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Friday, July 27, 2012

"UV Dosage Levels in Summer: Increased Risk of Ozone Loss from Convectively Injected Water Vapor," by James G. Anderson*, David M. Wilmouth, Jessica B. Smith and David S. Sayres, Science, DOI: 10.1126/science.1222978

Science DOI: 10.1126/science.1222978

UV Dosage Levels in Summer: Increased Risk of Ozone Loss from Convectively Injected Water Vapor

  1. David S. Sayres
  1. 1Department of Chemistry and Chemical Biology, Department of Earth and Planetary Sciences and School of Engineering and Applied Sciences, Harvard University, Cambridge, MA.
  1. *Correspondence:


The observed presence of water vapor convectively injected deep into the stratosphere over the United States fundamentally changes the catalytic chlorine/bromine free radical chemistry of the lower stratosphere by shifting total available inorganic chlorine into the catalytically active free-radical form, ClO. This chemical shift markedly affects total ozone loss rates and makes the catalytic system extraordinarily sensitive to convective injection into the mid-latitude lower stratosphere in summer. Were the intensity and frequency of convective injection to increase as a result of climate forcing by the continued addition of CO2 and CH4 to the atmosphere, increased risk of ozone loss and associated increases in UV dosage would follow.
Because the binary sulfate-water aerosols are ubiquitous in the lower stratosphere, if the necessary temperature and water conditions are met, then heterogeneous conversion of inorganic chlorine to free radical form can occur anywhere, not just in the polar regions. While the Arctic lower stratosphere is marginally colder than the mid-latitude lower stratosphere over the US in summer, what matters is the combination of water vapor concentration and temperature.
The in situ observations of H2O obtained from both the high altitude NASA ER-2 and WB-57 aircraft extending over a number of recent missions are summarized in Fig. 1B. The data shown were retrieved during flights originally selected to observe the outflow from typical convective storms over the US in summer. What proved surprising is the remarkable altitude to which large concentrations of water vapor are observed to penetrate. The convective injection of water into the stratosphere was also observed with surprising frequency, occurring in approximately 50% of the summertime flights over the US. The convective origin of this water vapor is established by simultaneous in situ observations of H2O and the HDO isotopologue (19, 20), the concentration of which differentiates between direct convective injection and other pathways linking the troposphere and stratosphere (19, 2123). The observed presence of water vapor enhancements reaching and occasionally exceeding 12 ppmv at temperatures in the vicinity of 200 K in the altitude region between 15 and 20 km, as displayed in Fig. 1B, has significant consequences.
The initiation of fundamental changes in the photochemistry of the lower stratosphere in summer is captured in Fig. 1C, that superimposes on the threshold plot for chlorine activation over the range of 2–10 μm2/cm3 for reactive surface area, the observed in situ H2O mixing ratios and temperatures at 90 ± 10 mb pressure. It is clear that, at observed water vapor concentrations and temperatures, the threshold for chlorine activation converting inorganic chlorine to free radical form is routinely crossed in the summertime. The result is that ClO can become a major component of the available inorganic chlorine budget within regions of high water vapor. Convective injection of water vapor to heights reported here can occur in storm systems that are ~50 km across, with smaller domains of high altitude injection embedded within them at their origin (24). The elevated concentrations of water can spread to 100 km or more in horizontal extent within a few days (19, 25), and remain at the elevated levels reported here over a period of days. This phenomenon has been analyzed by Newman et al. (26) using high altitude (70–100 mb) observations of rocket plume dispersion that defines the rate of horizontal spreading from a point source. Additionally, the circular flow pattern of air in the lower stratosphere over the US resulting from the North American summer monsoon provides the potential for repeated convective injection events into the summer lower stratosphere over the US.
There are a number of important considerations associated with the issue of convective injection of water vapor inducing chlorine activation and catalytic removal of ozone over mid-latitudes of the NH in summer. First is the fact that a remarkably dry stratosphere characterizes the current climate state. However, the paleorecord holds evidence that the stratosphere, under conditions of high CO2 concentrations, was characterized by significantly higher water vapor concentrations than is the case today (43, 44). If currently increasing concentrations of CO2, CH4 and other infrared active gasses force the stratospheric system to a state of increasing water vapor concentrations, the impact on ozone is of significant concern given the concentrations of chlorine and bromine in the stratosphere today.
Second, the loss of ice from the Arctic Ocean opens the possibility for significant increases in CO2 and CH4 release from melt zones in the Arctic. A release of just 0.5% per year of the carbon tied up in the soils of Siberia and Northern Alaska alone will double the carbon added to the atmosphere each year from the combustion of fossil fuels world-wide (45). This release of carbon from clathrates and permafrost will accelerate the forcing of the climate that is potentially linked to the intensity and frequency of convective injection of water into the stratosphere.
Third, engineering the climate by the addition of sulfates to increase reflective aerosol concentrations and thereby reduce climate forcing by reflecting sunlight back to space (46, 47) would significantly increase reactive surface area which would accelerate the processing of chlorine to free radical form (Fig. 1, A and C), thereby decreasing ozone concentrations. In the same vein, the convective injection of water vapor into the stratosphere increases the sensitivity of ozone loss to volcanic injection of sulfates into a stratosphere with current loading of chlorine and bromine. Evidence for this was presented for the eruption, in 1991, of Mt. Pinatubo by Salawitch et al. (35).
Fourth, from the perspective of human health, a primary concern is that decreasing ozone concentrations, particularly in summer over populated areas, results in increased UV dosage levels. Sustained increases in UV dosage levels are in turn associated with the increased incidence of skin cancer (48, 49), which is currently 1 million new cases a year in the US (49).
Lastly we emphasize that, because chlorine activation depends exponentially on water vapor and temperature, and in turn that the forcing of climate may well control the convective injection of water into the lower stratosphere, the idea that ozone “recovery” is in sight because we have controlled CFC and halon release is a potentially significant misjudgment.                

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