Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 4 2 2004 10.5194/acpd-4-1941-2004 http://www.atmos-chem-phys-discuss.net/4/1941/2004/ http://www.atmos-chem-phys-discuss.net/4/1941/2004/acpd-4-1941-2004.html http://www.atmos-chem-phys-discuss.net/4/1941/2004/acpd-4-1941-2004.pdf 1941 1975 2004-04-06 The annual cycle of hydrogen peroxide: is it an indicator of chemical instability? R. W. Stewart richard.w.stewart@nasa.gov NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA A box model has been used to study the annual cycle in hydrogen peroxide concentrations with the objective of determining whether the observed difference in summer and winter values reflects instability in the underlying photochemistry. The model is run in both steady-state and time-dependent modes. The steady-state calculations show that, for some range of NO_x background levels, two stable solutions to the continuity equations exist for a period of days in spring and fall. The corresponding time-dependent model indicates that, for sufficiently high background NO_x concentrations, the spring and fall changes in H₂O₂ concentration may be interpreted as a forced transition between the two underlying stable regimes. The spring transition is more rapid than that in fall, an asymmetry that becomes more marked as background NO_x increases. This asymmetry is related to the different time scales involved in chemical production and loss of H₂O₂. Observations of the spring increase in H₂O₂ concentration may therefore provide a better measure of the change in the underlying chemical regime than does the fall decrease.