Abstract. Many halocarbons are powerful greenhouse gases and also influence climate indirectly through depletion of stratospheric ozone which opposes their direct greenhouse effect. Changes in effective radiative forcing (ERF) from historical ozone depletion have been diagnosed from model experiments with perturbed halocarbons run under the sixth Coupled Model Intercomparison Project. This is more negative than the offline stratospheric-temperature-adjusted radiative forcing (SARF). Including effects of ozone depletion on the methane lifetime makes the historical net ERF of ozone depleting substances consistent with zero. The Integrated Ozone Depletion (IOD) metric has been used to apportion this ERF between the halocarbon species and thereby derive indirect 100-year Global Warming Potentials (GWP100s) for a suite of halocarbons. The indirect GWP100 for CFC-11 is enough to make the net GWP100 likely negative, whereas the indirect contribution for CFC-12 is smaller due to a combination of longer stratospheric lifetime and fewer chlorine atoms. use of the online ERF, rather than the offline SARF, allows the model physics to account for changes in stratospheric temperature (as well as tropospheric temperature, water vapour and clouds) rather than estimating stratosphere temperature changes using fixed dynamical heating. This online calculation of radiative forcing rather than offline leads to approximately double the indirect GWPs compared to World Meteorological Organization assessments. This formalism can be used with other estimates of ozone ERF, as the indirect GWPs scale linearly with this quantity.

Halogen (chlorine, bromine, and iodine) species actively interact with other atmospheric constituents such as nitrogen oxides, organic gases, and ozone. Previous field observations confirmed the ubiquitous existence of halogens in various environments and modeling simulations quantified halogen impacts on air pollutants at present time. However, the abundance and impacts of continental inorganic halogens (CIH) on global air quality throughout history remain unexplored. Here, we present a global inventory of CIH emissions from anthropogenic and biomass-burning sources from 1970 to 2015, with average fluxes of 4302, 207, and 98 kt/yr for chlorine, bromine, and iodine, respectively, representing ~15%, 5-10%, and ~3% of the corresponding global short-lived chlorine, bromine, and iodine emissions. Incorporating these emissions into a global chemistry-climate model reveals a substantial increase ( > 50%) in inorganic halogen levels over continents, helping to reproduce the observed halogen levels. Our results also suggest substantial effects of CIH on air pollutant concentrations with significant spatio-temporal variations, e.g., the largest perturbations in nitrogen oxides and secondary aerosols migrating from Europe and North America in the 1980s-1990s, to East Asia in the 2000s, then to South Asia in the 2010s, suggesting a movement of CIH-associated air quality impact hotspots from the developed regions to the developing areas. This study highlights the under-appreciated role of halogen chemistry in air quality and its evolution in the recent four decades and calls for attention to the potential CIH impacts in emerging regions where field observations are not yet available.

Atmospheric methane is a potent greenhouse gas that is photochemically active. The addition of chlorine to the atmosphere has been proposed to mitigate global warming through methane reduction by increasing its chemical loss. However, the potential environmental impacts of such climate mitigation remain unexplored. We explore the possible effects of increasing reactive chlorine emissions on the methane budget, atmospheric composition and radiative forcing. Due to non-linear chemistry we found that achieving effective methane reduction require a minimum 3-fold increase in chlorine atoms compared to present-day levels. Our highest scenario, 50-fold present-day chlorine levels, led to a reduction of the surface temperature by 0.6°C in the year 2050. Beyond the direct effects on methane and temperature, our results show significant alterations in other climate forcers, particularly a large decrease in tropospheric ozone. This translates into a reduction in radiative forcing of a similar magnitude as of the methane removed. Additionally, the Antarctic stratosphere ozone burden during September and October was reduced by up to 40% with the highest chlorine addition. Consequently, the implementation of such strategies requires careful consideration of various factors, including the quantity and method of chlorine addition, as well as potential environmental impacts on air quality and ocean acidity.

Abstract This study investigates the sources and regional attributions of nitrogen oxides (NOx) in the upper troposphere|upper tropospheric (UT) during the Asian Summer Monsoon (ASM). The importance of South Asia (SA) and East Asia (EA) contributions is the subject of main interest. Using artificial tracers in a chemistry‐climate model, simulations with tracers from surface anthropogenic and lightning sources in SA and EA are conducted. Model results are validated with airborne observations from the Asian Summer Monsoon Chemical and Climate Impact Project (ACCLIP) campaign in 2022 over the West Pacific. Good agreement between modeled and observed NOx is found in the UT. The results indicate that within the ASM anticyclone, both SA and EA sources significantly contribute to the UT NOx, with contributions of 41% and 36%, respectively. While in the ACCLIP region during 2022, EA sources play a more important role, accounting for 50% compared to 19% from SA sources.

Global modeling of the hydroxyl radical (OH) remains a significant challenge, pushing chemistry-climate models to rely on idealized scenarios with methane () concentrations rather than emission fluxes. In this study, we employ an emission-driven configuration in the Community Earth System Model Version 2.2 (CESM2.2) and demonstrate the effect of incorporating detailed Short-Lived Halogen (SLH) chemistry representation on both emission- and concentration-driven simulations in terms of global methane loss and overall chemical dynamics. The net impact of the updated SLH chemistry reduces ozone () and hydroxyl radical (OH) in both hemispheres, resulting in higher abundance and longer lifetime of carbon monoxide (CO) and . Comparisons with NASA's Atmospheric Tomography (ATom) mission data show joint improvements in OH, , CO and . Further evaluation against CO measurements from NASA's Measurement of the Pollution in The Troposphere (MOPITT), from JAXA's Greenhouse Gases Observing Satellite (GOSAT) confirms significant amelioration in modeled CO and , especially in the Northern Hemisphere during winter and spring, correcting a common wintertime underestimation. The annual tropospheric loss with OH is reduced from 573 to 504 Tg in 2017, resulting in an increase in lifetime of about 1.2 years, bringing it to approximately 10 years, which is well within the range of uncertainty in empirical estimates. In contrast, the estimated chlorine sink increases from 2 to around 15 Tg . Additionally, we find that the sensitivity of the 's chemical loss to CO emission changes is underestimated in the prescribed simulations.

The solar forcing dataset prepared for the 6th round of the Coupled Model Intercomparison Project (CMIP6) has been used extensively in climate model experiments. Recently, an International Space Science Institute (ISSI) Working Group was established to revisit the solar forcing recommendations in order to define a roadmap for building a revised solar forcing dataset for the upcoming 7th round of CMIP (Funke et al., 2023). This new dataset will introduce changes in the radiative forcing of climate either directly, or indirectly via changes in atmospheric composition. In CMIP6, the solar forcing consisted of both a total solar irradiance (TSI), along with a spectrally resolved solar irradiance (SSI). The TSI for solar minimum was set to 1360.8±0.5Wm-2 and the SSI covered the 10nm to 100mm spectral region. A similar approach is proposed for CMIP7 except for two major aspects of the reconstruction: 1) the definition of the reference spectrum for the quite Sun; 2) the temporal variability. The major difference between the proposed CMIP7 SSI quite sun reference spectrum and that used for CMIP6 is the spectral shape. The new SSI spectrum has an irradiance that is 1-5% higher in the visible band and lower by 1-2% in the Near-IR wavelength range (1000-2000nm). The solar temporal variability in the CMIP6 and CMIP7 reconstructions are based on both the NRLSSI2 and SATIRE reconstructions. These reconstructions have been improved in preparation for CMIP7 and the aim is for both reconstructions to use the same reference spectrum and be driven by the same solar proxies. In this work we used the Whole Atmosphere Community Climate Model (WACCM) to examine the chemical and climate implications of the proposed CMIP7 solar forcing updates compared to the CMIP6 approach. WACCM is a chemistry-climate model that extends from the surface to 140km. The horizontal resolution is ~1degree. WACCM has a detailed representation of chemical and dynamical processes from the troposphere through the lower thermosphere. We examined the “chemical only” impacts of the solar forcing choice by running WACCM in the specified dynamics mode using NASA Modern-Era Retrospective analysis for Research and Applications Version 2 (MERRA2). The “climate” impacts were derived by running the model with interactive dynamics coupled to a deep ocean. Conclusions from this work will support the development of the next version of WACCM for participation in the CMIP7 assessment.Funke, B., Dudok de Wit, T., Ermolli, I., Haberreiter, M., Kinnison, D., Marsh, D., Nesse, H., Seppälä, A., Sinnhuber, M., and Usoskin, I.: Towards the definition of a solar forcing dataset for CMIP7, Geosci. Model Dev. Discuss. https://doi.org/10.5194/gmd-2023-100.

We analyze tropical ozone (O3) and carbon monoxide (CO) distributions in the upper troposphere (UT) for 2005–2020 using Aura Microwave Limb Sounder (MLS) observations and simulations from the Whole Atmosphere Community Climate Model (WACCM) and two variants of the Community Atmosphere Model with Chemistry (CAM-chem), with each variant using different anthropogenic CO emissions. Trends and variability diagnostics are obtained from multiple linear regression. The MLS zonal mean O3 UT trend for 20° S–20° N is +0.39 ± 0.28 % yr−1; the WACCM and CAM-chem simulations yield similar trends, although the WACCM result is somewhat smaller. Our analyses of gridded MLS data yield positive O3 trends (up to 1.4 % yr−1) over Indonesia and east of that region, as well as over Africa and the Atlantic. These positive mapped O3 trends are generally captured by the simulations but in a more muted way. We find broad similarities (and some differences) between mapped MLS UT O3 trends and corresponding mapped trends of tropospheric column ozone. The MLS zonal mean CO UT trend for 20° S–20° N is −0.25 ± 0.30 % yr−1, while the corresponding CAM-chem trend is 0.0 ± 0.14 % yr−1 when anthropogenic emissions are taken from the Community Emissions Data System (CEDS) version 2. The CAM-chem simulation driven by CAMS-GLOB-ANTv5 emissions yields a tropical mean CO UT trend of 0.22 ± 0.19 % yr−1, in contrast to the slightly negative MLS CO trend. Previously published analyses of total column CO data have shown negative trends. Our tropical composition trend results contribute to continuing international assessments of tropospheric evolution.

Combining satellite data from HALOE (The Halogen Occultation Experiment, available from 1991-2005) and ACE-FTS (Atmospheric Chemistry Experiment - Fourier Transform Spectrometer, available from 2004-present), we quantified the stratospheric chlorine processes after the 2020 Australian wildfire and major volcanic eruptions (1991 Pinatubo, 2015 Calbuco, and 2022 Tonga). The 2020 Australian wildfire was the largest wildfire since the satellite era. This wildfire released of the order of 1 Tg of aerosols into the stratosphere, comparable to small-scale volcanic eruptions. Despite this rather small amount of stratospheric aerosol loading, its impact on the stratospheric chlorine reservoirs (HCl and ClONO2) was enormous. In contrast to volcanic eruptions, most of the aerosols from wildfires are organics, which could lead to different chemical processes from inorganic sulfates. We use these observations to demonstrate that wildfire aerosols uptake HCl much more efficiently than volcanic aerosols, especially at temperatures warmer than 200 K. Furthermore, while the 1991 Pinatubo eruption injected an order of magnitude more aerosol into the stratosphere than the 2020 Australian wildfire, we show that the two events led to a similar amount of HCl decrease in the mid-latitude and polar region. Most of the decrease in HCl after the 2020 Australian wildfire was balanced by an increase in ClONO2; whereas calculated ClONO2 remained unchanged after the 1991 Pinatubo eruption (indicated by model simulations). With current climate change projections, we are expected to have more frequent wildfires in the future, and this more efficient HCl loss pathway poses new threats to the recovery of the stratospheric ozone layer.

Short lived halocarbons (SLHs) have a range of implications on atmospheric chemistry, primarily over the lower troposphere. Nevertheless, deep convection could rapidly transport large amounts of these near surface pollutants to the upper troposphere and change the chemical composition and radiative balance of the atmosphere. The North American Monsoon Anticyclone (NAMA) and Asian Summer Monsoon Anticyclone (ASMA) dominate the circulation of the upper troposphere and lower stratosphere (UTLS) in the northern hemisphere. Two recent airborne missions, the 2021-2022 Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) and the 2022 Asian summer monsoon Chemical and Climate Impact Project (ACCLIP), investigated the dynamics and chemical composition of UTLS air masses in the outflow of these anticyclones. SLHs were measured using the University of Miami Whole Air Sampler (WAS) during DCOTSS, and both WAS and the NSF NCAR Trace Organic Gas Analyzer with Time of Flight mass spectrometer (TOGA-TOF) during ACCLIP. In this presentation, we use these measurements and a global model to evaluate the impact of convection on the transport of the SLHs to the UTLS. Specifically, we use the Community Earth System Modeling (CESM) framework, with updated halogen chemistry. In addition, we will estimate the contributions from different source regions using the forward CESM modeling using tagged analysis and will compare the results with back trajectory techniques.

Atmospheric gravity waves can play a significant role on atmospheric chemistry through temperature fluctuations. A recent modeling study introduced a method to implement subgrid‐scale orographic gravity‐wave‐induced temperature perturbations in the Whole Atmosphere Community Climate Model (WACCM). The model with a wave‐induced temperature parameterization was able to reproduce for example, the influence of mountain wave events on atmospheric chemistry, as highlighted in previous literature. Here we extend the subgrid‐scale wave‐induced temperature parameterization to also include non‐orographic gravity waves arising from frontal activity and convection. We explore the impact of these waves on middle atmosphere chemistry, particularly focusing on reactions that are strongly sensitive to temperature. The non‐orographic gravity waves increase the variability of chemical reaction rates, especially in the lower mesosphere. As an example, we show that this, in turn, leads to increases in the daytime ozone variability. To demonstrate another impact, we briefly investigate the role of non‐orographic gravity waves in cirrus cloud formation in this model. Consistent with findings from the previous study focusing on orographic gravity waves, non‐orographic waves also enhance homogeneous nucleation and increase cirrus clouds. The updated method used enables the global chemistry‐climate model to account for both orographic and non‐orographic gravity‐wave‐induced subgrid‐scale dynamical perturbations in a consistent manner.

There have been long concerns on the potential environmental impact of aviation, which is the second biggest source of transport greenhouse gas emissions after road transport. Direct emissions from aviation accounted 3.8% of total CO2 emissions, which is estimated to contribute ~3.5% to the anthropogenic effective radiative forcing of climate (IPCC). The environmental impact of emissions from space launches is currently receiving much attention due to the space industry being one of the fastest growing global economic sectors. Since the first assessment of the impact of rocket emissions by Cicerone and Stedman (1974), there have been many developments in rockets and modelling. Rocket emissions can inject significant quantities of gases and particles into the atmosphere (including chlorine compounds HCl, H2O, CO2, NOx, H2, Al2O3 and black carbon), potentially affecting ozone depletion, the dynamics of the atmosphere, and climate change. Feng et al. (2023) have investigated stratospheric ozone depletion due to the presence of small satellites (e.g., CubeSats) with an iodine propulsion system to keep them in orbit. They have shown that an increase in the number of small satellite launches could cause substantial ozone depletion in the Antarctic.In this work, we have incorporated the up-to-date aviation emission inventories (Teoh et al., 2024) and rocket emissions (Brown et al., 2023) into a state-of-the-art global chemistry-climate model (NCAR’s Community Earth System Model, CESM2) to explore how aviation and rocket emissions affect the stratospheric ozone layer and climate once the gases and particulates are injected into the atmosphere. The model includes dynamics, transport, aerosol microphysics, photochemistry, radiation, emissions, and their influences on stratospheric ozone depletion. We have carried out many model experiments in CEMS2 using different configurations (free running, specific-dynamic versions of Whole Atmosphere Community Climate Model) with different chemistry and NOx emissions scenarios from aircraft and rocket emissions (from zero NOx emissions, released NOx emission inventories and up to 100 times NOx emissions) to assess the atmospheric changes induced by these emissions under historical and future scenarios.

The Antarctic ozone 'hole' was discovered in 1985 (ref. 1) and man-made ozone-depleting substances (ODSs) are its primary cause2. Following reductions of ODSs under the Montreal Protocol3, signs of ozone recovery have been reported, based largely on observations and broad yet compelling model-data comparisons4. Although such approaches are highly valuable, they do not provide rigorous statistical detection of the temporal and spatial structure of Antarctic ozone recovery in the presence of internal climate variability. Here we apply pattern-based detection and attribution methods as used in climate-change studies5-11 to separate anthropogenically forced ozone responses from internal variability, relying on trend pattern information as a function of month and height. The analysis uses satellite observations together with single-model and multi-model ensemble simulations to identify and quantify the month-height Antarctic ozone recovery 'fingerprint'12. We demonstrate that the data and simulations show compelling agreement in the fingerprint pattern of the ozone response to decreasing ODSs since 2005. We also show that ODS forcing has enhanced ozone internal variability during the austral spring, influencing detection of forced responses and their time of emergence. Our results provide robust statistical and physical evidence that actions taken under the Montreal Protocol to reduce ODSs are indeed resulting in the beginning of Antarctic ozone recovery, defined as increases in ozone consistent with expected month-height patterns.

In January 2020, tropopause‐level ozone in the austral mid‐latitudes was the highest ever observed in the available Microwave Limb Sounder data record since 2004. Two extreme events preceded this anomaly: the Australian Black Summer fires and the 2019 sudden stratospheric warming (SSW), raising the question of how these disruptions influenced Southern Hemisphere ozone. Here, we investigate the dynamical and chemical contributions to the ozone anomaly using a chemistry‐climate model and satellite observations. We find that downward transport of polar ozone‐enriched air due to the SSW later spread equatorward. Such transport together with photochemical ozone production from emissions of wildfires (fueled by dry and hot conditions previously attributed to the SSW) increased tropopause‐level ozone by up to 30 ppb, with transport as the dominant factor (around 80%). While chemical ozone production from wildfires is well‐recognized, our results highlight that SSWs can greatly influence mid‐latitude ozone through dynamical effects.

Organic aerosol (OA) is an important constituent of the Earth's atmosphere, yet the extent of its destruction by photolysis remains an active research question. Recent laboratory studies reveal evidence for rapid short-term photolysis for secondary OA, but the rate declines to negligible levels over time. Here we use the stratosphere to investigate long-term OA photolysis because of the relatively simple sources and sinks of OA in this region. Airborne campaign observations show that the organic content in organic-sulfate aerosols remains stable with altitude and time in the stratosphere, indicating no significant photolysis. Satellite observations of the 2020 Australian wildfires reveal OA persists over a year in the stratosphere, consistent with model simulations excluding long-term photolysis. These findings suggest long-term OA photolysis is negligible in the real atmosphere. The current Community Earth System Model (CESM) significantly underestimates the abundance of stratospheric OA due to assumed rapid photolysis. We add this well-validated mechanism into CESM by turning off secondary OA photolysis after it is 50 days old, effectively simulating stratospheric OA consistent with observations. In summary, multiple lines of evidence confirm that the long-term photolysis of OA is negligible or extremely slow. Incorporating this mechanism into CESM addresses a key model deficiency, improving simulation of stratospheric OA.

The very large pyrocumulonimbus events that occurred during the Australian summer of 2019–2020 caused extremely unusual partitioning of stratospheric chlorine in the Southern Hemisphere midlatitude and Antarctic regions not only in 2020 but also in 2021. This was likely caused by enhanced HCl solubility in organic species that increased heterogeneous chemistry. Here, we show that observed HCl and ClONO2 values remain outside the pre-wildfire satellite range, measured from 2005 until just prior to the event, in both the Southern Hemisphere midlatitude and Antarctic regions in 2021. Through model simulations, we replicate this prolonged multiyear chemical perturbation, in good agreement with observations. This was achieved by calculating the HCl solubility in mixed wildfire and sulfate aerosols consistent with assumptions of (1) liquid–liquid phase separation and (2) linear dependence on organic and sulfate composition. The model simulations also suggest that the Australian pyrocumulonimbus organic aerosols contributed to low midlatitude ozone values in 2020 and 2021. A marked, photochemically controlled seasonality of the chemical perturbations and ozone depletion is also observed and simulated, and its underlying chemical drivers are identified. This work highlights that lower concentrations of smoke still had profound effects on stratospheric heterogeneous chemistry more than a year after the 2019–2020 wildfire event.

A new Stratospheric Aerosol Intervention (SAI) experiment has been designed for the Chemistry–Climate Model Initiative (CCMI-2022) to assess the impacts of SAI on stratospheric chemistry and dynamical responses and inter-model differences using a constrained setup with a prescribed stratospheric aerosol distribution and fixed sea surface temperatures and sea ice. This paper serves a dual purpose: first, it describes the details of the experimental setup and the prescribed aerosol distribution and demonstrates the suitability of the simplified setup to study SAI impacts in the stratosphere in a multi-model framework. The experiment allows attributing inter-model differences to the resulting impacts on atmospheric chemistry, radiation, and dynamics rather than the model uncertainty arising from differences in aerosol forcing and feedbacks from the ocean and sea ice under SAI. Second, we use the Whole Atmosphere Community Climate Model (WACCM6) to compare the interactive stratospheric aerosol configuration with coupling to land, ocean, and sea ice used to produce the stratospheric aerosol distribution with the results of the constrained SAI experiment. With this, we identify and isolate the stratosphere-controlled SAI-induced impacts from those influenced by the coupling with the ocean. Overall, this comparison facilitates an advanced process-level understanding of the drivers of SAI-induced atmospheric responses. For example, we confirm earlier suggestions that the SAI-induced positive phase of the North Atlantic Oscillation in winter, with the corresponding winter warming over Eurasia and related changes, is driven by stratosphere–troposphere coupling. Future multi-model comparisons will thus provide an important contribution to upcoming scientific assessments of ozone depletion.

Mercury (Hg) is a global pollutant with substantial risks to human and ecosystem health. By upward transport in tropical regions, mercury enters into the stratosphere, but the contribution of the stratosphere to global mercury dispersion and deposition remains unknown. We find that between 5 and 50% (passing through the 400-kelvin isentropic surface and tropopause, respectively) of the mercury mass deposited on Earth's surface is chemically processed in the lower stratosphere. Our results show the stratosphere as a unique chemical environment where elemental mercury is efficiently converted to long-lived oxidized species. Subsequent downward transport contributes substantially to the oxidized mercury burden in the troposphere. The results show that the stratosphere facilitates the global dispersion of large amounts of mercury from polluted source regions to Earth's remote environments. We find that stratospheric transport is as important as tropospheric transport in interhemispheric mercury dispersion. Future projections suggest that expected changes in atmospheric circulation will increase the transport of mercury into the stratosphere.

Abstract Following the Hunga Tonga–Hunga Ha'apai (HTHH) eruption in January 2022, stratospheric ozone depletion was observed at Southern Hemisphere mid‐latitudes and over Antarctica during the 2022 austral wintertime and springtime, respectively. The eruption injected sulfur dioxide and unprecedented amounts of water vapor into the stratosphere. This work examines the chemistry contribution of the volcanic materials to ozone depletion using chemistry‐climate model simulations with nudged meteorology. Simulated 2022 ozone and nitrogen oxide (NOx = NO + NO2) anomalies show good agreement with satellite observations. We find that chemistry yields up to 4% ozone destruction at mid‐latitudes near ∼70 hPa in August and up to 20% ozone destruction over Antarctica near ∼80 hPa in October. Most of the ozone depletion is attributed to internal variability and dynamical changes forced by the eruption. Both the modeling and observations show a significant NOx reduction associated with the HTHH aerosol plume, indicating enhanced dinitrogen pentoxide hydrolysis on sulfate aerosol.

Bromine monoxide (BrO) is relevant to atmospheric oxidative capacity, affecting the lifetime of greenhouse gases (i.e., methane, dimethylsulfide) and mercury oxidation. However, measurements of BrO radical vertical profiles are rare, and BrO is highly variable. As a result, the few available aircraft observations in different regions of the atmosphere are not easily reconciled. Autonomous multi-axis differential optical absorption spectroscopy (MAX-DOAS) instruments placed at remote mountaintop observatories (MT-DOAS) present a cost-effective alternative to aircraft, with the potential to probe the climate-relevant yet understudied free troposphere more routinely. Here, we describe an innovative full-atmosphere BrO and formaldehyde (HCHO) profile retrieval algorithm using MT-DOAS measurements at Mauna Loa Observatory (MLO - 19.536 degrees N, 155.577 degrees W; 3401 m a.s.l.). The retrieval is based on time-dependent optimal estimation and simultaneously inverts 190+ individual BrO (and formaldehyde, HCHO) SCDs (slant column densities; SCD = dSCD + SCDRef) from solar stray light spectra measured in the zenith and off-axis geometries at high and low solar zenith angles (92 degrees > SZA > 30 degrees) to derive BrO concentration profiles from 1.9 to 35 km with 7.5 degrees of freedom (DoFs). Two case study days are characterized by the absence (26 April 2017, base case) and presence of a Rossby-wave-breaking double tropopause (29 April 2017, RW-DT case). Stratospheric-BrO vertical columns are nearly identical on both days (VCD = (1.5 +/- 0.2) x 1013 molec. cm-2), and the stratospheric-BrO profile peaks at a lower altitude during the RW-DT (1.6-2.0 DoFs). Tropospheric-BrO VCDs increase from (0.70 +/- 0.14) x 1013 molec. cm-2 (base case) to (1.00 +/- 0.14) x 1013 molec. cm-2 (RW-DT) owing to a 3-fold increase in BrO in the upper troposphere (1.7-1.9 DoFs). BrO at MLO increases from (0.23 +/- 0.03) pptv (base case) to (0.46 +/- 0.03) pptv (RW-DT) and is characterized by an added time resolution (similar to 3.8 DoFs). Up to (0.9 +/- 0.1) pptv BrO is observed above MLO in the lower free troposphere in the absence of the double tropopause. We validate the retrieval using aircraft BrO profiles and in situ HCHO measurements aboard the NSF/NCAR GV aircraft above MLO (11 January 2014) that establish BrO peaks around 2.4 pptv above 13 km in the upper troposphere-lower stratosphere (UTLS) during a similar RW-DT event (0.83 x 1013 molec. cm2 tropospheric-BrO VCD above 2 km). The tropospheric-BrO profile measured using MT-DOAS (RW-DT case) and using the aircraft agree well (after averaging-kernel smoothing). Furthermore, these tropospheric-BrO profiles over the central Pacific Ocean are found to closely resemble those over the eastern Pacific Ocean (2-14 km) and are in contrast to those over the western Pacific Ocean, where a C-shaped tropospheric-BrO profile shape has been observed.