Atmospheric Environment (v.38, #17)

The emission of electrically charged soot particles by an aircraft gas-turbine combustor is investigated using a theoretical model. Particular emphasis is placed on the influence of the fuel sulfur content (FSC). The model considers the production of primary “combustion” electrons and ions in the flame zone and their following interaction with molecular oxygen, sulfur-bearing molecules (e.g. O2, SO2, SO3, etc.) and soot particles. The soot particle size distribution is approximated by two different populations of mono-dispersed large and small soot particles with diameters of 20–30 and 5–7 nm, respectively. The effect of thermal ionization of soot and its interaction with electrons and positive and negative ions is included in the model. The computed positive and negative chemiion (CI) concentrations at the combustor exit and relative fractions of small neutral and charged soot particles were found to be in satisfactory agreement with experimental data. The results show that the FSC indeed may influence the concentration of negative CI at low fuel flow into combustor. Importantly the simulation indicates a very efficient mutual interaction of electrons and ions with soot particles with a large effect on both ion and charged soot particle concentrations. This result may be interpreted as a possible indirect effect of FSC on the growth and size distribution of soot particles.
Keywords: Aircraft engine; Charged soot particles; Chemiions; Combustion;

Use of micrometeorological methods for measurement of surface NO and NO2 fluxes is problematic due to chemical reactions of the O3–NO–NO2 triad in the surface air layer resulting in violation of the assumption of flux constancy with height. Application of a flux-gradient approach close to a short-grass surface (<1 m) minimises this problem, and allows for continuous monitoring of flux and deposition velocity under varying environmental conditions. In this study, we evaluate data collected using a flux-gradient approach for NO and NO2 flux determination over turfgrass from May to August 2001 in Ontario, Canada. Gas concentrations were determined using two chemiluminescence analysers sampling at 2 Hz from two heights of 0.3 and 0.5 m. Deposition velocities for NO2 were determined by dividing the calculated flux by the measured atmospheric concentration. The unfertilised grass field studied was a very small source of NO (mean hourly flux 0.45±0.04 ng N m−2  s−1) and a net sink for NO2 (–1.76±0.06 ng N m−2  s−1). Largest NO2 downward flux occurred during a period with ideal grass growing conditions, and lowest fluxes were measured during the driest period of the summer. This same seasonal pattern was also noted for the NO2 deposition velocity (mean 2.7±0.17 mm s−1), which was mostly comprised by a surface resistance (daytime mean 436 s m−1). The flux-gradient approach proposed yielded NO2 flux and deposition velocities within the range of values presented in the literature, and responded to environmental variables in an expected manner.
Keywords: Nitric oxide flux; Nitrogen dioxide flux; NO2 deposition velocity; Flux-gradient method;

Simulating regional-scale ozone climatology over the eastern United States: model evaluation results by C Hogrefe; J Biswas; B Lynn; K Civerolo; J.-Y Ku; J Rosenthal; C Rosenzweig; R Goldberg; P.L Kinney (2627-2638).
To study the potential impacts of climate change on air quality and public health over the eastern United States, a coupled global/regional-scale modeling system consisting of the NASA-Goddard Institute for Space Studies Atmosphere–Ocean model, the MM5 mesoscale meteorological model and the Community Multiscale Air Quality (CMAQ) model for air quality has been developed. Evaluation results of the modeling system used to simulate climate and ozone air quality over the eastern United States during the five summers of 1993–1997 are presented in this paper. The results indicate that MM5 and CMAQ capture interannual and synoptic-scale variability present in surface temperature and ozone observations in the current climate, while the magnitude of fluctuations on shorter time scales is underestimated. A comparison of observed and predicted spatial patterns of daily maximum ozone concentrations shows best performance in predicting patterns for average and above-average ozone concentrations. The frequency distributions of the duration of extreme heat and ozone events show similar features for both model predictions and observations. Finally, application of a synoptic map-typing procedure reveals that the MM5/CMAQ system succeeded in simulating the average ozone concentrations associated with several frequent pressure patterns, indicating that the effects of synoptic-scale meteorology on ozone concentrations are captured by the modeling system. It is concluded that the GCM/MM5/CMAQ system is a suitable tool for the simulation of summertime surface temperature and ozone air quality conditions over the eastern United States in the present climate.
Keywords: Regional-scale air quality modeling; Model evaluation; Regional climate modeling; Interannual variability; Map-typing analysis;

We investigate the time scales of various dynamic processes during the evolution of the particle size distribution from its emission from a vehicle exhaust pipe through its dilution at kerbside and urban level. In addition, the situation in a road tunnel or near a highway is discussed. The derived formula framework allows the estimation of approximated time scales based on a given particle size distribution for the processes dilution with background air, coagulation, deposition and condensation. The variation of the time scales for these processes with “lifetime” of the exhaust particles and the dependence of the time scales on the particle size is shown. We identify the spatial or temporal scales under which the discussed processes are important and have to be included in operational particle pollution models.
Keywords: Aerosol modelling; Nanoparticles; Particle size distribution; Traffic source;

Wintertime sources and distribution of airborne lead in Korea by Vinit K. Mishra; Ki-Hyun Kim; Chang-Hee Kang; Kum Chan Choi (2653-2664).
The geographical distribution patterns of atmospheric lead (Pb) were investigated using the data sets acquired from three locations in Korea during the wintertime period (December 2002). As these sites were selected to represent different levels of anthropogenic activities in Korea, Pb concentrations of each site were found in the highly variable range of: 27.6±26.3 (8.42–96.7: Jeju), 128±60.9 (35–238: Busan), and 200±97.2  ng m −3 (38.7–401: Seoul). By comparing our Pb levels with those measured previously from other locations in Korea and around the world, we attempted to diagnose the general status of Pb pollution on the Korean peninsula. It was found that our Pb values in Seoul are quite compatible with those studied previously in many urban areas in Korea, while notably higher than those of urbanized regions in Europe and USA. The major processes responsible for controlling Pb levels in each area were also investigated using a series of statistical analysis. The results of the correlation analysis between Pb and other elements indicated the importance of either crustal (Jeju and Seoul) or vehicular source processes (in Seoul). The results of principle component analysis further suggested the possibly significant role of incineration (in the case of Seoul). The overall results of our study suggest differences in Pb concentration levels at different sites of the Korean peninsula in a tight relation with local or regional anthropogenic activities.
Keywords: PM10; Trace metals; Enrichment factor; Leaded gasoline; Meteorological parameters; Anthropogenic;

A parameterization of wave stress in the planetary boundary layer for use in mesoscale models by Carmen J. Nappo; Hye-Yeong Chun; Hyuk-Je Lee (2665-2675).
A parameterization of gravity wave stress generated by subgrid-scale topography is described and tested in a one-dimensional version of the Advanced Regional Prediction System (ARPS) model. It is argued that in the planetary boundary layer (PBL) where wave reflections occur, the so-called WKB method for evaluating wave stress may not be applicable. Gravity waves launched by a subgrid-scale Gaussian ridge are calculated on line using a linear wave model. The total flow is constrained to be convectively stable by using a terrain-height adjustment to decrease wave amplitudes and thereby prevent wave overturning. In this method when the waves grow large enough to overturn, the wave amplitudes are decreased by decreasing the maximum height of the terrain obstacle, H. At each time step, the ARPS model flow is modified by the divergence of the wave stress. The effects of wave-stress divergence on turbulence parameterization is examined using three turbulence closure schemes, K-theory with constant eddy diffusivity, the Smagorinsky closure, and the turbulence-kinetic energy closure. Also, the effects of vertical grid spacing are tested using spacings of 10, 20, 50 and 100  m . The model is initialized with a hyperbolic-tangent wind profile and constant Brunt–Väisälä frequency. It is shown that wave-stress divergence can lead to elevated layers of turbulence and diffusion where they would not occur in the absence of the wave-stress parameterization. It is also shown that if the vertical grid spacing is too great, then the effects of wave breaking are not fully realized.
Keywords: Gravity waves; Wave stress; Wave-stress parameterization; Stable PBL; Subgrid-scale terrain effects;

Quantifying the dry deposition of ammonia in ammonia-rich and ammonia-poor environments using a surrogate surface approach by Nadjoua Moumen; Seung-Muk Yi; Heather A. Raymond; YoungJi Han; Thomas M. Holsen (2677-2686).
In this study, dry deposition samples were collected with both an aerodynamically smooth water surface (WSS) and a knife-edge surrogate surface (KSS) in Potsdam, NY, Solomons, MD, and at a farm in upstate NY intermittently between March 2000 and August 2001. The WSS and the citric acid-coated paper filter on the KSS were used to measure total ammonium deposition (NH3 and p-NH4 +), while a greased Mylar disk on the KSS was used to measure particulate NH4 + (p-NH4 +) deposition. An annular denuder system (ADS) was used to measure NH3 gas phase and particulate NH4 + concentrations. In side-by-side experiments with two WSSs, no statistical difference was found between simultaneously measured total ammonium deposition at any of the sites. Average total NH3 fluxes measured with the WSS in Potsdam, Solomons, and at the farm were 3.1, 2.1, and 310 mg NH4 +  m−2 day−1, respectively. Similarly gaseous NH3 fluxes measured with the citric acid-coated paper on the KSS were 3.6, 2.1, and 500 mg NH4 +m−2 day−1, respectively. The p-NH4 + fluxes to the KSS-greased strip were negligible at all three locations. Fluxes measured by the citric acid paper filters were slightly greater than those measured with the WSS suggesting that the acidic dry surfaces had a higher affinity for ammonia than the water in the WSS.The average gas-phase ammonia concentrations in Potsdam, Solomons, MD and at the farm were 0.60×10−3, 2.1×10−3, and 0.25 mg NH4 +  m−3, respectively. The average ammonium concentrations were 0.50×10−3, 1.2×10−3, and 14×10−3  mg NH4 +  m−3, respectively.
Keywords: WSS; KSS; ADS; Citric acid-coated paper; Ammonia; Ammonium; Deposition flux; Concentration;

Quantifying the dry deposition of reactive nitrogen and sulfur containing species in remote areas using a surrogate surface analysis approach by Heather A Raymond; Seung-Muk Yi; Nadjoua Moumen; YoungJi Han; Thomas M Holsen (2687-2697).
In this study, the fluxes of gaseous (HNO3 and SO2) and particulate (NO3 and SO4 2−) species were measured using two recently developed surrogate surfaces, the water surface sampler (WSS) and the knife-edge surrogate surface (KSS). Sampling was conducted in Potsdam, NY, Solomons, MD, and at a farm in upstate NY intermittently between March 2000 and August 2001. The KSS contained both Nylasorb filters and greased Mylar disks. No statistical difference was found between measured total nitrate and sulfate deposition in side-by-side experiments with two WSSs. Average total nitrate fluxes measured with the WSS and Nylasorb filter on the KSS in Potsdam, Solomons, and at the farm were 3.6, 6.2, 2.9 mg m−2  day−1 and 2.5, 6.7, 2.3 mg m−2  day−1, respectively. Solomons had the highest total nitrate flux probably due to its proximity to large urban industrialized areas including Washington, DC, and Baltimore, MD. Average gaseous HNO3 fluxes measured with the WSS and Nylasorb filter on the KSS in Potsdam, Solomons, and at the farm (calculated by subtracting the fraction of particles deposited to the Mylar disk from the total deposition to the WSS and Nylasorb filter) were 2.4, 4.9, 1.9 mg m−2  day−1 and 1.3, 5.6, 1.3 mg m−2  day−1, respectively. In Potsdam, total nitrate deposition to the WSS was greater than to the Nylasorb filter whereas in Solomons the opposite was true. This increased flux at Solomons may be because nitrate salts were formed on the surface of the Nylasorb filter by reaction of HNO3 with previously deposited sea salt aerosols. Average p-NO3 fluxes to the greased Mylar for the three sampling sites were 1.3, 1.3, and 1.0 mg m−2  day−1. The measured SO2 deposition velocity to the WSS in Potsdam and Solomons was 2.1 and 1.5 cm s−1, respectively, and agreed well with other studies. The deposition velocity for HNO3 to the WSS measured in Potsdam (8.1 cm s−1) was greater than to the Nylasorb filter (4.6 cm s−1) whereas in Solomons the opposite was true (4.6 cm s−1 vs. 7.1 cm s−1).
Keywords: WSS; KSS; Nylasorb filter; Nitrate; Sulfate; Flux; Deposition velocity;

The effect of fuel sulfur on NH3 and other emissions from 2000–2001 model year vehicles by Thomas D. Durbin; John T. Pisano; T. Younglove; Claudia G. Sauer; Sam H. Rhee; Tao Huai; J.Wayne Miller; Gervase I. MacKay; Albert M. Hochhauser; Michael C. Ingham; Robert A. Gorse; Loren K. Beard; Dominic DiCicco; Neville Thompson; Richard J. Stradling; James A. Rutherford; James P. Uihlein (2699-2708).
Atmospheric ammonia (NH3) is an important precursor to secondary particulate matter formation and information is currently scarce on NH3 emissions from advanced low-emission vehicles using low-sulfur fuels. With the continuing reduction in the level of sulfur in gasoline, it is important to understand how this change could impact NH3 emissions, particularly for advanced vehicle technologies. For this study, a total of 12 California-certified low-emission vehicles were tested with a gasoline containing 5, 30, and 150 ppmw sulfur and with both as-received and bench-aged catalysts. Vehicles were tested on each fuel/catalyst configuration over the federal test procedure (FTP) and US06 test cycles. Both regulated and NH3 emissions were measured in real-time. NH3 emission rates were generally lower than those of other regulated emissions over the FTP and in the range 14–21 mg mi−1 for the fleet. NH3 emission rates were approximately five times higher over the more aggressive US06 cycle compared to the FTP. NH3 emissions were primarily observed during transients, with higher emissions for more aggressive accelerations. Overall, the NH3 emission factors for the newer technology vehicles tested were lower that than those found in previous studies of older vehicle technologies. Sulfur did not affect NH3 emissions over the FTP, but higher NH3 emissions were found for increasing fuel sulfur levels over the US06. Sulfur effects were also observed for nitrogen oxides over the FTP and for all regulated emissions over the US06.
Keywords: Automotive exhaust; Automotive emissions; Vehicle emissions; Chassis dynamometer; Emissions rates; Ammonia; Fuel sulfur;

The gas-phase reaction of ozone with α-cedrene (I) in the absence and presence of an OH radical scavenger (cyclohexane) has been studied in the outdoor 25 m3 Teflon film chambers at the UNC smog chamber facility. Twenty-two compounds were identified for the first time in this study. Important products identified and semi-quantified were α-cedronaldehyde (VI), α-cedronic acid (VIII), α-cedrinic acid (X), α-norcedronaldehyde (VII), 8-hydroxy-9-α-cedranone (III), and α-norcedronic acid (IX). Acetone, formaldehyde, and acetaldehyde also were identified in this study. On average, measured gas and particle phase products accounted for ∼56% of the reacted α-cedrene.The total yield of products bearing hydroxy groups (e.g. 8-hydroxy-9-α-cedranone (III)) and aldehydes (e.g. α-cedronaldehyde (VI), α-norcedronaldehyde (VII)) was found to be slightly dependent on the presence of an OH radical scavenger. Generally, the aldehyde yield was found to be higher when cyclohexane was present, and the hydroxy compounds yield was higher when cyclohexane was absent. All products identified in this study were found in the particle phase. α-Cedrinic acid (X), α-cedronic acid (VIII), and α-cedronaldehyde (VI) were observed in the early stage in the aerosol phase and may play an important role in the early formation of SOA. Detailed reaction schemes leading to most reaction products observed here are presented and discussed.
Keywords: Sesquiterpene; α-Cedrene; Secondary organic aerosol; Mechanism; Organic aerosol;

Nighttime production of elemental gaseous mercury in interstitial air of snow at Station Nord, Greenland by Christophe P. Ferrari; Aurélien Dommergue; Claude F. Boutron; Henrik Skov; Michael Goodsite; Bjarne Jensen (2727-2735).
The study of the global cycle of mercury in polar environments is of great importance as mercury appears to be highly accumulated in the snowpack before melting. Exchange rates between snow and atmosphere is of particular interest to better understand the dispersion of this pollutant in polar ecosystems. Continuous monitoring of gaseous mercury (Hg°) in the air of snow and in ambient air at Station Nord, Greenland were performed from 5 to 13 March, 2002. Hg° concentrations in the snow during the day were lower than atmospheric ones as a result of rapid oxidation of Hg° by bromine species (e.g. Br· and BrO·) to form Hg(II) species, subsequently deposited onto snow grains. During 4 successive nights, we recorded a Hg° production in the air of the snow at the depth of 20 cm below the snow surface. Moreover during the 5 following days, we also recorded a Hg° production in the air of the snow at the depth of 40 cm. During these periods, concentrations of Hg° in the snow started to increase after the sunset, and reached a maximum around 2:00 a.m., thereafter decreasing to a minimum just before sunrise. Resulting emission fluxes were however weak in the range of 0.06–0.40 ng m−2  h−1. We postulated that this phenomenon could be the result of the reduction of Hg(II) by HO2 · produced in the snow at night.
Keywords: Mercury; Polar regions; Snow; Oxidation; Reduction; Emission;

A generalized approach for estimating season-specific diurnal profiles of anthropogenic heating for cities is presented. Each profile consists of heat released from three components: building sector, transportation sector, and metabolism. In turn, the building sector is divided into heat released from electricity consumption and heat released from heating fuels such as natural gas and fuel oil. Each component is developed separately based on a population density formulation. The profiles are based on commonly available data resources that are mapped onto the diurnal cycle using seasonal profile functions. Representative winter and summer weekday profiles are developed and presented for six large US cities. The diurnal profiles have morning and evening peaks, with summertime maxima up to 60 W m−2. Anthropogenic heating in winter is generally larger, with maxima up to 75 W m−2. While these analyses were carried out at the city-scale the paper discusses how the same data sources could be applied at scales down to the individual census tract (or traffic analysis zone), resulting in high spatial resolution profiles and larger maxima corresponding to higher population densities in the urban core. Based on our analysis of San Francisco we find that the urban core region may have a daytime population density that is 5–10 times that of the city-scale value. Hence, the corresponding anthropogenic heating values in the urban core will be 5–10 times the magnitudes of the city-scale values presented in this paper.
Keywords: Heat islands; Energy balance; Waste heat; Energy analysis;

Effects of meteorology and tropospheric aerosols on UV-B radiation: a 4-year study by Gustavo G. Palancar; Beatriz M. Toselli (2749-2757).
Broadband Ultraviolet-B (UV-B, 280–315 nm) and total (300–3000 nm) radiation measurements and calculations have been carried out in Córdoba City, Argentina (31° 24′S, 64° 11′ W, 470 m above sea level) between November 1998 and December 2002. UV-B measurements were carried out with a YES UVB–1 radiometer while calculations were performed using the tropospheric ultraviolet visible model (TUV 4.1). Total measurements were taken using a YES TSP-700 pyranometer and the calculations using a parameterization. The site selected for the measurements represents semi-urban conditions and is along the most frequent wind direction (NE–SW) with respect to the city center. For clear sky days, calculations showed a good agreement with measurements. However, this study showed a substantial reduction (up to 50%) in the UV-B radiation on days with high levels of aerosols. The default aerosol properties included in the model did not account for the observed reductions. A four-day-aerosol-loading episode was analyzed. Relationships between measurements and calculations were used to derive specific information about the aerosols present on those days. In this work we analyzed the hourly, seasonal, and interannual variations of the measured UV-B solar noon values in order to assess the aerosol influence on each of them. Both UV-B and total irradiance showed a systematic reduction every winter–spring period. The origin of the aerosols and the causes for this behavior are also discussed.
Keywords: Córdoba city (Argentina); Seasonal and interannual variation; Aerosol loading; Radiative transfer modeling; Aerosol properties;

Comment on “Instantaneous secondary organic aerosol yields and their comparison with overall aerosol yields for aromatic and biogenic hydrocarbons” by Weimin Jiang by Eladio M. Knipping; Robert J. Griffin; Frank M. Bowman; Betty Pun; Christian Seigneur; Donald Dabdub; John H. Seinfeld (2759-2761).