No abstract

Abstract. Reactive halogen chemistry in the springtime Arctic causes ozone depletion events and alters the rate of pollution processing. There are still many uncertainties regarding this chemistry, including the multiphase recycling of halogens and how sea ice impacts the source strength of reactive bromine. Adding to these uncertainties are the impacts of a rapidly warming Arctic. We present observations from the CHemistry in the Arctic: Clouds, Halogens, and Aerosols (CHACHA) field campaign based out of Utqiag ̇vik, Alaska from mid-February to mid-April of 2022 to provide information on the vertical distribution of bromine monoxide (BrO), which is a tracer for reactive bromine chemistry. Data was gathered using the Heidelberg Airborne Imaging DOAS Instrument (HAIDI) on the Purdue University Airborne Laboratory for Atmospheric Research (ALAR) and employing a unique sampling technique of vertically profiling the lower atmosphere with the aircraft via "porpoising" ma- neuvers. Observations from HAIDI were coupled with radiative transfer model calculations to retrieve mixing ratio profiles throughout the lower atmosphere (below 1000 m), with unprecedented vertical resolution (50 m) and total information gathered (average of 17.5 degrees of freedom) for this region. A cluster analysis was used to categorize 245 retrieved BrO mixing ratio vertical profiles into four common profile shapes. We often found the highest BrO mixing ratios at the Earth’s surface with a mean of nearly 30 pmol mol−1 in the lowest 50 m, indicating an important role for multiphase chemistry on the snowpack in reactive bromine production. Most lofted BrO profiles corresponded with an aerosol profile that peaked at the same altitude (225 m above the ground), suggesting that BrO was maintained due to heterogeneous reactions on particle surfaces aloft during these cases. A majority, 11 of 15, of the identified lofted BrO cases occurred on a single day, March 19, 2022, over an area covering more than 24,000 km2, indicating that this was a large scale lofted BrO event. The clustered BrO mixing ratio profiles should be particularly useful for MAX-DOAS studies, where prior BrO profiles, needed for the optimal estimation retrieval, are not often based on previous observations. Future MAX-DOAS studies (and past reanalyses) could rely on the profiles provided in this work to improve BrO retrievals.

Abstract. Tropical Tropopause Layer cirrus clouds and their radiative effects represent a major uncertainty in the evaluation of Earth’s energy budget. High altitude aircraft offer an opportunity to provide observations at cirrus cloud altitudes, most commonly using in-situ measurements of ice particle optical properties and composition. In particular, remote sensing of scattering properties and near-IR ice water absorption in the limb can provide unique insights into thin and sub-visible cirrus clouds. Here we present novel spectroscopic observations of path-averaged ice water absorptions onboard NASA’s Global Hawk aircraft, during the Airborne Tropical TRopopause Experiment, which took place in 2011, 2013, and 2014. The University of California Los Angeles and University of Heidelberg mini-DOAS instrument provided multi-angle limb-scanning observations of scattered solar radiation in the near-IR (900–1726 nm), allowing the identification of ice and liquid water, O2, CO2 and H2O. The VLIDORT-QS radiative transfer (RT) code was specifically developed for this study and used to simulate high altitude limb observations for varied cloud scenarios. We performed a comprehensive sensitivity study, developing a fundamental understanding of airborne near-IR limb observations of cirrus clouds. We identified two general distinct cases: a linear regime for optically thin clouds, where the ice absorption is proportional to ice water content (IWC), and a regime for optically thick cirrus clouds, where ice absorption is in saturation and independent of IWC. Results also demonstrate how molecular oxygen absorption can be used to infer information on optical properties of ice particles in the second regime only, with minimal information for thin cirrus clouds. We also explored the feasibility of retrieving IWC from mini-DOAS path-averaged ice water absorption (SIWP) measurements. This innovative interpolation-based approach requires a small number of RT calculations per observation to determine the sensitivity of SIWP to IWC. Spectral retrievals were applied for a particularly interesting case during Science Flight 2 over Guam in February 2014, during which the aircraft flew in circles in the same general area for an extended period. Retrieved IWC results are consistent with independent in situ measurements from other instruments on board. The measurements of ice particle scattering and absorption at different azimuths relative to the sun and at different altitudes represents a unique opportunity to test our approach and to infer properties of the ice particles, together with information on cirrus cloud radiative transfer.