is a new modeling system that produces global forecasts of atmospheric composition at 25km 2 horizontal resolution. • GEOS-CF model output is freely available and offers a new tool for academic researchers, air quality managers, and the public.

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.

Bromine radicals (Br + BrO) are important atmospheric species owing to their ability to catalytically destroy ozone as well as their potential impacts on the oxidative pathways of many trace gases, including dimethylsulfide and mercury. Using space-based observations of BrO, recent studies have reported rapid enhancements of tropospheric BrO over large areas (so called "BrO explosions") connected to near-surface ozone depletion occurring in polar spring. However, the source(s) of reactive bromine and mechanism(s) that initiate these BrO explosions are uncertain. In this study, we investigate the relationships between Arctic BrO explosions and two of the proposed sources of reactive bromine: sea-salt aerosol (SSA) generated from blowing snow and first-year (seasonal) sea ice. We use tropospheric column BrO derived from the Ozone Monitoring Instrument (OMI) in conjunction with the Goddard Earth Observing System Version 5 (GEOS-5) data assimilation system provided by National Aeronautics and Space Administration Global Modeling and Assimilation Office. Case studies demonstrate a strong association between the temporal and spatial extent of OMI-observed BrO explosions and the GEOS-5 simulated blowing snow-generated SSA during Arctic spring. Furthermore, the frequency of BrO explosion events observed over the 11-year record of OMI exhibits significant correlation with a time series of the simulated SSA emission flux in the Arctic and little to no correlation with a time series of satellite-based first-year sea ice area. Therefore, we conclude that SSA generated by blowing snow is an important factor in the formation of the BrO explosion observed from space during Arctic spring. Key Points: • OMI-observed Arctic tropospheric BrO explosions are associated with GEOS-5 simulated sea-salt aerosols (SSAs) from wind-driven blowing snow • The 11-year time series analysis shows that the Arctic BrO explosion frequency and simulated blowing snow SSA emission flux are correlated • The satellite-based record of first-year sea ice area does not show significant correlation with the Arctic BrO explosion frequency Supporting Information: • Supporting Information S1

Abstract. We report measurements of bromine monoxide (BrO) and use an observationally constrained chemical box model to infer total gas-phase inorganic bromine (Br y ) over the tropical western Pacific Ocean (tWPO) during the CON-TRAST field campaign (January-February 2014). The observed BrO and inferred Br y profiles peak in the marine boundary layer (MBL), suggesting the need for a bromine source from sea-salt aerosol (SSA), in addition to organic bromine (CBr y ). Both profiles are found to be C-shaped with local maxima in the upper free troposphere (FT). The median tropospheric BrO vertical column density (VCD) was measured as 1.6×10 13 molec cm −2 , compared to model predictions of 0.9 × 10 13 molec cm −2 in GEOS-Chem (CBr y but no SSA source), 0.4 × 10 13 molec cm −2 in CAM-Chem (CBr y and SSA), and 2.1×10 13 molec cm −2 in GEOS-Chem (CBr y and SSA). Neither global model fully captures the Cshape of the Br y profile. A local Br y maximum of 3.6 ppt (2.9-4.4 ppt; 95 % confidence interval, CI) is inferred between 9.5 and 13.5 km in air masses influenced by recent convective outflow. Unlike BrO, which increases from the convective tropical tropopause layer (TTL) to the aged TTL, gas-phase Br y decreases from the convective TTL to the aged TTL. Analysis of gas-phase Br y against multiple tracers (CFC-11, H 2 O / O 3 ratio, and potential temperature) reveals a Br y minimum of 2.7 ppt (2.3-3.1 ppt; 95 % CI) in the aged TTL, which agrees closely with a stratospheric injection of 2.6 ± 0.6 ppt of inorganic Br y (estimated from CFC-11 correlations), and is remarkably insensitive to assumptions about heterogeneous chemistry. Br y increases to 6.3 ppt (5.6-7.0 ppt; 95 % CI) in the stratospheric "middleworld" and 6.9 ppt (6.5-7.3 ppt; 95 % CI) in the stratospheric "overworld". The local Br y minimum in the aged TTL is qualitatively (but not quantitatively) captured by CAM-Chem, and suggests a more complex partitioning of gas-phase and aerosol Br y species than previously recognized. Our data provide corroborating evidence that inorganic bromine sources (e.g., SSA-derived gas-phase Br y ) are needed to explain the gas-phase Br y budget in the upper free troposphere and TTL. They are also consistent with observations of significant bromide in Upper Troposphere-Lower Stratosphere aerosols. The total Br y budget in the TTL is currently not closed, because of the lack of concurrent quantitative measurements of gas-phase Br y species (i.e., BrO, HOBr, HBr, etc.) and aerosol bromide. Such simultaneous measurements are needed to (1) quantify SSA-derived Br y in the upper FT, (2) test Br y partitioning, and possibly explain the gas-phase Br y minimum in the aged TTL, (3) constrain heterogeneous reaction rates of bromine, and (4) account for all of the sources of Br y to the lower stratosphere.

A chemical ionization mass spectrometer was used to measure BrO and HOBr + Br2 over the Tropical West Pacific Ocean within the altitude range of 1 to 15 km, during the CONvective TRansport of Active Species in the Tropics (CONTRAST) campaign in 2014. Isolated episodes of elevated BrO (up to 6.6 pptv) and/or HOBr + Br2 (up to 7.3 pptv) were observed in the tropical free troposphere (TFT) and were associated with biomass burning. However, most of the time we did not observe significant BrO or HOBr + Br2 in the TFT and the tropical tropopause layer (TTL) above our limits of detection (LOD). The 1 min average LOD for BrO ranged from 0.6 to 1.6 pptv and for HOBr + Br2 ranged from 1.3 to 3.5 pptv. During one flight, BrO observations from the TTL to the extratropical lowermost stratosphere were used to infer a profile of inorganic bromine (Bry). Based on this profile, we estimated the product gas injection of bromine species into the stratosphere to be 2 pptv. Analysis of Bry partitioning further indicates that BrO levels are likely very low in the TFT environment and that future studies should target the measurement of HBr or atomic Br.

<p><strong>Abstract.</strong> We report measurements of bromine monoxide (BrO) and use an observationally constrained chemical box-model to infer total gas phase inorganic bromine (Br<sub>y</sub>) over the tropical Western Pacific Ocean (tWPO) during the CONTRAST field campaign (January–February 2014). The median tropospheric BrO Vertical Column Density (VCD) over the tWPO was measured as 1.6 × 10<sup>13</sup> molec cm<sup>−2</sup>, compared to model predictions of 0.4 × 10<sup>13</sup> in CAM-Chem, 0.9 × 10<sup>13</sup> in GEOS-Chem, and 2.1 × 10<sup>13</sup> in GEOS-Chem with a sea-salt aerosol (SSA) bromine source. The observed BrO and inferred Br<sub>y</sub> profiles is found to be C-shaped in the troposphere, with local maxima in the marine boundary layer (MBL) and in the upper free troposphere. Neither global model fully captures this profile shape. Between 6 and 13.5 km, the inferred Br<sub>y</sub> is highly sensitive to assumptions about the rate of heterogeneous bromine recycling (depends on the surface area of ice/aerosols), and the inclusion of a SSA bromine source. A local Br<sub>y</sub> maximum of 3.6 ppt (2.3–11.1 ppt, 95 % CI) is observed between 9.5 and 13.5 km in air masses influenced by recent convective outflow. Unlike BrO, which increases from the convective TTL to the aged TTL, gas phase Br<sub>y</sub> decreases from the convective TTL to the aged TTL. Analysis of gas phase Br<sub>y</sub> against multiple tracers (CFC-11, H<sub>2</sub>O / O<sub>3</sub> ratio, and θ) reveals a Br<sub>y</sub> minimum of 2.7 ppt (2.4–3.0 ppt, 95 % CI) in the aged TTL, which is remarkably insensitive to assumptions about heterogeneous chemistry. Br<sub>y</sub> increases to 6.3 ppt (5.9–6.7 ppt, 95 % CI) in the stratospheric middleworld, and 6.9 ppt (6.7–7.1 ppt, 95 % CI) in the stratospheric overworld. The local Br<sub>y</sub> minimum in the aged TTL is qualitatively (but not quantitatively) captured by CAM-chem, and suggests a more complex partitioning of gas phase and aerosol Br<sub>y</sub> species than previously recognized. Our data provide corroborating evidence that inorganic bromine sources (e.g., SSA derived gas phase Br<sub>y</sub>) are needed to explain the gas phase Br<sub>y</sub> budget in the TTL. They are also consistent with observations of significant bromide in UTLS aerosols. The total Br<sub>y</sub> budget in the TTL is currently not closed, because of the lack of concurrent quantitative measurements of gas phase Br<sub>y</sub> species (i.e., BrO, HOBr, HBr, etc.) and aerosol bromide. These simultaneous measurements are needed 1) to quantify SSA derived Br<sub>y</sub> aloft, 2) to test Br<sub>y</sub> partitioning, and explain the gas phase Br<sub>y</sub> minimum in the aged TTL, 3) to constrain heterogeneous reaction rates of bromine, and 4) to account for all of the sources of Br<sub>y</sub> to the lower stratosphere.</p>

The NASA Goddard Earth Observing System (GEOS) Composition Forecast (GEOS-CF) provides recent estimates and 5-day forecasts of atmospheric composition to the public in near-real time. To do this, the GEOS Earth system model is coupled with the GEOS-Chem tropospheric-stratospheric unified chemistry extension (UCX) to represent composition from the surface to the top of the GEOS atmosphere (0.01 hPa). The GEOS-CF system is described, including updates made to the GEOS-Chem UCX mechanism within GEOS-CF for improved representation of stratospheric chemistry. Comparisons are made against balloon, lidar, and satellite observations for stratospheric composition, including measurements of ozone (O 3 ) and important nitrogen and chlorine species related to stratospheric O 3 recovery. The GEOS-CF nudges the stratospheric O 3 toward the GEOS Forward Processing (GEOS FP) assimilated O 3 product; as a result the stratospheric O 3 in the GEOS-CF historical estimate agrees well with observations. During abnormal dynamical and chemical environments such as the 2020 polar vortexes, the GEOS-CF O 3 forecasts are more realistic than GEOS FP O 3 forecasts because of the inclusion of the complex GEOS-Chem UCX stratospheric chemistry. Overall, the spatial patterns of the GEOS-CF simulated concentrations of stratospheric composition agree well with satellite observations. However, there are notable biases-such as low NO x and HNO 3 in the polar regions and generally low HCl throughout the stratosphere-and future improvements to the chemistry mechanism and emissions are discussed. GEOS-CF is a new tool for the research community and instrument teams observing trace gases in the stratosphere and troposphere, providing near-real-time three-dimensional gridded information on atmospheric composition.

The MERRA‐2 Stratospheric Composition Reanalysis of Aura Microwave Limb Sounder (M2‐SCREAM) is a new reanalysis of stratospheric ozone, water vapor, hydrogen chloride (HCl), nitric acid (HNO3) and nitrous oxide (N2O) between 2004 and the present (with a latency of several months). The assimilated fields are provided at a 50‐km horizontal resolution and at a three‐hourly frequency. M2‐SCREAM assimilates version 4.2 Microwave Limb Sounder (MLS) profiles of the five constituents alongside total ozone column from the Ozone Monitoring Instrument. Dynamics and tropospheric water vapor are constrained by the MERRA‐2 reanalysis. The assimilated species are in excellent agreement with the MLS observations, except for HNO3 in polar night, where data are not assimilated. Comparisons against independent observations show that the reanalysis realistically captures the spatial and temporal variability of all the assimilated constituents. In particular, the standard deviations of the differences between M2‐SCREAM and constituent mixing ratio data from The Atmospheric Chemistry Experiment Fourier Transform Spectrometer are much smaller than the standard deviations of the measured constituents. Evaluation of the reanalysis against aircraft data and balloon‐borne frost point hygrometers indicates faithful representations of small‐scale structures in the assimilated water vapor, HNO3 and ozone fields near the tropopause. Comparisons with independent observations and a process‐based analysis of the consistency of the assimilated constituent fields with the MERRA‐2 dynamics and with large‐scale stratospheric processes demonstrate the utility of M2‐SCREAM for scientific studies of chemical and transport variability on time scales ranging from hours to decades. Analysis uncertainties and guidelines for data usage are provided.