Central California Ozone Study (CCOS) - Desert Research Institute

esponse to changes in wind speed as well as a low threshold for rotation for conditions of very
light winds. Acceptable precision levels for anemometers are ±0.5 m/s for most air quality
studies (e.g., Thullier et al., 1993).
Wind direction measurements are made with wind vanes that orient themselves with the
direction of the wind. High quality bearings are used for sensitivity and direction is resolved with
an internal potentiometer that has a reference location of zero which must be physically aligned
with true north when in position on the tower. The position of true north is determined with a
sensitive compass or solar reference and the north position of the wind vane is then aligned in this
direction. Care must be taken in the alignment procedure or in subsequent resolution of the wind
direction vectors to account for the magnetic declination at the observation site.
Averaging can be done over intervals of a few points to the entire data record. The
interval must be chosen with respect to the phenomena being observed. As shorter averaging
intervals are chosen, periodic motion may be revealed when the period is significantly greater than
the averaging period. The averaging process attenuates small-scale fluctuations due to turbulence.
The averaging interval must contain sufficient data points to constitute a reasonable average,
eliminating the small scale turbulent fluctuations and instrument derived errors such as response
sensitivity and anemometer run-up and run-down. Five minutes is a reasonable compromise
between the need to acquire a statistically valid number of samples and the nature of phenomena
such as wind gusts and dispersion.
The time average of horizontal wind direction standard deviation (σ θ ) can be calculated as
from one second observations over a period of 5 minutes or less (Greeley and Iversen, 1985).
This minimizes the influence of the wind flow which may be non-stationary in both speed and
direction over longer averaging periods.
C.1.2 Surface Relative Humidity
Humidity indicates the amount of water vapor in the air. The most common reference for
this property of the air is the relative humidity (RH), the ratio of the amount of water vapor
actually in the air to the amount the air could hold at a given temperature and pressure, that is, the
ratio of the actual to the saturated vapor pressure.
Accurate measurements at high RH (>85%) is important to atmospheric transformations
and to visibility reduction. An accuracy of 2-3% RH is needed to adequately estimate aerosol
properties. A 2-3% accuracy for RH over the 90-100% RH is attainable by several RH sensors
(e.g. Vaisala, Rotronics) and corresponds to ±0.5°C when expressed as dewpoint temperature.
Motor-aspirated radiation shields ensure adequate ventilation and exposure rates (U.S. EPA,
1989).
C.1.3 Surface Temperature
Equivalent resistance temperature detectors (RTD (e.g., Vaisala, Rotronics, Climatronics,
Campbell Scientific, etc.) are used at the meteorological sites. Motor-aspirated radiation shields
ensure adequate ventilation and exposure rates, according to U.S. EPA quality assurance
requirements (U.S. EPA, 1989). The accuracy (~0.15-0.5 °C) and functional precision (0.1°C) of
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