Sunday, 11 February 2018

Uncertainty about pollution from wood stoves

In my previous post on air pollution, especially fine particulates (PM2.5), in Cambridge I concentrated on traffic, hardly mentioning wood stoves  as a possible source. However, wood burning is cited as an important source of pollution in parts of London [1] so why not Cambridge? To be honest I steered clear of the issue because there is such a huge amount of uncertainty I felt unable to present any facts with confidence. The National Atmospheric Emissions Inventory (NAEI) estimates that 35% of PM2.5 in the air is from domestic wood burning[2] - or it could be 10 times less [3].

The issue is so uncertain that the government is running a call for evidence on the issue. This runs until 27th Feb.  In their call for evidence they say [4]:
  • Burning wet wood (i.e. not properly seasoned wood) generates at least twice the emissions from dry wood.
  • Estimates for the proportion of wood that is burnt wet range from 80% (from the wood industry) to 20% (from a survey by BEIS).
  • Estimates of how much wood is burnt in total range for 3 to 6 million tonnes per year
  • Burning on an open fire generates up to 10 times the emissions from a modern wood stove.

So on the supply side of the problem there is uncertainty in how much wood is burnt, whether it is dried first or not and whether it is burnt on an open fire or a stove.  The National Atmospheric Emissions Inventory (NAEI) includes estimates of air pollutions sources across a range of sectors but they admit that their estimates of domestic burning could be wrong by an order of magnitude. They say they have been conservative and probably overestimated the source [3].

Particulates are not the only emissions from wood burning. For example wood contains chlorine and treated wood contains a lot of chlorine which means that burning it can produce dioxins. NAEI estimates that 17% of dioxins in the air come from domestic wood burning [2] - but that estimate depends on how much treated waste wood is being burnt.

You would think it would be possible to get a good idea of the size of the issue from monitoring actual pollution, but this is also non-trivial because there are many different sources. One way to tell particulates apart is by their colour - which wavelengths they absorb and which they reflect. Using this technique, measurement at about 30 sites across the country has found the contribution of wood usually 5-7% in winter in cities, but up to 10% in one case. Considering the whole year, wood contribution is less and in rural areas, wood smoke is less concentrated [5]. The number of sample points is not very large but it does suggest that particulates from wood burning are not too much of a problem in most places.

In my analysis of air pollution at monitoring sites in Newmarket Road and Gonvillle Place, I looked for correlation with outdoor temperature, or with weekday/weekend patterns, as you would expect if there was a significant contribution from wood stoves. I found a little correlation with temperature and nothing significant on days of the week. This may mean it is not a problem in Cambridge, at least not yet. One thing that all the sources agree on is that emissions from wood fires and stoves are increasing.

[1] Air pollution: Sadiq Khan calls for ban on wood-burning stoves (Guardian) Sep 2017
[2] UK emissions data selector (NAEI) latest data is from 2015
[3] The Potential Air Quality Impacts from Biomass Combustion (DEFRA) 2017
[4] Call for evidence on Domestic Burning of House Coal, Smokeless Coal, Manufactured Fuel and Wet Wood (DEFRA) Jan 2018
[5] Airborne particles from wood burning in UK cities (UCL) March 2017

1 comment:

  1. Article in the Guardian about this today: