A discussion on the determination of atmospheric OH and its trends

Abstract. The global hydroxyl radical distribution largely determines the oxidation efficiency of the atmosphere and, together with their sources and atmospheric transport, the distributions and lifetimes of most trace gases. Because of the great importance of several of these gases for climate, ozone budget and OH itself, it is of fundamental importance to acquire knowledge about atmospheric OH and possible trends in its concentrations. In the past, average concentrations of OH and trends were largely derived using industrially produced CH3CCl3 as a chemical tracer. The analyses have given valuable, but also rather uncertain results. In this paper we describe an idealized computer aided tracer experiment which has as one of its goals to derive tracer concentration weighted, global average <k(OH)>, where he temporal and spatial OH distribution is prescribed and k is the reaction rate coefficient of OH with a hitherto never produced (Gedanken) tracer, which is injected at a number of surface sites in the atmosphere in well known amounts over a given time period. Using a three-dimensional (3D) time-dependent chemistry/transport model <k(OH)> can be accurately determined from 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 of surface sites. The results from this analysis are encouraging enough to actually think about the feasibility to carry out a global dedicated tracer experiment to derive <k(OH)> and its temporal trends. However, before that, we propose to test the methods which are used to derive <k(OH)>, so far largely using CH3CCl3, with an idealized tracer experiment, in which a global model is used to calculate the "Gedanken"  tracer distribution, representing the real 3-D world, from which we next derive <k(OH)>, using only the tracer information from a limited set of surface sites. We propose here that research groups which are, or will be, involved in global average OH studies to participate in such an inter-comparison of methods, organized and over-seen by a committee appointed by the International Global Atmospheric Chemistry (IGAC) program.