Artifacts associated with the measurement of particulate mercury in an urban environment: The influence of elevated ozone concentrations

The potential for ozone to cause an artifact during sampling of fine particulate mercury was investigated in southwest Detroit in July 2001 and July 2002. During the July 2001 sampling period, the use of KI and KCl denuders placed upstream to remove ozone and reactive gaseous mercury, Hg2+(g), resulted in more particulate mercury (maximum 33 pg m−3) on a denuded filter when compared to an undenuded one. Additionally, a KCl denuded quartz filter also showed more particulate mercury (maximum 12 pg m−3) compared to an undenuded filter when ozone concentrations were most elevated. Further sampling in July 2002 used a repeated sampling design and resulted in more particulate mercury on undenuded filters when compared to denuded ones. Conditions in July 2001 were characterized by elevated concentrations of ozone during which there were three ozone action days declared by the State of Michigan whereas ozone concentrations in July 2002 were 22% lower. One possible explanation for the observations may be that elemental mercury, Hg0(g), is oxidized to reactive gaseous mercury, Hg2+(g), during sampling leading to an artifact. The potential for this type of artifact may be greater during periods of high photochemical activity and this must be considered when sampling during the warmer seasons when high levels of oxidants are likely to be present in an urban atmosphere.