Meteorological data
Any study of air pollution should include a study of the weather patterns (meteorology) of the local area because the fate of air pollutants is influenced by the movements and characteristics of the air mass into which they are emitted.
If the air is calm and pollutants cannot disperse then the concentration of these pollutants will build up. Conversely, if a strong, turbulent wind is blowing any pollution generated will be rapidly dispersed into the atmosphere resulting in lower pollutant concentrations in the air.
The measurements of wind speed and direction, temperature, humidity, rainfall and solar radiation are important parameters used in the study of air quality and can assist in furthering understanding of the chemical reactions that occur in the atmosphere.
Meteorological data is used to predict air pollution events, such as inversions, high pollutant concentration days, and to simulate and predict air quality using computer models.
An example of air parcel movement and the reasoning behind the siting of air monitoring stations is shown in a simulated pollution event.
Wind speed and direction
Wind speed and direction measurements are important for air quality monitoring. If high pollutant concentrations are measured at a monitoring station the wind data recorded at the station can be used to determine the general direction and area of the emissions. When the likely source or sources have been identified they can then be managed to reduce the impacts on air quality.
Wind speed is measured using an instrument called an anemometer. The Air Quality Sciences branch of the Department of Environment and Resource Management currently uses sonic anemometers at most stations. Sonic anemometers operate on the principal that the speed of wind affects the time it takes for sound to travel from one point to another. If the sound is travelling in the direction of the wind then the transit time is decreased. Conversely, if it is travelling in the opposite direction to the wind, the transit time is increased. Sonic anemometers emit sound waves, record the time taken to receive the wave at the other end of the instrument, then convert the value into the wind speed. By monitoring the speed of sound along two different axes, wind direction is also able to be measured.
The measurement of meteorological parameters is important to gain an understanding of the impacts of a region's meteorology on air pollutant concentrations, the prediction of inversions, the study of wind fields and for dispersion modelling.
Temperature
Temperature and sunlight (solar radiation) play an important role in the chemical reactions that occur in the atmosphere to form photochemical smog from other pollutants. Favourable conditions can lead to increased concentrations of smog.
Temperature measurements are recorded as part of monitoring to support air quality assessment, air quality modelling and forecasting.
Humidity
Like temperature and solar radiation water vapour also plays an important role in many thermal and photochemical reactions in the atmosphere. As water molecules are small and highly polar, they can adsorb strongly to many substances. If attached to particles suspended in the air they can significantly increase the amount of light scattered by the particles (see monitoring of aerosols). If the water molecules attach to corrosive gases, such as sulfur dioxide, the gas will dissolve in the water and form an acid solution that can cause damage to health and property.
The amount of water vapour in the atmosphere is highly variable; it depends on geographic location, the nearness of water bodies, wind direction and ambient air temperature. Relative humidity is generally higher during summer when temperature and rainfall are also at their highest. The water vapour content of air is generally measured as a percentage of the saturation vapour pressure of water at a given temperature and is called relative humidity (%RH).
Humidity measurements are carried out using a thin polymer film that either absorbs or emits water vapour as the relative humidity of the ambient air changes. The absorption properties of the polymer film depend on the amount of water contained in it. As the relative humidity changes, so does the capacity of the polymer film to absorb or emit the water. A sensor measures these changes and converts them into a humidity reading.
Rainfall
Tipping bucket rain guage
Rain has a 'scavenging' effect when it washes particulate matter out of the atmosphere and dissolves gaseous pollutants. Reduced particle concentrations after rain improves visibility. The rain acts as a solvent for gaseous pollutants, such as sulfur dioxide, forming acid rain resulting in potential damage to materials or vegetation where it falls. Certain geographic areas, or particular times of the year, where there is frequent high rainfall are likely to have improved air quality.
Rainfall is measured using a tipping bucket rain gauge as illustrated (right). The gauge registers precipitation (rainfall) by counting small increments of rain collected. When rain falls into the funnel it runs into a container divided into two equal compartments by a partition.
The empty container has been designed to balance in stable equilibrium when resting against a stop. In its normal position it is tilted with one side or the other resting against a stop.
When a specified amount of rain has drained from the funnel into the upper compartment the bucket tilts the opposite way so that the compartment containing the rain comes to rest against the stop on the opposite side. The collected water then empties out and the other compartment starts to fill.
The number and rate of bucket movements are counted and logged electronically and together with the area of the funnel is converted into both the quantity and intensity of rainfall.
Solar radiation
The intensity of sunlight has an important influence on the rate of the chemical reactions that produce photochemical smog in the atmosphere. The intensity of sunlight can be affected by the cloudiness of the sky, time of day and geographic location. Therefore, it is important to monitor solar radiation for use in modelling photochemical smog events.
Solar radiation is measured using a device called a pyranometer. It measures the energy that radiates onto the surface of the instrument, or in more general terms—the intensity of sunlight.
Last updated 14 February 2011