Continuing the theme of wildfires in the Western Contiguous U.S (CONUS), we have this recent paper by Spracklen et.al.: Wildfires drive interannual variability of organic carbon aerosol in the western U.S. in summer.

Abstract: Forest wildfire area burned in the western U.S. has increased in recent decades resulting in a substantial organic carbon (OC) source with large interannual variability. We derive OC emissions from wildfires using data for area burned for 1980–2004 and ecosystem specific fuel loadings.For the period 1989–2004 we analyze OC observations in the western U.S. from the IMPROVE network and use a global chemical transport model to simulate OC concentrations. Modeled and observed OC concentrations are highly correlated when we use interannually varying fire emissions (R2 = 0.88); the correlation is smaller with climatological emissions (R2 = 0.4). We estimate that the observed increase in wildfire activity after the mid 1980s has caused mean OC concentrations in summer over the western U.S. to increase by 30% relative to 1970–1984. In the coming decades, climate change will likely cause further increases in wildfires resulting in increased OC concentrations with implications for health and visibility.

In the above figure, Spracklen has plotted the area burned on the left vertical axis and both the organic carbon (OC) emissions due to biomass burning and biogenic activities on the right axis as a function of year. The biogenic OC emissions are fairly constant from year-to-year; roughtly 0.2 teragrams (a teragram is 1,000,000,000,000 grams, which is 1 million metric tons). This is a lot of carbon.

Also notice that there is a wider variability in the biomass burned OC. The mean value of the biomass OC appears to be around 0.2 Tg, the same as the biogenic OC. However, there is a much higher variance in the time series. Values range from near zero up to 0.6 Tg. But perhaps most important of all, the burn area is correlated with the biomass OC. The more acres burned, the more biomass OC. It is very intuitive, but it only matters if the magnitude of the variations was of the same order of magnitude (or larger) than the background biogenic OC contributions. As it turns out, they are. Therefore, the interannual variability in the OC is largely due to biomass burning.

There are many implications for this research. In the previous post, I introduced a paper in the journal Science that linked the amount of wildfires in the Western CONUS to the rising temperatures. It showed that there is a positive correlation between the surface temperature and the frequency of wildfires; as the temperature rises, so does the occurence of fires. The Spracklen paper tells us that when we have more wildfires, there is more biomass OC. The carbon is not carbon dioxide. It is not a gas; it is a solid particle called an aerosol. Aerosols are not very well understood, but these organic carbon aerosols are thought be be “black” and absorb the incoming solar radiation. This tends to warm the lower atmosphere, contrasted with sulfate aerosols which are “white” and reflect the incoming solar radiation.

In addition to the potential effects on climate, increased aerosols have detrimental effects on human health and welfare. These particles can get into our lungs, and do bad things - I’m not exactly what they do or how they do it, but I’m told it’s not good. Think cigarette smoke; that’s an aerosol too. In addition to the health effects of increased aerosols, they also decrease the visibility. A decrease in the visibility causes a decrease in welfare. People like to see long distances - they don’t like to be able to see only a half-mile due to the high concentrations of aerosol particles.

Spracklen et.al. conclude:

We estimate that increases in wildfire area burned in the western U.S. after the mid-1980s has caused long-term mean summertime OC concentrations in the western U.S. to increase by 30% (from 1.0 ug/m3 between 1970 and 1984 to 1.3 ug/m3 in 1995–2004). Based on an analysis of the IMPROVE data, we find that the contribution of organic carbon mass to the total fine aerosol concentration is about 40% in low fire years, but increases to about 55% in high fire years. The increase in wildfires and OC concentrations we describe here will therefore have important implications for visibility degradation. Increasing temperature over the next few decades in the western U.S. could drive further increases in wildfire activity with potential impacts on OC concentrations. Further degradation in visibility due to increasing wildfires will make it increasingly difficult to meet targets set by the U.S. Environmental Protection Agency [2003] Regional Haze Rule. The impact of future climate change on forest fires in North America needs to be quantified to allow us to estimate the effects on future air quality.

Spracklen, D. V., J. A. Logan, L. J. Mickley, R. J. Park, R. Yevich, A. L. Westerling, and D. A. Jaffe (2007), Wildfires drive interannual variability of organic carbon aerosol in the western U.S. in summer, Geophys. Res. Lett., 34, L16816, doi:10.1029/2007GL030037. [PDF, requires subscription]

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