A discussion on the determination of atmospheric OH and its trends

[摘要] The oxidation efficiency of the troposphere is largely determined by thehydroxyl radical and its global distribution. Its presence limits the lifetime of most trace gases.Because of the great importance of several of these gases for climate, ozone budget andOH itself, it is of fundamental importance to acquire knowledge about atmosphericOH and possible trends in its concentrations. In the past, average concentrations ofOH and trends were largely derived using industrially produced CH₃CCl₃ as a chemicaltracer. The analyses have given valuable, but also rather uncertain results. In this paper wedescribe an idealized computer aided tracer experiment which has as one ofits goals to derive tracer concentration weighted, global average <k(OH)>, wherethe temporal and spatial OH distribution is prescribed and k is thereaction rate coefficient of OH with a hitherto never produced (Gedanken) tracer, which is injected at a number of surface sites in the atmosphere inwell known amounts over a given time period. Using a three-dimensional (3-D)time-dependent chemistry transport model, <k(OH)> can be accurately determinedfrom the calculated 3-D tracer distribution. It is next explored how well<k(OH)> can be retrieved solely from tracer measurements at a limited number ofsurface sites. The results from this analysis are encouraging enough to actually think about the feasibility to carry out a global dedicated tracerexperiment to derive <k(OH)> and its temporal trends. However, before that, wepropose to test the methods that are used to derive <k(OH)>, so far largelyusing CH₃CCl₃, with an idealized tracer experiment, in which aglobal chemistry transport model is used to calculate the ``Gedanken'' tracer distribution,representing the real 3-D world, from which <k(OH)> is derived, using onlythe tracer information from a limited set of surface sites. We propose herethat research groups which are, or will be, involved in global average OH studies to participate in such an inter-comparison of methods, organized andover-seen by a committee appointed by the International Global AtmosphericChemistry (IGAC) program.

[学科分类] 大气科学