Extent of H-atom abstraction from OH + p-cymene and upper limits to the formation of cresols from OH + m-xylene and OH + p-cymene

Aromatic hydrocarbons are important constituents of vehicle exhaust and of non-methane volatile organic compounds in ambient air in urban areas. It has recently been proposed that dealkylation is a significant pathway for the OH radical-initiated reactions, leading to the formation of phenolic compounds and/or oxepins (Noda, J., Volkamer, R., Molina, M.J., 2009. Dealkylation of alkylbenzenes: a significant pathway in the toluene, o-, m-, and p-xylene + OH reaction. Journal of Physical Chemistry A 113, 9658–9666.). We have investigated the formation of cresols from the reactions of OH radicals with m-xylene and p-cymene, and obtain upper limits of <1% for formation of each cresol isomer from OH + m-xylene and <2% for formation of each cresol isomer from OH + p-cymene. In addition, we have measured the formation yield of 4-methylacetophenone (the major product formed subsequent to H-atom abstraction from the CH(CH₃)₂ group) in the OH + p-cymene reaction to be 14.8 ± 3.2%, and estimate that H-atom abstraction from the CH₃ and CH(CH₃)₂ groups in p-cymene accounts for 20 ± 4% of the overall OH radical reaction. We also used a relative rate technique to measure the rate constant for the reaction of OH radicals with 4-methylacetophenone to be (4.50 ± 0.43) × 10^?12 cm³ molecule^?1 s^?1 at 297 ± 2 K.     

Feasibility of a simple laboratory approach for determining temperature influence on SPMD–air partition coefficients of selected compounds

Semipermeable membrane devices (SPMDs) are a widely used passive sampling methodology for both waterborne and airborne hydrophobic organic contaminants. The exchange kinetics and partition coefficients of an analyte in a SPMD are mediated by its physicochemical properties and certain environmental conditions. Controlled laboratory experiments are used for determining the SPMD–air (K_sa's) partition coefficients and the exchange kinetics of organic vapors. This study focused on determining a simple approach for measuring equilibrium K_sa's for naphthalene (Naph), o-chlorophenol (o-CPh) and p-dichlorobenzene (p-DCB) over a wide range of temperatures. SPMDs were exposed to test chemical vapors in small, gas-tight chambers at four different temperatures (−16, −4, 22 and 40 °C). The exposure times ranged from 6 h to 28 d depending on test temperature. K_sa's or non-equilibrium concentrations in SPMDs were determined for all compounds, temperatures and exposure periods with the exception of Naph, which could not be quantified in SPMDs until 4 weeks at the −16 °C temperature. To perform this study the assumption of constant and saturated atmospheric concentrations in test chambers was made. It could influence the results, which suggest that flow through experimental system and performance reference compounds should be used for SPMD calibration.     

Roadside BTEX and other gaseous air pollutants in relation to emission sources

Hourly concentrations of benzene, toluene, ethylbenzene, m,p-xylenes, and o-xylene (BTEX) plus CO, NO_x, SO₂ were monitored at roadsides simultaneously with the traffic volume during the dry season of 2004, in Hanoi, Vietnam. The selected three streets included Truong Chinh (TC) with high traffic volume, Dien Bien Phu (DBP) with low traffic volume, and Nguyen Trai (NT) with high traffic volume running through an industrial estate. BTEX were sampled by SKC charcoal tubes and analyzed by GC–FID. Geometric means of hourly benzene, toluene, ethylbenzene, m,p-xylenes and o-xylene are, respectively, 65, 62, 15, 43, and 22 μg m⁻³ in TC street; 30, 38, 9, 26, and 13 μg m⁻³ in DBP street; and 123, 87, 24, 56, and 30 μg m⁻³ in NT street. Levels of other gaseous pollutants including CO, NO_x, and SO₂, measured by automatic instruments, were low and not exceeding the Vietnam national ambient air quality standards. BTEX levels were comparatively analyzed for different downwind distances (3–50 m) from the street, between peak hours and off-peak hours, as well as between weekdays and weekend. Results of principal component analysis suggest that the gaseous pollutants are associated with different vehicle types.     

In situ measurements of volatile aromatic hydrocarbon biodegradation rates in groundwater

Benzene and alkylbenzene biodegradation rates and patterns were measured using an in situ microcosm in a crude-oil contaminated aquifer near Bemidji, Minnesota. Benzene-D6, toluene, ethylbenzene, o-, m- and p-xylenes and four pairs of C₃- and C₄-benzenes were added to an in situ microcosm and studied over a 3-year period. The microcosm allowed for a mass-balance approach and quantification of hydrocarbon biodegradation rates within a well-defined iron-reducing zone of the anoxic plume. Among the BTEX compounds, the apparent order of persistence is ethylbenzene > benzene > m,p-xylenes > o-xylene ≥ toluene. Threshold concentrations were observed for several compounds in the in situ microcosm, below which degradation was not observed, even after hundreds of days. In addition, long lag times were observed before the onset of degradation of benzene or ethylbenzene. The isomer-specific degradation patterns were compared to observations from a multi-year study conducted using data collected from monitoring wells along a flowpath in the contaminant plume. The data were fit with both first-order and Michaelis-Menten models. First-order kinetics provided a good fit for hydrocarbons with starting concentrations below 1 mg/L and Michaelis-Menten kinetics were a better fit when starting concentrations were above 1 mg/L, as was the case for benzene. The biodegradation rate data from this study were also compared to rates from other investigations reported in the literature.     

Gas/solid partitioning of semivolatile organic compounds (SOCs) to air filters. 2. Partitioning of polychlorinated dibenzodioxins,polychlorinated dibenzofurans,and polycyclic aromatic hydrocarbons to quartz fiber filters

Gas/particle distributions of atmospheric semi-volatile organic compounds (SOCs) are often measured using filter/sorbent samplers. Unfortunately, the adsorption of gaseous SOCs onto a filter can cause positive biases in the measured particle-phase concentrations, and negative biases in the measured gas-phase concentrations. When quartz fiber filters (QFFs) are used, surface-area-normalized gas/quartz partition coefficient (K_p,s, m³ m⁻²) values will be useful when estimating the magnitudes of these errors. Gas/QFF K_p,s values have been reported in the literature only for polycyclic aromatic hydrocarbons (PAHs) and n-alkanes. Gas/QFF K_p,s values were measured here for a series of polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), and also for a range of PAHs. Within each of the three individual compound classes, plots of log K_p,s vs. log p_L^o (sub-cooled liquid vapor pressure) were found to be linear with slopes of approximately −1. At relative humidity (RH)=25%, the pooled log K_p,s data at 25°C for the three compound classes were correlated with log p_L^o nearly as well (r²=0.95) as were the data for the individual compound classes (r²≈0.97). In general, the K_p,s values for the PAHs and PCDD/PCDFs studied were found to be about a factor of 2 larger for partitioning to clean QFFs at RH=25% than for TMFs at RH=21–52%. Backup QFFs used in filter/sorbent sampling in a suburban area yielded K_p,s values for PAHs at RH=37% that were significantly lower than for clean QFFs at the same RH. (This may have been the result of the adsorption of ambient organic compounds that at least partially blocked the direct adsorption of the SOCs to the QFF surface). Therefore, when QFFs are used to separate atmospheric gas- and particle-phase SOCs, corrections for compound-dependent gas adsorption artifacts for QFFs may need to be carried out using K_p,s values that were obtained with ambient backup QFFs.     

Sorption of perfluorooctane sulfonate to carbon nanotubes in aquatic sediments

To date, sorption of organic compounds to nanomaterials has mainly been studied for the nanomaterial in its pristine state. However, sorption may be different when nanomaterials are buried in sediments. Here, we studied sorption of Perfluorooctane sulfonate (PFOS) to sediment and to sediment with 4% multiwalled carbon nanotubes (MWCNTs), as a function of factors affecting PFOS sorption; aqueous concentration, pH and Ca²⁺ concentration. Sorption to MWCNT in the sediment–MWCNT mixtures was assessed by subtracting the contribution of PFOS sorption to sediment-only from PFOS sorption to the total sediment–MWCNT mixture. PFOS Log K_D values ranged 0.52–1.62 L kg^?1 for sediment and 1.91–2.90 L kg^?1 for MWCNT present in the sediment. The latter values are relatively low, which is attributed to fouling of MWCNT by sediment organic matter. PFOS sorption was near-linear for sediment (Freundlich exponent of 0.92 ± 0.063) but non-linear for MWCNT (Freundlich exponent of 0.66 ± 0.03). Consequently, the impact of MWCNT on sorption in the mixture was larger at low PFOS aqueous concentration. Effects of pH and Ca²⁺ on PFOS sorption to MWCNT were statistically significant. We conclude that MWCNT fouling and PFOS concentration dependency are important factors affecting PFOS–MWCNT interactions in sediments.     

Laboratory studies on the uptake of aromatic hydrocarbons by ice crystals during vapor depositional crystal growth

Uptake of aromatic hydrocarbons (AH) by ice crystals during vapor deposit growth was investigated in a walk-in cold chamber at temperatures of 242, 251, and 260 K, respectively. Ice crystals were grown from ambient air in the presence of gaseous AH namely: benzene (C₆H₆), toluene (methylbenzene, C₇H₈), the C₈H₁₀ isomers ethylbenzene, o-, m-, p-xylene (dimethylbenzenes), the C₉H₁₂ isomers n-propylbenzene, 4-ethyltoluene, 1,3,5-trimethylbenzene (1,3,5-TMB), 1,2,4-trimethylbenzene (1,2,4-TMB), 1,2,3-trimethylbenzene (1,2,3-TMB), and the C₁₀H₁₄ compound tert.-butylbenzene. Gas-phase concentrations calculated at 295 K were 10.3–20.8 μg m⁻³. Uptake of AH was detected by analyzing vapor deposited ice with a very sensitive method composed of solid-phase micro-extraction (SPME), followed by gas chromatography/mass spectrometry (GC/MS).Ice crystal size was lower than 1 cm. At water vapor extents of 5.8, 6.0 and 8.1 g m⁻³, ice crystal shape changed with decreasing temperatures from a column at a temperature of 260 K, to a plate at 251 K, and to a dendrite at 242 K. Experimentally observed ice growth rates were between 3.3 and 13.3×10⁻³ g s⁻¹ m⁻² and decreased at lower temperatures and lower value of water vapor concentration. Predicted growth rates were mostly slightly higher.Benzene, toluene, ethylbenzene, and xylenes (BTEX) were not detected in ice above their detection limits (DLs) of 25 pg g_ice⁻¹ (toluene, ethylbenzene, xylenes) and 125 pg g_ice⁻¹ (benzene) over the entire temperature range. Median concentrations of n-propylbenzene, 4-ethyltoluene, 1,3,5-TMB, tert.-butylbenzene, 1,2,4-TMB, and 1,2,3-TMB were between 4 and 176 pg g_ice⁻¹ at gas concentrations of 10.3–10.7 μg m⁻³ calculated at 295 K. Uptake coefficients (K) defined as the product of concentration of AH in ice and density of ice related to the product of their concentration in the gas phase and ice mass varied between 0.40 and 10.23. K increased with decreasing temperatures. Values of Gibbs energy (ΔG) were between −4.5 and 2.4 kJ mol⁻¹ and decreased as temperatures were lowered. From the uptake experiments, the uptake enthalpy (ΔH) could be determined between −70.6 and −33.9 kJ mol⁻¹. The uptake entropy (ΔS) was between −281.3 and −126.8 J mol⁻¹ K⁻¹. Values of ΔH and ΔS were rather similar for 4-ethlytoluene, 1,3,5-TMB and tert.-butylbenzene, whereas 1,2,3-TMB showed much higher values.     

Quantitative analysis of volatile organic compounds (VOCs) in atmospheric particles

There are a number of difficulties associated with the quantitative analysis of volatile organic compounds (VOCs) in atmospheric particles. Therefore, majority of the previous studies on VOCs associated with particles have been qualitative. Air samples were collected in Izmir, Turkey to determine ambient particle and gas phase concentrations of several aromatic, oxygenated and halogenated VOCs. Samples were quantitatively analyzed using thermal desorption–gas chromatography/mass spectrometry. Gas-phase concentrations ranged between 0.02 (bromoform) and 4.65 μg m⁻³ (toluene) and were similar to those previously measured at the same site. Particle-phase concentrations ranged from 1 (1,3-dichlorobenzene) to 933 pg m⁻³ (butanol). VOCs were mostly found in gas-phase (99.9±0.25%). However, the particulate VOCs had comparable concentrations to those reported previously for semivolatile organic compounds. The distribution of particle-phase VOCs between fine (d_p<2.5 μm) and coarse (2.5 μm<d_p<10 μm) fractions was also investigated. It was found that VOCs were mostly associated with fine particles.     

Emissions of PCDD/Fs,PCBs, and PAHs from legacy on-road heavy-duty diesel engines

Exhaust emissions of seventeen 2,3,7,8-substituted polychlorinated dibenzo-p-dioxin/furan (PCDD/F) congeners, tetra–octa PCDD/F homologues, 12 WHO 2005 polychlorinated biphenyl (PCB) congeners, mono–nona chlorinated biphenyl homologues, and 19 polycyclic aromatic hydrocarbons (PAHs) from three legacy diesel engines were investigated. The three engines tested were a 1985 model year GM 6.2 J-series engine, a 1987 model year Detroit Diesel Corporation 6V92 engine, and a 1993 model year Cummins L10 engine. Results were compared to United States’ mobile source inventory for on-road diesel engines, as well as historic and modern diesel engine emission values. The test fuel contained chlorine at 9.8 ppm which is 1.5 orders of magnitude above what is found in current diesel fuel and 3900 ppm sulfur to simulate fuels that would have been available when these engines were produced. Results indicate PCDD/F emissions of 13.1, 7.1, and 13.6 pg International Toxic Equivalency (I-TEQ) L⁻¹ fuel consumed for the three engines respectively, where non-detects are equal to zero. This compares with a United States’ mobile source on-road diesel engine inventory value of 946 pg I-TEQ L⁻¹ fuel consumed and 1.28 pg I-TEQ L⁻¹ fuel consumed for modern engines equipped with a catalyzed diesel particle filter and urea selective catalytic reduction. PCB emissions are 2 orders of magnitude greater than modern diesel engines. PAH results are representative of engines from this era based on historical values and are 3–4 orders of magnitude greater than modern diesel engines.     

Gas-phase absorption cross sections of 24 monocyclic aromatic hydrocarbons in the UV and IR spectral ranges

Absorption cross sections of 24 volatile and non-volatile derivatives of benzene in the ultraviolet (UV) and the infrared (IR) regions of the electromagnetic spectrum have been determined using a 1080 l quartz cell. For the UV a 0.5 m Czerny-Turner spectrometer coupled with a photodiode array detector (spectral resolution 0.15 nm) was used. IR spectra were recorded with an FT-IR spectrometer (Bruker IFS-88, spectral resolution 1 cm^-1). Absolute absorption cross sections and the instrument function are given for the UV, while for the IR, absorption cross sections and integrated band intensities are reported.The study focused primarily on the atmospherically relevant methylated benzenes (benzene, toluene, o-xylene, m-xylene, p-xylene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, ethylbenzene, styrene) and their ring retaining oxidation products (benzaldehyde, o-tolualdehyde, m-tolualdehyde, p-tolualdehyde, phenol, o-cresol, m-cresol, p-cresol, 2,3-dimethylphenol, 2,4-dimethylphenol, 2,5-dimethylphenol, 2,6-dimethylphenol, 3,4-dimethylphenol, 3,5-dimethylphenol, 2,4,6-trimethylphenol and (E,Z)- and (E,E)-2,4-hexadienedial).The UV absorption cross sections reported here can be used for the evaluation of DOAS spectra (Differential Optical Absorption Spectroscopy) for measurements of the above compounds in the atmosphere and in reaction chambers, while the IR absorption cross sections will primarily be useful in laboratory studies on atmospheric chemistry, where FT-IR spectrometry is an important tool.     

Use of thermal desorption–gas chromatography–mass spectrometry (TD–GC–MS) on identification of odorant emission focus by volatile organic compounds characterisation

Volatile organic compounds (VOCs) from several different municipal solid wastes’ treatment plants in Mallorca (Spain) have been analysed by thermal desorption–gas chromatography–mass spectrometry (TD–GC–MS). Ambient (immission) air was collected during February and March 2011 by active sampling onto sorbents Tenax™ TA and Carboxen™ 1000. The study presents the chemical characterisation of 93 volatile organic compounds (VOCs) from an overall set of 84 immission air samples. 70 VOCs were positively identified.The linear fit for all 93 external standard calibration, from 10 mg L⁻¹ to 150 mg L⁻¹ (n = 4), was within the range 0.974 < r² < 0.998. Limits of detection of the method (LOD) for all the standards were within the range 1.1–4,213 pg, as the absolute standard amount spiked into sorbent tubes in 1 μL standard mixture (dissolved in methanol).Overall results stated systematic correlation between waste’s nature and VOCs’ air composition. Organic wastes show main contribution of terpenes, waste water sludge residues’ of reduced sulphured compounds (RSCs) and municipal solid wastes show contribution of a wide sort of VOCs. The use of a chemometric approach for variable’s reduction to 12 principal components enables evaluation of similarities and dissimilarities between facilities. PCA clearly related samples to its corresponding facility on the basis of their VOCs composition and the ambient temperature.     

Bioaccumulation and uptake routes of perfluoroalkyl acids in Daphnia magna

Perfluoroalkyl acids (PFAs), one kind of emerging contaminants, have attracted great attentions in recent years. However, the study about their bioaccumulation mechanism remains scarce. In this research, the bioaccumulation of six kinds of PFAs in water flea Daphnia magna was studied. The uptake rates of PFAs in D. magna ranged from 178 to 1338 L kg^?1 d^?1, and they increased with increasing perfluoroalkyl chain length; the elimination rates ranged from 0.98 to 2.82 d^?1. The bioaccumulation factors (BAFs) of PFAs ranged from 91 to 380 L kg^?1 in wet weight after 25 d exposure; they increased with increasing perfluoroalkyl chain length and had a significant positive correlation with the n-octanol/water partition coefficients (log K_ow) of PFAs (p < 0.05). This indicated that the hydrophobicity of PFAs plays an important role in their bioaccumulation. The BAFs almost kept constant when the PFA concentrations in aqueous phase increased from 1 to 10 μg L^?1. Scenedesmus subspicatus, as the food of D. magna, did not significantly affect the bioaccumulation of PFAs by D. magna. Furthermore, the body burden of PFAs in the dead D. magna was 1.08–2.52 times higher than that in the living ones, inferring that the body surface sorption is a main uptake route of PFAs in D. magna. This study suggested that the bioaccumulation of PFAs in D. magna is mainly controlled by their partition between organisms and water; further research should be conducted to study the intrinsic mechanisms, especially the roles of protein and lipid in organisms.     

Levels of persistent organic pollutants in air in China and over the Yellow Sea

The occurrence of persistent toxic substances (PTS) in China and possibly their regional transport in the Yellow and East China Seas region was studied. Organochlorines in atmospheric gas-phase and particulate matter were collected by high-volume sampling (filters and polyurethane foams) during 2 weeks in June 2003 (dry season) simultaneously at a Yellow Sea coastal site in an urban area, Qingdao, China, and a rural island site, Gosan, Jeju Island, Korea. Using GC methods, the samples were analysed for 9 persistent organic pollutants (POPs) regulated under the global POP convention, namely aldrin, chlordane (cis- and trans-isomers CC and TC), DDT and metabolites (o,p′-DDT, p,p′-DDD, and p,p′-DDE), dieldrin, endrin, heptachlor, hexachlorobenzene (HCB), mirex and PCB (congeners number 28, 52, 101, 153 and 180), and for hexachlorocyclohexane (α-, β- and γ-isomers), a PTS and now considered for regulation under the convention, too. At the coastal site additionally o,p′-DDE and -DDD, β-endosulfan, isodrin, heptachlorepoxide and δ-HCH, and at the island site additionally p,p′-DDT and 12 additional PCB congeners were analysed. 9 samples were collected at the coastal and 15 (for PCBs 5) at the island site. Long-range advection pathways were determined based on analysed back-trajectory calculations.The mean concentrations of DDT and its metabolites, HCB, HCH, and PCB at the coast were in the 100–1000 pg m⁻³ range. Higher concentrations prevailed during nighttime. The levels were in general lower at the island site, but not for DDT. Local sources are likely. PCBs were even 2 orders of magnitude lower, suggesting that PCBs are not subject to regional transport but elevated concentrations in air are limited to the source areas. Organochlorine pesticide levels on the other hand were seemingly determined by regional transport over Mainland China rather than by emissions in the coastal area. The currently used pesticides mirex and chlordane were found at elevated levels, i.e. 79 (6.6–255) and 36 (<6–71) pg m⁻³, respectively, at the coast but not over the island. The POPs pesticides aldrin, dieldrin and endrin, never registered in China, were mostly found at <10 pg m⁻³ except for endrin at the coastal site (up to 400 pg m⁻³) and aldrin at the island site (up to 50 pg m⁻³).     

Radon exhalation and ultrafine fraction of radon progeny in closed room air

The influence of ²²²Rn exhalation from walls and air exchange (low ventilation rates ν<0.3 h^-1) upon its concentration in room air has been considered. It was found that the radon concentration reachs 84 Bq m^-3 at exhalation and ventilation rates of 66 Bq hm^-2 and 0.28 h^-1, respectively. The radon concentration and the ultrafine fraction f_p of potential α energy concentration as well as the equilibrium factor F of the short-lived radon progeny were also determined in three different completely closed rooms. An electroprecipitation method was applied for determining the ²²²Rn concentration while a single wire-screen technique was used for the determination of ultrafine radon progeny. During the measurements, the radon concentrations were varied between 33 and 134 with a mean value 89 Bq m^-3. A mean ultrafine fraction (f_p) of 0.16 was obtained at a mean aerosol particle concentration (Z) of 1700 cm^-3 and a mean equilibrium factor (F) of 0.33. The obtained mean value of f_p was found to be about five times higher than the value reported in the ICRP publication (f_p=0.03). The attachment rate (X), the deposition rate (q^f) and the deposition velocity (v^f_d) of the ultrafine radionuclide ²¹⁸Po were calculated. A mean value of X was found to be 49 h^-1 at a mean q^f of 46 h^-1 and a mean v^f_d of 4.6 m h^-1. The attachment coefficient β of ²¹⁸Po was found to vary between 0.016 and 0.047 with a mean value 0.028 cm³ h^-1.     

Measurement and analysis of atmospheric ammonia emissions from anaerobic lagoons

Ammonia-nitrogen flux (NH₃-N=(14/17)NH₃) was determined from six anaerobic swine waste storage and treatment lagoons (primary, secondary, and tertiary) using the dynamic chamber system. Measurements occurred during the fall of 1998 through the early spring of 1999, and each lagoon was examined for approximately one week. Analysis of flux variation was made with respect to lagoon surface water temperature (∼15 cm below the surface), lagoon water pH, total aqueous phase NH_x(=NH₃+NH₄⁺) concentration, and total Kjeldahl nitrogen (TKN). Average lagoon temperatures (across all six lagoons) ranged from approximately 10.3 to 23.3^°C. The pH ranged in value from 6.8 to 8.1. Aqueous NH_x concentration ranged from 37 to 909 mg N l⁻¹, and TKN varied from 87 to 950 mg N l⁻¹. Fluxes were the largest at the primary lagoon in Kenansville, NC (March 1999) with an average value of 120.3 μg N m⁻² min⁻¹, and smallest at the tertiary lagoon in Rocky Mount, NC (November 1998) at 40.7 μg N m⁻² min⁻¹. Emission rates were found to be correlated with both surface lagoon water temperature and aqueous NH_x concentration. The NH₃-N flux may be modeled as ln(NH₃-N flux)=1.0788+0.0406T_L+0.0015([NH_x]) (R²=0.74), where NH₃-N flux is the ammonia flux from the lagoon surface in μg N m⁻² min⁻¹, T_L is the lagoon surface water temperature in ^°C, and [NH_x] is the total ammonia-nitrogen concentration in mg N l⁻¹.     

Biogenic emission of dimethylsulfide (DMS) from the North Yellow Sea,China and its contribution to sulfate in aerosol during summer

Seawater, atmospheric dimethylsulfide (DMS) and aerosol compounds, potentially linked with DMS oxidation, such as methanesulfonic acid (MSA) and non-sea-salt sulfate (nss-SO₄^2?) were determined in the North Yellow Sea, China during July–August, 2006. The concentrations of seawater and atmospheric DMS ranged from 2.01 to 11.79 nmol l^?1 and from 1.68 to 8.26 nmol m^?3, with average values of 6.20 nmol l^?1 and 5.01 nmol m^?3, respectively. Owing to the appreciable concentration gradient, DMS accumulated in the surface water was transferred into the atmosphere, leading to a net sea-to-air flux of 6.87 μmol m^?2 d^?1 during summer. In the surface seawater, high DMS values corresponded well with the concurrent increases in chlorophyll a levels and a significant correlation was observed between integrated DMS and chlorophyll a concentrations. In addition, the concentrations of MSA and nss-SO₄^2? measured in the aerosol samples ranged from 0.012 to 0.079 μg m^?3 and from 3.82 to 11.72 μg m^?3, with average values of 0.039 and 7.40 μg m^?3, respectively. Based on the observed MSA, nss-SO₄^2? and their ratio, the relative biogenic sulfur contribution was estimated to range from 1.2% to 11.5%, implying the major contribution of anthropogenic source to sulfur budget in the study area.     

Henry’s law constant measurements for formaldehyde and benzaldehyde as a function of temperature and water composition

Henry’s law constants H of formaldehyde and benzaldehyde were determined using a dynamic system based on the water/air equilibrium at the interface within the length of a microporous tube. The measurements were conducted over the range 273–293 K in (i) deionized water, (ii) 35 g L^?1 solution of NaCl simulating seawater and (iii) two nitric acid solutions, i.e. 0.63 and 6.3 wt%.In pure water, the obtained data were used to derive the following Arrhenius expressions: ln H = (6423 ± 542)/T ? (13.4 ± 2.0) and ln H = (6258 ± 280)/T ? (17.5 ± 1.0) for formaldehyde and benzaldehyde, respectively. The H values, calculated at 293 K from Arrhenius expressions cited above were the following (in units of M atm^?1): H = 5020 ± 1170 (formaldehyde), H = 47 ± 5 (benzaldehyde). The temperature dependence of H permits then to derive the solvation enthalpies for both compounds: ΔH_solv = ?(53.4 ± 4.5) kJ mol^?1 and ΔH_solv = ?(52.0 ± 2.3) kJ mol^?1 for formaldehyde and benzaldehyde, respectively.In 35 g L^?1 salt solution, the H values were 27–66% and 12–21% lower than their respective determinations in deionized water, for formaldehyde and benzaldehyde respectively. The observed salt effect was used to estimate the following Setschenow coefficients at 293 K for 0.6 M NaCl: formaldehyde (0.21) and benzaldehyde (0.09).In 6.3 wt% nitric acid solution, H values of benzaldehyde were approximately 30% higher than those found in pure water although no significant influence was observed for formaldehyde.Finally, our experimental data were then used to estimate the fractions of formaldehyde and benzaldehyde in atmospheric aqueous phase and their derived atmospheric lifetimes.