What is a Pollution Rose?
A pollution rose is a radial plot used to visualise the relationship between wind direction and the concentration of a specific air pollutant, like a variation of the traditional wind rose.
Instead of plotting wind speed, it plots the average concentration of a pollutant for each wind direction sector, with the length of each "petal" or spoke indicating the measured concentration.
This helps identify the location of pollution sources by showing which wind directions lead to higher levels of a pollutant.
For example:
- Short, dark-coloured spokes - Infrequent winds but with high pollution
- Long, light-coloured spokes - Frequent winds bringing cleaner air
- Long, multi-coloured spokes - Frequent winds with variable pollution levels
*Please note that the data for wind speed and direction is taken from MET Stations
What is the purpose of a Pollution Rose?
Purpose: It helps identify the source and direction of air pollution. For example, if a pollution rose shows high concentrations of a specific pollutant when the wind is blowing from the industrial district, it suggests that the district is a source of that pollution.
Tooltip: Select different pollutants from the drop down menu in the toolbar.
If your preferred pollutant is not in the list. Please re-run Analysis to update them.
How does a Pollution Rose work?
- Structure: Similar to a traditional wind rose, it uses a polar coordinate system to show directional data.
- Data plotted: Wind direction is shown on the circular axis, based on count of hours the wind came from that direction. Each spoke is then further categorised by concentration.
- Comparison: In “Network View” Multiple pollution roses can be plotted on the same figure to compare data from different monitoring locations. Please note: If you cannot interact with the Pollution Rose please zoom in. The Rose Petals will then become interactive when appropriate.
What does a Pollution Rose show?
Interpreting the patterns on a pollution rose can provide valuable insights into local air quality:
- Source identification: A long spoke in a particular direction suggests that more wind comes from that direction. The colour then indicates what level of pollutant is present.
- Comparative analysis: You can plot pollution roses for multiple pollutants (e.g., PM10 and PM2.5) to see if a single source is affecting different types of emissions. For instance, a high PM10 reading from one direction but a low PM2.5 reading could suggest a non-combustion source, like wind-blown dust.
- Multiple sources: If a pollution rose shows high concentrations from several directions, it may indicate multiple contributing sources.
- Triangulation: By using several monitoring locations, analysts can compare the pollution roses to triangulate the precise location of a source. For example, if multiple sites show high pollution readings when the wind is from the direction of a known industrial site, it can help confirm that source is a major contributor.
What the Pollution Rose Petals mean…
Further Interpretation of the Concentration Bands and Spokes and Direction
- The concentration bands on the pollution rose show different concentration levels based on percentile ranges.
- The ranges go from the lowest reading upto the 90th percentile
- Each band gets a colour from light (low pollution) to dark (high pollution)
- The breakpoints (thresholds)are calculated from the minimum value upto the 90th percentile of the data (not the maximum). This prevents outliers from skewing the visualisation.
- The spoke length indicates how frequently wind comes from that direction (as a percentage of total observations)
- The spoke colour shows the concentration distribution when wind comes from that direction; Lighter colours = lower concentrations Darker colours = higher concentrations Multiple colour bands in one spoke show the range of concentrations observed.
- Also the stacked nature of the chart means each spoke shows both the frequency of winds and the distribution of concentrations from that direction simultaneously.
- The petal shows pollution when the wind blows from that direction toward the monitor.
- A long petal pointing east (E) means pollution is high when the wind comes from the east.
- The opposite side (west, W) does not represent exposure on the west. It simply corresponds to another directional bin (wind blowing from the west).
What the Pollution Rose does NOT show
- Where pollution goes
- Where people downwind are exposed
- What happens on the “other side” of the petal in a geometric sense
It only shows pollution arriving at the monitor from that direction.
What you cannot assume from a Pollution Rose alone
You cannot assume:
“…and therefore the pollution continues past the sensor and keeps travelling south beyond it.”
Why? Because:
1. The sensor is a point measurement
It only tells you what arrives at that point, not what happens afterward.
2. Concentrations change rapidly downwind
After passing the monitor the pollution could:
- Dilute
- Be chemically transformed
- Lift vertically
- Be blocked by terrain or buildings
- Mix with cleaner air
- Deposit to the ground
So the “plume” may or may not still exist further downwind.
3. There may be new sources or sinks beyond the monitor
You can’t assume uniform conditions on the far side.
4. Wind direction at ground level can shift over short distances
Buildings, trees, or terrain can alter flow patterns.
You can say:
“Under the measured wind conditions, air that passed the sensor was moving in a particular direction.”
But that’s not the same as proving that:
- The same polluted air reaches a specific place beyond the sensor
- The pollution concentration remains similar
To make those claims, you need dispersion modelling or additional sensors downwind.
So the safe interpretation is:
A Pollution Rose shows the direction polluted air arrives from — not where it ultimately goes after the monitor.
Enjoy!