Andrew P. Rutter

Insights into the Origin of Water Soluble Organic Carbon in Atmospheric Fine Particulate Matter

Concentrations of fine carbonaceous aerosols (PM2.5), including elemental carbon (EC), organic carbon (OC), and water-soluble organic carbon (WSOC) were measured from filters collected every 6th day for one year at three urban/industrial sites in the Midwestern United States: Cincinnati, Cleveland, and Mingo Junction, Ohio. The water-soluble fraction of fine particulate OC varied considerably from site to site and monthly averages were between 39–85% at Cincinnati, 35–68% at Cleveland, and 32–65% at Mingo Junction. Average monthly concentrations of WSOC were compared with measurements of organic source tracers, including levoglucosan, to better understand the spatial and temporal distribution and sources of WSOC. Two methods of predicting the non-biomass burning portion of WSOC (WSOCNB) were compared including estimation of secondary organic carbon (SOC) by the EC-tracer Method and comparison of the unapportioned OC from a chemical mass balance (CMB) source apportionment analysis. Poor correlations between SOC estimated by the EC-tracer method and WSOCNB suggested that the use of the EC-tracer method to estimate SOC may be significantly flawed with respect to low time-resolved measurements, such as one-in-six day measurements. Good correlations between CMB unapportioned OC and WSOCNB at all three sites (R2 = 0.68 to 0.91) suggested that direct measurements of levoglucosan and WSOC could provide a reasonable estimate of secondary organic carbon concentrations in some locations. However, application of this method to daily measurements made in Detroit, MI during July of 2007 and January/February of 2008 demonstrated that, on some individual days near large point sources, non-biomass burning sources of WSOC were important contributors to WSOC concentrations.

Evaluation of an Offline Method for the Analysis of Atmospheric Reactive Gaseous Mercury and Particulate Mercury

Abstract Reactive gaseous mercury (RGM) and particulate mercury (PHg) were collected in Milwaukee, WI, between April 2004 and May 2005, and in Riverside, CA, between July 25 and August 7, 2005 using sorbent and filter substrates. The substrates were analyzed for mercury by thermal desorption analysis (TDA) using a purpose-built instrument. Results from this offline-TDA method were compared with measurements using a real-time atmospheric mercury analyzer. RGM measurements made with the offline-TDA agreed well with a commercial real-time method. However, the offline TDA reported PHg concentrations 2.7 times higher than the real-time method, indicating evaporative losses might be occurring from the real-time instrument during sample collection. TDA combined with reactive mercury collection on filter and absorbent substrates was cheap, relatively easy to use, did not introduce biases due to a semicontinuous sample collection strategy, and had a dynamic range appropriate for use in rural and urban locations. The results of this study demonstrate that offline-TDA is a feasible method for collecting reactive mercury concentrations in a large network of filter-based samplers.

Sensitivity and bias of molecular marker-based aerosol source apportionment models to small conltibutions of coal combustion soot.

Carbonaceous atmospheric particulate matter (PM25) collected in the midwestern United States revealed that soot emissions from incomplete coal combustion were important sources of several organic molecular markers used in source apportionment studies. Despite not constituting a major source of organic carbon in the PM25, coal soot was an important source of polyaromatic hydrocarbons, hopanes, and elemental carbon. These marker compounds are becoming widely used for source apportionment of atmospheric organic PM, meaning that significant emissions of these marker compounds from unaccounted sources such as coal soot could bias apportionment results. This concept was demonstrated using measurements of atmospheric PM collected on a 1-in-6 day schedule at three monitoring sites in Ohio: Mingo Junction (near Steubenville), Cincinnati, and Cleveland. Impacts of coal sootwere measured to be significant at Mingo Junction and small at Cleveland and Cincinnati. As a result, biases in apportionment results were substantial at Mingo Junction and insignificant at Cleveland and Cincinnati. Misapportionments of organic carbon mass at Mingo Junction were significant when coal soot was detected in the particulate samples as identified bythe presence of picene, but when coal soot was not included in the model: gasoline engines (+8% to +58% of OC), smoking engines (0% to -17% of OC), biomass combustion (+1% to +11% of OC), diesel engines (-1% to -2% of OC), natural gas combustion (0% to -2% of OC), and unapportioned OC (0% to -47% of OC). These results suggest that the role of coal soot in source apportionment studies needs to be better examined in many parts of the United States and other parts of the world.

Climate Sensitivity of Gaseous Elemental Mercury Dry Deposition to Plants: Impacts of Temperature, Light Intensity, and Plant Species

Foliar accumulations of gaseous elemental mercury (GEM) were measured in three plant species between nominal temperatures of 10 and 30 °C and nominal irradiances of 0, 80, and 170 W m(-2) (300 nm-700 nm) in a 19 m(3) controlled environment chamber. The plants exposed were as follows: White Ash (Fraxinus americana; WA); White Spruce (Picea glauca; WS); and Kentucky Bluegrass (Poa partensis; KYBG). Foliar enrichments in the mercury stable isotope ((198)Hg) were used to measure mercury accumulation. Exposures lasted for 1 day after which the leaves were digested in hot acid and the extracted mercury was analyzed with ICPMS. Resistances to accumulative uptake by leaves were observed to be dependent on both light and temperature, reaching minima at optimal growing conditions (20 °C; 170 W m(-2) irradiance between 300-700 nm). Resistances typically increased at lower (10 °C) and higher (30 °C) temperatures and decreased with higher intensities of irradiance. Published models were modified and used to interpret the trends in stomatal and leaf interior resistances to GEM observed in WA. The model captured the experimental trends well and revealed that stomatal and internal resistances were both important across much of the temperature range. At high temperatures, however, stomatal resistance dominated due to increased water vapor pressure deficits. The resistances measured in this study were used to model foliar accumulations of GEM at a northern US deciduous forest using atmospheric mercury and climate measurements made over the 2003 growing season. The results were compared to modeled accumulations for GEM, RGM, and PHg using published deposition velocities. Predictions of foliar GEM accumulation were observed to be a factor of 5-10 lower when the temperature and irradiance dependent resistances determined in this study were used in place of previously published data. GEM uptake by leaves over the growing season was shown to be an important deposition pathway (2.3-3.7 μg m(-2) of one-sided leaf area; OSLA) when compared to total mercury wet deposition (1.2 μg m(-2) OSLA) and estimates of reactive mercury dry deposition (0.1-6 μg m(-2) OSLA). Resistance-Temperature-Irradiance relationships are provided for use in models.

Understanding the sources and composition of the incremental excess of fine particles across multiple sampling locations in one air shed

Well-designed health studies and the development of effective regulatory policies need to rely on an understanding of the incremental differences in particulate matter concentrations and their sources. Although only a limited number of studies have been conducted to examine spatial differences in sources to particulate matter within an air shed, routine monitoring data can be used to better understand these differences. Measurements from the US EPA Chemical Speciation Network (CSN) collected between 2002-2008 were analyzed to demonstrate the utility of regulatory data across three sites located within 100 km of each other. Trends in concentrations, source contribution, and incremental excesses across three sites were investigated using the Positive Matrix Factorization model. Similar yearly trends in chemical composition were observed across all sites, however, excesses of organic matter and elemental carbon were observed in the urban center that originated from local emissions of mobile sources and biomass burning. Secondary sulfate and secondary nitrate constituted over half of the PM2.5 with no spatial differences observed across sites. For these components, the excess of emissions from industrial sources could be directly quantified. This study demonstrates that CSN data from multiple sites can be successfully used to derive consistent source profiles and source contributions for regional pollution, and that CSN data can be used to quantify incremental differences in source contributions of across these sites. The analysis strategy can be used in other regions of the world to take advantage of existing ambient particulate matter monitoring data to better the understanding of spatial differences in source contributions within a given air shed.