Niveles y origen de gases reactivos y su relación con aerosoles en la proximidad a focos de emisión y en la troposfera libre de Tenerife

Resumen

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 range 0.2 - 0.7. In addition to these events, episodes of high O3 concentrations (47-87 ppb), high O3 / CO ratios (0.7-6.5) and low correlations between O3 and CO occur in late spring and summer. These events are associated with downward transport of air from the upper troposphere in the eastern side of depressions developing in the Eastern North Atlantic. The influence of long range transport on NOX and SO2 concentrations recorded under free troposphere conditions could not be studied due to the concentrations of these trace gases were lower than the detection limit of the analysers used (60 ppt for SO2 and 50 ppt for NO and NOX for 5 min averages) during the 37% and 92% of night-time period/year, respectively. At Izaña mountain observatory, the concentrations of UFPs are strongly influenced by the development of orographic buoyant upward flows during daylight. These ascending airflows perturbed the free troposphere and resulted in an increase in the concentrations of water vapour, SO2, NOX and UFPs during daylight. The data analysis shows that photo-chemically induced nucleation of SO2 (and probably other not measured gaseous precursors) plays a key role in the formation of UFPs in these ascending airflows. Because of this, the concentrations of nano-particles (3-10 nm) tend to reach high values during daylight. New particle formation was observed almost every day owing to the favourable conditions associated with the entry of boundary layer air in the low free troposphere, even if SO2 concentrations are rather low (tens to hundreds of ppt; i.e. 1 to 3 orders of magnitude lower than in the Santa Cruz Metropolitan Area). The low surface area of pre-existing particles, low temperature and high radiation intensity clearly favoured the formation of ultrafine and nano particles. At night, the concentrations of UFPs are similar to those recorded in other free troposphere sites around the world (~ 500 cm-3). These results clearly demonstrate that high mountains that enter in the free troposphere are active source regions of ultrafine particles. These results have important implications that should be considered by air quality managers. First, most of studies on UFPs and urban air quality have focused on vehicle exhaust emissions. In fact, vehicle exhaust is the only source subject to limit values for emissions of UFPs. The results of this research clearly demonstrate that SO2 emissions result in high concentrations of UFPs (frequently higher than those linked to vehicle exhaust emissions). Because of these high SO2 emissions, UFPs in Santa Cruz Metropolitan Area exhibit features (e.g. daily evolution, typical concentrations ranges, UFP to black carbon ratio) different to those observed in cities where vehicle exhausts are the predominant UFPs source (e.g. London, Milan, Lugano or Bern). The influence of SO2 emissions on UFPs concentrations is observed even at distant sites from the SO2 emission sources, as demonstrated with the data collected at Izaña observatory. Second; about the open debate on if the current limit values for PM10 and PM2.5 should be complemented, this study concludes that the simultaneous monitoring of ultrafine, black carbon, PM10 and PM2.5 particles is a suitable strategy of tracing aerosol pollutants of different nature (fresh vs. aged) and for difference sources: 1) UFPs are a better tracer of the fresh emissions of vehicle exhaust, ship and refinery, than PM2.5 and PM2.5-10, 2) PM2.5 is significantly influenced by aged aerosols, 3) black carbon was found to be the better tracer of solid particles emitted by vehicle exhausts. To include UFPs measurements is of especial relevance. A recent research, based on the data set produced in this study, concluded that there is an association between hospitalizations due to heart failure and exposure to UFPs in the ambient air of the study city.