Download Comments on wind direction and the influence on measured PM2.5

Comments on wind direction and the influence on measured PM2.5 (smoke)
pollution levels at Ti Tree Bend Air Station (Launceston)
Summary: This document explores the influence of prevailing winds on the measurement of PM2.5
(smoke) pollution at the Ti Tree Bend air station (Launceston). It is shown that high pollution levels
mostly occur on cold, calm nights. The prevailing wind under these conditions is from the south-toeast quadrant, which carries air from the Launceston urban and residential areas towards Ti Tree
Bend. North-West winds (which would bring air from rural areas up the Tamar to Ti Tree Bend) are
shown to preferentially occur during warmer weather, when domestic wood-heaters are not
commonly in operation. There is no indication to suggest that Ti Tree Bend is poorly located with
respect to the Launceston residential areas in terms of detecting winter-time smoke pollution.
Ti Tree Bend Air Quality station is located on the Tamar River, several kilometres north-west of the
Launceston CBD. Routine monitoring of PM10 and PM2.5 (particulate matter up to 10 µm and 2.5 µm
aerodynamic diameters respectively) has been carried out for a number of years. The site was
selected following field measurements and air dispersion modelling studies. In this report we
reassess the suitability of the site for measuring winter-time smoke pollution from Launceston’s
domestic wood heaters.
Figure 1 shows data derived from hourly measurements obtained at Ti Tree Bend from mid July 2007
(when the station was equipped with its own meteorological sensors) to early March 2009. We
show the proxy PM2.5 hourly estimates (derived from a dustrak optical scattering instrument) as well
as hourly wind direction and hourly air temperature.
The top left panel shows proxy PM2.5 plotted against air temperature. In general, the highest
pollution values occur during cold weather (due to the use of domestic solid fuel heating). The high
data points taken near T=20 C arise from bushfires in summer. The cluster of points near a
temperature of 10 C arises from smoke from planned burns in autumn. Apart from these, the
remaining data indicate smoke pollution is closely tied to ambient air temperature. This is a well
documented behaviour at Launceston.
Figure 1 - Launceston Ti Tree Bend station hourly data, July 2007 - March 2009. Top left: Proxy PM2.5 and
ambient temperature. High PM2.5 values tend to occur during the cold weather. Top right: probability
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distribution of wind directions. Two prominent peaks occur, a well defined peak near 300 (north-westerly)
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and a broader peak from around 80 to 180 (east to south). Bottom right: Ambient temperature and wind
direction. North-west winds are a feature of the warmer weather. Bottom left: PM2.5 and wind direction.
High PM2.5 values are associated with easterly-to-southerly winds.
The top left panel shows the distribution of wind directions (in the usual meteorological convention
of the direction of origin of the wind) plotted as a probability density function (i.e. the area under
the curve is equal to unity). There is a large peak near 300O, which is an ‘up-river wind’, with a
secondary, broader, peak around 70O to 170O. Ti Tree Bend is located a few kilometres north-west
of the Launceston city centre. Generally speaking a wind in the range from 70O to 170O means that
there will be air movement from the populated areas towards Ti Tree Bend.
Integration shows that the wind at Ti Tree Bend blows from 280O to 340O around 45% of the time,
and from 70O to 180O about 37% of the time.
The lower left panel shows the relationship between wind direction and ambient temperature.
Generally speaking the 70O to 170O direction winds occur in cold weather. The 280O to 340O winds
are associated with warmer temperatures.
The lower right panel shows air quality (measured by the PM2.5 proxy) and wind direction. It seems
that the largest pollution readings are obtained for the 70O to 170O winds, but high values can also
occur for the up-river winds at times, albeit much less commonly.
Important information is also available from wind speeds. The Figure 2 shows in the top panel the
measured hourly wind speed and the wind direction. The strongest winds come from the northwest
(up-river winds) and from the south-east (which would be down-river). The lower panel shows air
pollution plotted against wind speed. It is clear that the strongest pollution measurements occur
under conditions of low wind speed, usually less than 1 m s1
.
Figure 2 - Ti Tree Bend data. Top panel: hourly wind speed and direction. Strong winds are mainly either
north-westerly or south-easterly. Bottom panel: PM2.5 and wind speed. High PM2.5 measurements occur
almost exclusively under low wind speed conditions.
Taking these data into account, and excluding smoke from planned burns and bushfires, the
conclusion is that the biggest pollution signals seen at Ti Tree Bend occur under cold, calm
conditions (wind speed less than 1 m s-1), when the wind is moving air from the population areas
towards Ti Tree Bend. Given this, and the topography, this wind is likely to be a down-valley
drainage flow (katabatic).
Winds blowing from the north-west (up-river), which would move air from Ti Tree Bend to
Launceston, tend to occur in the warmer weather (when domestic wood heater use would be
reduced) and tend to be stronger. The stronger winds would mean greater mixing and dispersion of
any pollutants produced in Launceston.
If we consider the distribution of wind directions for data when the hourly temperature was below
8 C we obtained the histogram shown in Figure 3. Clearly the previous peak in the north-west wind
occurrence is greatly reduced. For these data, 66% of the wind directions lie between 70O and 180O,
while only 15% fall in the 280O to 340O range. Hence north-west (up-river) winds at Ti Tree Bend are
relatively rare in cold weather.
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Figure 3 - Histogram of wind speeds for cold weather (hourly temperature less than 8 ). The occurrence of
north-westerly winds is greatly reduced under these conditions.
Figure 4 shows hourly data of wind direction and wind speed for Ti Tree Bend, but only for instances
when the temperature was below 8O C. The plot focuses on behaviour under low wind-speed
conditions. The points are plotted with a symbol size proportional to the proxy PM2.5 measurement.
As noted, the highest PM2.5 measures occur under low wind speed conditions. The concentration of
high PM2.5 points in the 90O to 180O wind directions clearly shows that under calm cold conditions Ti
Tree Bend is sampling air that has moved from the urban areas of Launceston.
Figure 4 - Wind speed and wind direction for low wind speed conditions. The size of the symbol is
proportional to the PM2.5 value. High PM2.5 readings tend to preferentially occur for easterly-to-southerly
winds
This analysis shows that in winter Ti Tree Bend Air station is frequently sampling air that has
originated in the direction of the Launceston urban area, and hence provides a measure of
conditions likely to be experienced by residents of the city and surrounds.
JLI, March 2009.