Spatiotemporal distribution and source analysis of Arctic tropospheric reactive bromine and its role in ozone depletion

Ozone depletion associated with tropospheric bromine explosion in the Arctic
substantially modify the oxidative capacity of the atmospheric boundary layer and
promote the oxidation and deposition of gaseous elemental mercury (GEM), with
important implications for Arctic regional ecosystems and global climate change.
Currently, the lack of continuous observations in polar regions has resulted in a relative
scarcity of systematic studies on the spatiotemporal variability and source mechanisms
of tropospheric reactive bromine in the Arctic. In particular, the chemical evolution of
reactive bromine during extreme ozone depletion events (EODEs) and its impacts on
the accurate estimation of total inorganic bromine (Bry) remain unclear, greatly limiting
an in-depth understanding of the mechanisms governing polar atmospheric evolution
and hindering a precise understanding of changes in the polar atmospheric environment
under global change. Based on long-term observations from a Multi-Axis Differential
Optical Absorption Spectrometer (MAX-DOAS) at the Chinese Yellow River Station
(78.92°N, 11.93°E) from 2017 to 2023, this thesis integrates satellite remote sensing,
surface meteorological and atmospheric composition observations, together with
atmospheric chemistry and transport model simulations, to systematically investigate
the spatiotemporal distribution, source mechanisms, and chemical evolution of
tropospheric reactive bromine during EODEs in the Arctic.
First, this study investigated the multi-scale spatiotemporal characteristics and
environmental effects of springtime tropospheric bromine monoxide (BrO) in NyÅlesund, Arctic. The results show that March–May is the period with a high frequency
of bromine explosion events (BEEs) in the Arctic spring. Among these months, BrO 0–
4 km columns reached their highest peak in March, which also exhibited the most
pronounced interannual variability. Vertically, BrO was mainly distributed in the lower
atmosphere below 2.5 km and gradually decreased in concentration with increasing
altitude. Correlation analysis showed that BrO columns were significantly negatively
correlated with polar boundary-layer ozone and GEM concentrations, demonstrating
the key driving role of Arctic reactive bromine activation and explosion in boundarylayer ozone depletion and GEM oxidation and deposition.
Second, this study systematically explores the sources and controlling mechanisms
of springtime BEEs in Ny-Ålesund, Arctic. The coupled evolution of BrO and aerosol extinction exhibits distinct seasonal variations, and that heterogeneous reactivation of
bromine reservoir species on sea-salt aerosol (SSA) surfaces is a key mechanism
sustaining high BrO concentrations. Air-mass source analysis indicates that, during
BEEs, the contact time of air masses with multi-year sea ice regions was significantly
longer than that with first-year sea ice regions, with the bromine flux contributed
accounting for more than 50%; meanwhile, high-wind conditions promoted reactive
bromine release from sea-ice surfaces. The modeled sea-salt aerosols generated by
blowing snow over sea-ice surfaces and the corresponding bromine emission fluxes
were both significantly positively correlated with observed BrO concentrations, further
confirming that blowing snow, particularly blowing snow over multi-year sea ice
surfaces, is an important source of reactive bromine in the polar atmosphere.
Finally, this study reveals the chemical evolution of reactive bromine during Arctic
EODEs (with ozone below 1 ppbv) and its implications for polar bromine budget
estimation. The analysis shows that EODEs accounted for 8.1%–52.1% of all ozone
depletion events during the Arctic spring (with ozone below 10 ppbv). During EODEs,
ozone abundance became the dominant limiting factor for BrO formation, resulting in
a positive relationship between BrO and ozone. Analysis of a typical EODE case
revealed that, under stable boundary-layer conditions, a BrO-hole vertical structure
formed, which was fundamentally characterized by a dynamic shift in the BrO/Bry ratio
rather than a loss of the total bromine reservoir. Quantitative assessment showed that
the conventional estimation method using a fixed BrO/Bry ratio systematically
underestimated Bry in the Arctic spring, with an underestimation bias of 5.6%–36.1%.
In summary, this thesis establishes a long-term observational dataset of springtime
BrO 0–4 km column concentrations and vertical profiles in Ny-Ålesund, Arctic. The
thesis reveals the spatiotemporal evolution and source mechanisms of tropospheric
reactive bromine in the polar region, clarifies the key roles of heterogeneous
reactivation on SSA surfaces and blowing snow over multi-year sea ice in BEEs, and
quantitatively elucidates the bias in Bry estimation during EODEs. The findings refine
the theoretical framework of Arctic halogen chemistry and ozone depletion and provide
important scientific support for accurately assessing changes in polar atmospheric
oxidative capacity and their climatic and environmental effects.