Atmospheric Chemistry and Physics, vol. 13, issue 1 (2013) pp. 15-30
This work investigates the variability of ozone (O sub(3)), carbon monoxide (CO) and equivalent black carbon (BC) at the Italian Climate Observatory "O. Vittori" (ICO-OV), part of the Mt. Cimone global GAW-WMO station (Italy). For this purpose, ICO-OV observations carried out in the period January 2007-June 2009, have been analyzed and correlated with the outputs of the FLEXPART Lagrangian dispersion model to specifically evaluate the influence of biomass burning (BB) and anthropogenic emissions younger than 20 days. During the investigation period, the average O sub(3), CO and BC at ICO-OV were 54 plus or minus 3 ppb, 122 plus or minus 7 ppb and 213 plus or minus 34 ng m super(-3) (mean plus or minus expanded uncertainty with p < 95%), with clear seasonal cycles characterized by summer maxima and winter minima for O sub(3) and BC and spring maximum and summer minimum for CO. According to FLEXPART outputs, BB impact is maximized during the warm months from July to September but appeared to have a significant contribution to the observed tracers only during specific transport events. We characterised in detail five "representative" events with respect to transport scales (i.e. global, regional and local), source regions and O sub(3), CO and BC variations. For these events, very large variability of enhancement ratios O sub(3)/CO (from -0.22 to 0.71) and BC/CO (from 2.69 to 29.83 ng m super(-3) ppb super(-1)) were observed. CO contributions related with anthropogenic emissions (CO sub(ant)) contributed to 17.4% of the mean CO value observed at ICO-OV, with the warm months appearing particularly affected by transport events of air-masses rich in anthropogenic pollution. The proportion of tracer variability that is described by FLEXPART CO sub(ant) peaked to 37% (in May-September) for CO, 19% (in May-September) for O sub(3) and 32% (in January-April) for BC. During May-September, the analysis of the correlation among CO, O sub(3) and BC as a function of the CO sub(ant) indicated that ICO-OV was influenced by air-masses rich in anthropogenic pollution transported from the regional to the global scale. On the other side, CO and O sub(3) were negatively correlated during October-December, when FLEXPART does not show significant presence of recent anthropogenic emissions and only a few observations are characterized by enhanced BC. Such behaviour may be attributed to an ensemble of processes concurrent in enhancing O sub(3) with low CO (upper troposphere/lower stratosphere intrusions) and to O sub(3) titration by NO in polluted air-masses along with lower photochemical activity. An intermediate situation occurs in January-April when CO and O sub(3) were almost uncorrelated and BC enhancements were associated to relatively old (10 days) anthropogenic emissions.
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