Abstract

An analysis of a 3-yr database (1986–88) acquired new Mount Mitchell (35°44′05″N, 82°17′15″W, 2038 m MSL) where the forest consists primarily of Fraser fir and some red spruce stands is presented. The site was immersed in clouds for 28%–41% of the time during each of the three growing seasons (15 May–15 September). This study only investigated extreme episodes of wet (cloud-water pH% ≤.3:1)and dry (eg., an ozone concentration ≥ 70 ppb) acidic deposition. Extreme wet events occasionally relieved periods of high ozone (≥ 70 ppb) exposures during the final field intensive. Extreme wet and dry events could activate the decline mechanism in any above cloud-base forest, especially if the trees are exposed to such events during very early or very late stages of their Lives. The exposure of the forest to natural climatic stress, such as drought condition wintertime temperatures during the growing season, snow storm during early spring, etc., would also subject the forest to a stressful period during which the exposure to the aforementioned episodes of pollutant deposition might trigger a decline.

On the average, one of three cloud events that traverse this site is extreme. These extreme events usually last about 4 h. form during periods of high atmospheric pressure, have a liquid water content of 0.10 g m⁻³, and contain cloud droplets of mean diameter around 8.0 μm. During the dissipating stages, such cloud events result in maximum acidic deposition. When such events are preceded by very high ozone (≥ 100 ppb), they may prove oven more detrimental to forest health. A precipitating cloud event (pH = 4.4 on the average) preceded by periods of very high ozone concentrations will become an extreme episode. Extreme acidic events can occur in association with 1) an 850-mb closed low, situated just north of Montreal, Canada, that advances southward into New York State, and 2) an 850-mb high extending over the Gulf of Mexico (between Florida and Louisiana) to over eastern Kansas. In contrast, an 850-mb low over the Indiana-Illinois-Kentucky border seems to be associated with nonextreme cloud events as it sets up a southwesterly-southeasterly flow into the site. Similarly, high pressure systems located north of the site (which travel from northern Ohio into Indiana and southward), and low pressure systems (which move between southern Quebec north of Lake Ontario and the New York State–Canadian border) are conducive to measurements of ozone exceeding 100 ppb at our site. Extreme cloud events may be characterized into four unique temporal pH patterns, each with a subpattern that includes rain. These patterns are found to be related to the physical characteristics of the sampled clouds. and to the synoptic and local meteorological conditions associated with them. A relationship between liquid water content and pH appears to exist for these events and seems to be dependent on the dynamics of their formation.

Abstract

The effect of transient supersaturations on the concentration of cloud condensation nuclei (CCN) in a conventional parallel-plate type thermal diffusion chamber is experimentally investigated by controlling the relative humidity of the sample and the temperature of the top and bottom plates individually. It is found that the error in CCN counts is quite significant around 90% relative humidity and might be dominant at cloud bases where the relative humidity approaches 100%. The error is a combined result of thermal and hydrodynamical transients. It is suggested that a chamber having a continuous and laminar inflow of the sample which has the steady-state temperature profile already existing in it is but suited for reliable CCN measurements. The calculations based on the theory of Saxena, Burford and Kassner help explain the obtained results which also support the earlier theoretical predictions of Fitzgerald. The results seem to disagree with recent experimental observations reported by Radke and Hegg.

Abstract

A new cloud condensation nuclei (CCN) spectrometer capable of simulating the desired range of cloud supersaturation in a single chamber is developed. It can rapidly and continuously measure and display the spatial and temporal distributions of the CCN activity spectrum with high resolution and is suitable for airborne, field and laboratory studies. Details of the spectrometer design for producing a range of supersaturations in the sample air flow, controlled size of activated droplets for detection, and of mechanisms for continuous sampling, scanning and recording are presented. The dependency of the plateau in number concentration of the activated CCN on the sample air flow rate and the detection limit are analyzed. The droplet counting mechanism is compared with the direct photographic method and is found in reasonable agreement. The usefulness of the spectrometer is demonstrated by detecting the simulated smoke plume in the laboratory in real time. Measurements on an urban aerosol indicate a strong scavenging effect of rain and drizzle on CCN concentrations and a marked diurnal characteristic change in the slope of the CCN activation spectrum.

Abstract

A kinetic analysis of the fog formation process due to the contact of warm, moist air with a cold wall is presented. The analysis reveals that a wave in the nominal saturation ratio forms at and spreads away from the wall. The threshold conditions under which the maximum saturation ratio at the wave head becomes unity are derived. Comparison is made between the thermodynamic treatment of air mass mixing and the kinetic treatment. It is shown that the difference in diffusivities of vapor and temperature is responsible for the discrepancy of the two treatments. Results are applied to the problems of air sample processing, to avoid transient supersaturations, in cold chambers. The necessity of considering the kinetic effects of molecular diffusion in advection fog formation over the ocean is demonstrated.

Abstract

Cyclic temporal variations of pH and ionic concentration in sampled clouds which traversed the Mt. Mitchell State Park site (35°44′05″N, 82°17′15″W, 2006 m MSL) during the summers of 1986, 1987 and 1988 are reported. These clouds typically had a measured pH minimum during their initial and final stages. The cause of this basic cyclic pattern is attributed to sampling at different vertical levels of the cloud. This is substantiated by visual observations made while sampling. Our results also suggest that the measured pH patterns do not always exhibit minima during the formative and dissipative stages of the cloud, apparently in response to the underlying dynamical processes. The relationship between temporal pH measurements made at a stationary site to vertical cloud levels provides insights into the physical processes (e.g., nucleation scavenging near cloudbase, dry air entrainment near cloudtop) influencing the observed cloudwater chemistry on a real-time basis and would improve cloud chemistry models. The determination of the vertical profiles of acidity and of the position within a cloud is feasible but sometimes impractical. Hence, due to its relatively more cost effectiveness and versatility, a simple cloud chemistry model which explicitly estimated the vertical acidity profile of a cloud was sought to simulate the temporal acidity measurements at our site. A few of our event-averaged cloudwater pH values are compared with the results of such a cloud chemistry model and the two are found in excellent agreement. For the cloud event of 19 August 1987, the model predicted an average pH value of 3.03 compared with the average measured value of 3.09. The results of such comparisons between the observed and computed pH are discussed, and it is pointed out that the model could be used on an operational basis to predict the pH of cloudwater that would impact upon the montane forest. Such estimates could be strategically used to improve the long-term forest health either by protecting the new plant growth from such acidic deposition, or by cloud deacidification through cloud seeding.

Abstract

Variations in the chemical composition of cloud droplets of different sizes are predicted in models. Measurements made in natural clouds to verify this prediction are extremely limited, however. During the spring of 1995 and the summers of 1995 and 1996, a size-fractionating version of the California Institute of Technology active-strand cloud water collector was operated on a mountaintop platform in Mount Mitchell State Park, North Carolina (35°44′05"N, 82°17′15"W), to examine differences in drop chemistry between large and small cloud droplets. The size-fractionated measurements also were compared with the chemical composition collected from a passive string–type collector that collects bulk samples. Back-trajectory analysis was used to categorize the source of cloud-forming air masses that arrived at the site as polluted continental, continental, and marine. The differences in cloud drop acidity and chemical constituents were investigated for these different air masses. On average, smaller drops were more enriched in SO₄ ²⁻ , NO₃ ⁻ , NH₄ ⁺ , and H⁺; larger droplets had higher values of Na⁺, Ca²⁺, and Mg²⁺. Samples were collected for which the reverse was true, however. In this study, cloud droplet chemical inhomogeneity between droplet sizes and the effect of airmass origin on variations in the chemical composition were examined. Smaller droplets were found to be more acidic than were larger droplets for both marine and polluted continental air masses. The sodium content was the highest in the larger drops for marine events. The sulfate content in both the larger and smaller droplets was the highest for air masses that were from the polluted continental sector. Slightly higher solute concentrations for the larger droplet size range were found for events caused by orographic lifting; for cloud events influenced by frontal activity, higher solute concentrations were found for the smaller drop size range.

Abstract

In recent years the thermal diffusion chamber has found wide application in studying condensation nuclei (CCN) which are effective in natural cloud and fog formation. Explicit solution of the equations governing the transient behavior of the chamber suggests the necessity for precise control of the temperature or the relative humidity of the incoming sample if meaningful measurements are to be obtained. A recent conclusion of Fitzgerald, that transient supersaturations exceeding the steady-state peak value may arise if the incoming sample is saturated at a temperature less than that of the (hot) top plate, is also verified. Further, it is pointed out that as long as turbulence occurs while introducing the sample, the transient behavior of the chamber remains indeterminate. This defect may be eliminated by giving special consideration to the method of sample introduction. The measurements of CCN concentration as a function of time at Rolla taken with a thermal diffusion chamber are also presented. The CCN concentration, in general, is found to follow local meteorological conditions. The trend of our data shows a qualitative agreement with that available in the literature. It is suggested that an intercomparison of the data taken on a carefully fabricated thermal diffusion chamber with those taken on the other types of cloud chamber would help to decide the potential value of the former as a CCN counter.