The foci of studies on chemistry, aerosols, and dynamics interactions are to examine the effect of physical and dynamical processes on chemical species and to study the effect of chemistry on aerosols and cloud condensation nuclei. Ongoing projects within the program include studies on relating chemically reactive species concentrations and fluctuations to the species lifetimes, studies of the effects of boundary layer processes on chemical species distributions, cloud chemistry process studies, and prediction of chemical constituents on the cloud scale to mesoscale.

A Model for Relating Reactive Species Concentrations and Fluctuations to Lifetimes

Related website: http://www.mmm.ucar.edu/individual/lenschow/

Lenschow and David Gurarie (Case Western Reserve University) have developed a simple one-dimensional global model to predict mean vertical structure and fluctuations in trace gas concentrations, as a function of species lifetime in the atmosphere. A novel aspect is parameterization of transport across the top of the boundary layer, and across the tropopause, by an entrainment velocity. The three-layer analytical model is applied to species with surface sources that have lifetimes on the order of days to years, and generally compares well with observations from several long-range aircraft field deployments. A relation of this type is useful for estimating lifetimes of trace gases in the atmosphere; or conversely, if the lifetimes are known, average entrainment rates in the measurement region. The model also predicts a relationship among the surface emission, mean concentration, and lifetime, so that, given any two of these quantities, the third can be estimated.
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Effect of boundary layer processes on chemical species distributions

To understand why turbulence-induced segregation occurs between isoprene and hydroxyl radical (a reaction that plays an important role in ozone production) when using a simple chemical mechanism, while segregation does not occur with a more complex mechanism, Mary Barth (NCAR/MMM/ACD) and Edward Patton (Pennsylvania State University/MMM) have continued analyzing and documenting the results from several LES and box model simulations. Products of the isoprene oxidation mechanism (e.g., organic peroxy radicals such as methylvinylketone, methacrolein, and formaldehyde) that are not represented in the simple chemical mechanism play an important role in determining NOx levels. This, in turn, affects hydroxyl radical concentrations.

Barth, Patton, and Moeng plan to use the LES code that represents both clouds and chemistry to study the chemical transport and transformation in the environment of continental fair weather cumulus. This work will begin with examining the influence of cloud chemistry (both aqueous-phase chemistry and gas-phase chemistry altered by the separation of soluble and insoluble species). The influence of clouds upon chemical constituents in the boundary layer can be studied further by examining the importance of cloud-modified photolysis frequencies, and by examining the influence of cloud shading which reduces isoprene emissions and, therefore, isoprene concentrations in the CBL.

Cloud Chemistry Process Studies

Barth, Sanford Sillman (University of Michigan), Rynda Hudman (Harvard University), Mark Jacobson (Stanford University), Cheol-Hee Kim (National Institute of Environmental Research, Korea), Anne Monod (Laboratoire Chimie et Environnement, France), and Jinyou Liang (California Air Resources Board) performed an in-depth analysis of the results from the cloud chemistry photochemical box model intercomparison, which showed good agreement among models that are being used in the community. Because parcels of air usually flow in and out of cloud in a matter of minutes, the investigators examined whether the chemical species were affected by the manner in which cloud was introduced (either continuously, or intermittently). Formaldehyde and formic acid concentrations (Figure 48) were affected because of the timing of the formaldehyde production during clear-sky intervals and its destruction during cloudy intervals. The analysis also revealed that the time step used for the chemistry calculations should be a multiple of the cloud time step, otherwise deviations of the results from more accurate solutions occur.

Figure 48. Total (gas + aqueous phase) concentration of (a) ozone, (c) formaldehyde, and (d) formic acid as a function of time for the intermittent cloudy and continuous cloudy simulations for the Barth-Gear (black), Kim-VODE (green), Barth-EBI (red) at ? = 5 min, and Sillman-EB (blue) models.

Cloudscale and Mesoscale Prediction of Chemical Constituents

Barth performed an analysis of cloudscale numerical simulations of the 10 July 1996 Stratosphere-Troposphere Experiments, Radiation, Aerosols and Ozone (STERAO)-Deep Convection experiment. She investigated the relative importance of the production and destruction of the soluble and reactive species, hydrogen peroxide and formaldehyde, both of which play an important role in the net production of ozone. Hydrogen peroxide also is the key oxidant in converting sulfur dioxide to sulfate. Results from the analysis show that depletion of hydrogen peroxide in the cloud drops is offset by chemical production in the gas phase, thus creating very little effect on total hydrogen peroxide concentrations. Gas-phase chemistry controlled formaldehyde concentrations in the lower regions of the convective cloud, but in the upper regions of the convection, aqueous-phase chemistry destroyed formaldehyde while gas-phase chemistry produced it. This resulted in no effect in formaldehyde concentrations for this region.

The Weather Research and Forecasting (WRF) model must possess the capability of simulating the interactions between dynamics, radiation, and chemistry for applications in air quality prediction and abatement strategies, and for the planning, forecasting, and interpretation of research field campaigns. For the past two years investigators have been planning the development of WRF model components necessary for predicting chemical constituents and aerosols. Barth and Skamarock assisted in the development of a template, currently being created by NOAA/FSL scientists, for what is now known as the WRF-chem model. With the development of this template, the time has come to advance the WRF-chem model to a research quality tool.