Atmospheric Environment (v.38, #18)

Daytime dry deposition of nitric acid to a conifer forest with a LAI of 5.3 was measured using the relaxed eddy accumulation technique. The observations indicate a mean friction velocity of 0.45 m s−1and a mean dry deposition velocity (v d) of 7.5 cm s−1, with approximately equal aerodynamic (r a) and non-aerodynamic resistances (r b+r c). Mean r a is 10 s m−1, while mean r b+r c (derived from the difference between the observed v d and a value derived based solely on r a) is 13.2 s m−1. Assuming the surface resistance (r c) is zero, the viscous sub-layer resistance (r b) from a number of models capture the mean observationally derived value to within ±40%, but the models underestimate the variability inherent in the measurements. This discrepancy between the modeled and observed sample-to-sample variability of r b does not appear to be accounted for by leaf wetness, stomatal opening or an additional dependence on friction velocity.
Keywords: HNO3; Dry deposition; Flux; Relaxed eddy accumulation; Forest;

First detection of nitrogen from NO x in tree rings: a 15N/14N study near a motorway by M. Saurer; P. Cherubini; M. Ammann; B. De Cinti; R. Siegwolf (2779-2787).
Nitrogen isotope analysis (δ 15N) of tree rings is potentially useful for evaluating the temporal development of the nitrogen (N) deposition to forests and for studying the long-term effects of N accumulation in ecosystems. To test this hypothesis, we investigated three sites across a pollution gradient in differing distances (20,150,1000 m) from a motorway in Switzerland, which was built in 1965. We sampled four Picea abies trees per site, whereby we extracted the tree ring cores with hot water and solvents before the isotope analysis to remove mobile N storage compounds, and determined the isotope variations in the stem wood for the period 1928–2000. While tree ring growth was not affected by the construction of the motorway, the δ 15N values were increasing by up to 7.9‰ after 1965 at the most polluted site, indicating the uptake of NO x from car exhausts, although the signal was highly variable. Isotopically heavy NO x emissions were observed in an earlier study at the same location resulting in a δ 15N-gradient of recent needles from +1.3‰ to −4.4‰ with increasing distance from the motorway. This gradient was also reflected in the tree rings, but dampened by a factor of about 2 compared to the needles. For the trees near the motorway, the total nitrogen concentration in the tree rings varied in parallel with the δ 15N values (r 2=0.52). This enabled us to apply a mass balance equation for reconstructing the isotope signal of N originating from the car exhausts for the period 1965–2000, with the δ 15N of NO2 in the range +1.3‰–+6.4‰. The more distant sites were much less affected by the traffic and their isotope ratio reflected the influence of varying proportions of isotopically heavy (NO2) and light (NH x ) deposition. We conclude that the analysis of tree ring 15N variations is a promising tool for the detection of the role played by nitrogen deposition to the forests.
Keywords: Picea abies; Air pollution; Dendroecology; Nitrogen deposition; Stable isotopes; Nitrogen dioxide;

Influence of ammonia on sulfate formation under haze conditions by Janja Turšič; Axel Berner; Boštjan Podkrajšek; Irena Grgić (2789-2795).
In the presence of hygroscopic material, stable aqueous droplets form at relative humidity below 100%. In this work, we examined the role and importance of NH3 in SO2 oxidation under haze conditions. Synthetic deposits composed mainly of NaCl and NaNO3 were exposed to SO2/NH3/air gas mixture under controlled conditions, typical for heavily polluted atmosphere. In contrast to catalytic reaction by Mn(II) in the absence of NH3, which is self-limited, the production of sulfate in the presence of NH3 at RH 80% increases considerably and linearly with time. The catalytic effect of Mn(II) was observed only for NaNO3 deposits. Non-catalytic SO2 oxidation in the presence of NH3 is negligible below the deliquescent relative humidity, i.e. RH<75%. Above this value a considerable increase of sulfate formation in a narrow range of relative humidity (75–80%) was noticed. We found out that the reaction rate is not proportional to the volume of condensed water; we suggest that available surface play a role in limiting the reaction. The rate of conversion for both pure salts at RH⩾80% was found to be 1.1% h−1 ([SO2]=2 ppm, [NH3]=1 ppm, T=10°C), which is more than an order of magnitude higher in comparison with our previous results on MnCl2/NaCl and MnCl2/NaNO3 at RH=80% in the absence of NH3.
Keywords: Deliquescent aerosol; NaCl; NaNO3; NH3; SO2 oxidation;

A new concept to model a finite forest area in a boundary layer wind tunnel is proposed. A specific arrangement of rings made from metallic mesh is used in order to reproduce the effect of trees on the atmospheric boundary layer. The comparison between field data and wind tunnel data of wind profiles through the canopy is satisfying and that justifies the use of this model concept to manufacture the model of the finite inhomogeneous forest area surrounding the Research Centre of Jülich (Germany), as guaranteed within the research project ECHO.
Keywords: Forest canopy; Porosity; Wind turtulence; Deciduous forest;

Aerosol modeling with CHIMERE—preliminary evaluation at the continental scale by B. Bessagnet; A. Hodzic; R. Vautard; M. Beekmann; S. Cheinet; C. Honoré; C. Liousse; L. Rouil (2803-2817).
Aerosol modeling is a challenging scientific problem aimed at improving our knowledge in the many complex processes involved in multiphase chemistry and transport. Correct simulations of aerosols are also required in order to elaborate particle emission reduction strategies. The CHIMERE chemistry transport model (Atmos. Environ. 35 (2001) 6277) has been improved to account for particle transport, formation, deposition at the European scale. The aerosol model accounts both for inorganic (NO 3 , SO 4 2−,  NH 4 +) and organic species of primary or secondary origin. Secondary organic aerosols from biogenic and anthropogenic gas precursors are partitioned into gas and particulate phases through a temperature dependent partition coefficient. The modeling approach is presented in this paper with preliminary simulation results over Europe. Comparisons with available data at background stations give acceptable results on PM10, with correlation coefficients usually exceeding 0.5 and normalized errors in the 30–80% range in many regions. However, results on sulfate, nitrate and ammonium species display less correct error statistics. Comparisons on sulfate concentrations give normalized errors in the range 30–80% in summer and less correct in winter. Temporal correlation coefficients usually range from 0.30 to 0.70. Nitrate concentrations are better simulated during winter than during summer. Difficulties in simulating heterogeneous and aqueous phase processes could explain model deficiencies. Moreover, temperature dependence of gas/particle partitioning processes for nitrate, ammonium and secondary organic species could mainly explain the seasonal variability of biases. Model deficiencies are observed in Southern countries, certainly due to natural dust emissions and resuspended particles. Finally, sea salts seems to have a quite significant influence on error statistics in coastal areas.
Keywords: Aerosol model; Sectional approach; Heterogeneous and aqueous chemistry; Validation; Error statistics;

Seasonal differences in snow chemistry from the vicinity of Mt. Everest, central Himalayas by Shichang Kang; Paul A. Mayewski; Dahe Qin; Sharon A. Sneed; Jiawen Ren; Dongqi Zhang (2819-2829).
During August and September 1998, fresh snow samples were collected in the East Rongbuk (ER) Glacier on the northern slope of Mt. Everest over an elevation range 5800–6500 m. Three snowpits were sampled in the ER glacier at an elevation of 6400, 6500, and 6500 m in August 1998, May 2000 and October 2002, respectively. Snow chemical data from fresh snow and snowpit samples from the ER Glacier are shown to be consistent with earlier results reported by other researchers from the vicinity of Mt. Everest. Among major ions, Ca2+ has the most striking seasonal differences: non-monsoon snow Ca2+ concentration is one order of magnitude higher than monsoon value. A large seasonal difference characterizes fresh snow SO4 2− but does not seem to persist in snowpit samples probably as a consequence of post-depositional ion elution. Non-monsoon snow Na+, K+ and Cl are close to two times higher than monsoon snow for both fresh snow and snowpit samples. Magnesium has distinct seasonal variations in snowpit samples and a four-fold higher concentration in non-monsoon snow than that in monsoon snow. Seasonal differences in major chemical composition in snow over the Mt. Everest region provide a definitive indicator for precisely dating ice cores and as a consequence are essential in reconstructing the history of climate change and atmospheric chemistry in this region.
Keywords: Fresh snow; Snowpit; Major ions; Seasonal variations; Mt. Everest region;

To obtain a good estimation of particle size distribution and concentration encountered by car passengers during their journeys, an electrical low-pressure impactor was installed in a car. Two measurement campaigns were carried out. The first one covered various traffic conditions in urban and rural areas near Rouen (France). The second campaign covered several journeys up and down the same tunnel. Measurements were recorded in the range 30 nm–10 μm at 1 Hz frequency.The size distributions show two main shapes depending mainly on the concentration. For high concentrations the distributions correspond to diesel emissions with the mode comprised between 60 and 100 nm. For low concentrations the maximum of the distribution is located in the finest particle class of the device, between 30 and 60 nm.The concentrations observed are generally less than 50 000 part cm−3 but may reach more than 106 part cm−3 during specific road events. Likewise high-concentration values may be regularly observed at the end of the tunnel along which the concentration increases linearly, after a surprising decrease. One main conclusion is that on-road measurements are necessary for a correct evaluation of the conditions encountered by car passengers.
Keywords: On-road measurements; Vehicle emission; Particle size distribution; Tunnel; Emission factor;

Cleaning products and air fresheners: exposure to primary and secondary air pollutants by William W. Nazaroff; Charles J. Weschler (2841-2865).
Building occupants, including cleaning personnel, are exposed to a wide variety of airborne chemicals when cleaning agents and air fresheners are used in buildings. Certain of these chemicals are listed by the state of California as toxic air contaminants (TACs) and a subset of these are regulated by the US federal government as hazardous air pollutants (HAPs). California's Proposition 65 list of species recognized as carcinogens or reproductive toxicants also includes constituents of certain cleaning products and air fresheners. In addition, many cleaning agents and air fresheners contain chemicals that can react with other air contaminants to yield potentially harmful secondary products. For example, terpenes can react rapidly with ozone in indoor air generating many secondary pollutants, including TACs such as formaldehyde. Furthermore, ozone–terpene reactions produce the hydroxyl radical, which reacts rapidly with organics, leading to the formation of other potentially toxic air pollutants. Indoor reactive chemistry involving the nitrate radical and cleaning-product constituents is also of concern, since it produces organic nitrates as well as some of the same oxidation products generated by ozone and hydroxyl radicals.Few studies have directly addressed the indoor concentrations of TACs that might result from primary emissions or secondary pollutant formation following the use of cleaning agents and air fresheners. In this paper, we combine direct empirical evidence with the basic principles of indoor pollutant behavior and with information from relevant studies, to analyze and critically assess air pollutant exposures resulting from the use of cleaning products and air fresheners. Attention is focused on compounds that are listed as HAPs, TACs or Proposition 65 carcinogens/reproductive toxicants and compounds that can readily react to generate secondary pollutants. The toxicity of many of these secondary pollutants has yet to be evaluated. The inhalation intake of airborne organic compounds from cleaning product use is estimated to be of the order of 10 mg d−1  person−1 in California. More than two dozen research articles present evidence of adverse health effects from inhalation exposure associated with cleaning or cleaning products. Exposure to primary and secondary pollutants depends on the complex interplay of many sets of factors and processes, including cleaning product composition, usage, building occupancy, emission dynamics, transport and mixing, building ventilation, sorptive interactions with building surfaces, and reactive chemistry. Current understanding is sufficient to describe the influence of these variables qualitatively in most cases and quantitatively in a few.
Keywords: Indoor air quality; Hazardous air pollutants; Terpenes; Ozone; Hydroxyl radical; Nitrate radical;

Measurements of mass concentrations of 35 trace elements (TEs) and of total fine particulate matter (PM2.5) were conducted at 20 residences and six high school rooms in Mira Loma, California, from September 2001 to January 2002. Sulfur (S) and silicon (Si) were the most abundant TEs measured (excluding a residence with heavy smokers). On average, total TE concentrations were lower indoors relative to outdoors; the proportion of TEs in total PM2.5 was also lower indoors relative to outdoors. Among indoor sites, TE concentrations were found to be lower inside the schoolrooms relative to inside the residences. Environmental tobacco smoke (ETS) was found to contribute significantly to elevated levels of total TE inside residences; however, concentrations of carcinogenic TEs were not significantly different between residences with and without smokers. Potassium (K) and chlorine (Cl) were the most abundant species in a residence with frequent indoor smokers. Combustion-related elements were more enriched inside the residences relative to crustal elements.
Keywords: Trace elements; PM2.5; Environmental tobacco smoke; Indoor air quality; Outdoor-to-indoor ratio;

Positive and negative ions have been measured by the ion mobility analyzer in the exhaust of a jet aircraft engine behind the combustor exit at the ground-level. The operational conditions of the combustor were varied covering two fuel flows (FFs) and three fuel sulfur contents (FSCs). About 50% of the observed ions have mass number m (amu, atomic mass units) larger than 100 and the most massive ions have m up to about 1500–3000 depending on the FF and FSC. Considering such large m, many of these ions must be relatively large molecular ions, as for example polycyclic aromatic hydrocarbons (PAH). Labile cluster ions are unlikely due to the high temperature (about 1000 K). Measured ion concentrations referred to standard conditions are n p=4×107–1.7×108  cm−3 (positive ions) and n n=6×107–2.1×108  cm−3 (negative ions). The total ion emission index is E i=(E p+E n)=1.2×1016–2.0×1016 ions kg−1 fuel burnt. Emission index E i increases markedly with FF. For low FF, E i increases markedly with higher FSC. Our E i is the same order as was observed for large volatile particles in wakes of a jet aircraft in-flight formed by the ion-induced nucleation followed by the ion-assisted growth of freshly nucleated aerosols.
Keywords: Aircraft engine; Chemiions emission;

Validation of a semi-continuous instrument for elemental carbon and organic carbon using a thermal-optical method by Min-Suk Bae; James J Schauer; Jeffrey T DeMinter; Jay R Turner; David Smith; Robert A Cary (2885-2893).
A commercial semi-continuous elemental and organic carbon (ECOC) analyzer has been developed by Sunset Laboratory Inc. for the analysis of atmospheric particulate matter. The field deployable instrument is an alternative to off-line laboratory-based analysis of samples collected in the field. Although the Sunset Laboratory analyzer provides time-resolved particulate ECOC measurements using a methodology comparable to the laboratory-based NIOSH 5040 ECOC measurement, the performance of the semi-continuous ECOC analyzer has not been fully validated in the past.As part of the St. Louis-Midwest Supersite, two semi-continuous Sunset Labs ECOC field analyzers were operated to measure fine particle organic and elemental carbon for the entire year of 2002. The dual instrument operation provided a means to have a true continuous ECOC measurement, which would not be possible with only one instrument since the instrument cannot continue sample collection during the sample analysis period. In parallel with the semi-continuous ECOC analyzers, daily 24-h integrated PM2.5 samples were collected for ECOC analysis in the laboratory. Both the 24-h integrated filter samplers and the semi-continuous ECOC instruments were operated downstream of an organics denuder to remove positive adsorption artifacts associated with semi-volatile organic compounds. Comparison of 24-h averaged semi-continuous fine particle total carbon (TC), organic carbon (OC) and elemental carbon (EC) showed good agreement with the daily integrated off-line measurements. The linear regression of the averaged semi-continuous TC on the integrated off-line TC had a coefficient of regression (R 2) of 0.89 and a regression slope of 0.97±0.02 when at least 20 h of the day were measured. The OC measurements had a R 2 equal to 0.90 and a regression slope of 0.93±0.02. The EC measurements had a poorer R 2 due to the very low levels of EC at the sampling site but a regression slope of 0.95±0.05. The accuracy of using a single semi-continuous ECOC instrument, configured to measure ECOC every other hour to represent the daily 24-h average, was also evaluated.
Keywords: Carbonaceous aerosol; Organic carbon; Elemental carbon; NIOSH 5040; Particulate matter; Thermal-optical analysis;

Methods for imputation of missing values in air quality data sets by Heikki Junninen; Harri Niska; Kari Tuppurainen; Juhani Ruuskanen; Mikko Kolehmainen (2895-2907).
Methods for data imputation applicable to air quality data sets were evaluated in the context of univariate (linear, spline and nearest neighbour interpolation), multivariate (regression-based imputation (REGEM), nearest neighbour (NN), self-organizing map (SOM), multi-layer perceptron (MLP)), and hybrid methods of the previous by using simulated missing data patterns. Additionally, a multiple imputation procedure was considered in order to make comparison between single and multiple imputations schemes. Four statistical criteria were adopted: the index of agreement, the squared correlation coefficient (R 2), the root mean square error and the mean absolute error with bootstrapped standard errors. The results showed that the performance of interpolation in respect to the length of gaps could be estimated separately for each variable of air quality by calculating a gradient and an exponent α (Hurst exponent). This can be further utilised in hybrid approach in which the imputation has been performed either by interpolation or multivariate method depending on the length of gaps and variable under study. Among the multivariate methods, SOM and MLP performed slightly better than REGEM and NN methods. The advantage of SOM over the others was that it was less dependent on the actual location of the missing values. If priority is given to computational speed, however, NN can be recommended. The results in general showed that the slight improvement in the performances of multivariate methods can be achieved by using the hybridisation and more substantial one by using the multiple imputations where a final estimate is composed of the outputs of several multivariate fill-in methods.
Keywords: Missing data; Air quality; Multivariate; Imputing; Neural networks;

Evaluation of evaporative emissions from gasoline powered motor vehicles under South African conditions by H Van der Westhuisen; A.B Taylor; A.J Bell; M Mbarawa (2909-2916).
This paper reports the quantification of the evaporative emissions from gasoline-powered motor vehicles in South Africa under various conditions including high temperatures. Three vehicles with and without evaporative emission control systems have been tested to quantify gasoline evaporative emissions, including both diurnal and running loss emissions. Measurements were made in a Sealed Housing for Evaporative Determination over a range of ambient temperatures. The standard diurnal test shows that evaporative emissions increase progressively with increasing fuel temperature. South African gasoline powered motor vehicles exhibited evaporative emissions 10 times higher than that allowed by the United States Environmental Protection Agency. The extended-time diurnal test shows that the fuel emitted from gasoline-powered motor vehicles in South Africa is about 97 million litres of gasoline every year.
Keywords: Emissions measurement; Diurnal emissions; Evaporative emissions; Gasoline powered motor vehicles; Running loss;

The widely used mesoscale meteorological model, MM5 and a recently released US EPA air quality model, Models-3/CMAQ were employed to investigate the effect of mass inconsistency in air quality modeling. During a 24-h simulation using a hydrostatic MM5 output, the mass inconsistent error, as measured by trace species concentration, grew by 242%. The nonhydrostatic simulation produced approximately 6 times larger mass inconsistent error than the hydrostatic model. This is because the nonhydrostatic model is not strictly mass consistent by neglecting terms representing pressure increase due to heating in a pressure perturbation equation, another form of the continuity equation. In both the hydrostatic and nonhydrostatic models, mass inconsistency was produced due to inconsistent numerical schemes and integral time steps between the meteorological model and the air quality model. Temporal interpolation of meteorological data in air quality modeling partly caused the mass inconsistency as well. The mass inconsistent error was removed successfully by conserving the mixing ratio of the trace species in both the hydrostatic and the nonhydrostatic cases.
Keywords: Mass inconsistent error; Mass consistency; Air quality modeling; MM5; CMAQ;

Biogenic emission of dimethylsulfide from a highly eutrophicated coastal region, Masan Bay, South Korea by Gangwoong Lee; Sung-Hyun Kahng; Jae-Ryoung Oh; Kyung-Ryul Kim; Meehye Lee (2927-2937).
Atmospheric dimethylsulfide (DMS), water-soluble ionic species in aerosol such as non-seasalt sulfate and methanesulfonic acid (MSA), and seawater DMS were measured in highly eutrophicated Masan Bay, Korea in July–August 1997. Mean (median) concentrations of atmospheric DMS, seawater DMS, non-seasalt sulfate, and MSA during the experiment were 188 pptv (49 pptv), 6.3 nM (5.3 nM), 3.0 μg m−3 (2.3 μg m−3), and 0.010 μg m−3 (0.008 μg m−3), respectively. The vertical profiles of seawater, especially in inner bay, reveal that DMS concentrations were enhanced near the bottom coincidently with extremely low levels of chlorophyll-a and depleted oxygen. There were several episodes of high DMS mixing ratios up to a few ppbv, which was associated with strong wind and elevated DMS but very low chlorophyll-a and relatively low dissolved oxygen contents in the surface water. It indicates that DMS accumulated in anoxic bottom water was often transferred to the overlying water column, consequently leading to elevated DMS in the atmosphere. The mean (median) molar ratio of MSA to non-seasalt sulfate was 0.41% (0.30%), which implies the major contribution of anthropogenic SO2 to sulfur budget in the study area. The median flux of DMS from sea to air was estimated to 3.2 μm m−2  d−1.
Keywords: Dimethylsulfide; DMS; MSA; Non-seasalt sulfate; Anoxic bottom water; Bay; Biogenic emissions; Reduced sulfur compounds; Flux;