Evidence for a sampling artifact for particulate-phase mercury in the marine atmosphere

During a coastal environment study an artifact associated with a widely used particulate mercury (Hg) sampling method was discovered that produces inconsistent results for Hg in PM10 samples of different duration (12 versus 4-h). The artifact is attributed to loss of Hg from filters over the course of the longer sampling periods, perhaps due to chemical reaction, enhanced by high humidities common to the marine boundary layer. On the seven days on which three successive 4- and 12-h duration samples were obtained, the mean Hg concentration of PM10 was significantly higher for the 4-h samples (22.5 pg m−3) than for the 12-h samples (8.4 pg m−3) (p=0.012). The day-to-day differences between 4- and 12-h samples ranged from −1 to 29 pg m−3. No consistent difference was observed in the Hg concentration of PM2.5 of different durations. Adsorption of reactive gaseous mercury (RGM) onto PM2.5 was also evaluated by collection of samples with and without an upstream KCl denuder for RGM removal. Concentrations were not significantly different, indicating that RGM adsorption was not evident in this low aerosol, low RGM environment. The discovery of an artifact related to sample duration demonstrates the need for continued evaluation and validation of the currently accepted methods for atmospheric Hg measurements.