The Planetary Boundary Layer

CO MIXING RATIO (ppbv) H2 MIXING RATIO (ppmv) 

The importance of the Earth surface to atmospheric chemistry lies in the fact that it acts as an emitter and/or as a receiver of atmospheric trace substances. In this section we discuss concepts that have been found useful in describing these processes. The transport of gases across the ocean surface is reasonably well understood. The release of gases from soils, in contrast, cannot be modeled and must be explored by field studies. In the literature, much emphasis has been placed on the absorption of gases at the ground surface, a process referred to as dry deposition. 

The physical conditions of the atmosphere in the vicinity of the Earth surface are determined by friction and by heat transfer. Friction reduces wind velocity and turbulence as one approaches the surface, so that the eddy transport rate declines. Heating of the surface by solar radiation imparts energy to the overlying air, causing an enhancement of vertical motions and eddy transport owing to convection. An increase of temperature with height in the atmosphere for a certain distance instead of the normal, adiabatic decrease is called a temperature inversion layer. Temperature 

Boundary layer conditions are called labile when convection predominates in the vertical air exchange stable when the vertical air motions are dampened, such as by a temperature inversion and indifferent or neutral when the vertical turbulence is induced solely by the horizontal wind field. In the last case, the horizontal wind speed is generally found to increase with height in accordance with a logarithmic function 

If a trace gas in the atmosphere undergoes irreversible absorption or chemical reaction at the ground surface, the process will set up a vertical gradient of the mixing ratio leading to a downward flux  

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Typical concentrations of Hg species in the planetary boundary layer... 

Velocity and temperature gradients are confined to the surface layer defined by z < I-. Above L the wind velocity and potential temperature are virtually uniform with height. Venkatram (1978) has presented a method to estimate the value of the convective velocity scale w,. On the basis of this method, he showed that convective conditions in the planetary boundary layer are a common occurrence (Venkatram, 1980). In particular, the planetary boundary layer is convective during the daytime hours for a substantial fraction of each year ( 7 months). For example, for a wind speed of 5 m sec , a kinematic heat flux Qo as small as O.PC sec can drive the planetary boundary layer into a convective state. 

Vertical dispersion cannot be described in such simple terms (Kaimal et al., 1976). First, varies throughout the planetary boundary layer. In the region, L < z < O.lzi, o- , z, and a- scales with w, only above O.lzi. For Zi = 1500 m and an effective stack height of 400 m, the dispersing plume is controlled by an inhomogeneous region that is almost half the effective stack height. 

Lewellen, W. S., Teske, M., and Donaldson, C. duP. (1974). Turbulence model of diurnal variation in the planetary boundary layer. In Proceedings of the 1974 Heat Transfer and Fluid Mechanics Institute (L. R. Davis and E. R. Wilson, eds.), pp. 301-319. Stanford Univ. Press, Stanford, California. 

O Brien, J. (1970). On the vertical structure of the eddy exchange coefficient in the planetary boundary layer. J. Atmos. Sci. 27, 1213-1215. 

Wyngaard, J. C. (1975). Modeling the planetary boundary layer-extension to the stable case. Boundary Layer Meteorol. 9, 441-460. 

T. Smith, Jr., R. V. Arrieta, R. Rodriquez, and J. W. Birks, Vertical Profiling and Determination of Landscape Fluxes of Biogenic Nonmethane Hydrocarbons within the Planetary Boundary Layer in the Peruvian Amazon, J. Geophys. Res., 103, 25519-25532 (1998a). 

Altshuller, A. P and A. S. Lefohn, Background Ozone in the Planetary Boundary Layer over the United States, J. Air Waste Manage. Assoc., 46, 134-141 (1996). 

The structure of turbulence in the transition zone from a fully turbulent fluid to a nonfluid medium (often called the Prandtl layer) has been studied intensively (see, for instance, Williams and Elder, 1989). Well-known examples are the structure of the turbulent wind field above the land surface (known as the planetary boundary layer) or the mixing regime above the sediments of lakes and oceans (benthic boundary layer). The vertical variation of D(x) is schematically shown in Fig. 19.8b. Yet, in most cases it is sufficient to treat the boundary as if D(x) had the shape shown in Fig. 19.8a. 

In remote and unpolluted regions of the planetary boundary layer, natural sources of NOx (NO and N02) such as lightning, result in relatively small mixing ratios, typically being less than 20 pptv. In contrast, the amount of NOx in downtown city air is often above 100 ppbv. Thus N02 has a large tropospheric variability. Both the distribution of sources and the lifetime of N02 are very different in the troposphere compared to the stratosphere. 

Formation in the Planetary Boundary Layer, Atmos. Environ. 34 LVd d-lSTJ. 

The oxidative capabihty of the atmosphere is not simply a function of chemistry. Convective storms can carry short-lived trace chemicals from the planetary boundary layer (the first few hundred to few thousand meters) to the middle and upper troposphere in only a few to several hours. This can influence the chemistry of these upper layers in significant ways by delivering, e.g., reactive hydrocarbons to high altitudes. Conversely, the occurrence of very stable conditions in the boundary layer can effectively trap chemicals near the surface for many days, leading to polluted air. Larger-scale circulations serve to carry gases around latitude circles on timescales of a few weeks, between the hemispheres on timescales of a year, and between the troposphere and stratosphere on timescales of a few years. 

All species are readily removed by atmospheric deposition. NH3 is primarily removed by dry deposition (often close to its source). Aerosol NH4 is primarily removed by wet deposition in fact, the hydroscopic aerosol is a cloud-condensation nuclei. If NH t- (NH3 and NH4) is lifted above the planetary boundary layer (PEL), it can be transported large distances (1,000 km or more). Emissions in one location can impact receptors far downwind. 

The dominant transport mechanism for both aerosol and gaseous agents in the atmosphere is advection associated with the bulk motion of the atmosphere. Since airflows in the planetary boundary layer exhibit signihcant turbulence under most conditions (though turbulence may be suppressed under conditions of temperature inversion), this will cause aerosol releases to disperse into a plume or puff that expands... 

In order to understand atmospheric dispersion processes, it is accordingly necessary to understand the structure and characteristics of the planetary boundary layer... 

The MCM is a near-explicit chemical mechanism that describes the detailed degradation of a series of emitted VOC, and the resultant generation of ozone and other secondary pollutants, under conditions appropriate to the planetary boundary layer. Version 3 of the Master Chemical Mechanism (MCM v3) considers the oxidation of 125 VOC. The complete mechanism comprises 12,691 reactions of 4,351 organic species and 46 associated inorganic reactions, which were defined on the basis of the MCM scheme writing protocols (Jenkin et al, 1997, Saunders et al, 2003 Jenkin et al, 2003). Although MCM v3 has recently been superseded by v 3.1, the chemistry for non-aromatic VOC remains unchanged. 

Fig. 13-5 The sulfur cycle in the remote marine boundary layer. Within the 2500 m boundary layer, burden units are ng S/m and flux units are ng S/m h. Fluxes within the atmospheric layer are calculated from the burden and the residence time. Dots indicate that calculations based on independent measurements are being compared. The measured wet deposition of nss-SO " (not shown) is 13 7 //g S/m /h Inputs and outputs roughly balance, suggesting that a consistent model of the remote marine sulfur cycle within the planetary boundary layer can be constructed based on biogenic DMS inputs alone. Data (1) Andreae (1986) (2) Galloway (1985) (3) Saltzman et al. (1983) (4) sulfate aerosol lifetime calculated earlier in this chapter based on marine rainwater pH the same lifetime is applied to MSA aerosol. Modified from Crutzen et al. (1983) with the permission of Kluwer Academic Publishers.

Simple dispersion behaviour - as described above - is scale-independent, so that, for example, Gaussian variances estimated from small-scale experiments in a wind tunnel are applicable in the prediction of the atmospheric dispersion of smoke from a large chimney. However, the specification of a dispersion environment (the planetary boundary layer (PBL), terrain, obstacles, closed spaces, etc.) and particular assets (i.e. people or property that we might wish to protect) introduces absolute spatial and temporal scales. 

Here, H = 8500 m denotes the atmospheric scale height. Radon obviously would be a good tracer for turbulent mixing in the troposphere. Unfortunately, it is hard to measure. It should be noted that the assumption of a constant Kz in the troposphere is a reasonable approximation only for the region outside the planetary boundary layer (zs=2km). The air motions close to the earth surface are greatly influenced by friction, which causes the eddy coefficients to decrease from their values in the free troposphere. 

The resulting residence times, which are shown in Fig. 7-28 by the dashed line, should still be considered upper limits in view of the fact that values for K2 in the planetary boundary layer generally are smaller than 20 m2/s. Junge (1957) and Fabian and Junge (1970) have shown how one can, in principle, incorporate the variation of Kz in the boundary layer. This refinement will not be discussed, however, because meteorological conditions in the boundary layer are too variable for a generalized model to be applicable. 

FIGURE 4 Relationship between the seasonal cycles of CO, and O, in the interior of the Eurasian continent (at Zotino, 60°N, 90°E) within the planetary boundary layer. The left diagram shows the seasonal signal components in 0. induced from the terrestrial and oceanic seasonal sources the right-hand panel shows the modeled relation between the seasonal cycles of O, and CO2 (monthly averages). 

FIGURE 5. Slope of the modeled relationship between the seasonal cycles of O, and CO, in the planetary boundary layer. The color scale has been selected. such that values in the Northern Hemisphere are highlighted, where the relationship between the seasonal signals of the two tracers is essentially linear. The black dot indicates the location of the Zotino (60°N, 90°E) station displayed in Figure 4. 

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