A Conceptual Model for PM2.5 over Southeast Michigan: Intraurban Variability and High Concentration 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 standard from 65 to 35 micrograms per cubic meter. This revision profoundly increased the frequency of exceedance days observed in Southeast Michigan with fourteen of the fifteen compliance monitors violating the standard for the period 2004-2006. An understanding of the drivers for high PM2.5 days is needed to support the development of effective controls strategies and a study was undertaken to provide insights into such drivers. This analysis exploited PM2.5 routine compliance monitoring data as supplemented by fine particle speciation data and surfaces winds. At most sites in Southeast Michigan, exceedances occur predominantly – and in some cases exclusively – on days when the entire Southeast Michigan network is exhibiting high PM. In most cases, exceedances were observed when the area was experiencing a multiday high PM episode. In contrast, two monitors – Dearborn and Wyandotte – frequently had exceedances on days when the entire network was not exhibiting high PM with 38% of the Dearborn exceedances and 27% of the Wyandotte exceedances occurring on such days. 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 of excess mass at each site, relative to the base concentration, was examined using nonparametric wind regression (NWR). This approach clearly identified the urban plume emanating from Detroit including a plume associated with a large industrial region west/southwest of the central business district. NWR was used to identify site-specific temporal changes in the expected excess mass concentration as a function of wind direction over the past nine years, and the observed decreases in expected concentrations are consistent with known point source emission reductions.