Atmospheric Environment (v.42, #28)

Mercaptans emissions in diesel and biodiesel exhaust by Sérgio Machado Corrêa; Graciela Arbilla (6721-6725).
Biodiesel and ethanol are fuels in clear growth and evidence, basically due to its relation with the greenhouse effect reduction. There are several works regarding regulated pollutants emissions, but there is a lack of reports in non-regulated emissions. In a previous paper (Corrêa and Arbilla, 2006) the emissions of aromatic hydrocarbons were reported and in 2007 another paper was published in 2008 focusing carbonyls emissions (Corrêa and Arbilla, 2008). In this work four mercaptans (methyl, ethyl, n-propyl and n-butyl mercaptans) were evaluated for a heavy-duty diesel engine, fueled with pure diesel (D) and biodiesel blends (v/v) of 2% (B2), 5% (B5), 10% (B10), and 20% (B20). The tests were carried using a six cylinder heavy-duty engine, typical of the Brazilian fleet of urban buses, during a real use across the city. The exhaust gases were diluted near 20 times and the mercaptans were sampled with glass fiber filters impregnated with mercuric acetate. The chemical analyses were performed by gas chromatography with mass spectrometry detection. The results indicated that the mercaptans emissions exhibit a reduction with the increase of biodiesel content, but this reduction is lower as the mercaptan molar mass increases. For B20 results the emission reduction was 18.4% for methyl mercaptan, 18.1% for ethyl mercaptan, 16.3% for n-propyl mercaptan, and 9.6% for n-butyl mercaptan.
Keywords: Mercaptan; Vehicular emissions; Biodiesel; Diesel;

Long-term variation in midweek/weekend cloudiness difference during summer in Korea by Yong-Sang Choi; Chang-Hoi Ho; Byung-Gon Kim; Sun-Kyong Hur (6726-6732).
A 58-year record (1950–2007) of cloudiness measured at 11 ground stations in South Korea revealed that summertime cloudiness for Wednesday through Friday versus that for Saturday through Monday exhibited a bell-shaped curve; the midweek/weekend difference increased up to 6% in the 1960s and early 1970s, and decreased in the mid and late 1970s. Since 1980, the anomaly has become as small as before 1960 (within ±2%). This bell-shaped interdecadal variation in midweek/weekend difference in cloudiness had a strong negative correlation with those in insolation and diurnal temperature range and a positive correlation with those in relative humidity and number of days with light rain (<5 mm day−1) from coincident ground measurements. Unlike the meteorological variables, the aerosol concentrations from the stations in the same cities appear to be consistently higher in the midweek than on the weekend in recent years (1990–2004). Therefore, the long-term variations in midweek/weekend differences in cloudiness and relevant meteorological variables suggest that the regional weekly weather variations are unpredictable despite the rapid industrial growth in South Korea in the past few decades. This study discusses plausible interpretations related to aerosol effects.
Keywords: Weekend effect; Cloud; Aerosol; Korea;

Although the upgraded criteria air quality standards (O3, SO x , NO x and PM10) and more stringent stack emission standards have significantly improved the overall air quality in Taiwan, the southern region of Taiwan frequently exhibits undesirable PSI (pollutant standard index), due to intensive industrial outputs and heavy traffic volume. For example, the percent of the time with PSI >100 in Taiwan has been reduced from 7% in 1994 to 4.3% in 2005, but there is still about 6.5% of the time with PSI > 100 in the southern region in 2005, of which 77% is due to O3 contribution.The main goal of the present study is to develop a control strategy for reduction of NO x and NMHCs for reducing O3 levels. The maximum 1-h O3 isopleth profiles for southern Taiwan were first established based on 40 cases of different emission quantities of NO x and NMHCs using Taiwan Air Quality Model (TAQM) with four-level nested domains. The emission rates of O3 precursors from other regions are assumed to be constant and exert the same effect on the air quality under different weather conditions in southern Taiwan. To meet the air quality goals, the allowable capacity for NO x and NMHCs in this region was determined by the following two methods: (a) linear and (b) cumulated frequency. The allowable capacity of NO x and NMHC to meet a certain goal can be easily demonstrated by plotting arbitrary reduction lines in the O3 isopleth profiles. To meet the goal in the year 2011 (or O3 151 ppb from the cumulated frequency method), for example, the emission reduction NO x /NMHC for 1:4 case amounts to 75 and 300 ton d−1, respectively, for NO x and NMHC; the corresponding emission rates are 290 and 430 ton d−1 (or 21 and 41% reduction, respectively). Clearly, there are many possibilities for achieving the desirable goal by manipulating reduction quantities in both NO x and NMHC.
Keywords: Ozone control strategy; Ozone precursor; Allowable capacity; Photochemical grid model;

Personal coarse particulate matter exposures in an adult cohort by Ron Williams; Martin Case; Karin Yeatts; Fu-Lin Chen; James Scott; Erik Svendsen; Robert Devlin (6743-6748).
Volunteers associated with the North Carolina Adult Asthma and Environment Study (NCAAES) participated in an investigation of personal daily exposures to coarse and fine particulate matter size fractions (PM10–2.5, PM2.5). Data from these personal measurements were then compared to community-based measures that might typically represent surrogate measurements of exposure often used in epidemiological assessments. To determine personal exposures to various particulate matter (PM) size fractions, a recently evaluated personal PM monitor capable of direct PM10–2.5 size fraction collection was used. Participants living in the central region of North Carolina and enrolled in the NCAAES were asked to wear the monitor attached to a supporting backpack for 24-h collection periods. These volunteers were monitored for 2 to 4 days with subsequent gravimetric analysis of their PM samples. Personal PM10–2.5 mass concentrations were observed to be highly variable and ranged from 7.6 to 40.2 μg/m3 over an 8-month period. The median for this measurement from all participants (50th percentile) was 13.7 μg/m3. A coefficient of determination (r 2) of 0.02 was established for community-based PM10–2.5 mass concentrations versus personal exposures. Similar coefficients established for PM2.5 mass revealed only a modest improvement in agreement (r 2  = 0.12). Data from the exposure findings are reported here.
Keywords: Coarse particulate matter; PM10–2.5, personal monitoring;

The morphology of ultrafine particles on and near major freeways by Teresa L. Barone; Yifang Zhu (6749-6758).
The morphology of ultrafine particles (UFPs; diameter < 100 nm) collected on and near major Los Angeles freeways in April 2006 is reported. Samples were size selected with a differential mobility analyzer, collected by a nanometer aerosol sampler, and analyzed using a transmission electron microscope. Typical observed morphologies included aggregates, spheres, irregularly shaped particles, and particles with multiple inclusions. For freeway aerosols with 50 nm electrical mobility diameter, most (>90%) electron-opaque particles were surrounded by an electron-transparent material. This suggests that much of these particles were heterogeneously internally mixed. The fraction of UFPs in a given morphology class collected on and at increasing downwind distance from the I-405 freeway was compared. The fraction of aggregates measured 90 m downwind of I-405 was significantly less than the fraction measured on the freeway (p-value < 0.01). Because aggregates are a primary aerosol (directly emitted), this may indicate that secondary aerosol (formed in the atmosphere) becomes more prevalent with increasing distance from the freeway. The fraction of particles with multiple inclusions measured 90 m downwind of I-405 was significantly greater than the fraction measured on the freeway (p-value < 0.01). The increase in the number of particles with multiple inclusions with increasing distance from the freeway suggests that dilution does not prevent particles from colliding and merging which may alter the particle size distribution.
Keywords: Ultrafine particles; Morphology; Particulate matter; Size distribution; Vehicular emissions; Near roadway environment;

The biogenic VOC emissions from the forests in Finland are dominated by monoterpenes. Annually, 4000–5500 km2 of forests are felled, which corresponds to 2–3% of the total forest land. Damaging plant organs has been found to increase the terpene emissions of conifers, and therefore tree cuttings can be expected to increase monoterpene emissions from a managed site. The aim of this study was to evaluate the effects of felling of Scots pine timber on the ambient monoterpene concentration of forest air below the canopy layer. To achieve this, three experimental plots (diameter 25 m) established in an even aged Scots pine stand (ca. 40 years) were felled manually in June 8–9 2004 with different intensities: one was clear-cut and two thinned with removal of 30% and 60% of the tree basal area. The felled biomass was left at the plots for the measurement period of June–September. The aerial monoterpene concentration was increased 2–3 fold by clear-cutting during the first 7 weeks after the felling. The increment of concentration due to the clear-cut was greater than in the thinnings, which also increased the concentration significantly compared to the control plot in untouched forest. The last concentration observations in late-September (15 weeks after felling) showed negligible differences between the plots. The amount of logging residue left at the site was respectively largest at the clear-cut plot, and the residue is considered to be the most important factor explaining the increment of the monoterpene concentration. The aerial monoterpene concentration cannot directly be used to describe the monoterpene flux of the site, and the actual increment of emitted monoterpenes due to the fellings remain unclear. However, the significant increase in the ambient concentration induced by the felling implies that there is a great potential impact on local or even regional atmospheric chemistry.
Keywords: Pinus sylvestris; Biogenic VOC; Monoterpene; Logging; Silviculture;

Traditional culture-dependent methods to quantify and identify airborne microorganisms are limited by factors such as short-duration sampling times and inability to count non-culturable or non-viable bacteria. Consequently, the quantitative assessment of bioaerosols is often underestimated. Use of the real-time quantitative polymerase chain reaction (Q-PCR) to quantify bacteria in environmental samples presents an alternative method, which should overcome this problem. The aim of this study was to evaluate the performance of a real-time Q-PCR assay as a simple and reliable way to quantify the airborne bacterial load within poultry houses and sewage treatment plants, in comparison with epifluorescence microscopy and culture-dependent methods. The estimates of bacterial load that we obtained from real-time PCR and epifluorescence methods, are comparable, however, our analysis of sewage treatment plants indicate these methods give values 270–290 fold greater than those obtained by the “impaction on nutrient agar” method. The culture-dependent method of air impaction on nutrient agar was also inadequate in poultry houses, as was the impinger-culture method, which gave a bacterial load estimate 32-fold lower than obtained by Q-PCR. Real-time quantitative PCR thus proves to be a reliable, discerning, and simple method that could be used to estimate airborne bacterial load in a broad variety of other environments expected to carry high numbers of airborne bacteria.
Keywords: Bioaerosols; Epifluorescence; Occupational health; Poultry farm; Q-PCR; Sewage treatment plant;

Transient contaminant dispersion is of high importance to human comfort, health and safety. Lumped parameter model is usually applied to predict the evolution of contaminant dispersion, which, however, will not be effective when the contaminant distribution is not uniform. When airflow field is steady, the contaminant concentration distribution versus time will be determined by the concentration of supply air, contaminant source and initial concentration distribution in the ventilated space. An analytic expression of distributed contaminant concentration was developed using the accessibility of supply air (ASA), accessibility of contaminant source (ACS) and accessibility of initial condition (AIC). The ASA, ACS and AIC describe the contribution of supply air, contaminant source and initial condition to any point in ventilated space, and are determined by the airflow field if the contaminant sources and initial distribution are given. The analytic expression was validated by the tracer gas measurement in a test chamber. Based on the application of the expression in typical cases, the differences as well as connections between the analytic expression and lumped model were discussed. It is shown that the analytic expression is helpful for the understanding of the contaminant evolution process and may be used to obtain effective ventilation strategy for comfort and emergency.
Keywords: Gas contaminant; Transient dispersion; Ventilation; Accessibility of initial condition (AIC); Analytic expression; Distributed parameter model;

Based on the time-series analyses of bulk-aerosol samples, we report on the large-scale temporal variability in the atmospheric abundances of elemental carbon (EC) and organic carbon (OC) at two high-altitude sites, Manora Peak (1950 m asl in north India) and Mt. Abu (1680 m asl in western India). The total suspended particulate (TSP) mass concentration in the ambient atmosphere also exhibits large seasonal variability at both the sites; varying from 13.4 to 432.3 μg m−3 at Mt. Abu and 12.7 to 271.7 μg m−3 at Manora Peak. The relatively high abundance of TSP, occurring during Apr–Jun, is associated with enhanced contribution from mineral dust. Both, OC and EC abundances at Manora Peak are nearly 2–3 times higher than those at Mt. Abu; the minimum concentrations occurring during the high-dust season (Apr–Jun) and monsoon season (Jul–Aug) and maximum in winter months (Dec–Mar). At Mt. Abu, annual-average abundances of OC (range: 0.9–12.3 μg m−3; Av = 3.7 μg m−3) and EC (range: 0.06–2.3 μg m−3; Av = 0.5 μg m−3) account for about 10 and 2% of the TSP, respectively. In contrast, annual-average concentrations of OC and EC at Manora Peak are 8.7 μg m−3 (range: 2.0–22.3 μg m−3) and 1.1 μg m−3 (range: 0.14–2.7 μg m−3), respectively; and account for about 14 and 2% of the TSP. The OC/EC ratios at the two sites (Manora Peak, range: 4.8–14.9 and Mt. Abu, range: 3.0–11.5) are significantly higher compared to those reported in the literature (2.0–3.0) for the urban regions. The high OC/EC ratios and low EC concentrations are attributed to relative dominance of organic carbon derived from biomass burning (crop waste). The average contribution of total carbonaceous aerosols (TCA; TCA = 1.6 × OC + EC) to TSP is ∼24% at Manora Peak and that at Mt. Abu is only 15%. The relatively high contribution of TCA, at Manora Peak, is influenced by the regional emission sources in north India. The contribution of secondary organic carbon (SOC) to OC, calculated based on minimum OC/EC ratio method, averages around 27% at Manora Peak and ∼16% at Mt. Abu; and brings to focus its significant role on a regional scale. The low EC concentration together with significant contribution of OC and SOC to TCA and their temporal variability suggests reassessment of relative amounts of absorbing (BC) and scattering (OC) species used in the radiative forcing models on a regional scale.
Keywords: Aerosols; Organic carbon; Elemental carbon; Secondary organic carbon; OC/EC ratio;

Ammonia emissions from a beef cattle feedyard on the southern High Plains by Richard W. Todd; N. Andy Cole; R. Nolan Clark; Thomas K. Flesch; Lowry A. Harper; Bok H. Baek (6797-6805).
Concentrated animal feeding operations (CAFOs) are major sources of ammonia emitted into the atmosphere. There is considerable literature on ammonia emissions from poultry and swine CAFO, but few comprehensive studies have investigated large, open lot beef cattle feedyards. Ammonia emission rates and emission factors for a 77-ha, 45 000-head commercial beef cattle feedyard on the southern High Plains were quantified using measured profiles of ammonia concentration, wind speed and air temperature, and an inverse dispersion model. Mean summer emission rate was 7420 kg NH3  d−1, and winter emission rate was about half that, at 3330 kg NH3  d−1. Annual NH3–N emission rate was 4430 kg NH3–N d−1, which was 53% of the N fed to cattle. Daily per capita NH3–N losses increased by 10–64% after the daily per capita N in feed rations increased by 15–26%. Annual emission factors for the pen area of the feedyard were 19.3 kg NH3 (head fed)−1, or 70.2 kg NH3  Mg−1 biomass produced. Annual emission factors for the retention pond of the feedyard were estimated to be 0.9 kg NH3 (head fed)−1, or 3.2 kg NH3  Mg−1 biomass produced.
Keywords: Ammonia; Emission rate; Emission factor; Beef cattle; Feedyard; Feedlot; Micrometeorology; Inverse dispersion model;

Episodic ozone air quality in Jakarta in relation to meteorological conditions by Didin Agustian Permadi; Nguyen Thi Kim Oanh (6806-6815).
Surface O3 air quality in Jakarta, Indonesia, was analyzed using hourly monitoring data during January 2002–March 2004 from the five automatic monitoring stations with the aim to provide the first insight into the ozone formation and accumulation leading to the high ozone levels over the city. The city location near the equator with the intensive emission sources is of especial interest in this regard. The surface O3 levels in Jakarta were high which frequently exceeded the hourly national ambient air quality standard (120 ppb), i.e. over 450 hourly measurements in 2002 and 2003 or 0.7% over 66,000 hourly ozone measurements at the five stations during 2002–2003. The monthly average of O3 was maximum in October and minimum in February. Selected days of episodic high O3 in April, May, and October, and low ozone days in February were comparatively analyzed in relation to local and synoptic meteorological conditions. The high ozone days were characterized by more intense solar radiation, higher temperature, and lighter surface wind which are favorable for photochemical production of O3. Low pressure gradients on synoptic charts of the high ozone days linked to the low wind and more stagnant air that are favorable for ozone build-up over the city. Further studies, including photochemical modeling, are required to understand better the conditions leading to high ozone in the city in order to formulate the ozone management strategies.
Keywords: Surface ozone; Episode; Meteorology; Hotspots; Indonesia;

Ship cruise observations of aerosol optical properties have been carried out over oceanic areas around India during pre-monsoon season of 2006. The results reveal rather significant day-to-day variability in aerosol optical thickness (AOT). Aerosol loading is found to be relatively high over the Bay of Bengal (BoB) i.e., AOT at 500 nm is 0.36 ± 0.12 which is higher than those over Arabian Sea (AS) i.e., 0.23 ± 0.09 and North Indian Ocean (NIO) i.e., 0.26 ± 0.10. Dominance of fine-mode (α  = 1.21 ± 0.11) and coarse-mode (α  = 0.86 ± 0.20) aerosol particles has been observed, respectively, over the BoB and AS regions. Second order Angstrom exponent shows predominant positive and negative curvatures over BoB and AS, respectively. High fine-mode aerosol loading over BoB is found to be associated with air masses originating from northeastern Indo-Gangetic plains and southeastern Myanmar. The observed short wave solar flux decrease due to aerosol extinction is found to be 24, 19 and 21 W m−2 for the BoB, AS and NIO, respectively.
Keywords: Sun photometer; Oceanic regions; Aerosol optical thickness; Angstrom parameters; Fine- and coarse-mode particles;

Emission and outflow of polycyclic aromatic hydrocarbons from wildfires in China by Huishi Yuan; Shu Tao; Bengang Li; Chang Lang; Jun Cao; Raymond M. Coveney (6828-6835).
On the basis of burned area, biomass density, burn efficiency and emission factor, annual emissions of polycyclic aromatic hydrocarbons (PAHs) from wildfires in China are estimated for the period from 1950 to 2005. During that period, 7.8 × 106 and 7.5 × 106  Mg of biomass are burned annually by forest and grassland fires, respectively. The average annual emission of 16 PAHs was estimated at 256 ± 185 Mg (average ± standard deviation). Among the 16 PAHs studied, NAP contributed 55% of the total emissions from wildfires. Emission rates varied erratically and the annual emissions declined significantly since 1988. Wildfire PAH emission in China has shown strong seasonality with peaks in spring and fall. The provinces of Inner Mongolia, Heilongjiang and Yunnan ranked as the most prolific PAH source areas with respect to both density and amount of PAH emission. Spatial variations seem to be primarily related to vegetation cover which has been used in a regression model to predict a county-resolved emission inventory in China. Forest fires in Heilongjiang and Russia led to visible haze over Japan during October 2004. In order to trace the potential influence of PAHs, numerical simulation of potential receptor influence function (PRIF) based on forward trajectory calculation was implemented to address the general outflow pattern.
Keywords: PAHs; Wildfires; Emission; Outflow;

Speciation and temporal characterization of dicarboxylic acids in PM2.5 during a PM episode and a period of non-episodic pollution by Li-Ying Hsieh; Chien-Lung Chen; Meng-Wei Wan; Cheng-Hsien Tsai; Ying I. Tsai (6836-6850).
PM2.5 aerosol was collected during a PM episode and a non-episodic pollution period between September and November 2004 in suburban southern Taiwan and dicarboxylic acid and inorganic species content and provenance were investigated. Oxalic acid was the dominant dicarboxylic acid species, followed by succinic acid and malonic acid. Tartaric acid concentrations were the lowest. There was 49.3% more dicarboxylic acid in PM episode aerosol than in non-episodic aerosol. However, daily oxalic acid concentration increased 72.7% in PM episode aerosol, while succinic acid fell 20.9% and malonic acid fell 21.6%, indicating higher conversion of these acids into oxalic acid in PM episode aerosol. Dicarboxylic acid concentrations, particularly oxalic acid, peaked at night during the PM episode, due to accumulation of daytime oxalic acid combined with low wind velocity and low mixing layer height at this time. SO4 2−, NO3 and NH4 + were the dominant ionic species in both PM episode and non-episodic aerosols. NO3 concentration increased 106% during the PM episode, while HNO3, a gaseous precursor of NO3 , fell 57%, indicating a large conversion of HNO3 to aerosol phase NO3 in PM episode aerosol. Backward trajectory data indicate that burning of paddy fields may also contribute to oxalic acid content in PM episode aerosol in the study area, especially during nighttime. Principal component analyses showed that succinic acid, malonic acid and oxalic acid were important factors in both pollution periods and also that there was high correlation between tartaric acid and NOx or HNO2, indicative of vehicle emissions. The mass ratio of oxalic acid to sulfate in this aerosol was as high as 60.3‰, 35.5% higher than in non-episodic aerosol. Principal component analyses showed that photochemical aerosols contributed by succinic acid, malonic acid and oxalic acid were important factors in both periods. There was high correlation between tartaric acid and NOx or HNO2, indicative of vehicle emissions.
Keywords: Sampling artifacts; Diurnal variations; Oxalic acid; Malonic acid; Succinic acid; Tartaric acid; Inorganic salts;

2-Methyltetrols produced by the oxidation of isoprene have been recently found to contribute toward the formation of atmospheric secondary organic aerosol (SOA). However, the oxidation mechanism relevant to the formation of these polyols has not been completely understood. In this study, the photooxidation of four conjugated dienes (isoprene, 1,3-butadiene, 2,3-dimethyl-1,3-butadiene and 2,4-hexadiene) in the presence of 0.2–1 ppmv NO was examined by a series of laboratory experiments, and the polyols, organic acids and nitric acid in an aqueous solution of the resulting SOA were analysed by using ion-exclusion liquid chromatography/mass spectrometry (LC–MS). In the experiments performed using isoprene, 2-methyltetrols (comprising 0.5–2% of aerosol mass), methylnitrooxybutanetriols (comprising 1–7% of aerosol mass), methyldinitrooxybutanediols (comprising 0.3–8% of aerosol mass) and nitric acid (comprising 4–9% of aerosol mass) were found in the aqueous solution of the SOA samples. Three days after the extraction, the concentrations of nitrooxypolyols (i.e. methylnitrooxybutanetriols and methyldinitrooxybutanediols) decreased, whereas the concentrations of polyols and nitric acid increased. Similar results were obtained for all the four dienes. Nitrooxypolyols, which are produced by the gas-phase oxidation of dienes in the presence of NO x , contribute toward the SOA formation, and these compounds can decompose to polyols and nitric acid in an aqueous solution. The polyols and the nitric acid present in the aqueous solution are hydrolysis products, and not real constituents of aerosol. The direct gas-phase formation of polyols from the diene oxidation is suppressed in the presence of NO x .
Keywords: Urban atmosphere; Laboratory smog chamber; Polyol; Isoprene tetrol; Isoprene nitrate; Cloud processing;

This paper makes the case for computing model performance statistics that can be used explicitly in characterizing the uncertainty in model estimates. Using a framework that relates model estimates to corresponding observations, we show that the relevant statistics are the geometric mean, m g, and the geometric standard deviation, s g, of the ratio of the observed to the model estimates of the variable of interest. The second part of the paper describes a graphical representation of the relationship between model estimate and observation that is used to derive model performance statistics. This diagram builds upon that proposed by Taylor [A., 2001. Summarizing multiple aspects of model performance in a single diagram. Journal of Geophysical Research, 106, 7183–7192] but represents the error in terms of two components: one that is correlated to the model estimate and can be thus reduced in principle through model improvement, and a component that can be reduced only by expanding the model input set.
Keywords: Model evaluation; Model performance statistics; Air quality model; Model error; Taylor diagram;

Due to the apparent environmental omnipresence of perfluorocarboxylic acids (PFAs), an increasing number of researchers are investigating their ambient particle- and gas-phase concentrations. Typically this is done using a high-volume air sampler equipped with Quartz Fiber Filters (QFFs) or Glass Fiber Filters (GFFs) to sample the particle-bound PFAs and downstream sorbents to sample the gas-phase PFAs. This study reports that at trace, ambient concentrations gas-phase PFAs sorb to QFFs and GFFs irreversibly and hardly pass through these filters to the downstream sorbents. As a consequence, it is not possible to distinguish between particle- and gas-phase concentrations, or to distinguish concentrations on different particle size fractions, unless precautions are taken. Failure to take such precautions could have already caused reported data to be misinterpreted. Here it is also reported that deactivating QFFs and GFFs with a silylating agent renders them suitable for sampling PFAs. Based on the presented study, a series of recommendations for air-sampling PFAs are provided.
Keywords: PFOA; Filter-artefacts; Gas-particle partitioning; Silylation;