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We quantify the stratospheric injection of brominated very short-lived substances (VSLS) based on aircraft observations acquired in winter 2014 above the Tropical Western Pacific during the CONvective TRansport of Active Species in the Tropics (CONTRAST) and the Airborne Tropical TRopopause EXperiment (ATTREX) campaigns. The overall contribution of VSLS to stratospheric bromine was determined to be 5.0 ± 2.1 ppt, in agreement with the 5 ± 3 ppt estimate provided in the 2014 World Meteorological Organization (WMO) Ozone Assessment report (WMO 2014), but with lower uncertainty. Measurements of organic bromine compounds, including VSLS, were analyzed using CFC-11 as a reference stratospheric tracer. From this analysis, 2.9 ± 0.6 ppt of bromine enters the stratosphere via organic source gas injection of VSLS. This value is two times the mean bromine content of VSLS measured at the tropical tropopause, for regions outside of the Tropical Western Pacific, summarized in WMO 2014. A photochemical box model, constrained to CONTRAST observations, was used to estimate inorganic bromine from measurements of BrO collected by two instruments. The analysis indicates that 2.1 ± 2.1 ppt of bromine enters the stratosphere via inorganic product gas injection. We also examine the representation of brominated VSLS within 14 global models that participated in the Chemistry-Climate Model Initiative. The representation of stratospheric bromine in these models generally lies within the range of our empirical estimate. Models that include explicit representations of VSLS compare better with bromine observations in the lower stratosphere than models that utilize longer-lived chemicals as a surrogate for VSLS.

Air parcels with mixing ratios of high O and low HO (HOLW) are common features in the tropical western Pacific (TWP) mid-troposphere (300-700 hPa). Here, using data collected during aircraft sampling of the TWP in winter 2014, we find strong, positive correlations of O with multiple biomass burning tracers in these HOLW structures. Ozone levels in these structures are about a factor of three larger than background. Models, satellite data and aircraft observations are used to show fires in tropical Africa and Southeast Asia are the dominant source of high O and that low HO results from large-scale descent within the tropical troposphere. Previous explanations that attribute HOLW structures to transport from the stratosphere or mid-latitude troposphere are inconsistent with our observations. This study suggest a larger role for biomass burning in the radiative forcing of climate in the remote TWP than is commonly appreciated.

Hydroxyl radical (OH) is the main daytime oxidant in the troposphere and determines the atmospheric lifetimes of many compounds. We use aircraft measurements of O, HO, NO, and other species from the Convective Transport of Active Species in the Tropics (CONTRAST) field campaign, which occurred in the tropical western Pacific (TWP) during January–February 2014, to constrain a photochemical box model and estimate concentrations of OH throughout the troposphere. We find that tropospheric column OH (OH) inferred from CONTRAST observations is 12 to 40% higher than found in chemical transport models (CTMs), including CAM-chem-SD run with 2014 meteorology as well as eight models that participated in POLMIP (2008 meteorology). Part of this discrepancy is due to a clear-sky sampling bias that affects CONTRAST observations; accounting for this bias and also for a small difference in chemical mechanism results in our empirically based value of OH being 0 to 20% larger than found within global models. While these global models simulate observed O reasonably well, they underestimate NO (NO + NO) by a factor of 2, resulting in OH ~30% lower than box model simulations constrained by observed NO. Underestimations by CTMs of observed CHCHO throughout the troposphere and of HCHO in the upper troposphere further contribute to differences between our constrained estimates of OH and those calculated by CTMs. Finally, our calculations do not support the prior suggestion of the existence of a tropospheric OH minimum in the TWP, because during January–February 2014 observed levels of O and NO were considerably larger than previously reported values in the TWP.

We derive tropospheric column BrO during the ARCTAS and ARCPAC field campaigns in spring 2008 using retrievals of total column BrO from the satellite UV nadir sensors OMI and GOME-2 using a radiative transfer model and stratospheric column BrO from a photochemical simulation. We conduct a comprehensive comparison of satellite-derived tropospheric BrO column to aircraft in-situ observations of BrO and related species. The aircraft profiles reveal that tropospheric BrO, when present during April 2008, was distributed over a broad range of altitudes rather than being confined to the planetary boundary layer (PBL). Perturbations to the total column resulting from tropospheric BrO are the same magnitude as perturbations due to longitudinal variations in the stratospheric component, so proper accounting of the stratospheric signal is essential for accurate determination of satellite-derived tropospheric BrO. We find reasonably good agreement between satellite-derived tropospheric BrO and columns found using aircraft in-situ BrO profiles, particularly when satellite radiances were obtained over bright surfaces (albedo >0.7), for solar zenith angle

The goal of this presentation is to assess our quantitative understanding of the polar stratospheric ozone loss process. We present lagrangian observations of chemical ozone loss rates and of the temporal evolution of key chlorine species during sunset. These observations allow us to derive detailed information about the kinetics of the underlying chemical processes. The results are compared with highly constrained model calculations for these air masses. The space of kinetic parameters that lead to model results reproducing the observations within combined uncertainties is explored. The consistency with laboratory measurements of the parameters is assessed. In this approach good consistency suggests a complete understanding of the chemical processes while the lack of consistency would suggest gaps in the system of reactions in the model.In particular uncertainties of the photolysis cross sections of ClOOCl have long been a limiting factor in our theoretical understanding of the rate of polar stratospheric ozone losses. While previous observationally based work suggested values slightly larger than current recommendations from the laboratory, recent laboratory work suggested values that are nearly an order of magnitude smaller than what would be required to explain the observations of ozone loss and ClO in the atmosphere. This discrepancy triggered significant new laboratory work on the photolysis cross sections of this species. We will show what the impact of these new results on our understanding of polar ozone chemistry is. We will also discuss observational constraints for any unknown ozone loss chemistry in the polar winter stratosphere.

Uncertainties of the photolysis cross sections of ClOOCl have long been a limiting factor in our theoretical understanding of the rate of polar stratospheric ozone losses. Previous work suggested that values slightly larger than current recommendations, which are based on laboratory measurements, result in improved agreement between model calculations of polar stratospheric ozone loss rates and observations while at the same time also leading to improved agreement between observations of the diurnal variation of ClO and model calculations of this species. But new laboratory work (Pope et al, 2007) on the cross sections of ClOOCl suggest that its photolysis under polar stratospheric winter/spring conditions is nearly an order of magnitude slower than what would be required to explain the observations of ozone loss and ClO in the atmosphere and a factor of six slower than a value based on the current recommendations. We show what the impact of these new results on our understanding of polar ozone chemistry is and will discuss the potential for unknown chemical mechanisms that would lead to improved agreement between calculations of ozone loss based on the new cross sections with in-situ observations of ClO and ozone loss rates in the stratosphere.

Ozone loss in the polar regions is driven by halogen catalytic loss cycles. Central to the large ozone losses experienced every polar spring is the ClO dimer cycle. The recent ClOOCl photolysis cross-section measured by Pope et al. questions the role of this ClOOCl cycle in these large ozone losses.

Uncertainties of the photolysis cross sections of ClOOCl have long been a limiting factor in our theoretical understanding of the rate of polar stratospheric ozone losses. Previous work suggested that values slightly larger than current recommendations, which are based on laboratory measurements, result in improved agreement be- tween model calculations of polar stratospheric ozone loss rates and observations while at the same time also leading to improved agreement between observations of the diurnal variation of ClO and model calculations of this species. But new laboratory work (Pope et al, 2007) on the cross sections of ClOOCl suggest that its photolysis under polar stratospheric winter/spring conditions is nearly an order of magnitude slower than what would be required to explain the observations of ozone loss and ClO in the atmosphere and a factor of six slower than a value based on the current recommendations. We show what the impact of these new results on our understand- ing of polar ozone chemistry is. In model calculations that are based on the new cross sections and for typical Arctic conditions ratios of ClO/ClOx decrease by about a factor of two. The ozone loss rate by the ClO-dimer cycle, so far believed to be the most efficient ozone loss cycle, drops by about a factor of four and the loss rate by the coupled ClO-BrO cycle by nearly a factor of two. Overall ozone loss rates calculated based on the known ozone loss mechanisms drop by a factor of two to three and become much smaller than ob- servations. Also the calculated levels of ClO become much smaller than those ob- served in the stratosphere. These results demonstrate the tremendous uncertainty of current ozone loss calculations that comes from the broad range of the published cross sections for ClOOCl. In particular they suggest that, if the most recent publica- tion of the cross sections (Pope et al., 2007) is correct, a major fraction of observed polar ozone loss is due to a currently unknown

Utilizing in-situ measurements of ClO made in the same airmass more than once under different photochemical conditions during the Geophysica self-match flight during Euplex 2003, enabled us to examine our understanding of the kinetics of chlorine dimer cycle in detail. The kinetics of the chlorine dimer catalytic ozone loss cycle rely upon the thermal equilibrium between the monomer ClO and the dimer ClOOCl and the photolysis rate of the dimer ClOOCl. In this paper, we illustrate the kinetic constraints able to be determined by such a methodology. Our results suggest a thermal equilibrium constant a factor of 5 smaller and a ratio of photolysis rate to formation rate of ClOOCl a factor of 2 larger than the values based on the JPL recommendations.

The photolysis of ClOOCl is crucial in determining the rate of polar ozone loss due to the ClO+ClO cycle. New laboratory measurements of the ClOOCl cross section suggest that its photolysis is about a factor of six slower than a value based on current recommendations.

New laboratory measurements of the photolysis cross sections of ClOOCl result in values much smaller than previously published cross sections. Various presentations during this workshop will show that using the smaller cross sections in models based on standard chemistry results in calculated ozone loss rates, ClO, ClOOCl and OClO abundances that are not in agreement with atmospheric observations.

The ClO dimer cycle is thought to be the dominant ozone loss cycle in the polar winter/spring lower to mid stratosphere. The sensitivity of the efficiency of this cycle on key kinetic parameters is explored.

The photolysis of ClOOCl limits loss of polar ozone by the ClO+ClO cycle. New laboratory measurements of the ClOOCl cross section suggest that its photolysis is about a factor of six slower than a value based on current recommendations. We show the incorporation of these new cross sections into a photochemical model leads to poor agreement with values of ClO and ClOOCl measured during the SOLVE campaign, with the model under-estimating measured ClO and over-estimating measured ClOOCl by amounts that are much larger than the measurement uncertainties. These comparisons indicate that a model using the new cross section, and no other changes, provides a poor description of the partitioning of ClO and ClOOCl. Such a simulation also results in much slower ozone loss rates compared to a model using standard chemistry. It should be noted that, in the absence of the new cross section, there is still inconsistencies in our understanding of stratospheric chlorine when compared to stratospheric observations. We use the SOLVE data set to test a variety of postulated processes, discussed in a companion presentation, that could be invoked to resolve these discrepancies. The SOLVE data set is noteworthy because it provides a comprehensive description of the diurnal variation ClO and ClOOCl partitioning. For example, sufficient data was obtained during morning and evening to test hypothesis that result in predicted diurnal asymmetries in the partitioning. Implications of proposed new chemical mechanisms for ozone loss rates will also be discussed.

The chlorine chemistry under highly activated polar stratospheric spring conditions is critical to understanding the dramatic ozone losses that lead to the formation of the stratospheric ozone hole'. A particular flight pattern of the Geophysica high altitude aircraft during VINTERSOL-EUPLEX/SOLVE II on the 30th January 2003 in the Arctic was designed to test our theoretical understanding of the ClO/ClOOCl system. The outbound and inbound flight legs followed a calculated pattern such that the same air masses were sampled before and after sunset. During the inbound flight three encounters of the contrail from the outbound flight confirmed the success of the flight planning. In-situ ClO measurements were examined to study the conversion of ClO into its nighttime reservoir during sunset. Applying a trajectory Match technique, traditionally used to examine ozone-sonde concentrations, we examined this 3 hour period of ClO observations. 72 ClO Match pairs were identified with temperatures ranging between 200-206 K and SZAs between 84o and 95o. For a given Match pair the total active chlorine ClOx is assumed to be constant, but allowed to vary between Matches. This allowed the kinetic parameters controlling the ClO concentration - Keq, kf and J with differing SZA and temperatures to be closely examined. Due to the particular design of the flight pattern we are able to retrieve robust values for Keq based on ClO measurements alone. Hence our results for Keq are independent of the more uncertain dimer measurements (ClOOCl). We find values for Keq that are about a factor of 3-4 smaller than current JPL recommendations relatively independent of kf and J, as long as kf is larger than 70% of its recommended value. Derived values for J and ClOx are strongly anticorrelated. Hence, without dimer observations the retrieved J is very sensitive to assumptions about ClOx. However, using total available chlorine as an upper limit for ClOx, we find that the upper limits of the current J