title: On the construction, comparison, and variability of airsheds for interpreting semivolatile organic compounds in passively sampled air.
abstract: Air mass origin as determined by back trajectories often aids in explaining some of the short-term variability in the atmospheric concentrations of semivolatile organic contaminants. Airsheds, constructed by amalgamating large numbers of back trajectories, capture average air mass origins over longer time periods and thus have found use in interpreting air concentrations obtained by passive air samplers. To explore some of their key characteristics, airsheds for 54 locations on Earth were constructed and compared for roundness, seasonality, and interannual variability. To avoid the so-called "pole problem" and to simplify the calculation of roundness, a "geodesic grid" was used to bin the back-trajectory end points. Departures from roundness were seen to occur at all latitudes and to correlate significantly with local slope but no strong relationship between latitude and roundness was revealed. Seasonality and interannual variability vary widely enough to imply that static models of transport are not sufficient to describe the proximity of an area to potential sources of contaminants. For interpreting an air measurement an airshed should be generated specifically for the deployment time of the sampler, especially when investigating long-term trends. Samples taken in a single season may not represent the average annual atmosphere, and samples taken in linear, as opposed to round, airsheds may not represent the average atmosphere in the area. Simple methods are proposed to ascertain the significance of an airshed or individual cell. It is recommended that when establishing potential contaminant source regions only end points with departure heights of less than ∼700 m be considered.
authors: Department of Physical and Environmental Sciences, University of Toronto Scarborough, Toronto, Ontario, Canada.
authors: John N
authors: JN
authors: Westgate
authors: Frank
authors: F
authors: Wania
mesh_terms: D000393:Air Pollutants / Q000032:analysis*; D004784:Environmental Monitoring; D009930:Organic Chemicals / Q000032:analysis*; D055549:Volatile Organic Compounds / Q000032:analysis*
chemical_list: D000393:Air Pollutants; D009930:Organic Chemicals; D055549:Volatile Organic Compounds