[1] We have developed a global three-dimensional chemistry-meteorology model for studies of ozone and hydrocarbons in the troposphere, called Model of Atmospheric Transport and Chemistry-Max-Planck-Institute for Chemistry Version (MATCH-MPIC). The model currently calculates the distributions of 54 species and 141 reactions using a new flexible chemical integration method in connection with a fast general Rosenbrock solver. The reactions can be easily expanded for future studies with the model. The model includes updated emission inventories, an explicit dry deposition scheme, online photolysis rates, extensive budgeting capabilities and a correction for the so-called ''mass-wind inconsistency'' problem. One-year simulations at two different horizontal resolutions, approximately 1.9°Â 1.9°(T63) and 5.6°Â 5.6°(T21), both with 28 vertical levels, are extensively evaluated with available observations from surface stations, ozonesondes, and field campaigns. The model is generally able to reproduce the observations of ozone to within 10 nmol/mol, but it overestimates upper tropospheric ozone at northern high-latitude stations in winter and spring and tends to underestimate the summer maximum in the free troposphere. In the tropics the chemical tropopause is sometimes too low. Generally, the low-resolution run yields only slightly worse agreement with observations compared to the higher-resolution run, thus making it suitable for further sensitivity studies. In a simulation using different meteorological data, O 3 agrees much better with observations in the upper troposphere, possibly because of the higher resolution near the tropopause. The net ozone production integrated over all tropospheric regions with a net production and loss separately reveals that the calculated chemically induced redistribution of ozone in the troposphere is 2-3 times larger than the net stratospheric influx. Even in the upper troposphere, photochemical production is of similar magnitude to the stratospheric influence.

Abstract.Recently several field campaigns and satellite observations have found strong indications for the presence of bromine oxide (BrO) in the free troposphere. Using a global atmospheric chemistry transport model we show that BrO mixing ratios of a few tenths to 2 pmol mol −1 lead to a reduction in the zonal mean O 3 mixing ratio of up to 18% in widespread areas and regionally up to 40% compared to a model run without bromine chemistry. A lower limit approach for the marine boundary layer, that does not explicitly include the release of halogens from sea salt aerosol, shows that for dimethyl sulfide (DMS) the effect is even larger, with up to 60% reduction of its tropospheric column. This is accompanied by dramatic changes in DMS oxidation pathways, reducing its cooling effect on climate. In addition there are changes in the HO 2 : OH ratio that also affect NO x and PAN. These results imply that potentially significant strong sinks for O 3 and DMS have so far been ignored in many studies of the chemistry of the troposphere.

Abstract. The global mean OH concentration ([OH]GM ) has been used as an indicator of the atmospheric oxidizing efficiency and its changes over time. It is also used for evaluating the performance of atmospheric chemistry models by comparing with other models or with observationally-based reference [OH] GM levels. We contend that the treatment of this quantity in the recent literature renders it problematic for either of these purposes. Several different methods have historically been used to compute [OH] GM : weighting by atmospheric mass or volume, or by the reaction with CH 4 or CH 3 CCl 3 . In addition, these have been applied over different domains to represent the troposphere. While it is clear that this can lead to inconsistent [OH] GM values, to date there has been no careful assessment of the differences expected when [OH] GM is computed using various weightings and domains. Here these differences are considered using four different 3D OH distributions, along with the weightings mentioned above applied over various atmospheric domains. We find that the [OH] GM values computed based on a given distribution but using different domains for the troposphere can result in differences of 10% or more, while different weightings can lead to differences of up to 30%, comparable to the uncertainty which is commonly stated for [OH] GM or its trend. Thus, at present comparing [OH] GM values from different studies does not provide clearly interpretable information about whether the OH amounts are actually similar or not, except in the few cases where the same weighting and domain have been used in both studies. We define the atmospheric oxidizing efficiency of OH with respect to a given gas as the inverse of the lifetime of that gas, and show that this is directly proportional to the [OH] GM value weighted by the reaction with that gas, where the proportionality constant depends on the temperature distribution and the domain. We find that the airmass-weighted

[1] Ozone is an air quality problem today for much of the world's population. Regions can exceed the ozone air quality standards (AQS) through a combination of local emissions, meteorology favoring pollution episodes, and the clean-air baseline levels of ozone upon which pollution builds. The IPCC 2001 assessment studied a range of global emission scenarios and found that all but one projects increases in global tropospheric ozone during the 21st century. By 2030, near-surface increases over much of the northern hemisphere are estimated to be about 5 ppb (+2 to +7 ppb over the range of scenarios). By 2100 the two more extreme scenarios project baseline ozone increases of >20 ppb, while the other four scenarios give changes of À4 to +10 ppb. Even modest increases in the background abundance of tropospheric ozone might defeat current AQS strategies. The larger increases, however, would gravely threaten both urban and rural air quality over most of the northern hemisphere.