What exactly led to the wonderful clearance of air pollution last night? The Defra forecast for today says, "Air pollution levels will be Low for the whole of the UK today due to brisk westerly winds". That makes all kinds of sense. And so I was surprised when I looked at the data my sensors gathered last night and realised that the steep drop in PM2.5 pollution began hours before wind speeds began to increase.
Below is a bit of a busy, untidy graph, but bear with me. It's a quick mash up using a number of different graphs all displaying data for the same period of time, and superimposed on top of each other.
A puzzling picture
The first thing to note is the dark blue line for PM2.5 dropping from a peak of 50ug/m³ at about 2100hrs yesterday down to less than 5ug/m³ by 1000hrs today. The steepest drop occurs in the period highlighted in pink on the graph.
The second thing to note is the time at which wind speed (yellow line for gusts, light blue line for average, wind scale on left axis in mph) begins to pick up, about midnight. In fact wind speeds do not begin to increase above their previous (high PM2.5 concentration) values until after the pink region, ie. after the steepest drop in PM2.5 has occured. If anything the wind speed is lower in the pink region, although not by much. This is counter-intuitive!
The third thing to note is that the wind speed does actually pick up after midnight and the drop in PM2.5 continues in that period with peaks in wind speed correlating with dips in PM2.5 concentration. This is as expected.
It's a puzzler. How has the pollution dropped so rapidly without wind increasing? That leads to ...
The fourth thing to note. Atmospheric pressure - the green line (there is no scale for this, but peak is about 1032hPa, and minimum around 1022hPa) - begins its drop at precisely the same time that PM2.5 hits its peak and begins to fall. I do not want to clutter up the graph above any more, so I have not included the temperature readings on it. However, you can see from the set of graphs below (from which I extracted the pressure and wind lines for my mashup) that the beginning of the change in pressure correlates with a marked change in air temperature too.
Atmospheric pressure is very revealing, we find it hard to observe without instrumentation of some kind (apart from those who can feel a storm coming "in their bones"), but monitoring it reveals that there is a lot of activity going on around us that we cannot see or feel directly. I need to do some checking, but I would imagine that this change in air pressure was associated with a warm front moving over Eastbourne, introducing a fresh mass of air. This would correlate with the rapid drop in PM2.5 with the pressure change. The Met office pressure chart for noon on Saturday includes a warm front over northern France which could quite easily be the culprit we are looking for.
The wind picked up after front moved past which led to new pollution being blown away 3 hours later. Speculation at this point. I need to brush up on my weather fronts meteorology.