Thursday, October 8, 2015

Up in smoke

Gene Towne and I were working on revising a paper reviewing what is known about the effects of differences in the timing of burning on grasslands and grazers.

We had written "burning in late-April after the green vegetation has emerged, exacerbates smoke production and accompanying air pollution, which is at the forefront of the burning controversy."

This sentence seemed obvious to us. If you burn green biomass, it's smoky as heck.  Still, the sentence did not have a citation. Rightly so, it should have.

I spent some time going through papers to see if what we had observed empirically had basis in the literature.

After a few hours reading papers, it seems that the statement wasn't wrong, but I did make a mistake in not reading these papers sooner. Probably the best paper was Andrae and Merlet from 2001, which is frustrating because I apparently could have learned all of this 15 years ago.

1) Smoke is complex. It contains O3, CO, water vapor, NOx, HCN (!), SO2, CH4, C2H2, xylene, benzene, etc. as well as particulates of certain sizes.

**Technical point: smoke researchers talk a lot about emission ratios (amount of a product produced in a fire relative to a standard like CO2) and emission factors (same, but relative to amount of biomass burned). They also talk about CE, combustion efficiency, with is an emission ratio of all the products besides CO2 relative to CO2 produced in a fire.

2) Fire is complex. It has stages: ignition, flaming + glowing + pyrolysis, glowing + pyrolysis (a.k.a. smoldering), glowing, and extinction. Each has different chemistry and emissions.

3) Flaming involves relatively complete oxygenation (burning) of products. Smoldering does not. Smoldering (burning without flame) is more likely to produce some products like CO and NH3 than flaming, which is more likely to produce products like CO2 and NOx.

4) It's interesting to read how stuff catches fire. When biomass starts to burn, the first step is the drying/distillation step. This releases water and volatiles. Then comes pyrolysis with "thermal cracking" of the molecules which produces char, tar, and volatiles. Here, stuff is breaking up, but not burning. As the biomass gets hotter, the tar and gas begin to oxidize, which produces the flame.
occurs. Once the volatiles have burned off, then the biomass begins to smolder (glowing fire) and many of the incomplete oxidation products are produced.

For us, we're likely to rewrite the sentence a bit to acknowledge the complexity of fire and smoke.

In all, when green grass is burned, it's nitrogen concentration is higher than senesced grass, which leads to greater production of NOx, which is a precursor to ozone production that causes health problems. Whether green grass has a lower combustion efficiency hasn't quite been resolved (Mebust and Cohen 2013). It should be lower with wetter biomass, but this apparently hasn't been definitively demonstrated. If so, then burning green vegetation is going to produce a lot more junk.

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