. Furthermore our measurements indicate a modest decrease of the total concentration of entrained negative ions in the plume compared to the ambient atmosphere outside the plume. This decrease may be due to ion‐removal by ion‐attachment to aerosol‐particles and/or ion‐recombination with positive chemiions. We propose that the observed entrained ions can serve as probes for important plume components including gaseous sulfuric acid, aerosol particles and chemiions. Making use of this analytical potential we infer upper limits for the gaseous sulfuric acid concentration, total aerosol surface area density, and positive chemiion concentration. We conclude that initially formed gaseous sulfuric acid must have experienced rapid gas‐to‐particle conversion already in the very early plume at plume ages < 1.6 s.

Sulfuric acid and soot particle formation in aircraft exhaust

Size analyses of Ames wire impactor aircraft exhaust aerosol samples resulted in a continuous particle size distribution between 0.01 µm and 1.0 µm particle radius. The two orders of magnitude size range covered by the measurements correspond to 6–7 orders of magnitude particle concentration. The data determined a nucleation mode, composed of aircraft‐emitted sulfuric acid aerosol, that contributed between 62% and 85% to the total aerosol surface area and between 31% and 34% to its volume. Soot aerosol contributed 0.3% to the surface area and 0.07% to the volume of sulfuric acid aerosol. Emission indices were: EIH2SO4=0.09 g/kgFUEL and (0.2‐0.5) g/kgFUEL (for 75 ppmm and 675 ppmm fuel‐S, respectively), 1.7E‐4<EISOOT<1.3E‐3 g/kgFUEL, and EIN>0.01=1.7E14 and (7.7E14 and 5.2E15) particles/kgFUEL (for 75 and 675 ppmm fuel‐S, respectively). The sulfur (gas) to sulfuric acid (particle) conversion efficiency was between 10% and 37%.

New particle formation at a remote continental site: Assessing the contributions of SO2 and organic precursors

Ultrafine aerosols, with diameters less than 10 nm, nucleate from gas phase species. The composition of newly formed ultrafine atmospheric aerosols is not known with certainty; new particles have variously been conjectured to be sulfates, organic compounds, and sulfate/organic mixtures. The 1993 Tropospheric OH Photochemistry Experiment at Idaho Hill, Colorado, provided an opportunity to examine the question of which class of compounds, i.e., sulfates or organics, make the major contribution to new particle formation in the unpolluted troposphere. This study compared the production rates of sulfuric acid (from the oxidation of sulfur dioxide) and oxidized organic compounds to gauge their relative contributions to the formation of ultrafine particles. Potential organic precursor species examined in this study were the naturally occurring terpenes α– and β pinene, and the anthropogenic hydrocarbons toluene, m‐xylene, ethyl benzene, 1,2,4 trimethyl benzene, and methylcyclohexane. The calculated production of oxidized organics appeared well correlated with total particle surface area and volume, suggesting that at least some of the organic compounds formed in gas phase reactions condensed upon the preexisting aerosol. New particle formation was found to be more highly associated with elevated production of gas phase sulfuric acid (via the SO2‐OH reaction) than with production of oxidized organic products, although data from one day, during which sulfuric acid production and total aerosol surface area were both lower than usual, provided evidence for the involvement of terpene species in new particle formation. The results suggest that for this continental site, sulfuric acid was probably responsible for most of the observed new ultrafine particle formation. Low‐volatility organic compounds may have caused particle formation under the right conditions, but were more likely to condense upon preexisting particles.

Measurement of Expected Nucleation Precursor Species and 3–500-nm Diameter Particles at Mauna Loa Observatory, Hawaii

Abstract Atmospheric measurements of expected homogeneous nucleation precursors and aerosols were made at the Mauna Loa Observatory, Hawaii, from 28 June to 27 July 1992. Large molecular clusters and gas phase species including sulfuric acid (H2SO4), methane sulfonic acid (MSA), and OH were measured with a mass spectrometer. Aerosol measurements consisted of ultrafine particle concentrations (∼3–4 nm diameter) and fine particle size distributions (15–500 nm diameter). The altitude of the observatory (average pressure 680 mb) and the presence of mountain–valley winds permitted sampling of both up- and downslope air. Downslope air was found to have the highest concentrations of MSA but low ultrafine concentrations, whereas, upslope air had the highest H2SO4 and ultrafine concentrations. Episodes of substantial increase in ultrafine particles were observed during periods of rapid increase in H2SO4 concentrations. Total aerosol surface area and H2SO4 concentrations had the strongest influence on ultrafine par...

Further evidence for particle nucleation in clear air adjacent to marine cumulus clouds

Observational evidence is presented for the nucleation of condensation nuclei (CN) in the clear air adjacent to an isolated, marine, cumulus cloud. Two separate regions of particle nucleation are identified: one located above the cloud top, and the second located downwind of the cloud near the level of the anvil outflow. The regions of high CN concentrations were located in extremely clean marine air, with unactivated aerosol surface area (excluding the nucleation mode) <2 μm2 cm−3, air temperature −31°C, and higher relative humidities than the undisturbed environment. Vertical profile measurements downwind of the cloud showed that CN concentrations at the level of the anvil outflow (4.9 km) were 8 times greater than at any other level between the surface and 5.3 km. A conceptual model is formulated in which aerosol particles, sulfur dioxide (SO2), sulfuric acid vapor (H2SO4), dimethyl sulfide (DMS), and ozone (O3) from the boundary layer are entrained into the cumulus cloud. Total aerosol number concentrations and unactivated aerosol surface area decrease with height in the cloud due to Brownian diffusion and diffusiophoresis of cloud interstitial aerosol to hydrometeors, coalescence scavenging by cloud droplets, collisional scavenging by ice particles, and subsequent removal by precipitation. The air that is detrained from the cloud raises the relative humidity and vents the clean air, SO2, H2SO4, DMS, and O3 to the near‐cloud environment. Hydroxyl radicals then oxidize the SO2 and DMS to H2SO4. Under the conditions of high relative humidity, low total aerosol surface area, low temperatures, and high SO2 concentrations near cloud top, significant concentrations of new particles can be produced by homogeneous‐bimolecular nucleation of sulfuric acid solution droplets from H2SO4 and H2O vapor molecules. The concentration of CN as a function of time is calculated for the case described in this paper using a bimodal integral nucleation model. The model results show that significant numbers of CN could have been produced within a few hours by the homogeneous‐bimolecular nucleation of sulfuric acid solution droplets under the observed conditions provided the concentration of SO2 near cloud top was enhanced by vertical transport.

Antarctic stratospheric ice crystals

Ice crystals were replicated over the Palmer Peninsula at approximately 72°S on six occasions during the 1987 Airborne Antarctic Ozone Experiment. The sampling altitude was between 12.5 and 18.5 km (45–65 thousand ft pressure altitude) with the temperature between 190 and 201 K. The atmosphere was subsaturated with respect to ice in all cases. The collected crystals were predominantly solid and hollow columns. The largest crystals were sampled at lower altitudes where the potential temperature was below 400 K. While the crystals were larger than anticipated, their low concentration results in a total surface area that is less than one tenth of the total aerosol surface area. The large ice crystals may play an important role in the observed stratospheric dehydration processes through sedimentation. Evidence of scavenging of submicron particles further suggests that the ice crystals may be effective in the removal of stratospheric chemicals.