Time-resolved measurements of water-soluble micro and trace elements in ambient air particulate matter

Time-resolved concentration measurements of atmospheric pollutants are a powerful tool for carrying out any detailed study about the dependence of the measured concentration on emission fluxes, deposition processes and meteorological conditions [1]. Up to now, only a very few studies have been addressed to time-resolved measurements of chemical species in atmospheric particulate matter (PM), mainly because of the high costs of the sampling activities and the very small dimensions of the collected samples. In this work we optimized and validated a quantitative multi-elemental analytical method able to determine the water-soluble elemental content of PM in 2-hour samples (As, Ba, Cd, Co, Cr, Cu, Fe, Mg, Mn, Na, Ni, Pb, Sb, Sn, Sr, V). The method, suitable for analyzing conventional low-volume samples collected at 2.3 m3/h on Teflon membrane filters, was applied to a ten-day monitoring campaign carried out in Rome. During the campaign the main meteorological parameters were acquired, including natural radioactivity for the evaluation of the dilution properties of the lower atmosphere[1]. The temporal profiles of the elemental concentrations allowed us to highlight some interesting correlations among tracers of specific emission sources. A perfect agreement between the temporal profiles of Na and Mg concentrations (R2 = 0.99) was observed during the whole monitoring campaign, confirming the reliability of soluble Mg as a tracer of sea-salt events. Two sea-salt transport events were clearly identified[2]. Some trace elements of mainly anthropogenic origin (Tl, V, Pb and Zn) showed very different temporal profiles during the two sea-spray events, according to the different back-trajectories of the air masses (Hysplit model). Other relevant information concerned the temporal profiles of some tracers of traffic-related road dust resuspension (Cu, Sb, Mn, Fe, Zn)[3]. The time pattern of their contribution to PM was compared with the traffic flow and with the mixing properties of the lower atmosphere (estimated by monitoring natural radioactivity). During atmospheric stability days, these elements showed high correlation (R2 > 0.9) and concentration peaks in the evening and in the early morning, when the traffic flow is high and the atmospheric mixing is weak. During the night (weak mixing and low traffic flow) a concentration decrease was observed, probably due to particle dry deposition. A decrease of the measured concentrations was observed also during the warm hours, when traffic emission is diluted by an efficient mixing of the atmosphere. These examples indicate the potential of time-resolved analytical results for identifying the high number of processes that are responsible for PM time and space variations and for evaluating the relevance of their specific contributions. [1] C. Perrino, A. Pietrodangelo, A. Febo, Atmos. Environ., 35 (2001) 5235–5244 [2] C. Perrino, S. Canepari, E. Cardarelli, M. Catrambone, T. Sargolini, Environ. Monit. Assess., 128 (2007) 133–151 [3] S. Canepari, C. Perrino, F. Olivieri, M.L.Astolfi, Atmos. Environ., 42 (2008) 8161-8175