1 May 2012
A smoke-related chemical, isocyanic acid, may be a significant air pollutant in some parts of the world, especially where forest fires and other forms of biomass burning are common, according to new research by NOAA scientists and colleagues.
In a modeling study, the scientists found that relatively high levels of isocyanic acid likely occur in parts of tropical Africa, Southeast Asia, China, Siberia, and the Western Amazon Basin.
"This is the first study to model the global distribution of isocyanic acid, and the concentrations we estimated for many regions are high enough to warrant more research to understand people's exposures," said Paul Young, lead author of the paper published online on April 30 in the Journal of Geophysical Research. Young is an atmospheric scientist with NOAA ESRL CSD and CIRES (Cooperative Institute for Research in Environmental Sciences) at the University of Colorado at Boulder.
Isocyanic acid is emitted by burning biomass including forest fires, and people who cook or heat their homes using inefficient stoves could get significant exposure to the chemical. In the body, isocyanic acid can trigger certain kinds of protein damage which, in turn, leads to cataracts and inflammation associated with cardiovascular disease and rheumatoid arthritis, according to published health research.
NOAA scientists published a study last year that was one of the first to observe the acid in the air. Using a NOAA-built custom instrument, the researchers found the chemical in the air of downtown Los Angeles, downwind of a Colorado wildfire, and in cigarette smoke. That study also suggested that if the acid was present in the air at 1 part per billion by volume (ppbv) or higher, it could trigger chemical reactions associated with negative health effects.
In the new study, Young and his colleagues investigated further, using a sophisticated computer model of the atmosphere to better understand the chemical's distribution around the globe. Their model used the limited information available on isocyanic acid to include processes that remove the chemical from the atmosphere and to estimate its emissions.
The researchers found that annual average levels of isocyanic acid are likely highest in parts of China, where modeled concentrations reached 0.470 ppbv (about half of the 1 ppbv suspected to trigger health concerns). But at certain times, the pollutant spiked significantly higher in regions where episodic fires occurred: Isocyanic levels periodically reached 4 ppbv in parts of tropical Africa and the Western Amazon Basin, and up to 10 ppbv in southeast Asia and Siberia.
In parts of Siberia and tropical Africa, modeled isocyanic acid levels surpassed 1 ppbv for more than 60 days out of the year, the team reported. In more highly populated areas, however, exposure is potentially more significant: For Southeast Asia, the scientists estimated that more than 50 million people could be exposed to levels of isocyanic acid above 1 ppbv for more than seven days in one year.
Isocyanic acid is difficult to detect in the atmosphere with conventional measurement techniques, and the emissions estimates in the new paper are likely significantly underestimated in some parts of the world (where inefficient stoves are common), and possibly overestimated in others (where fossil fuel burning for electricity is more common).
"Our estimates are preliminary," said co-author Jim Roberts, a NOAA chemist. "But given the potentially high concentrations of isocyanic acid in those hotspots, and the numbers of people potentially affected, we hope that more atmospheric scientists and health researchers will be encouraged to look into this issue."
Young, P. J., L. K. Emmons, J. M. Roberts, J.-F. Lamarque, C. Wiedinmyer, P. R. Veres, and T. C. VandenBoer, Isocyanic acid in a global chemistry transport model: Tropospheric distribution and budget, and identification of regions with potential health impacts, Journal of Geophysical Research, doi:10.1029/2011JD017393, 2012.
This study uses a global chemical transport model to estimate the distribution of isocyanic acid (HNCO). HNCO is toxic, and concentrations exceeding 1ppbv have been suggested to have negative health effects. Based on fire studies, HNCO emissions were scaled to those of hydrogen cyanide (30%), resulting in yearly total emissions of 1.5Tg for 2008, from both anthropogenic and biomass burning sources. Loss processes included heterogeneous uptake (pH dependent), dry deposition (like formic acid), and reaction with the OH radical (k = 1 x 10-15 molecule-1 cm3 s-1). Annual mean surface HNCO concentrations were highest over parts of China (maximum of 470pptv), but episodic fire emissions gave much higher levels, exceeding 4ppbv in tropical Africa and the Amazon, and exceeding 10ppbv in Southeast Asia and Siberia. This suggests that large biomass burning events could result in deleterious health effects for populations in these regions. For the tropospheric budget, using the model-calculated pH the HNCO lifetime was 37 days, with the split between dry deposition and heterogeneous loss being 95%:5%. Fixing the heterogeneous loss rate at pH=7 meant that this process dominated, accounting for ~70% of the total loss, giving a lifetime of 6 days, and resulting in upper tropospheric concentrations that were essentially zero. However, changing the pH does not notably impact the high concentrations found in biomass burning regions. More observational data is needed to evaluate the model, as well as a better representation of the likely underestimated biofuel emissions, which could mean more populations exposed to elevated HNCO concentrations.