CHARACTERIZATION OF TRANSPORT AND TRANSFORMATION OF AIR POLLUTANTS OBSERVED IN THE FREE TROPOSPHERE OVER THE CENTRAL NORTH ATLANTIC

Long-range transport of continental pollution emissions has been found to affect air quality in the downwind regions. The Pico Mountain Observatory (PMO), which was established in 2001 in the Azores Islands, has been a perfect site to study long range transport from North America to the central North Atlantic. The dissertation uses the observations and modeling products created since 2001 to provide insights on ozone production tendency during transport, trace gases observations for varying transport patterns, and aerosol ages in the free troposphere over the central North Atlantic. The first phase of this dissertation research focused on determining the causes for the high d[O3]/d[CO] values (∼ 1 ppbv/ppbv) observed in the summers of 2009 and 2010 at PMO. The folded retroplume technique, developed by Owen and Honrath (2009), was applied to combine upwind FLEXPART transport pathways with GEOS-Chem chemical fields. This technique was used to simulate ozone chemistry during the two selected transport events from North America. Ozone production was found at 2 kma.s.l. above the Azores area in the first event plume, apparently driven by PAN decomposition during subsidence of the air mass in the Azores-Bermuda High. In the second event, net ozone loss occurred during transport in the lower free troposphere, yet observed d[O3]/d[CO] was high. In both events, CO loss through oxidation contributed significantly to d[O3]/d[CO] enhancement. The second phase looked into chemical signatures in several transport pathways. The observations associated with different transport patterns were viewed as chemical signatures, which exhibit the consequences of photochemical reactions and physical processes during long-range transport. Enhancement of trace gases at PMO is driven by transport of pollution emissions. Ozone and nitrogen oxides enhancements were also found associated with subsidence of air masses from the upper troposphere and stratosphere The d[O3]/d[CO] slope and non-methane hydrocarbon concentrations were also studied for the main transport patterns, which indicated that differently aged and polluted plumes intersect PMO as a result of a combination of photochemical oxidation and mixing with North Atlantic free tropospheric background air. The third phase of this dissertation used radionuclide tracers associated with aerosol samples during summer 2013 at PMO to estimate age of aerosols collected over the central North Atlantic. Due to different rates of decay, the activity ratio of the atmospheric radionuclide tracers, 210Po to 210Pb, can be used to estimate age of the carrier aerosols. Analysis of 18 unbiased samples showed an estimated aerosol atmospheric residence time of 20.7 days at PMO. The long age of the aerosols was attributed to less efficient removal processes in the free troposphere compared to the boundary layer