Research Projects

Project at a Glance

Project Status: complete

Title: Mathematical modeling of cloud chemistry in the California South Coast Air Basin 1988.

Principal Investigator / Author(s): Seigneur, Christian

Contractor: Bechtel Environmental, Inc. and Sonoma Technology, Inc.

Contract Number: A732-042


Research Program Area: Atmospheric Processes

Topic Areas: Acid Deposition, Modeling


Abstract:

A mathematical model of cloud chemistry was evaluated with the cloud chemistry data collected by Richards et al. from 1981 to 1985 in stratus clouds in the Los Angeles basin. The model simulates atmospheric chemistry including gas-phase chemistry, aqueous-phase chemistry and gas / droplet mass transfer, vertical transport, rainfall and turbulent diffusion.

Evaluation of the model entailed evaluation of the equilibrium chemistry. chemical kinetics and transport processes. The equilibrium chemistry was evaluated with a total of 52 case studies. Comparison of measured and predicted hydrogen ion concentrations showed. on average. an underprediction of 3% and an absolute error of 20%. The equilibrium chemistry of sulfur (IV) suggests that there was sulfur (IV) present as hydroxymethane sulfonic acid. There was good agreement between measured and calculated gas/liquid distributions of formaldehyde. However. there was a large discrepancy between the measured and calculated gas/liquid distributions of acetaldehyde suggesting that the aqueous-phase acetaldehyde concentrations are underestimated by about a factor of 40. The chemical kinetics was evaluated with five case studies for which cloud chemistry data w~re collected in the same air parcel at different times. covering periods of up to three hours. The results showed that the model underestimated nitrate formation in three cases and gave good agreement with the data in two cases. Concentrations of sulfur dioxide were too low to allow evaluation of the model for sulfate. These results suggested that vertical transport was a major process governing the chemical composition of stratus clouds and needs to be included in a cloud chemistry model. The model with treatment of cloud chemistry. vertical transport and diffusion was evaluated with two case studies. For one case study. which corresponded to an air parcel moving from offshore toward Los Angeles. there were discrepancies between measured and calculated concentrations of major species. This result suggests that transport processes may be more complex than the one dimensional transport simulated by the model. For the other case study, which corresponded to an air parcel moving over the eastern Los Angeles basin. there was very good agreement between measured and calculated concentrations of major species. This result suggests that the chemistry updraft and rainfall were reasonably well simulated with the model.

A total of 19 sensitivity analysis simulations was performed with this latter case study. The sensitivity of sulfate. nitrate. ammonium and hydrogen ion concentrations to percursor levels. various chemical kinetic and cloud microphysical parameters and various environmental conditions were discussed.

A mechanism for the chemistry of organic acids was developed and applied to this case study.


 

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