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  • Author or Editor: EDWIN C. FLOWERS x
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EDWIN C. FLOWERS and NORBERT F. HELFERT

Abstract

More precise radiation measurements in the Antarctic are required for studies designed to assess the radiation and heat budgets of that area. These requirements inspired the present investigation which is aimed at the experimental determination of certain of the instrumental characteristics and their consequences. Laboratory investigations of a group of Eppley pyranometers verifies previous findings that the temperature response is a unique characteristic of each instrument which can produce radiation-values 14 percent too high under the extreme Antarctic temperature conditions and differences between instruments at the same temperature of 3 to 8 percent. Tests made on the effect of inverting the pyranometer for the measurement of reflected solar radiation show that, the instrument sensitivity decreases by 4 to 6 percent in this position. A test on one normal incidence pyrheliometer indicates virtually no temperature effect. Weather Bureau calibrations of several pyrheliometers give results in excellent agreement with the calibrations performed by Eppley Laboratory. Field shade calibrations and simultaneous exposure comparisons of a group of Eppley pyranometers at the South Pole indicate that the computation of the total solar radiation should probably be made using separate direct and diffuse calibration factors for the pyranometers.

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James T. Peterson, Edwin C. Flowers, and John H. Rudisill III

Abstract

Atmospheric turbidity (aerosol optical thickness) was measured with sunphotometers across the Los Angeles Basin. Automobiles were used for east-west traverses of the metropolitan area (a distance of ∼100 km) on two days with distinctly different meteorological conditions: a hazy, relatively humid day and a warmer, dryer, less hazy day with easterly Santa Ana wind flow. Additionally, incident global UV and total solar irradiance were measured at six sites (five urban and one rural) and nephelometer measurements of aerosol concentrations were made at two locations.

On the hazy day turbidity was remarkably uniform across the Los Angeles Basin. In contrast, significant variation of turbidity from west to east occurred on the less polluted day. Solar radiation measurements also reflected the day-to-day and spatial turbidity differences. During the hazy day the urban sites received only 64–76% as much UV energy as did the rural mountain site. With easterly Santa Ana wind flow, San Bernardino received 17% more total solar energy than on the hazy day.

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James T. Peterson, Edwin C. Flowers, and John H. Rudisill

Abstract

As part of the Los Angeles Reactive Pollutant Project (September–November 1973) continuous measurements were made of incident ultraviolet and total solar irradiance at six sites (five urban and one nonurban) over the Basin. Turbidity and nephelometer measurements were made periodically at two sites. Incident irradiance during cloudless conditions was significantly depleted by the smoggy urban atmosphere. The largest daily measured differences of UV energy between the nonurban and urban sites was about 50%; greater differences were measured for shorter periods. Variations between urban sites of up to 40% for short periods were noted. Less energy was attenuated by the urban atmosphere on weekends than weekdays. For the complete 68-day experiment, the average absolute effect of the urban atmosphere was to reduce ultraviolet irradiance from 11–20% and total irradiance from 6–8% over the Basin.

Atmospheric turbidity decreased from September through October as meteorological conditions changed. The August–September urban mean (0.287 decadic base) is among the highest summertime values in the United States. The greatest daily average was 0.492. Turbidity data were related to simultaneous global total irradiance transmission measurements at the nonurban site. For all data over solar zenith angles from 25–70° a 0.01 turbidity increase corresponded to a global transmission decrease of 0.84%. Comparison of turbidity and diffuse and direct UV and total irradiance showed that the diffuse component directly depended on turbidity and solar zenith angle and inversely depended on wavelength and station elevation. Measurements were applied to several air pollution problems in Los Angeles. Study of coincident irradiance and daily maximum ozone concentrations at downtown Los Angeles showed that low and moderate amounts of radiation define an upper limit for maximum surface ozone concentration. For higher radiation levels, other factors governed ozone levels. Depletion of UV irradiance by the urban atmosphere at EI Monte was strongly related to local visibility for visibilities ≲10 km. Finally, from turbidity and nephelometer data, the thickness of urban air flow over the mountains north of Los Angeles was estimated to average 170 m.

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James T. Peterson, Edwin C. Flowers, and John H. Rudisill

Abstract

The formation of dew or frost on the outer glass hemisphere of pyranometers was found to cause erroneous values of incident hemispheric solar radiation. An air flow system was designed to continually ventilate the instrument and thereby prevent moisture formation on the outer hemisphere. Comparison of instantaneous early morning readings from ventilated and non-ventilated pyranometers indicate that substantial errors can occur because of dew or frost. Since the moisture from dew or frost naturally evaporates by mid-morning, daily totals of solar energy are not likely to be significantly affected on clear days.

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James T. Peterson, Edwin C. Flowers, Guillermo J. Berri, Cheryl L. Reynolds, and John H. Rudisill

Abstract

Some 8500 observations of atmospheric turbidity, taken at Raleigh, North Carolina from July 1969 to July 1975 are analyzed for within-day and day-to-day variations and their dependence on meteorological parameters. The annual average turbidity of 0.147 (0.336 aerosol optical thickness) is near the highest non-urban turbidity in the United States. A distinct diurnal turbidity cycle was evident with a maximum in early afternoon. Annually, highest turbidity and day-to-day variation occurred during summer with lowest values and variation during winter. Daily averages revealed an asymmetric annual cycle, with a minimum on 1 January and a maximum on 1 August. Turbidity showed a slight inverse dependence on surface wind speed. Aside from winter, highest turbidities occurred with southeast surface winds. Turbidity was directly proportional to both humidity and dew point. Correlations between turbidity and local visibility were best for visibilities <7 mi. Air mass trajectories arriving at Raleigh were used to study the dependence of turbidity on synoptic air mass. Air masses with a southern origin had greatest turbidities. Turbidity of an air mass significantly increased as the residence time of that air mass over the continental United States increased, with the most rapid changes during summer. A combination of Raleigh (1969–present) and Greensboro, North Carolina (1965–76) records showed a distinct summer increase through 1976, but no change during winter. A linear regression of annual averages for the complete record gave an 18% per decade turbidity increase.

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