Intensity, diurnal variation

As discussed in previous subchapters, the rate of the photochemical reductive dissolution of iron(III)(hydr)oxides depends on the concentration and type of surface complexes present and on the light intensity and its energy. Because the light intensity varies diurnally, also a diurnal variation in the iron(II) concentration can be expected in surface waters. This has been observed in acidic waters (McKnight and Bencala, 1988 Sulzberger et al., 1990). Fig. 10.17 shows such a diurnal variation in the concentration of dissolved Fe(II) in a slightly acidic alpine lake (Lake Cristallina) of Switzerland. 

Diurnal variation of the concentration of dissolved Fe(II) and of the incident light intensity O in Lake Cristallina. (The maximal measured light intensity is arbitrarily set to one). 

The diurnal variations in mean hourly average oxidant concentration are illustrated in Figures 4-26 and 4-27. Several factors influence the shapes of these curves. The primaiy influence is that of sunlight intensity, inasmuch as photons in the ultraviolet are responsible for the primary photochemical process that leads to ozone formation. Note that the St. 

These differences in light intensity, and in its diurnal variation at different latitudes and seasons, are critical because they alter the atmospheric chemistry at various geographical locations due to the fact that photochemistry is the major source of the free radicals such as OH that drive the chemistry. 

Figure 8.24 shows a typical diurnal variation of NO concentration (solid line) observed for the roadside air on a fine winter day. The high NO concentration was caused by busy traffic (especially large, diesel engine trucks). The dashed lines indicate the NO concentration in the air treated with the sheets from the windowed panels. Fig. 8.24 clearly shows that the PTFE sheets removed NO from the polluted air between 7 am and 5 pm. The UV-A intensity shown by the bell-shaped curve centered at noon was over 0.1 mW cm-2. The average removal percentage for NO with the windowed panels during the field test was 31-69%. 

Variations in the biochemistry and physiology of fish from one year to another must be accepted as real. Such a cycle would be linked to long-term changes in the climate resulting from solar activity (Chizhevsky, 1976). The trouble is that observations are insufficiently representative to yield clear patterns. As with diurnal variation, much of the published work relates to terrestrial organisms rather than fish, and much of the study has centred on fluctuations in the abundance of species which tend to develop outbreaks - sudden marked increases in numbers. However, fish such as salmon, cod, herring, sardines and other species have also been shown to exhibit long-term fluctuations in their numbers. Klyashtorin (1996) has found a close correlation between the velocity of rotation of the earth, which affects the intensity of circulation of the water in the oceans, and the abundance of stocks of many species of fish. 

In the absence of direct field measurements of pesticide fluxes eminating from a sprayed forest a series of suppositions may be drawn from similar observations of losses from treated agricultural crops. The volatilization of dieldrin and heptachlor from a grass pasture was characterized by rather marked diurnal variations in vertical flux intensities of both insecticides during the initial days post application (12). The authors concluded that the volatilization ceased or was greatly reduced with decreased solar radiance. Estimated relative vapour concentrations of dieldrin rapidly declined from saturation 2 hours post application to 10% by evening. This parameter reached a maximum of 30 - 40% on day 2 and 20 - 25% on day 3. Although the saturated vapour concentration of heptachlor is approximately fifty... 

Figure 2. Diurnal variation of DMS in air and surface seawater observed at a station in Gerlache Strait during a high pressure weather situation with intense daylight and strong katabatic winds at night. (Reprinted with permission from Ref. 12. Copyright 1987 by the American Geophysical Union).

Other factors relating to the light source that are significant in the application of this model to atmospheric conditions are the diurnal variation of intensity and spectral distribution. This should also add a complex nonlinear aspect to the analysis. 

In clean surface waters the concentration of dissolved oxygen is about 85-95% of saturation and varies throughout the day. These diurnal variations depend on the intensity of photosynthesis and changes in the temperatures. In the case of intensive photosynthetic assimilation of green organisms or in turbulent stretches in streams, water can easily become oversaturated by oxygen. At 15°C water is saturated by oxygen at a concentration of about 10 mg 1 . ... 

Variability may be defined as reflecting fluctuations in the atmosphere, of natural origin, with both temporal and spatial scales examples are diurnal, seasonal, solar activity-related variations impulsive events such as volcano eruptions and solar proton events fluctuations linked to some peculiar meteorological conditions, for example, intense cyclonic activities and jet streams. Variability by itself is a whole program to be conducted ideally on a four-dimensional basis (latitude, longitude, altitude, and time) by space vehicles, for example, satellites or from the space shuttle. This area of research is certainly the most urgent one to be de-... 

Diurnal and seasonal variations in solar intensity are, of course, of utmost importance to ecosystems. In the extreme polar regions there is no direct solar radiation at all for more than four months of the year, whereas near the equator the overall intensity of sunlight fluctuates less than 10% annually. The spectral energy distribution also varies with the season. For example, in July in the middle latitudes (ca. 40 ), the fraction of shorter-wave UV (290-315 nm) in the total solar radiation is more than three times higher than it is in December, due to the shorter path these easily scattered wavelengths have to traverse through the atmosphere. For similar reasons, shortwave UV is more intense at high elevations, particularly in the tropics where stratospheric ozone is less concentrated (Caldwell et al., 1980). 

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