Our paper on isocyanic acid (HNCO) is in press at the Journal of Geophysical Research-Atmospheres. HNCO is emitted from fires, and some other sources, and is potentially harmful at high concentrations (> 1 ppb). The study is the first to investigate the global distribution of HNCO, following on from Jim Roberts et al.'s work in measuring the chemical in the atmosphere and understanding some of its physical and chemical properties - see their paper in PNAS.
We modeled high concentrations in several regions with large fire sources. The first figure below shows a map of the number of days where the model predicts surface concentrations of HNCO greater than 1 ppb, on a grid cell (2°x2°) basis. The second figure weights the data in a different way, showing the size of the population potentially impacted. The populations are for areas where HNCO is greater than 1 ppb for 7 days or more. Click on the images for larger versions.
Overall, we hope this study encourages atmospheric and health scientists to make more measurements and better understand any potential risks from exposure.
Some more information can be found from the NOAA Research article, CIRES press release or AGU blog post.
Young, P. J., L. K. Emmons, J. M. Roberts, J.-F. Lamarque, C. Wiedinmyer, P. R. Veres and T. C. VandenBoer (2012). 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.
Just finished my poster for next week's AGU Fall Meeting in San Francisco (coauthored with Jean-François Lamarque, Andrew Conley, Doug Kinnison and Francis Vitt, all at NCAR).
We have run several 150 year (1850-2000) CAMChem simulations where one of several "forcings" (NOx, VOC, CO emissions, methane emissions, aerosols, CFC concentrations, and climate) was kept at its pre-industrial (PI) conditions, allowing the others to evolve to their present day (PD) conditions as normal. The goal was to isolate the contribution of a given forcing to (e.g.) the overall tropospheric ozone radiative forcing - i.e. how much of the Pi to PD ozone change is due to methane emissions?. This was done by comparing these simulations against one where everything was allowed to evolve. Shindell et al. (2009) did something similar to this, but they kept everything at PI conditions save the one forcing that they allowed to evolve to its PD values.
The bar chart shows the contribution of the different forcings to the overall PI to PD tropospheric ozone radiative forcing, with the insert comparing the results to those from Shindell et al. (and our simulations are detailed in table). Clearly, for quantifying the contribution of different forcings it matters if you change one in a PI background (Shindell et al.) or keep one at PI conditions and allow the others to change. E.g., increasing NOx emissions with PI VOCs is different to increasing NOx emissions with PD VOCs (...we are just finishing a simulation that looks at NOx emissions alone).
Overall, separating out the contributions of a given forcing to a chemically-active greenhouse gas like ozone is complicated!
"Changes in stratospheric temperatures and their implications for changes in the Brewer-Dobson circulation, 1979-2005", by Young, P. J., K. H. Rosenlof, S. Solomon, S. C. Sherwood, Q. Fu, and J.-F. Lamarque has been accepted and can be found on the Journal of Climate in press page.
From analyzing at the "annual cycle" of temperature trends from the satellite-mounted MSU and SSU instruments, and IUK radiosonde data, we found evidence of a significant strengthening of the Northern Hemisphere (NH) branch of the Brewer-Dobson circulation (BDC) in December, extending throughout the depth of the stratosphere. Similarly, we found a significant strengthening of the Southern Hemisphere branch of the BDC during August, extending to the middle stratosphere, The results also suggested a weakening of the lower stratosphere NH branch of the BDC during March, although there is some suggestion that this is not indicative of a secular trend (see also Free, 2011).
Models predict an increase in the strength of the BDC with increased greenhouse gas concentrations (e.g. Butchart et al., 2010), so any significant change could be a fingerprint of climate change.
The results are consistent with the MSU temperature/BDC studies of Fu et al. (2010) and Lin et al. (2010), but extend the analysis to the middle and upper stratosphere through the use of the SSU data.
Application to Chemistry-climate models (CCMVal2)?
In the paper we defined a "BDC index", which tracked the strength in the BDC by subtracting tropical temperatures from extratropical temperatures (see Yulaeva et al. (1994) and Young et al. (2011) for more on the tropical/high-latitude "see-saw" in temperatures). As a part of the response to reviewers (but not in the paper) we looked at how trends in the our BDC index correlated with modeled tropical vertical velocity trends, which are a more direct measure of BDC strength. The above figure shows that trends in the BDC index correlate quite closely with 21st century tropical (20°S/N and 30°S/N) vertical velocity trends using data from three chemistry-climate models (CCMs) from the recent CCMVal-2 exercise. Although not perfect for all altitudes/models, it perhaps illustrates the utility of our index for monitoring changes in the strength of the BDC.
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