Abstract
Abstract. Following polar sunrise in the Arctic springtime, tropospheric ozone episodically decreases rapidly to near-zero levels during ozone depletion events (ODEs). Many uncertainties remain in our understanding of ODE characteristics, including the temporal and spatial scales, as well as environmental drivers. Measurements of ozone, bromine monoxide (BrO), and meteorology were obtained during several deployments of autonomous, ice-tethered buoys (O-Buoys) from both coastal sites and over the Arctic Ocean; these data were used to characterize observed ODEs. Detected decreases in surface ozone levels during the onset of ODEs corresponded to a median estimated apparent ozone depletion timescale (based on both chemistry and the advection of O3-depleted air) of 11 h. If assumed to be dominated by chemical mechanisms, these timescales would correspond to larger-than-observed BrO mole fractions based on known chemistry and assumed other radical levels. Using backward air mass trajectories and an assumption that transport mechanisms dominate observations, the spatial scales for ODEs (defined by time periods in which ozone levels ≤15 nmol mol−1) were estimated to be 877 km (median), while areas estimated to represent major ozone depletions (<10 nmol mol−1) had dimensions of 282 km (median). These observations point to a heterogeneous boundary layer with localized regions of active, ozone-destroying halogen chemistry, interspersed among larger regions of previously depleted air that retain reduced ozone levels through hindered atmospheric mixing. Based on the estimated size distribution, Monte Carlo simulations showed it was statistically possible that all ODEs observed could have originated upwind, followed by transport to the measurement site. Local wind speed averages were low during most ODEs (median of ~3.6 m s−1), and there was no apparent dependence on local temperature.
Highlights
Global tropospheric oxidation is generally controlled by ozone (O3), a major greenhouse gas (Gauss et al, 2006) and the most important precursor to the primary atmospheric oxidant, hydroxyl radical (OH) (Seinfeld and Pandis, 2006; Thompson, 1992)
The O-Buoy was developed in part to enable the observation of ozone depletion events (ODEs) at the hypothesized location of their initiation, the frozen Arctic Ocean surface
Surface measurements of ambient O3, bromine monoxide (BrO), temperature, and wind speed from five separate O-Buoy deployments were utilized to gain insights into the characteristics of ODEs observed over the Arctic Ocean
Summary
Global tropospheric oxidation is generally controlled by ozone (O3), a major greenhouse gas (Gauss et al, 2006) and the most important precursor to the primary atmospheric oxidant, hydroxyl radical (OH) (Seinfeld and Pandis, 2006; Thompson, 1992). In a study of long-term Arctic coastal measurements, Tarasick and Bottenheim (2002) observed that ODEs most often occurred at temperatures of less than 253 K, leading to the proposal that such low temperatures could be necessary for the initiation of ozone depletion This hypothesis was strengthened by Adams et al (2002), who reported that frozen NaCl / NaBr surfaces efficiently uptake and react with HOBr to both form and release gas phase Br2 at temperatures below 253 K. Using this unique data set, we estimate the timescales of O3 depletion, examine the state of our understanding of the chemistry involved, and estimate the spatial extents and meteorological conditions supporting O3-depleted air masses
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