Cloud chemistry, liquid water content

Laboratory simulations of aqueous-phase chemical systems are necessary to 1) verify reaction mechanisms and 2) assign a value and an uncertainty to transformation rates. A dynamic cloud chemistry simulation chamber has been characterized to obtain these rates and their uncertainties. Initial experimental results exhibited large uncertainties, with a 26% variability in cloud liquid water as the major contributor to measurement uncertainty. Uncertainties in transformation rates were as high as factor of ten. Standard operating procedures and computer control of the simulation chamber decreased the variability in the observed liquid water content, experiment duration and final temperature from 0.65 to 0.10 g nr3, 180 to 5.3 s and 1.73 to 0.27°C respectively. The consequences of this improved control over the experimental variables with respect to cloud chemistry were tested for the aqueous transformation of SO2 using a cloud-physics and chemistry model of this system. These results were compared to measurements made prior to the institution of standard operating procedures and computer control to quantify the reduction in reaction rate uncertainty resulting from those controls. 

The cloud chemistry simulation chamber (5,6) provides a controlled environment to simulate the ascent of a humid parcel of polluted air in the atmosphere. The cloud forms as the pressure and temperature of the moist air decreases. By controlling the physical conditions influencing cloud growth (i.e. initial temperature, relative humidity, cooling rate), and the size, composition, and concentration of suspended particles, chemical transformation rates of gases and particles to dissolved ions in the cloud water can be measured. These rates can be compared with those derived from physical/chemical models (7,9) which involve variables such as liquid water content, solute concentration, the gas/liquid interface, mass transfer, chemical equilibrium, temperature, and pressure. 

The characterization of the factors which control the accuracy, precision, and validity of measurements made in a simulation facility for studying in-cloud chemical processes was described. An analysis of a large number of experimental data collected under widely varying conditions was performed. Cloud liquid water content, an observable principally dependent on cooling rate and reaction time, was found to be the most influential of the physical factors controlling the resultant chemistry. In order to precisely control and reproduce the physical conditions in the simulation facility, standard operating procedures and computer control were instituted. This method reduced the uncertainty of the SO2 to sulfate transformation rate by a factor of 4.4. 

Several areas in which chemical measurement technologies have become available and/or refined for airborne applications have been reviewed in this paper. It is a selective review and many important meteorological and cloud physics measurement capabilities of relevance to atmospheric chemistry and acid deposition (e.g., measurement of cloud liquid water content) have been ignored. In particular, we have not discussed particle size spectra measurements for various atmospheric condensed phases (aerosols, cloud droplets and precipitation). Further improvements in chemical measurement technologies can be anticipated especially in the areas of free radicals, oxidants, organics, and S02 and N02 at very low levels. Nevertheless, major incremental improvements in the understanding of acid deposition processes can be anticipated from the continuing airborne application of the techniques described in this review. 

As a first step in assessing the potential importance of nanoparticle reactions, we compare the volume and surface areas of these particles with the same values from other condensed phases with known chemical effects. We first consider nanoparticle volumes. As an upper limit, we consider an urban air parcel containing 20-nm diameter nanoparticles at a number concentration of 10 cm. Under this scenario, the nanoparticle volume is a small fraction (10 of the total air parcel volume. Thus the nanoparticle reaction rate (in units of mol m -air s ) would have to be ca. 10 times as fast as the equivalent gas phase reaction (mol m -air s ) to have a comparable overall rate in the air parcel. For comparison, clouds typically have liquid water contents of 10 to 10 (volume fraction) and can have significant effects upon atmospheric chemistry (Seinfeld and Pandis 1998). For simplicity of argument, if the medium of the cloud droplets and nanoparticles are assumed similar (e.g., dilute aqueous), then the fundamental rate constants in each medium are similar. Under this condition, reactant concentrations in the nanoparticles would need to be enhanced by 10, as compared to the cloud droplets, to have equal rates. Based on this analysis, it appears unlikely that reactions occurring in the bulk of nanoparticles could affect the composition of the gas phase. 

Liquid water is one of the most important cloud characteristics for atmospheric chemistry. The liquid water content of an air parcel starts from almost zero during cloud formation, reaches a maximum for a mature cloud, and returns to zero during cloud evaporation. 

Only a very small percentage of all the water in the climate system is actually present in the atmosphere (Table 2.12). Of the atmospheric water, most is in the vapor phase (Fig. 2.37) the liquid water content (LWC) of clouds is only in the order 1 g the cloud ice water content (IWC) still less, down to 0.0001 g m . But clouds play a huge role in the climate system, whereas precipitation closes the cycle for water and also for substances dissolved in it (wet deposition). Some of the processes (droplet formation, transfer processes, deposition, and chemistry) will be described later. The aim of this chapter is to describe the phenomenology of water in the atmosphere so far as we need it for an understanding of chemical processes. 

FIGURE 10 Aqueous phase concentrations (nmol dm ) of several species in a marine cloud (liquid water content 0.3 g/m ). Fluxes to and from the gas phase are given in molecule cm s , fluxes in the aqueous phase are gas-phase equivalents in the same units. Orders of magnitude are shown in parentheses. [From Zellner, R., ed. (1999). Global Aspects of Atmospheric Chemistry, Steinkopff/Springer, Darmstadt, Germany.]... 

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