Observations and analysis of tropospheric BrO in the Arctic region

Tropospheric bromine monoxide (BrO), as an intermediate product of an autocatalytic reaction cycle, plays an important role in observing and analysing so-called “bromine explosion events” (BEEs), that are commonly observed in the Arctic and Antarctic during polar spring. These events involve the release of inorganic bromine from saline snowpack and sea ice surfaces into the atmosphere through a heterogeneous, autocatalytic, chemical chain reaction cycle, in which bromine reacts with tropospheric ozone, leading to the formation of BrO. As a result, BEEs often coincide with ozone depletion events (ODEs), since bromine catalytically destroys ozone molecules, sometimes to concentrations below the detection limit of the instruments. Because tropospheric ozone is a primary source of the important oxidizing agent OH, these events significantly impact the boundary layer chemistry by changing the oxidizing capacity and radiative forcing.
In addition to cold temperatures, which are needed for bromine explosion reactions, two types of meteorological conditions have been observed during ODEs. The first is low wind speeds and a stable boundary layer, which allows bromine to accumulate near the ground and efficiently deplete ozone there. The second condition is characterized by high wind speeds above approximately 10 m/s leading to blowing snow and a higher, unstable boundary layer. This condition often occurs in connection with polar cyclones, where bromine can be transported and recycled at higher altitudes on snow and aerosol surfaces, extending ozone depletion to higher altitudes as well.
The observation of bromine monoxide has been an ongoing research topic since the 1980s, when ODEs were first observed in combination with elevated bromide concentrations. Since the mid-1990s, BrO has also been monitored from satellite-based UV-visible instruments (e.g. GOME, SCIAMACHY, GOME-2), enabling observation of BrO on a global scale, including the large-scale tropospheric BrO plumes resulting from BEEs in polar regions. The launch of the TROPOspheric Monitoring Instrument (TROPOMI) in October 2017 enables high-resolution daily measurements of BrO, significantly improving our ability to monitor and study these events.

In this study, two approaches were taken to improve the observation and understanding of tropospheric BrO, with a focus on BEEs and ODEs in the Arctic spring. The first part focuses on deriving troposheric BrO from measurements of total BrO columns from TROPOMI by applying different stratospheric separation methods, so-called stratospheric corrections. While the majority of BrO is located in the stratosphere, only a small fraction is located in the troposphere, except during BEEs, when the tropospheric BrO columns can occasionally exceed the stratospheric BrO columns. To evaluate the quality of different stratospheric separation methods in this study, five different approaches are applied to the TROPOMI BrO dataset: (1) a constant stratospheric BrO value, (2) a high pass filtering method applied in near real time processing, (3) an empirical multilinear regression model from Seo (2020), (4) a climatology-based method developed by Theys et al. (2011), and (5) a recently developed method for the OMPS instrument by Chong et al. (2024). The different separation methods are compared to each other and the results of all five methods are validated utilizing airborne tropospheric BrO measurements. The airborne data was taken from the “Heidelberg Airborne Imaging DOAS Instrument” (HAIDI) during the “Chemistry in the Arctic, Clouds, Halogens and Aerosols” (CHACHA) campaign, which was conducted in Alaska in spring 2022 (Brockway et al., 2024). The comparison with the CHACHA data showed a fairly good agreement between the constant stratospheric BrO value, the Theys et al. (2011), and the Seo et al. (2020) methods, with the latter showing the best correlation with the HAIDI data. The near real time method significantly underestimated the amount of tropospheric BrO and the Chong et al. (2024) method showed large differences in the attribution of BrO to the troposphere.

The second part of this study focuses on the influence of meteorological conditions and the amount of BrO on ODEs observed in Ny-Ålesund, Svalbard. For this purpose, two long-term ozone data sets were evaluated: one from ozone sondes launched in Ny-Ålesund, and the second one from in-situ measurements taken on Zeppelin Mountain, located near Ny-Ålesund. Both data sets were analysed for the March to May periods of the years 2010 to 2021, and ozone concentrations below a certain threshold were identified as ODEs. To examine the prevailing weather conditions during these events, ERA5 reanalysis data were used and categorized according to whether ODEs were present or not. The evaluation of both data sets produced consistent results: during ODEs, lower pressure is observed east of Svalbard and higher pressure over Greenland, resulting in the transport of cold polar air from the north towards Ny-Ålesund. Additionally, higher wind speeds and a deeper boundary layer were identified, supporting the assumption that ODEs frequently occur in conjunction with polar cyclones. The analysis of tropospheric BrO VCDs from satellite showed elevated BrO values throughout the Arctic during ODEs in Ny-Ålesund, with particularly high concentrations north of Svalbard. This analysis demonstrates, that ODEs in Ny-Ålesund are to a large extent driven by meteorology and the transport of cold polar air masses to the measurement sites.