BRIDGING THE GAP BETWEEN ATMOSPHERIC PHYSICS AND CHEMISTRY IN STUDIES OF SMALL-SCALE TURBULENCE BY JORDI
VILÀ-GUERAU
DE
ARELLANO
Experimental evidence for the role played by small-scale atmospheric processes in chemical reactions is necessary to verify the theoretical and numerical findings.
T
he articles inspired by the workshops Observations, Experiments and Large-Eddy Simulation (Stevens and Lenschow 2001) and Future Directions for Research on Meter- and Submeter-Scale Atmospheric Turbulence (Muschinski and Lenschow 2001) have indicated yet again that, in future studies of the small-scale turbulent flows, there is a need for greater collaboration between the disciplines of atmospheric physics and chemistry. Although, in their introduction, Muschinski and Lenschow (2001) mention the possible importance of these scales for the performance of second-order chemical transformations, they do not elaborate on which research
AFFILIATION: VILÀ-GUERAU
DE ARELLANO—Department of Meteorology and Air Quality, Wageningen University, Wageningen, Netherlands CORRESPONDING AUTHOR: Jordi Vilà-Guerau de Arellano, Dept. of Meteorology and Air Quality, Wageningen University, Duivendaal 2, 6701 AP Wageningen, Netherlands E-mail:
[email protected] DOI: 10.1175/BAMS-84-1-51
In final form 3 September 2002 © 2003 American Meteorological Society
AMERICAN METEOROLOGICAL SOCIETY
questions still need to be addressed if we are to gain a better understanding of reactive scalars in the atmospheric boundary layer (ABL). My intention is to extend the debate initiated during these two workshops to the study of reactive species in the ABL. In this research essay, I explain that it should be possible to gain greater insight into the role played by the atmospheric processes in the turbulent reacting flows by combining observations with simulation studies. Investigation in this area is necessary because, by omitting the effect of small-scale atmospheric processes on the chemical transformations, we are likely to reduce the accuracy of predictions of reactant concentrations like ozone and hydroxyl radical. Furthermore, I attempt to summarize briefly the current state of our knowledge and propose a comprehensive field experiment that will complement the experiments proposed by Muschinski and Lenschow (2001). My aim is to find observational evidence that will corroborate the theoretical findings and the results of the numerical simulations. THEORETICAL BASIS. Since the seminal papers by Donaldson and Hilst (1972) and Fitzjarrald and Lenschow (1983), the role that certain physical proJANUARY 2003
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cesses, and in particular turbulence, plays in the transformation of reactants has become clearer. Atmospheric turbulence is the process that brings together the reactants and it is the main mixing mechanism in clear and cloudy boundary layers. Following these pioneering studies, other authors (Brost et al. 1988; Kramm et al. 1991; Hamba 1993; Gao and Wesely 1994; Galmarini et al. 1997a,b; Verver et al. 1997; Petersen et al. 1999) have investigated various ABL reacting flows in order to find out under which conditions and for which parameters chemistry is limited by turbulence. In their research, they studied the influence of turbulence on chemistry by including the chemical terms in the governing equations. Their main conclusions are that under specific turbulent mixing and chemical conditions, the distribution and evolution of the average concentration (first-order moment) and the fluxes and covariances (secondorder moments) can be modified by chemical transformation. As a consequence, the behavior of reactants can differ from the behavior observed and modeled for inert scalars. More specifically, turbulence limits the reactivity of species that are transported, mixed, and transformed at similar timescales. A well-studied example of this limitation is the decrease that occurs in the reaction rate of the depletion of ozone as a result of the reaction with nitric oxide in an atmospheric convective boundary layer. Another physical process that changes the reactivity is the disturbance of the transfer of ultraviolet radiation by aerosols and cloud droplets, leading to variations in the photostationary rate. Similarly, the discontinuous emission and removal of species by nonuniform sources and sinks from the ABL, a process driven by turbulence, introduces variability into the concentration fields that can lead to an enhancement or a reduction of the chemical reactivity. The key dimensionless number that determines whether the chemical terms are of the same order as the thermodynamic terms in the governing equations is the ratio of the characteristic dynamic scale to the chemical reaction scale, namely the Damköhler number (Da) (Damköhler 1940). Since atmospheric flows are characterized by a large number of spatial and temporal scales, and to assure a proper classification of the turbulent reacting flows, two Damköhler numbers have to be defined (Bilger 1980):
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The first number, Dat , accounts for the mixing driven by the large scales. For instance, in the convective boundary layer, τt is the turnover time of the large eddies generated by convection and is defined as the ratio of the boundary layer depth to a convective velocity scale. The second number, Dak, relates the mixing activity of the smallest atmospheric energy containing turbulent scales defined by the Kolomogorov timescale to the chemical reaction rate. The timescale τk is a function of the kinematic viscosity of the air (ν ) and the turbulent kinetic energy dissipation rate (∈). For a second-order chemical reaction, the characteristic timescale of chemistry is defined as τc = (kC)−1, where k is the reaction rate and C is the concentration of species. On the basis of these numbers, the following classification of turbulent reacting flows is proposed (Bilger 1980; Molemaker and Vilà-Guerau de Arellano 1998): • Dat Ⰶ 1—turbulence uniformly mixes the species before a reaction occurs (slow chemistry); • Dak < 1 < Dat—turbulence modifies the chemical reactivity (moderate chemistry); • Dak Ⰷ 1—providing that turbulence brings the species together, chemical species achieve a balance much more rapidly than turbulence (fast chemistry). Stockwell (1995) calculated the Damköhler number Dat for a complete gas phase atmospheric chemistry set of reactions. He found that for a large number of species, Dat > 1. In particular, reactions involved hydroxyl radicals and reactions of peroxy radicals with nitric oxide belong to the categories of moderate and fast chemistry. However, the condition Dat > 1 is a necessary one, but it is not sufficient to determine the extent to which turbulence limits chemistry. For moderate and fast chemical mechanisms, the complementary condition that defines the state of the turbulent reacting flow is whether the reactants are in or out of chemical equilibrium. In a situation where chemistry is in balance, species are produced and depleted at similar chemical rates. Chemical mechanisms adjust continuously to reach equilibrium. Nevertheless, these equilibria can be prevented or disturbed by driving atmospheric processes like the separation of species by updrafts and downdrafts in the convective boundary layer or by insufficient mixing in a stably stratified boundary layer. Before closing this section, it is worth noticing that the influence of turbulence on chemistry on the average concentration differs from the influence on the high-order moments of the concentration distribu-
tion, namely fluxes and covariances. The reason is that the chemical terms have different expressions in the respective governing equations. Consequently, different Damköhler numbers and chemical equilibria can be found for the average concentration and for the fluxes and/or covariances. WHAT HAVE WE LEARNED FROM LARGE EDDY SIMULATIONS? Almost all the studies mentioned above are based on modeling the discretized governing equations and analyzing how the presence of the chemical terms cause deviations from inert scalar behavior. A shortcoming of these studies is that they require a parameterization of the vertical turbulent mixing, which is normally based on weak physical assumptions concerning the mixing length scales. The research carried out by means of large eddy simulation (LES) has helped to overcome this problem. Stevens and Lenschow (2001) stated that LES allows us, by means of control flow simulations, to confine the relevant aspects of a given flow found previously in theoretical studies. It is in this context that LES has enabled us to investigate further the governing parameters and the main characteristics of turbulent reacting flows. The Damköhler number, used previously in the cited modeling studies, proved essential in classifying the archetypal flows simulated by LES. The majority of the LES studies have simulated ABL driven by convection by using different, but rather simple chemical schemes. A brief summary of the most important LES for atmospheric reactive flows is given below. Schumann (1989) carried out the first LES with reactive species in the atmospheric boundary layer. He focused mainly on the segregation of species caused by the coherent structures generated in a convective boundary layer (CBL) and showed that transformation proceeds at a slower rate because of insufficient mixing. Under a similar CBL flow, his work was extended by Krol et al. (2000) for a more complex chemical mechanism. They found that because of the balance between production and destruction rates, and although the species are heterogeneously mixed, the effect of turbulence on the chemical reactions is almost negligible. However, the perturbations in this balance caused by nonuniform emissions, once again, causes departures from equilibrium and therefore a decrease in the reaction rate. These disturbances can have a larger effect close to the sources and sinks of the reactants, as Patton et al. (2001) showed in their LES study of a turbulent reacting flow in and above a plant canopy. Recent spectral analysis of reactants based on LES results (Jonker et al. 2002, manuscript AMERICAN METEOROLOGICAL SOCIETY
submitted to J. Atmos. Sci.) also confirms that the variance power spectra of the reactive compounds depends also on the Damköhler number and if the reactants are in or out of a chemical equilibrium. Turbulent fluctuations play an important role in plume dispersion and its transformation. Plumes are characterized by high concentration releases with large gradients and therefore the turbulent characteristics not only drive the plume dispersion and mixing, but they also limit the transformation. Sykes et al. (1992) and Meeder and Nieuwstadt (2002) demonstrated that the incomplete turbulent mixing significantly affects the depletion rate of the emitted species. The presence of clouds at the top of boundary layer introduces two additional physical processes that can influence the chemical reactions: the transformations of species within and on the surface of cloud droplets and the disturbance of the ultraviolet radiation field that largely modifies the first-order reaction rates. By means of LES, Vilà-Guerau de Arellano and Cuijpers (2000) studied the combined effect of radiation and turbulent mixing. In and around the cloud, the variation with height of the photodissociation rates and the differences of mixing intensity prevent the chemistry from reaching an equilibrium, and consequently modify the reactivity of the system. Moreover, at much smaller scales, turbulent mixing can also perturb the aerosol or cloud droplet equilibrium (Brost et al. 1988). The above-mentioned studies failed to deal directly with the question of how to represent mixing on the subgrid scales of LES. Currently, all the LES studies assume perfect mixing within the grid volume. However, important atmospheric radicals such as the hydroxyl radical (HO) are transformed at a faster rate than the characteristic turbulent scale (Dak Ⰷ 1). Consequently, the assumption of homogeneous mixing at subgrid scales could lead to erroneous results. Subgrid models previously used to simulate combustion flows (Menon et al. 1993) can help us understand how to treat the effect of turbulent mixing on reactions that occur on the smallest turbulent scales. These subgrid models can also play a crucial role at the interface regions (surface and entrainment zone) where reactants are normally introduced and removed and where small eddies play a predominant role. A FIELD EXPERIMENT DESIGNED TO COMBINE ATMOSPHERIC PHYSICS AND CHEMISTRY STUDIES. Up until now there have been far more atmospheric turbulent reacting flow studies based on modeling than studies based on obJANUARY 2003
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servations. The theoretical hypothesis and the LESderived predictions need to be verified by observations. Muschinski and Lenschow (2001) have proposed that coherent strategy research should be carried out on the smallest atmospheric scales; we believe that it would be interesting to set up a field experiment that could provide observational evidence for the distinct behavior of reactive species and inert species. Such an experiment however will have its limitations. It is difficult to measure reactive species with high sensitivity and high frequency resolution. However, since, in turbulence studies, we analyze fluctuating quantities, the absolute accuracy of the concentration measurement is of secondary importance. In spite of these difficulties, flux and covariance reactant measurements (or related quantities) for ozone, nitric oxide, nitrogen dioxide, nitic acid vapor, nitrate aerosol, and isoprene have been made in the past (Delany et al. 1986; Huebert et al. 1988; Coe and Gallagher 1992; Davis 1992; Duyzer and Fowler 1994; Pearson et al. 1998). Particularly promising are experiments that were carried out in a wind tunnel with fast response instruments in order to study the dispersion of a nitric oxide plume in an ozone environment (Brown and Bilger 1996). Recently it has become possible to measure isoprene fluxes, by a chemiluminiescene technique (Guenther and Hills 1998), and the fluxes of other biogenic hydrocarbons like acetone and methanol, by means of protontransfer-reaction mass spectrometry (Warneke et al. 2002). In my opinion, micrometeorologists and atmospheric chemists need to collaborate in order to draw up a program to deal with the following specific questions. 1) Do some fluxes of reactive species diverge with height in the surface layer as calculated by the models (Fitzjarrold and Lenschow 1983; Kramm et al. 1991; Kristensen et al. 1997), and do the flux profiles of reactants depart from the predicted linear profile of inert scalars in a convective boundary layer (Schumann 1989; Gao and Wesely 1994; Verver et al. 1997)? 2) Can we apply the relations based on surface layer or mixed layer similarity theory to the reactants (Hamba 1993; Galmarini et al. 1997b; Kristensen et al. 1997; Petersen et al. 1999)? 3) How large is the covariance term between species, and is it comparable to the product of their means (Schumann 1989; Galmarini et al. 1997a,b; Molemaker and Vilà-Guerau de Arellano 1998; Krol et al. 2000)? 54
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The three issues are closely linked and relevant for the accurate representation in large atmospheric models of the influence of turbulence on chemical reactions in the whole atmospheric boundary layer. Additionally, upper air observations are also necessary to determine the role played by physical processes within and around the clouds. It is also essential that all the atmospheric chemistry measurements are supported by detailed observations of the thermodynamic variables. Furthermore, we need to measure species with different concentration levels, for instance, the higher concentration of O3 compared to a relatively low NO concentration. As advocated by Stevens and Lenschow (2001), there is a need for laboratory experiments to complete the field and modeling studies. For instance, in a mixed layer with two reactives (Komori et al. 1991) and in the dispersion of a chemically reactive plume (Brown and Bilger 1996) measured in laboratory experiments, the covariance between reactants showed that, for a binary irreversible reaction, the transformation can be largely slowed down. The ultimate goal is to combine experiments and simulation studies in order to devise a comprehensive theory for reactive species that meshes current scaling theories developed for thermodynamic variables. If the experimental evidence confirms previous theoretical and simulation findings, then an immediate practical application would be improved parameterizations. The above-mentioned question (number 1) addresses the validity of the concept of deposition velocity, which is widely used in atmospheric chemistry models to remove the species from the ABL and to represent the vertical transport of species to the free troposphere. Question number 2 deals with whether the vertical turbulent transport can be parameterized for reactants in the same manner as for inert scalars, using similarity arguments. Finally, question number 3 aims to quantify how the mixing of species affects their reactivity. The segregation of species represented by a large negative covariance value causes a decrease in the reaction rate. A positive value for the covariance indicates that species are premixed before being introduced into the ABL. As a consequence, the reactivity rate is enhanced or decreased compared to the well-mixed assumption. If this covariance value is found to be comparable to the product of the average concentration, then atmospheric chemistry models applied to larger spatial and temporal scales will have to include the covariance between reactants in the form of a subgrid parameterization. In conclusion, it is desirable that studies of reactive species become more frequent in the meteo-
rological community and vice versa. It is particularly important that such studies are conducted at the spatial and temporal scales discussed during the two workshops. ACKNOWLEDGMENTS. I thank the reviewers for their critical remarks, which have helped considerably to improve the quality of the paper.
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