Potential role of stabilized Criegee radicals in sulfuric acid production in a high biogenic VOC environment.

We present field observations made in June 2011 downwind of Dallas-Fort Worth, TX, and evaluate the role of stabilized Criegee radicals (sCIs) in gaseous sulfuric acid (H2SO4) production. Zero-dimensional model calculations show that sCI from biogenic volatile organic compounds composed the majority of the sCIs. The main uncertainty associated with an evaluation of H2SO4 production from the sCI reaction channel is the lack of experimentally determined reaction rates for sCIs formed from isoprene ozonolysis with SO2 along with systematic discrepancies in experimentally derived reaction rates between other sCIs and SO2 and water vapor. In general, the maximum of H2SO4 production from the sCI channel is found in the late afternoon as ozone increases toward the late afternoon. The sCI channel, however, contributes minor H2SO4 production compared with the conventional OH channel in the mid-day. Finally, the production and the loss rates of H2SO4 are compared. The application of the recommended mass accommodation coefficient causes significant overestimation of H2SO4 loss rates compared with H2SO4 production rates. However, the application of a lower experimental value for the mass accommodation coefficient provides good agreement between the loss and production rates of H2SO4. The results suggest that the recommended coefficient for the H2O surface may not be suitable for this relatively dry environment.

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.

Atmospheric photolytic reduction of Hg(ii) in dry aerosols.

A laboratory reactor system was developed to examine the role of light and aerosol composition in the reduction of oxidized mercury (Hg(ii)) in laboratory-generated aerosols. Aerosolized sodium chloride, doped with mercury chloride, was exposed to light in a fixed-bed flow-through reactor. Three spectral ranges (UV, visible and a simulated solar spectrum) were examined, along with dark experiments, to investigate the role of light conditions in mercury reduction. In addition, the role of iron in the aerosol matrix was examined. The effluent from the reactor was analyzed for Hg(0) as evidence of reduction of Hg(ii) in the reactor. Significant reduction of Hg(ii) (1.5-9.9%) was observed for all three light sources and the rate of mercury reduction was proportional to the light irradiance. The presence of iron in the aerosol matrix inhibited the reduction rate and the degree of inhibition was dependent on the chemical form of the iron in the aerosol. The observed reduction reactions may be important chemical processes in the atmosphere and could be incorporated in atmospheric transport models that are used to understand the fate of atmospheric mercury.

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.

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.

A comparison of summertime secondary organic aerosol source contributions at contrasting urban locations.

Primary and secondary sources contributing to atmospheric organic aerosol during the months of July and August were quantitatively assessed in three North American urban areas: Cleveland, Ohio, and Detroit, Michigan, in the Midwest region and Riverside, California, in the Los Angeles Air Basin. Organic molecular marker species unique to primary aerosol sources and secondarytracers derived from isoprene, alpha-pinene, beta-caryophyllene, and toluene were measured using gas chromatography-mass spectrometry. Source contributions from motor vehicles, biomass burning, vegetative detritus, and secondary organic aerosol (SOA) were estimated using chemical mass balance (CMB) modeling. In Cleveland, primary sources accounted for 37 +/- 2% of ambient organic carbon, measured biogenic and anthropogenic secondary sources contributed 46 +/- 6%, and other unknown sources contributed 17 +/- 4%. Similarly, Detroit aerosol was determined to be 44 +/- 5% primary and 37 +/- 3% secondary, while 19 +/- 7% was unaccounted for by measured sources. In Riverside, 21 +/- 3% of organic carbon came from primary sources, 26 +/- 5% was attributed to measured secondary sources, and 53 +/- 3% came from other sources that were expected to be secondary in nature. The comparison of samples across these two regions demonstrated that summertime SOA in the Midwestern United States was substantially different from the summertime SOA in the Los Angeles Air Basin and indicated the need to exert caution when generalizing about the sources and nature of SOA across different urban areas. Furthermore, the results of this study suggestthatthe contemporary understanding of SOA sources and formation mechanisms is satisfactory to explainthe majority of SOA in the Midwest Additional SOA sources and mechanisms of formation are needed to explain the majority of SOA in the Los Angeles Air Basin.

Development and application of a novel acridinium ester for use as a chemiluminescent emitter in nucleic acid hybridisation assays using chemiluminescence quenching.

Chemiluminescent acridinium esters (AEs) permit the development of high sensitivity ligand binding assays due to a combination of high intensity light emission and very low backgrounds. Here these advantages are exploited for use in homogeneous nucleic acid hybridisation assays using quenched chemiluminescence. AE chemiluminescence is conventionally initiated at highly alkaline pH. Novel "active" AEs were designed that permit initiation under conditions compatible with maintenance of nucleic acid hybrids (i.e. pH less than 9). Methyl red was found to be a dark quencher species capable of functioning at this pH. Practical application of the chemiluminescence quenching assay system has been demonstrated using two model nucleic acid hybridisation assays based on intra- and intermolecular emitter/quencher pairs.

Evaluation of an offline method for the analysis of atmospheric reactive gaseous mercury and particulate mercury.

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.

A comparison of speciated atmospheric mercury at an urban center and an upwind rural location.

Gaseous elemental mercury (GEM), particulate mercury (PHg) and reactive gaseous mercury (RGM) were measured every other hour at a rural location in south central Wisconsin (Devil's Lake State Park, WI, USA) between April 2003 and March 2004, and at a predominantly downwind urban site in southeastern Wisconsin (Milwaukee, WI, USA) between June 2004 and May 2005. Annual averages of GEM, PHg, and RGM at the urban site were statistically higher than those measured at the rural site. Pollution roses of GEM and reactive mercury (RM; sum of PHg and RGM) at the rural and urban sites revealed the influences of point source emissions in surrounding counties that were consistent with the US EPA 1999 National Emission Inventory and the 2003-2005 US EPA Toxics Release Inventory. Source-receptor relationships at both sites were studied by quantifying the impacts of point sources on mercury concentrations. Time series of GEM, PHg, and RGM concentrations were sorted into two categories; time periods dominated by impacts from point sources, and time periods dominated by mercury from non-point sources. The analysis revealed average point source contributions to GEM, PHg, and RGM concentration measurements to be significant over the year long studies. At the rural site, contributions to annual average concentrations were: GEM (2%; 0.04 ng m(-3)); and, RM (48%; 5.7 pg m(-3)). At the urban site, contributions to annual average concentrations were: GEM (33%; 0.81 ng m(-3)); and, RM (64%; 13.8 pg m(-3)).

The impact of aerosol composition on the particle to gas partitioning of reactive mercury.

A laboratory system was developed to study the gas-particle partitioning of reactive mercury (RM) as a function of aerosol composition in synthetic atmospheric particulate matter. The collection of RM was achieved by filter- and sorbent-based methods. Analyses of the RM collected on the filters and sorbents were performed using thermal extraction combined with cold vapor atomic fluorescence spectroscopy (CVAFS), allowing direct measurement of the RM load on the substrates. Laboratory measurements of the gas-particle partitioning coefficients of RM to atmospheric aerosol particles revealed a strong dependence on aerosol composition, with partitioning coefficients that varied by orders of magnitude depending on the composition of the particles. Particles of sodium nitrate and the chlorides of potassium and sodium had high partitioning coefficients, shifting the RM partitioning toward the particle phase, while ammonium sulfate, levoglucosan, and adipic acid caused the RM to partition toward the gas phase and, therefore, had partitioning coefficients that were lower by orders of magnitude.

Beta-ketoacyl-acyl carrier protein synthase III from pea (Pisum sativum L.): properties, inhibition by a novel thiolactomycin analogue and isolation of a cDNA clone encoding the enzyme.

A beta-ketoacyl-acyl carrier protein (ACP) synthase III (KAS III; short-chain condensing enzyme) has been partly purified from pea leaves. The enzyme, which had acetyl-CoA:ACP acyltransferase (ACAT) activity, was resolved from a second, specific, ACAT protein. The KAS III enzyme had a derived molecular mass of 42 kDa (from its cDNA sequence) and operated as a dimer. Its enzymological characteristics were similar to those of two other plant KAS III enzymes except for its inhibition by thiolactomycin. A derivative of thiolactomycin containing a longer (C8 saturated) hydrophobic side-chain (compound 332) was a more effective inhibitor of pea KAS III and showed competitive inhibition towards malonyl-ACP whereas thiolactomycin showed uncompetitive characteristics at high concentrations. This difference may be due to the better fit of compound 332 into a hydrophobic pocket at the active site. A full-length cDNA for the pea KAS III was isolated. This was expressed in Escherichia coli as a fusion protein with glutathione S-transferase in order to facilitate subsequent purification. Demonstrated activity in preparations from E. coli confirmed that the cDNA encoded a KAS III enzyme. Furthermore, the expressed KAS III had ACAT activity, showing that the latter was inherent. The derived amino acid sequence of the pea cDNA showed 81-87% similarity to that for other plant dicotyledon KAS IIIs, somewhat less for Allium porrum (leek, 71%) and for Porphyra spp. (62%), Synechocystis spp. (65%) and various bacteria (42-65%). The pea KAS III exhibited four areas of homology, three of which were around the active-site Cys(123), His(323) and Asn(353). In addition, a stretch of 23 amino acids (residues 207-229 in the pea KAS III) was almost completely conserved in the plant KAS IIIs. Modelling this stretch showed they belonged to a peptide fragment that fitted over the active site and contained segments suggested to be involved in substrate binding and in conformational changes during catalysis, as well as an arginine suggested to participate in the acid-base catalytic mechanism.

Oxygen induction of a novel fatty acid n-6 desaturase in the soil protozoon, Acanthamoeba castellanii.

Induction of fatty acid desaturation is very important for the temperature adaptation of poikilotherms. However, in oxygen-limited late-exponential-phase Acanthamoeba castellanii cultures, oxygen alone was able to induce increased activity of a fatty acid desaturase that converts oleate into linoleate and which has been implicated in the temperature adaptation of this organism. Experiments with Delta(10)-nonadecenoate showed that the enzyme is an n -6 desaturase rather than a Delta(12)-desaturase. It also used preferentially 1-acyl-2-oleoyl-phosphatidylcholine as substrate and NAD(P)H as electron donor. The involvement of cytochrome b (5) as an intermediate electron carrier was shown by difference spectra measurements and anti-(cytochrome b (5)) antibody experiments. Of the three protein components of the desaturase complex, oxygen only increased the activity of the terminal (cyanide-sensitive) protein during n -6 desaturase induction. The induction of this terminal protein paralleled well the increase in overall oleate n -6 desaturation. The ability of oxygen to induce oleate desaturase independently of temperature in this lower eukaryotic animal model is of novel intrinsic interest, as well as being important for the design of future experiments to determine the molecular mechanism of temperature adaptation in poikilotherms.