Air quality measurements at Look Rock, Tennessee – on the western edge of the Great Smoky Mountains National Park – were begun in 1980 and expanded during the 1980s to a National Park Service (NPS) IMPROVE network station. Measurements were expanded again by the Tennessee Valley Authority (TVA, 1999–2007) to examine the effects of electric generating unit (EGU) emission reductions of SO2 and NOx on air quality at the station. Analysis of temporal trends (1999–2013) has been conducted at the site in collaboration with activities related to the 2013 Southeast Atmosphere Study (SAS) at Look Rock and other southeastern US locations. Key findings from these trend studies include the observation that primary pollutant levels have consistently tracked emission reductions from EGUs and other primary sources in the region, but reductions in secondary pollutants such as particulate sulfate and, specifically, ozone have been smaller compared to reductions in primary emissions. Organic carbonaceous material (OM) remains a major contributor (30–40 % in the period 2009–2013) to fine particulate mass at the site, as confirmed by ACSM measurements at the site in 2013. A large portion (65–85 %) of carbon in OM derives from modern carbon sources based on 14C measurements. Important parameters affecting ozone levels, fine mass, and visibility also include the specific diurnal meteorology at this ridge-top site, its location in a predominantly mixed-deciduous forest, and the presence of primary sources of precursors at distances of 50–500 km from the site in all directions.

The secondary organic aerosol (SOA) yield of β-caryophyllene photooxidation is enhanced by aerosol acidity. In the present study, the influence of aerosol acidity on the chemical composition of β-caryophyllene SOA is investigated using ultra performance liquid chromatography/electrospray ionization-time-of-flight mass spectrometry (UPLC/ESI-TOFMS). A number of first-, second- and higher-generation gas-phase products having carbonyl and carboxylic acid functional groups are detected in the particle phase. Particle-phase reaction products formed via hydration and organosulfate formation processes are also detected. Increased acidity leads to different effects on the abundance of individual products; significantly, abundances of organosulfates are correlated with aerosol acidity. To our knowledge, this is the first detection of organosulfates and nitrated organosulfates derived from a sesquiterpene. The increase of certain particle-phase reaction products with increased acidity provides chemical evidence to support the acid-enhanced SOA yields. Based on the agreement between the chromatographic retention times and accurate mass measurements of chamber and field samples, three β-caryophyllene products (i.e., β-nocaryophyllon aldehyde, β-hydroxynocaryophyllon aldehyde, and β-dihydroxynocaryophyllon aldehyde) are suggested as chemical tracers for β-caryophyllene SOA. These compounds are detected in both day and night ambient samples collected in downtown Atlanta, GA and rural Yorkville, GA during the 2008 August Mini-Intensive Gas and Aerosol Study (AMIGAS).

Overnight atmospheric transport and chemical evolution of photochemically aged Houston urban and petrochemical industrial plume were investigated in July 2005. We report here on the 26 July episode in which the aged plume was tagged 1.5 h before sunset with a pair of free‐floating controlled meteorological balloons, which guided quasi‐Lagrangian aircraft sampling in the plume as it was advected 300 km to the north over 8 h. The aged plume around sunset was well mixed within a 1600 m residual layer, and was characterized by enhanced levels of aerosol, O3, CO, olefins, acetaldehyde, total odd nitrogen compounds (NOy), and relatively small amounts (<1 ppbv) of NOx. The plume experienced appreciable shearing overnight due to the development of a low‐altitude nocturnal jet between 300 and 500 m above mean sea level (MSL). However, the plume above 600 m MSL remained largely undiluted even after 8 h of transport due to lack of turbulent mixing above the jet. About 40–60% of the NOx present in the aged plume around sunset was found to be depleted over this 8 h period. A constrained plume modeling analysis of the quasi‐Lagrangian aircraft observations suggested that by dawn this NOx was converted to nitric acid, organic nitrates, and peroxy acyl nitrates via reactions of NO3 radicals with enhanced levels of olefins and aldehydes in the plume. Sensitivity of NOx depletion to heterogeneous hydrolysis of N2O5 on aerosols was examined. These results have significant implications for the impacts of urban and industrial pollution on far downwind regions.

Isoprene-derived epoxydiols (IEPOX) are identified in ambient aerosol samples for the first time, together with other previously identified isoprene tracers (i.e., 2-methyltetrols, 2-methylglyceric acid, C(5)-alkenetriols, and organosulfate derivatives of 2-methyltetrols). Fine ambient aerosol collected in downtown Atlanta, GA and rural Yorkville, GA during the 2008 August Mini-Intensive Gas and Aerosol Study (AMIGAS) was analyzed using both gas chromatography/quadrupole mass spectrometry (GC/MS) and gas chromatography/time-of-flight mass spectrometry (GC/TOFMS) with prior trimethylsilylation. Mass concentrations of IEPOX ranged from approximately 1 to 24 ng m(-3) in the aerosol collected from the two sites. Detection of particle-phase IEPOX in the AMIGAS samples supports recent laboratory results that gas-phase IEPOX produced from the photooxidation of isoprene under low-NO(x) conditions is a key precursor of ambient isoprene secondary organic aerosol (SOA) formation. On average, the sum of the mass concentrations of IEPOX and the measured isoprene SOA tracers accounted for about 3% of the organic carbon, demonstrating the significance of isoprene oxidation to the formation of ambient aerosol in this region.

Laboratory experiments suggest that strong acids promote formation of enhanced levels of secondary organic aerosol (SOA), and organic aerosols may contribute to the health impacts of fine PM. We report results from examining hourly speciated fine particle data for evidence of ambient aerosol acidity-catalyzed SOA formation, as indicated by larger increases in the concentrations of organic aerosol mass occurring on days and in locations where more acidic aerosol (lower NH4+/SO4= molar ratios) exists. Data sets from the southeastern U.S. were examined for which hourly acidity of PM2.5 aerosols could be estimated, and for which hourly organic carbon (OC) content had been measured simultaneously. Within-day organic aerosol changes during selected periods were statistically related to concurrent aerosol acidity levels estimated from nitrate-corrected ammonium-to-sulfate ratios. Data from the Look Rock, TN, TVA/IMPROVE site for mid-July to mid-August 2004 showed average compositions frequently as acidic as NH4HSO4, however, no apparent increases in OC levels with increasing aerosol acidity were observed, even when [OC] changes were compared with time-delayed aerosol acidity estimates. SEARCH network data (2003–2004) for rural Centreville, AL (CTR) and Yorkville, GA (YRK) sites were also examined. Warm-season acidity levels were higher at CTR than at YRK, and daytime levels exceeded those at night at both sites. At the YRK site no consistent positive correlations were found between changes in OC or TC levels and aerosol acidity, even with time lags up to 6 h. Aerosol acidity at this site, however, is relatively low due to nearby agricultural sources of NH3. In contrast, during selected periods from April to October 2004, at CTR, 6-h lagged OC changes were weakly correlated with daytime, nitrate-corrected NH4+/SO4= molar ratios, but distinguishing this apparent relationship from meteorological effects on measured OC levels is challenging.

A late afternoon polluted air parcel transported from the Houston metropolitan area was monitored by an instrumented aircraft throughout the night of 21–22 July, 2005. Sampling was conducted during three flight segments over several downwind areas that were identified by a controllable meteorological balloon released from the Houston area at sundown. Samples were taken for approximately 2 h over each area. Using carbon monoxide as a tracer of the urban plume, it was revealed that the dilution inside the plume was relatively small. Ozone levels of up to 120 ppb were found in the plume at the furthest downwind distance, some 250 km northwest of Houston, with plume transport in the direction of the Dallas metropolitan area. The data further suggest that the nighttime conversion of NOx to NOz was very rapid, with complete (∼100%) conversion by the end of the night. At two locations the urban plume mixed with fresh emissions from power plants. At these sampling points ∼50% of the NOy had already been converted to NOz, thus indicating very rapid oxidation at night.

Twelve research fights were performed during August 2005 over the Dallas (Texas) metropolitan area. The primary objective was to estimate the relative contribution of primary emissions from large point sources, i.e., major power plants, compared with mobile sources in terms of O3 production. The distinction between the source types was derived from concurrent measurements of SO2 (tracer for point sources) and CO (tracer for mobile sources) relative to levels of O3 and NOx (the O3 precursor). The flights also examined the vertical structure of the atmosphere and its effect on the dispersion/dilution of the trace gases. During the first half of the study the O3 levels in the Dallas area were relatively low and only during the second half were significantly elevated O3 levels observed. For the latter period the relationship between maximum O3 levels, the air mass chemical age and the O3 yield are evaluated. The results also revealed that mobile sources are the main contributors to the elevated O3 levels in the Dallas area.

Improvements in measurement technology are permitting development of a more detailed scientific understanding of the cycling of mercury in the global atmospheric environment. Critical to advancing the state of knowledge is the acquisition of accurate measurement of speciated mercury (gaseous and particulate) at ground research stations in a variety of settings located around the globe. This paper describes one such research effort conducted at TVA's Look Rock air quality monitoring site in Tennessee—a mountain top site (813m elevation) just west of the Great Smoky Mountains National Park. The Great Smoky Mountains National Park is the largest National Park in the eastern US and it receives environmental protection under a variety of US statutes. Gaseous and particle mercury species along with some additional trace gases were measured at Look Rock during two field studies totaling 84 days in the spring and summer of 2004. Average results for the entire sampling period are: gaseous elemental mercury Hg(0): 1.65ngm−3, reactive gaseous mercury RGM: 0.005ngm−3, particulate mercury Hg(p): 0.007ngm−3. Literature review indicates that these levels are within the range found for other rural/remote sites in North America and worldwide. Reactive and particulate mercury comprised together less than 1%, on average, of total airborne mercury at Look Rock. When compared to the global background mercury literature, the Look Rock measurements demonstrate that the atmospheric mercury levels in the vicinity of the Great Smoky Mountains National Park are clearly dominated by the global atmospheric pool, not by local or regional sources.

[1] In their article Novakov et al. [2005] analyze the spatial distribution of OC/EC ratios for a variety of data reported worldwide, compare them to OC and EC emissions inventories, and then evaluate the effect of their “corrected” OC/EC values on the relative scattering and absorption by carbonaceous aerosols as it affects calculations of climate forcing potential. The authors claim that OC has been widely overestimated by large factors due to sorption of organic gases onto the generally used quartz filter matrix. The large value of this overestimate is “verified” by the need to reduce OC/EC ratios to match predominantly urban-based emissions inventories of OC and EC. [2] There are three major flaws in this evaluation which render the authors' conclusions suspect when applied to the global atmosphere. First, the net filter artifact during aerosol OC sampling is known to derive from both positive and negative artifacts [Eatough et al., 2003a]. The positive artifact is correctly reported by the authors as due to sorption of organic vapors on the filter and already collected particles, leading to a measurement of organic carbon mass which is positively biased by gaseous organic material. The negative artifact is due to the evaporation of organic species from already collected particles on the filter, which can occur because of pressure drop across the filter, and changes in the concentrations of semivolatile species in the gas phase, as well as changes in temperature, pressure, and other factors affecting the gas-particle equilibrium of semivolatile species. What is then measured on the front filter is the net amount of artifact from the combination of vapor sorption less aerosol vaporization. Techniques which “correct” for positive artifact by subtracting organic carbon found on a backup filter are in error (overcorrect the OC value) if significant volatilization of collected semivolatile species followed by collection on the backup filter occurs. Furthermore, this volatilization loss results in underestimation of OC even if some of the lost material is not collected on the backup filter. There is abundant evidence that at times the negative artifact can be significant and varies widely at different locations [Eatough et al., 2003a, 2003b; Huebert et al., 2004], so an approach which corrects for positive artifact but not negative artifact will result in measured OC/EC ratios which are biased low. This is true even if the net artifact is positive. [3] Second, the authors use predominantly urban emission data for global estimates of the artifact-induced errors in reported OC/EC ratios. This implicitly assumes that the correction for gaseous organics adsorbed on the filter is the same for urban as for rural areas. The relative amounts of positive and negative artifact OC are clearly dependent on the relative amounts and composition of gaseous organics vis à vis particulate carbon, that is, samples from urban areas with relatively much higher gaseous organic concentrations [Finlayson-Pitts and Pitts, 1999] but only up to about 50% higher particulate carbon levels [Edgerton et al., 2005] will report higher positive artifact, and hence larger OC/EC corrections will be needed to obtain “true ambient” OC/EC ratios compared to samples from rural areas. The latter have nearly as high particulate carbon levels contributing to negative artifact as urban areas, but much lower positive artifact. There may also be changes in the amount of artifact from gaseous adsorption or particulate volatilization during storage, and some research has found that a field blank correction can correct for most of the positive artifact. [4] Third, and most seriously, the authors appear to have made the extraordinary assumption that OC/EC ratios should remain constant during aerosol transport from urban or source-rich regions to rural and background locations. This appears to ignore the frequently large contribution made by secondary organic aerosol formation, especially during warm seasons of the year. Numerous laboratory studies have confirmed that primary emissions of gaseous organics can be converted to organic aerosols over timescales substantially shorter than the atmospheric lifetimes of organic aerosols [Pandis et al., 1995]. There is also abundant evidence that biogenic emissions in rural areas contribute substantially to secondary organic aerosol formation in rural atmospheres, based on source allocation and 14C-based studies [Tanner et al., 2004]. Further, the contribution of secondary organics to total aerosol carbon clearly varies widely, both seasonally and spatially [Zheng et al., 2002]. Therefore there is no sound reason to expect that OC/EC ratios should be consistent between urban and rural/background locations—indeed, we have every expectation that they should increase during transport from urban to rural locations due to secondary aerosol formation. As a result, we have no reason to expect that these ratios will necessarily agree with emissions inventories for primary organic and elemental carbon. [5] In summary, the magnitude of the corrections required by the authors' assertions of regionally continuous OC/EC are simply inconsistent with observed measurements of aerosol carbon in organic gas-denuded sample streams, measurements of field blanks, and observed stability of organic aerosol samples stored at low temperatures. They are also inconsistent with numerous estimates of the magnitude of contributions from secondary organic aerosol formation. Yes, there are artifacts in the measurement of filter samples of organic aerosol, both positive and negative. Yes, the parsing of OC and EC in analyses of ambient aerosols is operationally defined, and has significant uncertainty. However, the authors' claim that observed OC/EC ratios based on filter measurements are high by factors of two or more on the basis of the contention of continentally uniform OC/EC emission ratios is simply not consistent with a broad body of scientific evidence. [6] We note that the authors' principal focus was to use OC/EC values to evaluate relative scattering and absorption by carbonaceous aerosols as they affect calculations of climate forcing potential. Thus it might be suggested that when using models to determine the net cooling or heating effects of carbonaceous aerosols, scientists should first use the global emissions estimates of OC and EC from urban areas to estimate net effects of primary emissions, then use estimates of secondary organic aerosol (SOA) production to adjust the primary emissions' effects to the mix of primary and secondary carbonaceous aerosol found in the global atmosphere. We note that this will likely increase the modeled cooling produced by aerosol scattering (direct and indirect forcings) compared to using only the primary emissions estimates. The magnitude of this correction is also affected by uncertainties (noted by the authors) in determining the mass of organic matter (OM) from the measured value of OC, since the correction factor may differ for primary and secondary OC components.

Sources of carbonaceous aerosols collected from three sites of Chattanooga, TN (CH), Muscle Shoals, AL (MS), and Look Rock, TN (LR) in the Tennessee Valley Region (TVR) were apportioned using both organic tracer-based chemical mass balance (CMB) modeling and radiocarbon (14C) measurement and the results were compared. Eight sources were resolved by CMB, among which wood combustion (averaging 0.92μgm−3) was the largest contributor to primary organic carbon (OC) concentrations, followed by gasoline exhaust (0.35μgm−3), and diesel exhaust (0.18μgm−3). The identified primary sources accounted for 43%, 71%, and 14% of measured OC at CH, MS, and LR, respectively. Contributions from the eight primary sources resolved by CMB could explain 107±10% of ambient elemental carbon (EC) concentrations, with diesel exhaust (66±32%) and wood combustion (37±33%) as the most important contributors. The fossil fractions in total carbon determined by 14C measurements were in reasonably good agreement with that in primary (OC+EC) carbon apportioned by CMB in the MS winter samples. The comparison between the 14C and CMB results revealed that contemporary sources dominated other OC in the TVR, especially in summertime (84% contemporary).

Current fine particle NAAQS specify 24h integrated mass measurements as the compliance metric. However, the value of continuous short-time resolution sampling (1h or less) is recognized and being included in US EPA-monitoring strategies. An extensive body of fine mass concentration data has been acquired using continuous PM2.5 monitoring by TEOM at Look Rock, TN, augmented by trace gas measurements and, during enhanced monitoring periods, continuous sampling for aerosol sulfate (2 methods) and elemental (black) carbon. Continuous data are compared to the 24h-averaged values of mass and composition from integrated samplers at the site, and for fine mass and sulfate are found to be excellent (r2=0.97–0.99). We then report the diurnal variations in concentrations from extensive continuous monitoring during 3 summer periods (2000–2002) and for more limited periods throughout calendar year 2001. Levels of the several continuously measured gases and particulate constituents vary in magnitude diurnally according to expected patterns based on their emissions, their formation and loss processes, their lifetimes toward wet and dry deposition, the dynamics of up-slope—down-slope circulation, and solar-driven boundary layer growth and decay. Thus, concentrations of short-lifetime gases have minima at sunrise and maxima in the evening. Long-lived primary species (CO and black carbon) increase in mid-morning as the boundary layer height reaches the site and decay thereafter. Long-lived secondary species (sulfate and fine mass) show little diurnal variability. Factors controlling the observed diurnal variability thus significantly influence the extent and timing of human exposure to pollutant species, as well as affecting visibility impairment in complex terrain environments near the Great Smoky Mountains National Park.