Measuring and Modeling Short-Term Variability of PCBs in Air and Characterization of Urban Source Strength in Zurich, Switzerland

Abstract

Diel (24-h) variations of concentrations of polychlorinated biphenyls (PCBs) in air are reported at two sites in Zurich, Switzerland, a city surrounded by hills. One site was located in the valley near the city center and the other site was on a hill called Uetliberg, 411 m higher and about 5 km distant from the city center site. Air samples were collected simultaneously at both sites over 4-h time periods for 3 consecutive days under stable meteorological conditions during a high pressure system in August 2007. PCB concentrations at the city site were markedly higher than those at Uetliberg, indicating that the city site is influenced by urban sources of PCBs. Concentrations measured at both sites show a clear diel cycle but have opposite phases: in the city concentrations were lower during the day and higher at night, while at Uetliberg concentrations were higher during the day and lower at night. These observations are explained and interpreted using a multimedia mass balance model that includes a stable night-time inversion layer that formed over the city but below the hilltop site. At Uetliberg the concentration of PCBs is consistent with background levels and the diel concentration pattern can be explained bytemperature-mediated air-surface exchange and the influence of nearby woodland canopies. The diel pattern and concentrations in the city are attributable to volatilization from urban sources. We have developed a new method to estimate the strength of the urban PCB source using the model scenarios for the two sampling sites. Emission estimates derived from this method are in good agreement with earlier estimates derived from PCB production, consumption, and usage estimates. This study demonstrates the potential for estimating the strength of diffuse, regional sources of persistent organic pollutants (POPs) using a combined measurement and modeling approach. Such studies can provide important information to derive regional and national POPs emission inventories as required under the Stockholm Convention, and to quantify the effectiveness of actions to reduce POP emissions.

Atmospheric emissions and long-range transport of persistent organic chemicals

“…At the hilltop site, measured PCB concentrations were on the order of several 10 pg/m 3 during the day and lower at night; for this site, it was assumed that PCBs enter the model domain with inflowing air at background concentrations (on the order of several 10 pg/m 3 ). With this set-up, the model well reproduces phase and amplitude of PCB concentrations at the hilltop site [22]; analysis of the model behavior shows that during the night PCBs are removed from the lower air compartment by increased deposition to soil and vegetation (lower vapor pressure caused by lower temperature), while there is no inflow of fresh PCBs from the upper air compartment (air compartments assumed to be separated during the night). During the day, PCBs from the upper air compartment are mixed into the lower air compartment, which causes the concentrations to rise again.…”

“…In August 2007, PCB concentrations were measured in 4-h intervals at two locations in the Zürich, Switzerland [22]. The measurements were performed during a stable high-pressure system at two sites: one on a hilltop outside the city ("Uetliberg") and one in the city center; several different PCB congeners (from tri-chloro to hepta-chloro) were measured.…”

Sources, Fate and Effects of Contaminant Emissions in Urban Areas

“…), as well as modelled estimates of Eisenreich (2000) and Totten et al (2004) of 0.4-3.6 and 0.4 mg/m 2 year for Chicago and New York City respectively. Expressed on a per capita basis, PCB emissions have been reported as 35-350, 360 and 1600 mg/capita year for Toronto, Wilrijk, Belgium and Zurich respectively Van Gerven et al, 2004;Gasic et al, 2009).…”

“…Offenberg et al, 2005;Totten et al, 2006;Gasic et al, 2009). This information is of critical importance to risk assessment studies and the design of effective policies and programmes aimed at emissions reductions.…”

The Chemicals that will not Go Away: Implications for Human Exposure to Reservoirs of POPs

“…Several studies have estimated PCB release rates from cities ranging from 0.14 to 7.3 mg/m 2 y or 335 to 1600 mg/c y Van Gerven et al, 2004;Gasic et al, 2009). For Toronto, this release rate is the equivalent of $0.01-0.3% annually of the total documented stock.…”