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Observations of methyl chloroform combined with an atmospheric transport model predict a Northern to Southern Hemisphere hydroxyl ratio of slightly less than 1, whereas commonly used atmospheric chemistry models predict ratios 15-45% higher. The north-south distribution of atmospheric OH The hydroxyl radical is an important atmospheric oxidant, but our knowledge of its global distribution remains imprecise, with estimates for the ratio of Northern Hemisphere to Southern Hemisphere hydroxyl radical concentration varying from 0.85 to 1.4. These authors use a three-dimensional chemistry-transport model that has been well validated for interhemispheric transport using sulphur hexafluoride measurements, to obtain an interhemispheric hydroxyl radical ratio of 0.97[plus or minus]0.12. This information can help improve our understanding of the fate of atmospheric pollutants and greenhouse gases. The hydroxyl radical (OH) is a key oxidant involved in the removal of air pollutants and greenhouse gases from the atmosphere.sup.1,2,3. The ratio of Northern Hemispheric to Southern Hemispheric (NH/SH) OH concentration is important for our understanding of emission estimates of atmospheric species such as nitrogen oxides and methane.sup.4,5,6. It remains poorly constrained, however, with a range of estimates from 0.85 to 1.4 (refs 4, 7,8,9,10). Here we determine the NH/SH ratio of OH with the help of methyl chloroform data (a proxy for OH concentrations) and an atmospheric transport model that accurately describes interhemispheric transport and modelled emissions. We find that for the years 2004-2011 the model predicts an annual mean NH-SH gradient of methyl chloroform that is a tight linear function of the modelled NH/SH ratio in annual mean OH. We estimate a NH/SH OH ratio of 0.97 [plus or minus] 0.12 during this time period by optimizing global total emissions and mean OH abundance to fit methyl chloroform data from two surface-measurement networks and aircraft campaigns.sup.11,12,13. Our findings suggest that top-down emission estimates of reactive species such as nitrogen oxides in key emitting countries in the NH that are based on a NH/SH OH ratio larger than 1 may be overestimated., Author(s): P. K. Patra [sup.1] [sup.2] , M. C. Krol [sup.3] , S. A. Montzka [sup.4] , T. Arnold [sup.5] , E. L. Atlas [sup.6] , B. R. Lintner [sup.7] [...]

We use NAME, a trajectory model, to investigate the routes and timescales over which air parcels reach the tropical tropopause layer (TTL). Our aim is to assist the planning of aircraft campaigns focussed on improving knowledge of such transport. We focus on Southeast Asia and the Western Pacific which appears to be a particularly important source of air that enters the TTL. We first study the TTL above Borneo in November 2008, under neutral El Niño/Southern Oscillation (ENSO) conditions. Air parcels (trajectories) arriving in the lower TTL (below ∼15 km) are most likely to have travelled from the boundary layer (BL; <1 km) above the West Pacific. Few air parcels found above ∼16 km travelled from the BL in the previous 15 days. We then perform similar calculations for moderate El Ni ño (2006) and La Ni ña (2007) conditions and find year- to-year variability consistent with the phase of ENSO. Under El Niño conditions fewer air parcels travel from the BL to the TTL above Borneo. During the La Niña year, more air parcels travel from the BL to the mid and upper TTL (above ∼15 km) than in the ENSO-neutral year, and again they do so from the BL above the West Pacific. We also find intra-month variability in all years, with day-to-day differences of up to an order of magnitude in the fraction of an idealised short lived tracer travelling from the BL to the TTL above Borneo. These calculations were performed as a prelude to the SHIVA field campaign, which took place in Borneo during November 2011. So finally, to validate our approach, we consider measurements made in two previous campaigns. The features of vertical profiles of short-lived species observed in the TTL during CR-AVE and TC4 are in broad agreement with calculated vertical profiles of idealised short-lived tracers. It will require large numbers of observations to fully describe the statistical distribution of short-lived species in the TTL. This modelling approach should prove valuable in planning flights for the long-duration aircraft now capable of making such measurements. [ABSTRACT FROM AUTHOR]

Emission of bromine from sea-salt aerosol, frost flowers, ice leads, and snow results in the nearly complete removal of surface ozone during Arctic spring. Regions of enhanced total column BrO observed by satellites have traditionally been associated with these emissions. However, airborne measurements of BrO and O3 within the convective boundary layer (CBL) during the ARCTAS and ARCPAC field campaigns at times bear little relation to enhanced column BrO. We show that the locations of numerous satellite BrO “hotspots” during Arctic spring are consistent with observations of total column ozone and tropopause height, suggesting a stratospheric origin to these regions of elevated BrO. Tropospheric enhancements of BrO large enough to affect the column abundance are also observed, with important contributions originating from above the CBL. Closure of the budget for total column BrO, albeit with significant uncertainty, is achieved by summing observed tropospheric partial columns with calculated stratospheric partial columns provided that natural, short-lived biogenic bromocarbons supply between 5 and 10 ppt of bromine to the Arctic lowermost stratosphere. Proper understanding of bromine and its effects on atmospheric composition requires accurate treatment of geographic variations in column BrO originating from both the stratosphere and troposphere., Engineering and Applied Sciences