A Conceptual Model for PM2.5 over SoutheastMichigan: Intraurban Variability and HighConcentration Days

摘要

In 2006, the USEPA promulgated revisions to the National Ambient Air Quality Standard(NAAQS) for particulate matter (PM) which tightened the 24-hour (daily) PM2.5 standardfrom 65 to 35 micrograms per cubic meter. This revision profoundly increased thefrequency of exceedance days observed in Southeast Michigan with fourteen of thefifteen compliance monitors violating the standard for the period 2004-2006. Anunderstanding of the drivers for high PM2.5 days is needed to support the development ofeffective controls strategies and a study was undertaken to provide insights into suchdrivers. This analysis exploited PM2.5 routine compliance monitoring data assupplemented by fine particle speciation data and surfaces winds. At most sites inSoutheast Michigan, exceedances occur predominantly – and in some cases exclusively –on days when the entire Southeast Michigan network is exhibiting high PM. In mostcases, exceedances were observed when the area was experiencing a multiday high PMepisode. In contrast, two monitors – Dearborn and Wyandotte – frequently hadexceedances on days when the entire network was not exhibiting high PM with 38% ofthe Dearborn exceedances and 27% of the Wyandotte exceedances occurring on suchdays. On these days, these sites are significantly influenced by nearby emission sources.A network-wide base concentration was defined for each day and the distribution ofexcess mass at each site, relative to the base concentration, was examined usingnonparametric wind regression (NWR). This approach clearly identified the urban plumeemanating from Detroit including a plume associated with a large industrial regionwest/southwest of the central business district. NWR was used to identify site-specifictemporal changes in the expected excess mass concentration as a function of winddirection over the past nine years, and the observed decreases in expected concentrationsare consistent with known point source emission reductions.