Differential Absorption Lidar Measurements of Troposheric Ozone in the Arctic
Seabrook, Jeff Adams
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A differential absorption lidar was constructed at the laboratory at York University and deployed in field campaigns to measure vertical profiles of tropospheric ozone. Profiles of ozone concentration were derived from the range-resolved simultaneous detection of backscatter from two or more wavelengths of laser radiation. By analyzing the absorption differences due to ozone between the two lidar returns, an ozone profile along the optical path of the laser was determined. This method is capable of resolving ozone concentrations between a range of 300 m to 8 km from the lidar. During the spring in the polar region, tropospheric ozone depletion events occur due to the presence of inert halide salt ions such as Br- in the atmosphere. After polar sunrise, this Bromine photochemistry can cause ozone concentrations near the ice surface to drop to near zero levels. Outstanding questions addressed by the lidar measurements were (a) whether significant ozone depletion occurs in layers not connected to the surface, and, (b) how local topography can influence ozone concentrations measured at land based locations such as Eureka NU. Measurements were made during three field campaigns. The first was on the Amundsen icebreaker ship of the Canadian Coast Guard as part of the circumpolar Flow Lead study. For the second campaign the lidar was installed on the Polar-5 aircraft for flights over the sea ice north of Barrow Alaska. The third campaign involved ground based measurements from Eureka Weather Station on Ellesmere Island in the Canadian High Arctic. All of the measured ozone depletions were connected to the surface, and no evidence of ozone depleted air detached from the boundary layer was found. The lack of free tropospheric depletions indicate that such events are likely rare, and not a significant ozone sink. While measuring tropospheric ozone from a land based location, the measured depletions were found to be mainly confined to the atmospheric boundary layer except in instances where surrounding topography enabled the transport of ozone depleted air into the free troposphere. This effect was common at the Eureka weather station on Ellesmere Island, which is surrounded by a number of mountain ranges.