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The abundance of chlorine in the Earth's atmosphere increased considerably during the 1970s to 1990s, following large emissions of anthropogenic long-lived chlorine-containing source gases, notably the chlorofluorocarbons. The chemical inertness of chlorofluorocarbons allows their transport and mixing throughout the troposphere on a global scale1, before they reach the stratosphere where they release chlorine atoms that cause ozone depletion2. The large ozone loss over Antarctica3 was the key observation that stimulated the definition and signing in 1987 of the Montreal Protocol, an international treaty establishing a schedule to reduce the production of the major chlorine-and bromine-containing halocarbons. Owing to its implementation, the near-surface total chlorine concentration showed a maximum in 1993, followed by a decrease of half a per cent to one per cent per year4, in line with expectations. Remote-sensing data have revealed a peak in stratospheric chlorine after 19965, then a decrease of close to one per cent per year6, 7, in agreement with the surface observations of the chlorine source gases and model calculations7. Here we present ground-based and satellite data that show a recent and significant increase, at the 2σ level, in hydrogen chloride (HCl), the main stratospheric chlorine reservoir, starting around 2007 in the lower stratosphere of the Northern Hemisphere, in contrast with the ongoing monotonic decrease of near-surface source gases. Using model simulations, we attribute this trend anomaly to a slowdown in the Northern Hemisphere atmospheric circulation, occurring over several consecutive years, transporting more aged air to the lower stratosphere, and characterized by a larger relative conversion of source gases to HCl. This short-term dynamical variability will also affect other stratospheric tracers and needs to be accounted for when studying the evolution of the stratospheric ozone layer. Disciplines Medicine and Health Sciences | Social and Behavioral Sciences Publication DetailsMahieu, E., Chipperfield, M. P., Notholt, J., Reddmann, T., Anderson, J., Bernath, P. F., Blumenstock, T., Coffey, M. T., Dhomse, S. S., Feng, W., Franco, B., Froidevaux, L., Griffith, D. W. T., Hannigan, J. W., Hase, F., Hossaini, R., Jones, N. B., Morino, I., Murata, I., Nakajima, H., Palm, M., Paton-Walsh, C., Russell III, J. M., Schneider, M., Servais, C., Smale, D. & Walker, K. A. (2014). Recent Northern Hemisphere stratospheric HCl increase due to atmospheric circulation changes. Nature, 515 (7525), 104-107. The large ozone loss over Antarctica 3 was the key observation which stimulated the definition and signing of the Montreal Protocol in 1987, an international treaty establishing a schedule to reduce the production of the major chlorine-and brominecontaining halocarbons. Owing to its implementation, the near-surface total chlorine concentration showed a maximum in 1993, followed by a decrease of 0.5-1 %/yr 4 , in line with expectations. Remote-sensing data have revealed a peak in stratospheric...

The structure and variability of the middle atmosphere of Venus (60 to 140 km) were studied from the Pioneer Venus orbiter by using an infrared remote sensing instrument developed from those on terrestrial weather satellites. The wavelengths observed were selected to allow the vertical temperature profile, the albedo, the cloud opacity profile, and the far infrared opacity due to water vapor to be inferred from the data. The measured temperature field has been used to model the dynamics of the region, and the thermal and solar fluxes have been used to compute the planetary radiation budget. The results for the diurnal variation of temperature at a given height show fairly small amplitudes up to an altitude of about 95 km, above which the day to night contrast increases rapidly with height. At the equator the dependence of temperature in the stratosphere (65 to 95 km) on solar longitude is dominated by a wave number 2 solar tide with an amplitude of about 10 K. Transient features including traveling waves are also present on a wide range of scales. The equator to pole gradients are larger than expected, and the stratosphere is typically 15 to 20 K warmer at the pole than at the equator. Nightside temperatures in the mesosphere (95 to 140 km) are generally low except for a local maximum near the antisolar point, and breakdown of local thermodynamic equilibrium is evident above about 120 km. The winds forced by the measured temperature field in a diagnostic circulation model show the ‘4‐day’ zonal wind decreasing rapidly with height above the clouds and becoming very small by 80 or 90 km. altitude. The mean meridional component reverses at about the same altitude and pole‐to‐equator winds as high as 100 m s−1 are produced above 100 km. The most significant discovery concerning the cloud morphology is a dramatic ‘dipole’ structure, consisting of two clearings in the cloud at locations straddling the pole and rotating around it every 2.7 days. The clearings are thought to be evidence for subsidence of the atmosphere at the center of a polar vortex. The absence of corresponding evidence for descending motions elsewhere suggests that a single large circulation cell may fill the northern hemisphere at levels near the cloud tops. A crescent‐shaped ‘collar’ region, consisting of anomalous and variable temperature and cloud structure, surrounds the pole at about 70°N and rises perhaps 15 km above the mean cloud top elevation; it has a solar‐fixed component and sometimes contains spiral streaks. This feature, and the double vortex eye, are large, persistent deviations from the mean circulation due to planetary‐scale waves of unknown origin. No explanation is offered at present for the dominance of wave number 2 structures at equatorial and polar latitudes, while the mid‐latitudes are dominated by a wave number 1 feature (the polar collar). A thin, ubiquitous haze is found covering the northern hemisphere, including the polar features. The far‐infrared opacity of the atmosphere is greater in the afternoon than at any other lo...

The budget of nitrogen oxides (NO x) in the arctic free troposphere is calculated with a constrained photochemical box model using aircraft observations from the Tropospheric O 3 Production about the Spring Equinox (TOPSE) campaign between February and May. Peroxyacetic nitric anhydride (PAN) was observed to be the dominant odd nitrogen species (NO y) in the arctic free troposphere and showed a pronounced seasonal increase in mixing ratio. When constrained to observed acetaldehyde (CH 3 CHO) mixing ratios, the box model calculates unrealistically large net NO x losses due to PAN formation (62 pptv/day for May, 1-3 km). Thus, given our current understanding of atmospheric chemistry, these results cast doubt on the robustness of the CH 3 CHO observations during TOPSE. When CH 3 CHO was calculated to steady state in the box model, the net NO x loss to PAN was of comparable magnitude to the net NO x loss to HNO 3 (NO 2 reaction with OH) for spring conditions. During the winter, net NO x loss due to N 2 O 5 hydrolysis dominates other NO x loss processes and is near saturation with respect to further increases in aerosol surface area concentration. NO x loss due to N 2 O 5 hydrolysis is sensitive to latitude and month due to changes in diurnal photolysis (sharp day-night transitions in winter to continuous sun in spring for the arctic). Near NO x sources, HNO 4 is a net sink for NO x ; however, for more aged air masses HNO 4 is a net source for NO x , largely countering the NO x loss to PAN, N 2 O 5 and HNO 3. Overall, HNO 4 chemistry impacts the timing of NO x decay and O 3 production; however, the cumulative impact on O 3 and NO x mixing ratios after a 20-day trajectory is minimal.

Local ozone production and loss rates for the arctic free troposphere (58-85 • N, 1-6 km, February-May) during the Tropospheric Ozone Production about the Spring Equinox (TOPSE) campaign were calculated using a constrained photochemical box model. Estimates were made to assess the importance of local photochemical ozone production relative to transport in accounting for the springtime maximum in arctic free tropospheric ozone. Ozone production and loss rates from our diel steady-state box model constrained by median observations were first compared to two point box models, one run to instantaneous steady-state and the other run to diel steady-state. A consistent picture of local ozone photochemistry was derived by all three box models suggesting that differences between the approaches were not critical. Our model-derived ozone production rates increased by a factor of 28 in the 1-3 km layer and a factor of 7 in the 3-6 km layer between February and May. The arctic ozone budget required net import of ozone into the arctic free troposphere throughout the campaign; however, the transport term exceeded the photochemical production only in the lower free troposphere (1-3 km) between February and March. Gross ozone production rates were calculated to increase linearly with NO x mixing ratios up to ∼300 pptv in February and for NO x mixing ratios up to ∼500 pptv in May. These NO x limits are an order of magnitude higher than median NO x levels observed, illustrating the strong dependence of gross ozone production rates on NO x mixing ratios for the majority of the observations. The threshold NO x mixing ratio needed for net positive ozone production was also calculated to increase from NO x ∼ 10 pptv in February to ∼25 pptv in May, suggesting that the NO x levels needed to sustain net ozone production are lower in winter than spring. This lower NO x threshold explains how wintertime photochemical ozone production can impact the build-up of ozone over winter and early spring. There is also an altitude dependence as the threshold NO x needed to produce net ozone shifts to higher values at lower altitudes. This partly