Conceptual Model - Causes of Haze in Mingo Area (MING1)

Secondary sulfate from east and south of the site during the summer time is the major aerosol component during most of the worst visibility days in the Mingo Wilderness Area. Secondary nitrate from the central U.S. also contributes significantly to haze and is the main contributor to haze during winter 20% worst visibility days. time.

As shown in Figure 1, The Mingo Wilderness occupies 7,730 acres on the western half of the Mingo in southeastern . Terrain is marshy flatlands, surrounded by low hills. The MING1 IMPROVE site is located ~3 km northeast of the town Puxico near the southeastern corner of the Wildlife Refuge. The site elevation is 112 m (367 ft). Surrounding terrain is hilly and groundcover is predominantly forest and lakes. Based on all the valid aerosol measurements during 2001-2004 at 3 MING1, the average PM2.5 mass concentration is 11.0 µg/m . The average total light -1 extinction coefficient (Bext) is 92.8 Mm (Visual Range ~ 54 Km; Deciview ~ 21). The average contributions of the major aerosol components to Mingo haze are particulate sulfate 47.8%, nitrate 15.7%, organic matter (OMC) 13.3%, elemental carbon (light absorbing carbon, LAC) 4.2%, fine soil 0.9%, sea salt 0.3%, and coarse mass (CM) 4.9%.

Figure 1. Terrain and land features surrounding the Mingo sampling site

Sulfate is the largest aerosol contributor to light extinction during the 20% worst days, with a contribution of ~ 57%. Nitrate and OMC contribute about 16% and 12%, respectively, to light extinction during the 20% worst visibility days. Figure 2 shows that the highest occurrence of the 20% worst days happened in September, in which ~ 42% of the sampling days are 20% haziest days at Mingo. As shown in Figure 3, on the 20% worst visibility days, sulfate is the largest aerosol contributor to haze (from ~30% in November to ~80% in July) except in January and December when nitrate dominates the aerosol light extinction with a contribution of ~50%. Nitrate also contributes about 25- 35% during the 20% worst days in February and March, and OMC contributes about 20- 30% during the 20% worst days in October and November. Figure 4 indicates that during the 20% best days, air usually comes from northwest of the site; while during the 20% worst haze days, air may come from every directions around the site. It also shows that the prevailing transport wind directions in MING1 are predominantly from the northwest in winter and from the south-southwest in the summer.

Figure 2. Percentage of sampling days that are 20% worst days in each month

Based on the PMF receptor modeling, six source factors are identified for MING1. Figure 5 illustrates the contribution of each PMF resolved source factor to PM2.5 mass at the site. Sulfate-rich secondary aerosol is the biggest contributor to PM2.5 mass, with a total contribution of ~55%. Two sulfate-rich secondary aerosol source factors are identified by PMF. Difference maps of the PMF factor score weighted and un-weighted residence times (Figure 6) shows one with transport mainly from northeast of the site (i.e. Ohio River Valley), while the other has transport mostly from south. Figure 6 also shows that secondary nitrate mostly comes from northwest of the site (i.e. central U.S.)

Figure 3. Average contributions of major aerosol chemical components to light extinction during 20% worst days in each month

Figure 4. Normalized residence time for 20% best (top left) and 20% worst (top right) days, and for January (bottom left) and August (bottom right) (air mostly transported from the blue area under the given sampling days)

Figure 5. Average contributions of PMF resolved source factors to PM2.5 mass concentration.

Figure 6. Difference maps of the PMF source factor (Sulfate-rich secondary source factor I on the top left, Sulfate-rich secondary source factor II on the top right, nitrate-rich secondary source factor on the bottom left) weighted and un-weighted residence times.