Temporal and Vertical Distribution of Acidity and Ionic Composition in Clouds: Comparison between Modeling Results and Observations

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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.

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