Clouds, chemistry

Several instniments have been developed for measuring kinetics at temperatures below that of liquid nitrogen [81]. Liquid helium cooled drift tubes and ion traps have been employed, but this apparatus is of limited use since most gases freeze at temperatures below about 80 K. Molecules can be maintained in the gas phase at low temperatures in a free jet expansion. The CRESU apparatus (acronym for the French translation of reaction kinetics at supersonic conditions) uses a Laval nozzle expansion to obtain temperatures of 8-160 K. The merged ion beam and molecular beam apparatus are described above. These teclmiques have provided important infonnation on reactions pertinent to interstellar-cloud chemistry as well as the temperature dependence of reactions in a regime not otherwise accessible. In particular, infonnation on ion-molecule collision rates as a ftmction of temperature has proven valuable m refining theoretical calculations. 

When NMHC are significant in concentration, differences in their oxidation mechanisms such as how the NMHC chemistry was parameterized, details of R02-/R02 recombination (95), and heterogenous chemistry also contribute to differences in computed [HO ]. Recently, the sensitivity of [HO ] to non-methane hydrocarbon oxidation was studied in the context of the remote marine boundary-layer (156). It was concluded that differences in radical-radical recombination mechanisms (R02 /R02 ) can cause significant differences in computed [HO ] in regions of low NO and NMHC levels. The effect of cloud chemistry in the troposphere has also recently been studied (151,180). The rapid aqueous-phase breakdown of formaldehyde in the presence of clouds reduces the source of HOj due to RIO. In addition, the dissolution in clouds of a NO reservoir (N2O5) at night reduces the formation of HO and CH2O due to R6-RIO and R13. Predictions for HO and HO2 concentrations with cloud chemistry considered compared to predictions without cloud chemistry are 10-40% lower for HO and 10-45% lower for HO2. 

The estimate of the distance must now consider the estimate of the interstellar extinction Av, best estimated by the reddening Av can take several values and in calculations of molecular cloud chemistry typical values are of order 1 but may be as much as 5. The distance calculation in Equation 5.1 can be significantly perturbed so that an A v of 2.4 can reduce the apparent distance by a factor of 3. 

There are some advantages of the temporal models of cloud chemistry associated with the concentrations of molecules at different times. Can we learn about the age of the cloud by its chemical composition or the age of an embedded star by the chemistry observed towards the object Can the molecular environment be understood from the inventory of chemicals Are there chemical diagnostics for planetary formation, star formation or even black holes All of these questions are at the frontier of Astrochemistry. 

Since 1990, Ramanathan has been Victor C.AIderson Professor of Applied Ocean Sciences and Professor of Climate and Atmospheric Sciences at the Scripps Institution of Oceanography in San Diego. In 1991 he was appointed director of the Center for Clouds, Chemistry, and Climate at Scripps, and in 1996, he became director of the Center for Atmospheric Sciences at the institution. 

Liang, J., and D. J. Jacob, Effect of Aqueous Phase Cloud Chemistry on Tropospheric Ozone, J. Geophys. Res., 102, 5993-6001 (1997). 

Crutzen, P. J., and V. Ramanathan, Clouds, Chemistry and Climate, NATO ASI Series, Series I, Vol. 35, Springer-Verlag, Berlin, 1996. 

Now that we have seen how methane can be formed from atomic constituents, let us consider how more complex hydrocarbons can be produced. We note at the outset that methane is efficiently synthesized from C+ and C, both of which are more abundant at early stages of the cloud chemistry than at steady state, at which time most of the carbon is in the form of CO. It should not be surprising therefore that, as shall be discussed below, the calculated abundances of methane and species formed from methane are found to peak at times well before steady state conditions are achieved. 

The FREZCHEM model can be used to simulate what would happen theoretically to cloud droplets and their chemistries as they are lofted (convected) to higher (colder) altitudes. We used two aqueous datasets to simulate atmospheric chemistries. The first dataset consist of mean annual concentrations of ions in precipitation from the Hubbard Brook ecosystem (1.0 km altitude) (table 4 in Likens et al. 1977). The second dataset is from Mt. Sonnblick, Austria (3.1km altitude) and is a direct measure of cloud chemistry (table II, May 1991 in Brantner et al. 1994). In both cases, the chemistries are similar in relative concentrations with H+, NHj, Na+, Cl-, NOg and SO4- as the dominant ions. 

It is clear from the information presented above that liquid phase oxidation of reduced sulfur compounds play an important role in natural processes in water and in wastewater treatments. However, further work is needed to clarify the role of these compounds in cloud chemistry and precipitation acidity. Areas of further investigations should include ... 

Henry s law coefficients and effective Henry s law coefficients for gases of concern in cloud chemistry are summarized in Figure 3. 

As noted below, recent experimental studies have yielded measurements of this important quantity for systems of interest in cloud chemistry. 

Nitrogen Oxide Reactions. Examination of possible aqueous-phase reactions of nitrogen dioxide and peroxyacetyl nitrate has revealed no reactions of importance to cloud chemistry (21,22). This situation is a consequence of the low solubilities and/or low reactivities of these gases with substances expected to be present in cloudwater, although studies with actual precipitation samples would be valuable in confirming this supposition. NO2 has been shown (23) to react with dissolved S(IV), but the details of the mechanism and rate of this reaction remain to be elucidated. 

These inferences from field measurements provide support for the applicability of evaluations of cloud chemistry based upon laboratory studies. 

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. 

Describe a cloud chemistry simulation facility to emulate atmospheric aqueous phase interactions among gases, particles, and liquid water droplets. 

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. 

A functional representation of the cloud chemistry simulated chamber is shown in Figure 1. The chamber is constructed of a cylindrical aluminum inner shell 1.8 m in diameter, 2.5 m in height with dome-shaped ends that provide a total volume of 6.6 m. The... 

The cloud chemistry simulation chamber is a complicated measurement system which cannot be totally controlled, and it is necessary to... 

The main question when assessing uncertainties in the cloud chemistry simulation results is What system parameters have the greatest influence on the observed chemistry Uncertainties in previously observed transformation rates of SO2 to sulfate with this facility are as large as a factor of ten (5). If we assume the first order rate of transformation of SO2 to SO4, Rs02 t0 be... 

In contrast to the dark cloud chemistry, the molecules in circumstellar envelopes (IRC -f 10216) seem to be created continuously in a small, high temperature high density layer- which allow fast thermodynamic equilibrium- and subsequently expelled into the lower density cool envelope. There they are observed with an... 

However, the present discussion pertains to dark cloud chemistry. The experimental interstellar observations clearly indicate that the distribution of carbon chain molecules is correlated, and that the column densities of the longer chain members decreases about linearly with increasing chain length. Several mechanisms have been proposed for the chain building. For cool dark clouds Churchwell et al. (1978) and in further detail Walmsley et al. (1979) have proposed a formation scheme by which the longer chain molecules are formed via the acetylene backbone reaction ... 

Schwartz S. E. and Slingo A. (1996) Enhanced shortwave cloud radiative forcing due to anthropogenic aerosols. In Clouds, Chemistry, and Climate—Proceedings of NATO Advanced Research Workshop (eds. P. Crutzen and V. Ramanathan). Springer, Heidelberg, pp. 191-236. 

The cosmic ray ionization driving dark cloud chemistry could be different. The production and energy distribution of cosmic rays are thought to depend on a high-energy event, such as a supernova or accretion onto a black hole. How fortuitous... 

Deister, U., Neeb, R., Helas, G., Warneck, P. "The Equilibrium CFt)0H)S03-+ H2O in Aqueous Solution Temperature Dependence and Importance in Cloud Chemistry," J. Phys. Chem., 1986, in press. 

Chamber Simulations of Cloud Chemistry The AIDA Chamber... 

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