| Summary: | This study is focused on identifying the sources and processes that contribute to the concentrations of reactive gases (NOx, SO2, CO and O3) and ultrafine particles (UFPs) and their relationship in ambient air. Because of their impact on human health and their influence on climate, particular attention is paid to ultrafine particles. The study is based on experimental data of reactive gases and ultrafine (diameter < 0.1 µm), black carbon, PM10 (diameter < 10 µm) and PM2.5 (diameter < 2.5 µm) particles concentrations measured between 2006 and 2010 in the ambient air of two different environments: i) the Metropolitan Area of Santa Cruz de Tenerife, where important emissions of pollutants occur, and ii) the Izaña Global Atmospheric Watch (GAW) observatory, a remote mountain top site located in the low free troposphere (~2,400 meters above sea level), well above the stratocumulus layer characteristic of the subtropical oceanic regions. In the Metropolitan Area of Santa Cruz de Tenerife, the main sources of UFPs are: vehicle exhausts, ships and a crude oil refinery. The urban background concentration of UFPs is dominated by vehicle exhaust emissions, whereas UFPs events are induced by the emissions of SO2 from ships and the refinery. The concentration of UFPs linked to vehicle exhaust emissions maximized in the morning (07:00-09:00 GMT, 5,000-25,000 cm-3 = 25th - 75th percentile), whereas those linked to ships (15,000-45,000 cm-3) and refinery (25,000-95,000 cm-3) emissions maximized in the 10:00-17:00 GMT period, due to the effects of meteorology and photochemistry. When UFPs (24-h mean) were present in concentrations within the range 2,000- 20,000 cm-3 (0.1th - 70th percentile), vehicle exhausts were the predominant source, accounting for 85% of the observed UFPs. Higher concentrations (i.e., up to 56,000 cm-3, >percentile 70th, 24-h mean) were prompted by the contributions of ships and refinery emissions. The highest UFP concentrations were recorded in pollution events occurred from 10:00 to 17:00 GMT. In this period, ship emissions accounted for 39% of UFPs when their concentrations were within the range 19,500 -46,700 cm-3 (percentile 55th-90th), whereas the refinery accounted for 60% of UFPs with concentrations within the range 46,700-99,100 cm-3 (percentile 95th-100th). Below 19,500 cm-3 (percentile 55th), vehicle exhausts accounted for 91% of UFPs. At Izaña mountain observatory, the concentrations of reactive gases are influenced by two main processes: upward transport from the boundary layer and a synoptic scale long range transport. The concentrations of reactive gases show a strongly marked daily cycle, induced by the upward transport of air from boundary layer during daylight and downward transport of air from the free troposphere at night. This diurnal airflow is induced by the development of buoyant air flows around Tenerife. The mean daylight to night-time ratios were ~3 for NOX, ~2 for SO2, ~1 for CO and <1 for O3. As average, the 75% of NOX, 50% of SO2 and 4% of the CO recorded during daylight are attributed to the upward transport from the boundary layer to Izaña. This contribution results in NOX, SO2 and CO concentrations above the free troposphere nocturnal-background during daylight. In contrast, the concentrations of O3 are regularly lower during daylight than at night due to the strong vertical gradient of this trace gas in this region. The processes that contribute to the concentrations of CO and O3 in the free troposphere were studied by the analysis of the night-time concentrations of these gases with products determined with back-trajectories (transport pathways and potential source regions). The analysis of these products and the correlation observed between O3 and CO during the four study years (2007- 2010) point that the eastward exportation of pollutants from North America modulates the variability in the concentrations of O3 and CO in the North Atlantic free troposphere. In this scenario, O3 typically shows concentrations within the range 34 - 76 ppb and O3 / CO ratios within the r
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