Earth’s boundary layer

This report deals primarily with the origins and effects of ozone and other photochemical oxidants. It is limited, more or less, to the problem of urban pollution and to such closely related topics as natural background in the earth s boundary layer. No consideration is given to the stratospheric ozone layer and the effects produced by supersonic transport (sst) emission or halocarbons. 

Kellogg L. H. (1997) Growing the Earth s D layer effect of density variations at the core-mantle boundary. Geophys. Res. Lett. 24, 2749-2752. 

Another widely used concept is that of a planetary boundary layer (PBL) in contact with the surface of the Earth above which lies the "free atmosphere." This PBL is to some degree a physically mixed layer due to the effects of shear-induced turbulence and convective overturning near the Earth s surface. 

The solar radiation incident on the atmosphere and the Earth s surface represents the largest external energy contribution. The optical irradiation at the upper boundary layers of the atmosphere is 4,435 kJ/cm2/year, of which around 1,108 kJ/cm2/year reaches the Earth s surface. 

In Gubbio, Italy, a 1 cm layer of clay between extensive limestone formations marks the boundary between the Cretaceous and Tertiary Periods. This clay layer was known to have been deposited about 65 million years ago when many life forms became extinct, but the length of time associated with the deposition was not known. In an attempt to measure this time with normally deposited meteoritic material as a clock, extensive measurements of iridium abundances (and those of many other elements) were made on the Gubbio rocks. Neutron activation analysis was the principal tool used in these studies. About 50 elements were searched for in materials like the earth s crust, about 40 were detected and about 30 were measured with useful precision [26-28]2. 

The boundary layer is the lowest part of the atmosphere, closest to the earth s surface. Stull (1988) defines the boundary layer as that part of the troposphere that is directly influenced by the presence of the earth s surface, and responds to surface forcings [such as heat transfer, pollutant emissions, evaporation etc.] with a timescale of about an hour or less. Typical boundary layer heights range from 100 to 3000 m in altitude. The rest of the overlying troposphere is called the free troposphere. 

Figure 2.20 summarizes the role of inversions and the boundary layer in terms of typical changes in mixing of the atmosphere close to the earth s surface at various times of the day (Stull, 1988). At midday, there is generally a reasonably well-mixed convective layer... 

FIGURE 2.20 Schematic of mixing processes in atmosphere close to the earth s surface as a function of time of day. (Adapted, with kind permission from Kluwer Academic Publishers, from R. B. Stull, 1988, An Introduction to Boundary Layer Meteorology, Fig. 1.7. 1988 by Kluwer Academic Publishers.)... 

Figure 3.25 shows the results of one set of calculations of the effects of aerosol particles whose properties were judged to be characteristic of continental or urban situations, respectively, on the transmission of UV and visible radiation to the earth s surface (Erlick and Frederick, 1998). The ratio of the transmission with particles to that without is plotted in two wavelength regions, one in the UV and one in the visible. Two different relative humidity scenarios are shown. The average summer relative humidity was 70% RH in the boundary layer and 20% RH in the free troposphere. The high relative humidity case assumes 90% RH in the boundary layer and 30% in the free troposphere. (The RH in the stratosphere was taken to be 0% in both cases see Chapter 12.)... 

Use the data in Tables 3.7 and 3.11 to calculate the ratio of the actinic flux at the earth s surface for an 80% surface albedo compared to the best estimate albedo at solar zenith angles of 0 and 78° for the following wavelength regions 298-300, 318-320, and 400-405 nm. Comment on the expected effects on photochemistry in the boundary layer. 

Most studies of the chemical composition of particles in the troposphere to date have used analysis of bulk samples, which are usually collected in the boundary layer close to the earth s surface. As discussed in Chapter 6. J.3, there is a great deal of interest in the chemistry of the upper troposphere. Much less is known about the chemical composition in this region, particularly of particles. However, it appears that organics are also important constituents of particles in this region as well. For example, Novakov et al. (1997) in studies of particles both onshore and offshore of the eastern United States found that the mass fraction of the particles due to carbon compounds increased as a function of altitude. In the boundary layer, the fraction was typically 10-40%, increasing to 50-90%atan altitude of 2-3 km. 

Long term observations indicate that UV-B radiation reaching the earth s surface may have decreased by 5-18% since the industrial revolution in the industrialised midlatitudes of the Northern Hemisphere (NH). However, on a global basis, this may have been offset by the stratospheric ozone layer reduction. It is not possible to estimate the net effect from both, attenuation and increase, because of the limited amount of spatial and temporal coverage of measurements (Liu et al., 1991). In an attempt to present calculated and modelled effects of aerosol on UV flux the authors used the Discrete Ordinate Radiative Transfer Model (DISORT Stammes et al. 1988) for different visual ranges and boundary layer depths (Figure 1). The decrease at 310 nm is 18% and 12 % for a 2km and 1km PBL respectively. 

Pa PACE PAGES PAHO PALE PAR PARCS PBL PCM PDV PhA PIK PIRA PIRATA POC POLDER Partial pressure in the atmosphere Permafrost And Climate in Europe Pilot Analysis of Global EcoSystems Pan American Health Organization Paleoclimates of Arctic Lakes and Estuaries Photosynthetic Active Radiation Paleoenvironmental ARCtic Science Planetary Boundary Layer Parallel Climate Model Pacific Decadal Variability Phytogenic Aerosol Potsdam-Institut fur Klimafolgenforschung Petroleum Industry Research Associates Pilot Research moored Array in the Tropical Atlantic Permanganate Oxidizable Carbon POLarization and Directionality of the Earth s Reflectances Princeton Ocean Model... 

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