Climatology of Springtime Surface Ozone Depletion Events at Alert, Nunavut, Canada from 1992-2007.

Arctic surface ozone depletion events (ODEs) are springtime phenomena that occur when ambient ozone levels of 35-40 parts per billion (ppb) quickly decrease to a few ppb in a matter of hours. ODEs are created by catalytic reactions with bromine that occur in the near - surface of the atmosphere. These occurrences are driven not only by complex atmospheric chemistry but also by daily weather conditions and long - range transport. Events can last for several days and have a strong effect upon the seasonal ozone signature - there is a discernable minimum of mean ozone levels when examining the annual ozone time series. This study aims to identify important weather variables and transport patterns that affect ODEs at daily and seasonal time scales for Alert, Nunavut, Canada from 1992 - 2007.

Hourly ozone data were gathered from Environment Canada; these data establish the dependent variables at daily and seasonal timescales. On a daily basis, they identify days related to ODEs (their initiation, occurrence, and termination). Seasonally, these data define the bounds and strength of each ODE season - there is clear inter - annual variability across the ODE seasons.

Weather data are examined for Alert, and these compose the independent variables. Daily surface weather variables are gathered from the National Climatic Data Center. Twice daily radiosonde observations from the upper air station were obtained via the data archive of the Department of Atmospheric Sciences at the University of Wyoming University. The surface and radiosonde data are used to indentify specific weather variables that influence ODEs. They are also used in concert to develop a daily weather type for seasonal days; they are input for a two - step clustering procedure that groups their principal components. This establishes a holistic classification of daily weather that can be linked to ODEs. Finally, HYSPLIT back - trej ectories are calculated for the period of record based upon the NCEP/N CAR Reanalysis data. These trajectories are grouped via two - step cluster analysis using their 3 - dimensional shape. They create a categorical variable that distinguishes transport patterns for each day.

Weather and transport patterns are examined for their linkages to ODE - related days; these identify important day-to-day variations that affect ODEs. The daily weather types are also used in this manner to examine the holistic daily weather related to ODEs.

To assess inter-annual variability of ODEs, a few features are inspected. First, seasonal differences in weather and air flow patterns are compared to ODE seasonal strength -- this identifies the components that are important to within-season variability. Second, weather in the weeks preceding each season's start and end are compared to find favorable conditions that spark and prolong the ODE season. Third, transport patterns from the previous winter are inspected to find linkages between winter flow and ODE strength the following season.

During springtime, ODEs exhibit significantly different weather and transport patterns than non - ODE days. Cool, moderate and maritime weather conditions are most associated with ODEs. More specifically, ODEs are associated with lower than normal temperatures, decreased static stability, elevated boundary layer height, increased ventilation, and Eurasian transport. Stagnant air flow also promotes their sustenance as low - ozone air masses are able to reside over the region. Decreased static stability offers the most counter - intuitive relationship, since stable conditions should promote nearsurface 5 chemistry. Decreased stability may be most indicative of a transition in air masses that contain more favorable ODE conditions.

Seasonally, a low temperature relationship is also apparent. Lower than normal dew point temperatures (highly collinear with temperature) are positively related to stronger ODE seasons. Transport from Eurasia contains an expected, though relatively weak, positive relationship to increased seasonal ODEs. However, increased air flow from the northern island of Canada shows a more uniform positive relationship to the strength of ODE season. This finding provides evidence for a new, localized source region of ODEs, which likely contributes bromine from young sea ice and polynyas.

Earlier ODE seasons are preceded by both lower than normal boundary layers and decreased static stability. These represent different stability modes. Lower than normal boundary layers are able to create a confined near-surface layer that supports ODE chemistry. On the other hand, conditions of decreased stability are similar to those at daily timescales—more favorable ODE air masses are advected into the Alert region.

ODE seasons are prolonged as a few favorable conditions occur later in the season. Lower than normal temperatures and sea level pressure late in the season, likely associated with cold-core lows or Arctic cold fronts, prolong the ODE season into early summer. These are conditions most similar to spring, when ODEs occur most frequently.

Winter transport patterns are also important to ODEs in the following spring— transport from Eurasia and the northern islands of Canada are positively related to stronger ODE seasons. Both source regions contribute important materials into the local snowpack to be utilized for ODEs in spring. Eurasia likely delivers anthropogenic material, while the high-Arctic Canadian islands transport localized bromine.