Quantifying Denitrification and Its Effect on Ozone Recovery

 

Azadeh Tabazadeh

Upper Atmosphere Research Satellite (UARS) observations indicate that denitrification occurs in the Antarctic, without significant dehydration, during mid-to-late June (see figure). In contrast, UARS data show no indication for the presence of large-scale denitrification in the Arctic even during the coldest winters of the last decade (see figure). The fact that denitrification occurs in the Antarctic in a relatively warm month raises concern about the possibility for the occurrence of this event in a future colder and possibly more humid lower stratosphere, as a result of climate change and/or natural variability, and its subsequent effect on ozone recovery. Polar Stratospheric Cloud (PSC) lifetimes required for Arctic denitrification to occur in the future are presented and contrasted against the current Antarctic cloud lifetimes during early and mid-to-late June. Ozone sensitivity calculations show that widespread denitrification can enhance future Arctic ozone loss by about 40 % during the coldest winters of the next century.

It is well-known that PSCs play an important role in the formation of the springtime Antarctic "ozone hole" by activating chlorine and denitrifying the stratosphere. Since similar levels of active chlorine concentrations have been measured in both polar vortices, the lack of extensive denitrification observed in the Arctic has been speculated to be one of the main factors currently preventing the formation of an Arctic "ozone hole". Here, we present for the first time a quantitative study on the denitrification process that explains why this event currently occurs extensively in the Antarctic and not the Arctic, using data obtained from a number of different instruments onboard the UARS.

Up to now a few studies have implied that Arctic denitrification, during the coldest winters of the last decade, has already contributed significantly to the depletion of ozone inside the Arctic vortex. On the other hand there is a wealth of information available suggesting that the apparent and simultaneous loss of both ozone and reactive nitrogen in the Arctic is often purely a result of dynamical mixing of different air masses with high and low values of both species that is unrelated to denitrification. Also, the majority of in situ and balloon data sets showing denitrification in the Arctic are collected either directly over or downwind of mountainous terrain (mainly over Greenland and Norwegian mountains), where lee waves could strongly affect the local reactive nitrogen (and/or water vapor) profile through small-scale cloud processing. However the fact that satellite data show no indication of widespread denitrification in the Arctic at present suggests that the local perturbations caused by lee waves are not of global or even regional significance.

To resolve the controversy between space, in situ and balloon observations regarding denitrification in the Arctic, we introduce a new concept by comparing and contrasting "PSC lifetimes" between the two hemispheres to investigate whether "PSC lifetimes" could have been long enough in the past Arctic winters to have led to widespread denitrification. The concept of "PSC lifetime" provides new insights into how long a PSC needs to persist in the winter for the denitrification process to occur, and why the event currently occurs only in the Antarctic. Further, we plan to show that future forecasted perturbations in temperature and water vapor can increase Arctic "PSC lifetimes" to the point where denitrification can occur during the coldest winters of the next century with important implications for Arctic ozone recovery.

 

Collaborators: Michelle Santee, NASA JPL; Patrick Hamill, San Jose State University

Point of Contact: Azadeh Tabazadeh, 650/604-1096, atabazadeh@mail.arc.nasa.gov


[UARS Data Plot]
Minimum temperatures for a selected number of polar winters at the 50 mb pressure level, which is near the 450 K potential temperature surface. The periods for ternary PSC formation and denitrification during the Antarctic winter of 1992 are marked in the figure as blue boxes. The NAT (nitric acid trihydrate) and ICE envelopes (shaded gray) show temperatures at which nitric acid and water saturate to form nitric acid trihydrate and water ice particles, respectively. The ICE nucleation envelope marks the temperature when ice clouds can nucleate in the stratosphere. The critical temperature envelopes are calculated for nitric acid and water vapor mixing ratios of 9 to 12 ppbv and 4 to 5 ppmv, respectively, based on in situ observations. Two Arctic study periods are also marked in the plot as orange and green boxes for the 1994-1995 and 1995-1996 winters, respectively. Symbols shown are the time series of MLS gas phase nitric acid averaged and binned between the 75-80o S and 75-80o N for the Antarctic and the two Arctic winters, respectively.