Cloudwater Deposition

Modeling studies (Lovett 1984 Lovett and Reiners 1986 Mueller 1990) have suggested that cloud droplet removal rate depends critically on characteristics of the forest canopy. Mueller (1990) calculated that cloudwater deposition at the edge of a forest was four to five times greater than in a closed forest. The cloud LWC and the size distribution of cloud droplets (especially the concentration of large droplets) also influence significantly the overall wet deposition rate. 

A combination of measurements and simulations have indicated that cloudwater sulfate, acidity, nitrate, and ammonium deposition exceeds wet deposition via precipitation for some NE U.S. sites located above 400 m. While the current large uncertainties that are associated with cloudwater deposition models make firm estimates impossible, the available data indicate that cloudwater deposition is a substantial contributor to wet deposition inputs to high elevation sites in the eastern United States. Similar conclusions have been reached by European studies. 

Cloud droplets are typically far more acidic than precipitation droplets collected at the ground. In essence, cloud drops are small and have not been subjected to the dilution associated with growth to the size of raindrops, snowflakes, and so on, nor the neutralization associated with the capture of surface-derived NH3 and alkaline particles held in layers at lower altitudes. Interception of these droplets therefore provides a route by which 

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Heath J. A. and Huebert B. J. (1999) Cloudwater deposition as a source of fixed nitrogen in a Hawaiian montane forest. Biogeochemistry 44, 119—134. 

SO4 in cloudwater Washout, rainout Dry deposition Cloud nucleation COS + OH multistep - HzS MSA - sol by some mechanism... 

Cloud droplet capture in the form of intercepted fog appears to be a seasonably important sink for pollutant emissions in the LA Basin. At Henninger Flats up to 50% of the total wet deposition of H", NOo", and 04" may be due to cloud interception low intensity springtime drizzle accounted for 20% of the deposition measured in precipitation. The intercepted cloudwater that deposited on pine... 

The first challenge concerns the involvement of multiple phases in wet deposition. Not only does one deal with the three usual phases (gas, aerosol, and aqueous), but the aqueous phase can be present in several forms (cloudwater, rain, snow, ice crystals, sleet, hail, etc.), all of which have a size resolution. To complicate matters even further, different processes operate inside a cloud, and others below it. Our goal will initially be to create a mathematical framework for this rather complicated picture. To simplify things as much as possible we consider a warm raining cloud without the complications of ice and snow. There are four media or phases present, namely, air, cloud droplets, aerosol particles, and rain droplets. A given species may exist in each of these phases for example, nitrate may exist in air as nitric acid vapor, dissolved in rain and cloud droplets as nitrate, and in various salts in the aerosol phase. Nonvolatile species like metals exist only in droplets and aerosols, while gases like HCHO exist only in the gas phase and the droplets. The size distribution of cloud droplets, rain droplets, and aerosols provides an additional complication. Let us initially neglect this feature. For a species i, one needs to describe mathematically its concentration in air C(,air, cloudwater C,[C 0ud, rainwater C .rain, and the aerosol phase Qpan- We assume that all concentrations are expressed as moles of i per volume of air (e.g., mol m 3 of air). These concentrations will be a function of the location (x,y,z) and time and can be described by the atmospheric diffusion equation... 

Waldman, J. M., J. W. Munger, D. L. Jacob and M. R. Hoffmann (1985) Chemical characterization of stratus cloudwater and its role as a vector for pollutant deposition in a Los Angeles pine forest. Tellus 37B, 91-108... 

Forms of precipitation other than rainfall may contain excess acidity. Acidic fog can be especially damaging because it is very penetrating. In early December 1982, Los Angeles experienced a severe, 2-day episode of acid fog. This fog consisted of a heavy concentration of acidic mist particles at ground level that reduced visibility and were very irritating to breathe. The pH of the water in these particles was 1.7, much lower than ever before recorded for acid precipitation. Another source of precipitation heavy in the ammonium, sulfate, and nitrate ions associated with atmospheric acid is acid rime. Rime is frozen cloudwater that may condense on snowflakes or exposed surfaces. Rime constitutes up to 60% of the snowpack in some mountainous areas, and the deposition of acidic constituents with rime may be a significant vector for the transfer of acidic atmospheric constituents to Earth s surface in some cases. 

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