| Topic: | Re:Air pollution in Putney | |
| Posted by: | Richard Hodges | |
| Date/Time: | 03/03/14 14:14:00 |
| First question has to be whether you trust HAL modelling (HAL standing for Heathrow Airport Ltd.) - not exactly a pristine record there. Then, as you spotted, an incredible (sic) allocation of 2/3 of production to "background". What I have tried to show is that NO2 production by aircraft on approach is conservatively of a comparable magnitude to the assessed figure for road traffic, more likely a magnitude or more higher. What follows is dispersion, direction and conversion. - Dispersion will be near-total (partly depending on wind), which would lead to a fairly consistent background during operational hours - which is what I am suggesting is apparent in the metering data. - Direction is a combination of the course of the planes and the direction the wind takes the exhaust. Best combination has all emissions forced away from the town, but worst combination has everything all the way to Heathrow blown back across the town. Worst case also happens to be the normal prevailing condition. The full maths is complex, but it is not beyond the imagination to maintain a simple matrix of flight line, wind direction and speed. My contention is that we can expect to see a significant correlation day-by-day, particularly on Site 3 (off-High Street) - Conversion depends on so many features (temperature, wind, UV levels and more) that it cannot be practically modelled. I can only comment on the metering data published by the London Air Quality Network - daily, weekly and other extended periods. Particularly on Site 3 data, but also on Site 2 (Facade) I believe there is evidence of: - A secondary pattern, distinct from road traffic in time signature - A pattern that appears greater on some days over others. - A pattern that, whilst secondary to that of road traffic (and we know the explicit source here) is significant, taking up a significant proportion of the 200ug/m3 trigger level - Dramatic, common anomalies in all the data, not explainable by the generally published theory: low readings at peak traffic, high readings at low traffic (particularly on the supposedly 'traffic-free' Site 3) The evidence is clear: something other than road traffic (perhaps more than one thing) is impacting significantly on measured NO2 figures. It may be contributing 25-50% to the hourly value, which strongly suggests it has to be addressed if the >200ug/m3 peak limit is to be met. More than that, it looks like the annual mean cannot be met, even if the High Street were closed. I suspect aircraft, and it's doubly important if that is going to grow dramatically. We need access to the LAQN numbers to analyse and average out the data. And we need to secure or create a record of flight path, wind direction and speed to isolate the impact of aircraft. It's disappointing because this sort of analysis should have been in place four years ago. You should be very interested in the data for the Shepherd's Bush site (now apparently closed): similar geography, worse traffic, but noticeably better NO2 readings. I would suggest that the biggest difference between the two sites is that one often has planes, one doesn't. |