The seasonal evolution of NMHCs and light alkyl nitrates at middle to high northern latitudes during TOPSE

The Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment was designed to follow the role of photochemistry in the evolution of the springtime maximum of tropospheric ozone (O₃) in the Northern Hemisphere (NH) at high latitudes. Determination of the composition and seasonal evolution of volatile organic carbon (VOC) species, which take part in and are good indicators for photochemical processes in the troposphere, was an important part of this study. We report measurements of a large number Of C₂-C₁₀ nonmethane hydrocarbons (NMHCs), selected C₁-C₂ halocarbons, and C₁-C₄ alkyl nitrates. These gases were quantified in whole air samples collected aboard the National Center for Atmospheric Research (NCAR) C-130 aircraft at altitudes between 30 m and 8 km. Seven TOPSE sampling trips were flown between early February and mid-May 2000 covering the region from Colorado (40°N) to Churchill (in Manitoba, Canada), Thule (in northern Greenland), and as far north as 85°N. These measurements represent the most comprehensive spatial characterization of the North American Arctic to date and revealed strong latitudinal, vertical, and temporal NMHC gradients. In the midtroposphere north of Churchill (58°N), ΣNMHCs decreased by ∼6.2 ppbC between February and May (1.6 ppbC month^-1) and the magnitude of this change diminished with altitude. Over the same period, midtropospheric O₃ levels increased by ∼16 ppbv (4.2 ppbv month^-1). Free tropospheric NMHC decreases were consistent with removal by hydroxyl (OH) radicals at an average mixing ratio for mid-March to mid-May of 4.1 × 10⁵ mol cm^-3 . The alkyl nitrates, which are a reservoir species for tropospheric reactive odd nitrogen (NO_Y), revealed similar latitudinal, vertical, and temporal gradients to their parent NMHCs. Their total decreased by ∼4 pptv month ^-1, representing 10% or less of NO_Y. In conjunction with meteorological trajectory analysis, different trace gas signatures provided significant clues to the origins of individual polluted air masses. Several of these air masses were rapidly advected over the Pole from source regions in northeastern and western Europe as well as an air mass that originated over the southwestern United States/Baja California that contained unusually high levels of alkanes. In addition, episodes of low boundary layer (BL) O₃ associated with low NMHC mixing ratios and trajectories from over the Arctic Ocean were frequently sampled toward the latter part of the experiment. The TOPSE data described here provide a unique picture of NH trace gas evolution from winter to summer that will be invaluable to models investigating the role that anthropogenic emissions play in high latitude O₃ chemistry.