Solar radiation intensity, measurement

The kinetics of the various reactions have been explored in detail using large-volume chambers that can be used to simulate reactions in the troposphere. They have frequently used hydroxyl radicals formed by photolysis of methyl (or ethyl) nitrite, with the addition of NO to inhibit photolysis of NO2. This would result in the formation of 0( P) atoms, and subsequent reaction with Oj would produce ozone, and hence NO3 radicals from NOj. Nitrate radicals are produced by the thermal decomposition of NjOj, and in experiments with O3, a scavenger for hydroxyl radicals is added. Details of the different experimental procedures for the measurement of absolute and relative rates have been summarized, and attention drawn to the often considerable spread of values for experiments carried out at room temperature (-298 K) (Atkinson 1986). It should be emphasized that in the real troposphere, both the rates—and possibly the products—of transformation will be determined by seasonal differences both in temperature and the intensity of solar radiation. These are determined both by latitude and altitude. 

The solar constant (intensity of solar radiation outside the Earth s atmosphere at die mean distance between die earth and the sun) has been determined by measurements from satellites and high-altitude aircraft and is 1.353 kilowatts per square meter. This extraterrestrial radiation,... 

The Earth s elliptical orbit causes the distance between the Earth and the Sun (the Earth s radius vector) to vary by 3.39% from perihelion (closest) to aphelion (farthest). These variations in distance cause the intensity of solar radiation at the top of the atmosphere to vary as 1/R2, where R is the radius vector. Thus the solar input at the top of the atmosphere varies from 1414 Wm 2 (in December) to 1321 Wm 2 (in July). Additional variations in solar intensity, or brightness, result from the solar sunspot cycle, and even solar oscillations. These slight variations in the solar output are usually accounted for in the calculation of solar energy available at the top of the atmosphere, or the total extraterrestrial solar radiation, referred to as ETR. The ETR has only been monitored from space since the early 1970 s, or almost three solar sunspot cycles. Excellent histories of ETR measurements and analysis are provided in Frohlich3 and Gueymard.4... 

Solar Radiation. Of all the factors which collectively determine the amount and spectral distribution of the radiation entering a surface layer of the atmosphere, the best established appear to be the spectral irradiance outside the atmosphere and the attenuation by molecular scattering. The absorption coefficients of ozone are well established, but no easy method exists for determining the amount of ozone in a vertical profile of the atmosphere at a given time. The measurement of the particulate content of the atmosphere and its correlation with atmospheric transmission is a field in which much remains to be accomplished. Surprisingly few data exist on the spectral distribution of sky radiation and its variation with solar elevation and atmospheric conditions. The effect of clouds is of secondary importance, as intense smog generally occurs under a clear sky. 

Generally, spectrophotometric methods are used to measure the atmospheric ozone concentration. By means of suitable optical devices15 one detects the intensity of solar radiation in two narrow wavelength bands. In one of these bands ozone strongly absorbs the radiation while, in the other, 03 has little absorption. The ozone content of the air can be calculated by comparing the two values measured. In this way the total ozone quantity in an air column is determined. This parameter expressed in cm is termed the total ozone. It represents the thickness of the layer which the same amount of ozone would form if it were separated from the air and held at normal temperature and pressure. 

In the solar occultation method a space-borne detector (e.g., photomultiplier tube, photolytic array detector, Fourrier transformed interferometer) points towards the Sun, and during brief periods (sunrise, sunset), when the optical path penetrates into the atmosphere (see Figure 4.19a), measures the attenuation of the solar radiation by the absorbing compounds. The intensity at frequency v detected by the spacecraft is... 

The absorption spectrum of water vapor consists of a continuum from 145 to 186 nm, diffuse bands from 69 to 145 nm, and a continuum below 69 nm. A maximum in the intensity of a diffuse band (121.8 nm) is located in the vicinity of the Lyman-a line (121.6 nm). Figure 4.39 shows the shape of the absorption cross section of water vapor above 120 nm. Measurements of this cross section were reported by Watanabe and Zelikoff, 1953, Thompson et al. (1963), Laufer and McNesby (1965), Schurgers and Welge (1968), Yoshino et al. (1996b), and Cantrell et al. (1997). In particular, the value of cross section longward of 175 nm. Beyond this wavelength, the penetration of solar radiation is determined by the 02... 

With a heat intensity of 2,000 Btu/hr./sq.ft. (six times the intensity of solar radiation) the pain threshold is 8 seconds. Therefore, to allow a man time to run to safety, he should not be subjected to a heat intensity higher than approximately 1,500 Btu/hr./sq.ft. in the event of a major refinery or chemical plant failure which sends large amounts of combustible gas to the flare stack. A stack of sufficient height can be selected to satisfy this condition. But if this is impractical, adequate protective measures should be taken to ensure his safety. 

Pyrheliometer - A device that measures the intensity of direct beam solar radiation. 

The solar constant is defined as the radiation intensity received at right angles to the sun s radiation at the earth s mean distance, 1.49 x 10 km. This is the average between largest distance (aphelion), in July, and smallest distance (perihelion), on January 4. Aphelion distance exceeds perihelion, at present, by 3%. The solar constant is 1370 W/m. It is a measure of the sun s heat output... 

The direct physical measurement of the spectral actinic flux F(X) is not easy, although attempts have been made (Shetter and Muller 1999 Hofzumahaus et al. 1999). Generally, irradiance E(k) (radiation flux per unit area, W nm ) is measured by radiometers, and experiments to compare solar spectral intensity in the field with radiative transfer models have been made in order to convert the spectral irradiance E(X) to F(X). In these analyses, downward actinic flux Fd (A) is obtained by upper-hemispherical integration of observed spectral radiance L(k,9,4>) (radiation flux per solid angle, W sr m nm ), and F (A) is expressed as the sum of the flux of direct radiation Fq (X) and downward diffusive... 

Eventually, the goal is to find Ip, and from that cloud or surface properties or the planetary albedo, provided we know the instrument response, the intensity of the solar radiation reflected from the diffuser, /d, and, of course, the measured values Sp and Sd- The task of finding the planetary albedo from such measurements is discussed in Section 8.6. The disadvantages of calibration with a diffusor plate are... 

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