Atmospheric chemistry of 1-methyl-2-pyrrolidinone

Rate constants for the atmospheric reactions of 1-methyl-2-pyrrolidinone with OH radicals, NO₃ radicals and O₃ have been measured at 296±2 K and atmospheric pressure of air, and the products of the OH radical and NO₃ radical reactions investigated. Using relative rate techniques, rate constants for the gas-phase reactions of OH and NO₃ radicals with 1-methyl-2-pyrrolidinone of (2.15±0.36)×10^-11 cm³ molecule^-1 s^-1 and (1.26±0.40)×10^-13 cm³ molecule^-1 s^-1, respectively, were measured, where the indicated errors include the estimated overall uncertainties in the rate constants for the reference compounds. An upper limit to the rate constant for the O₃ reaction of <1×10^-19 cm³ molecule^-1 s^-1 was also determined. These kinetic data lead to a calculated tropospheric lifetime of 1-methyl-2-pyrrolidinone of a few hours, with both the daytime OH radical reaction and the nighttime NO₃ radical reaction being important loss processes. Products of the OH radical and NO₃ radical reactions were analyzed by gas chromatography with flame ionization detection and combined gas chromatography–mass spectrometry. N-methylsuccinimide and (tentatively) 1-formyl-2-pyrrolidinone were identified as products of both of these reactions. The measured formation yields of N-methylsuccinimide and 1-formyl-2-pyrrolidinone were 44±12% and 41±12%, respectively, from the OH radical reaction and 59±16% and ∼4%, respectively, from the NO₃ radical reaction. Reaction mechanisms consistent with formation of these products are presented.     

Kinetics of the gas-phase reactions of NO3 radicals with ethyl acrylate,n-butyl acrylate,methyl methacrylate and ethyl methacrylate

The relative rate method has been used to determine the rate constants for the gas-phase reactions of NO₃ radicals with a series of acrylate esters: ethyl acrylate (k₁), n-butyl acrylate (k₂), methyl methacrylate (k₃) and ethyl methacrylate (k₄) at 298 ± 1 K and 760 Torr. The obtained rate constants are k₁ = (1.8 ± 0.25) × 10^?16 cm³ molecule^?1 s^?1, k₂ = (2.1 ± 0.33) × 10^?16 cm³ molecule^?1 s^?1, k₃ = (3.6 ± 1.2) × 10^?15 cm³ molecule^?1 s^?1, k₄ = (4.9 ± 1.7) × 10^?15 cm³ molecule^?1 s^?1. The experimental rate constants are in good agreement with theoretical rate constants calculated by an algorithm of the correlation between the rate constants and the orbital energies for the reactions of unsaturated VOCs with NO₃ radicals. In addition, the atmospheric lifetimes of the compound against NO₃ attack are estimated and the results show that NO₃ reactions contribute little to the atmospheric losses of acrylate esters except in polluted regions.     

Night-time tropospheric chemistry of the unsaturated alcohols (Z)-pent-2-en-1-ol and pent-1-en-3-ol: Kinetic studies of reactions of NO3 and N2O5 with stress-induced plant emissions

The night-time tropospheric chemistry of two stress-induced volatile organic compounds (VOCs), (Z)-pent-2-en-1-ol and pent-1-en-3-ol, has been studied at room temperature. Rate coefficients for reactions of the nitrate radical (NO₃) with these pentenols were measured using the discharge-flow technique. Because of the relatively low volatility of these compounds, we employed off-axis continuous-wave cavity-enhanced absorption spectroscopy for detection of NO₃ in order to be able to work in pseudo first-order conditions with the pentenols in large excess over NO₃. The rate coefficients were determined to be (1.53±0.23)×10⁻¹³ and (1.39±0.19)×10⁻¹⁴ cm³ molecule⁻¹ s⁻¹ for reactions of NO₃ with (Z)-pent-2-en-1-ol and pent-1-en-3-ol. An attempt to study the kinetics of these reactions with a relative-rate technique, using N₂O₅ as source of NO₃ resulted in significantly higher apparent rate coefficients. Performing relative-rate experiments in known excesses of NO₂ allowed us to determine the rate coefficients for the N₂O₅ reactions to be (5.0±2.8)×10⁻¹⁹ cm³ molecule⁻¹ s⁻¹ for (Z)-pent-2-en-1-ol, and (9.1±5.8)×10⁻¹⁹ cm³ molecule⁻¹ s⁻¹ for pent-1-en-3-ol. We show that these relatively slow reactions can indeed interfere with rate determinations in conventional relative-rate experiments.     

Gas-phase chemistry of benzyl alcohol: Reaction rate constants and products with OH radical and ozone

A bimolecular rate constant, k_{OH+Benzyl alcohol}, of (28 ± 7) × 10^?12 cm³ molecule^?1 s^?1 was measured using the relative rate technique for the reaction of the hydroxyl radical (OH) with benzyl alcohol, at (297 ± 3) K and 1 atm total pressure. Additionally, an upper limit of the bimolecular rate constant, k_{O3+Benzyl alcohol}, of approximately 6 × 10^?19 cm³ molecule^?1 s^?1 was determined by monitoring the decrease in benzyl alcohol concentration over time in an excess of ozone (O₃). To more clearly define part of benzyl alcohol's indoor environment degradation mechanism, the products of the benzyl alcohol + OH were also investigated. The derivatizing agents O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) and N,O-bis(trimethylsilyl) trifluoroacetamide (BSTFA) were used to positively identify benzaldehyde, glyoxal and 4-oxopentanal as benzyl alcohol/OH reaction products. The elucidation of other reaction products was facilitated by mass spectrometry of the derivatized reaction products coupled with plausible benzyl alcohol/OH reaction mechanisms based on previously published volatile organic compound/OH gas-phase reaction mechanisms.     

Measurements of JNO3− in snow by nitrate-based actinometry

Chemical actinometry was used to measure nitrate photolysis rate coefficients, J_NO3⁻, on and in snowpack at Summit, Greenland. Sealed glass tubes containing nitrate and a hydroxyl radical trapping system were buried in snow and exposed for between 2 and 24 h. Average J_NO3⁻ values for 2-h midday exposures in early June on surface snow were 10–14×10⁻⁷ s⁻¹. Averages over 24 h were 3.5–4.5×10⁻⁷ s⁻¹. These values reflect the integrated photon flux and also any variation of the nitrate photolysis rate with temperature. Attenuation of J_NO3⁻ within the firn was 0.03–0.04 cm⁻¹ for 24-h exposures and 0.08 cm⁻¹ for a 2-h exposure. Different attenuation coefficients may relate to differential light penetration due to changes in sun angle over the course of 24 h.     

OH and HO2 radical chemistry in a forested region of north-western Greece

This paper explores several aspects of the chemistry of a forested region in north-western Greece, from data collected during the AEROBIC97 campaign. An observationally constrained box model has been constructed to enable comparisons between modelled concentrations of OH and HO₂ and those determined by the fluorescence assay by gas expansion (FAGE) technique. These results represent the first comparison of measured and modelled OH concentrations in such an environment. The modelled OH concentrations are, on average,∼50% of those measured (range of 16–61%) over 4 days of model and measurement comparison. Possible reasons for the model-measurement discrepancy are discussed. A rate of production analysis illustrates the dominance of isoprene and the monoterpenes on OH loss, as well as the significance of the ozonolysis of biogenic species as an OH source. The measured and modelled [HO₂]/[OH] ratio averaged between 11:00 and 15:00 h is much higher than has been found previously for similar NO_x concentrations,∼75 and 340, respectively, cf. 10–20. The high ratio reflects the rapid recycling through the OH–HO₂ oxidation chain, involving biogenic species. The high biogenic concentrations result in a midday OH lifetime of∼0.15 s. Finally, for the conditions encountered during the campaign, there is high net photochemical ozone production, peaking at∼20 ppbv h⁻¹ around 09:00 h.     

Chemistry of HOx radicals in the upper troposphere

Aircraft observations from three recent missions (STRAT, SUCCESS, SONEX) are synthesized into a theoretical analysis of the factors controlling the concentrations of HO_x radicals (HO_x=OH+peroxy) and the larger reservoir family HO_y (HO_y=HO_x+2H₂O₂+2CH₃OOH+HNO₂+HNO₄) in the upper troposphere. Photochemical model calculations capture 66% of the variance of observed HO_x concentrations. Two master variables are found to determine the variance of the 24 h average HO_x concentrations: the primary HO_x production rate, P(HO_x), and the concentration of nitrogen oxide radicals (NO_x=NO+NO₂). We use these two variables as a coordinate system to diagnose the photochemistry of the upper troposphere and map the different chemical regimes. Primary HO_x production is dominated by the O(¹D)+H₂O reaction when [H₂O]>100 ppmv, and by photolysis of acetone (and possibly other convected HO_x precursors) under drier conditions. For the principally northern midlatitude conditions sampled by the aircraft missions, the HO_x yield from acetone photolysis ranges from 2 to 3. Methane oxidation amplifies the primary HO_x source by a factor of 1.1–1.9. Chemical cycling within the HO_x family has a chain length of 2.5–7, while cycling between the HO_x family and its HO_y reservoirs has a chain length of 1.6–2.2. The number of ozone molecules produced per HO_y molecule consumed ranges from 4 to 12, such that ozone production rates vary between 0.3 and 5 ppbv d⁻¹ in the upper troposphere. Three chemical regimes (NO_x-limited, transition, NO_x-saturated) are identified to describe the dependence of HO_x concentrations and ozone production rates on the two master variables P(HO_x) and [NO_x]. Simplified analytical expressions are derived to express these dependences as power laws for each regime. By applying an eigenlifetime analysis to the HO_x–NO_x–O₃ chemical system, we find that the decay of a perturbation to HO_y in the upper troposphere (as from deep convection) is represented by four dominant modes with the longest time scale being factors of 2–3 times longer than the steady-state lifetime of HO_y.     

Heterogeneous reactivity of pyrene and 1-nitropyrene with NO2: Kinetics,product yields and mechanism

The heterogeneous reactivity of nitrogen dioxide with pyrene and 1-nitropyrene (1NP) adsorbed on silica particles has been investigated using a fast-flow-tube in the absence of light. Reactants and products were extracted from particles using pressurised fluid extraction (PFE) and concentration measurements were performed using gas chromatography/mass spectrometry (GC/MS). The pseudo-first order rate constants were obtained from the fit of the experimental decay of particulate polycyclic compound concentrations versus reaction time. Experiments were performed at three different NO₂ concentrations and second order rate constants were calculated considering the oxidant concentration. The following rate constant values were obtained at room temperature: k(NO₂ + pyrene) = (9.3 ± 2.3) × 10^?17 cm³ molecule^?1 s^?1 and k(NO₂ + 1NP) = (6.2 ± 1.5) × 10^?18 cm³ molecule^?1 s^?1, showing that the reactivity of 1NP was slower by a factor of 15 than that of pyrene. 1NP was identified as the only NO₂-initiated oxidation product of pyrene and all the three dinitropyrenes were identified in the case of the 1NP reaction. The product quantification allowed showing that the kinetics of oxidation product formation was equal to that measured for parent compounds degradation, within uncertainties, confirming the validity of the reaction kinetics measurements.     

Relative rate measurements of reactions of unsaturated alcohols with atomic chlorine as a function of temperature

The kinetics of two structurally similar unsaturated alcohols, 3-butene-2-ol and 2-methyl-3-butene-2-ol (MBO232), with Cl atoms have been investigated for the first time, as a function of temperature using a relative method. As far as we know, the present work also provides the first value for 3-buten-2-ol. The coefficient at room temperature was also obtained for 2-propene-1-ol (allyl alcohol). The reactions were investigated using a 400 L Teflon reaction chamber coupled with gas chromatograph-coupled with flame-ionization detection (GC-FID) detection. The experiments were performed at atmospheric pressure and at temperatures between 256 and 298 K in air or nitrogen as the bath gas. The obtained kinetic data were used to derive the Arrhenius expressions, k_MBO232=(2.83±2.50)×10⁻¹⁴ exp (2670±249)/T, k_3-buten-2-ol=(0.65±1.60)×10⁻¹⁵ exp (3656±695)/T (in units of cm³ molecule⁻¹ s⁻¹). Finally, results and atmospheric implications are discussed and compared with the reactivity with OH and NO₃ radicals.     

Adsorption of NO2 on carbon aerosol particles in the low ppb range

The interaction of NO₂ on carbonaceous aerosol particles in an NO₂ concentration range relevant for the troposphere was studied. The adsorption as a function of NO₂ concentration (2.5–65 ppb) was investigated along with the dependence on time (1–600 s) and particle concentration. The results exhibit a zero-order process in NO₂ for the chemisorption over the measured time and concentration range. The results suggest that the chemisorption reaction is limited by a rapidly established steady-state coverage of a precursor in the form of reversibly adsorbed NO₂ which seems to be constant over the whole investigated NO₂ concentration range. Within the first 20 s, a chemisorption rate of 2.5×10¹¹ molecules cm^-2 s^-1 was calculated. To estimate a saturation value for the NO₂ adsorption on carbonaceous aerosol particles, bulk experiments were performed where the aerosol was deposited on a filter before exposure to NO₂. This gives a lower limit for the total NO₂ adsorption of about 1×10¹⁴ molecules cm^-2 of particle surface area. The measurements show that the concept of the often used sticking coefficient γ (i.e. the number of adsorbed molecules per number of the total gas–surface collisions) is not a useful parameter to describe the chemisorption of NO₂ at low ppb concentration on such complex surfaces as carbonaceous aerosol particles.     

Observations of hydroxyl and the sum of peroxy radicals at Summit,Greenland during summer 2003

The first measurements of peroxy (HO₂+RO₂) and hydroxyl (OH) radicals above the arctic snowpack were collected during the summer 2003 campaign at Summit, Greenland. The median measured number densities for peroxy and hydroxyl radicals were 2.2×10⁸ mol cm⁻³ and 6.4×10⁶ mol cm⁻³, respectively. The observed peroxy radical values are in excellent agreement ($R^2=0.83$, $\mathrm{M}/\mathrm{O}=1.06$) with highly constrained model predictions. However, calculated hydroxyl number densities are consistently more than a factor of 2 lower than the observed values. These results indicate that our current understanding of radical sources and sinks is in accord with our observations in this environment but that there may be a mechanism that is perturbing the (HO₂+RO₂)/OH ratio. This observed ratio was also found to depend on meteorological conditions especially during periods of high winds accompanied by blowing snow. Backward transport model simulations indicate that these periods of high winds were characterized by rapid transport (1–2 days) of marine boundary layer air to Summit. These data suggest that the boundary layer photochemistry at Summit may be periodically impacted by halogens.     

FT-IR kinetic and product study of the Br-radical initiated oxidation of α, β-unsaturated organic carbonyl compounds

Using the relative kinetic technique the kinetics of the gas-phase reactions of Br radicals with acrolein, methacrolein and methylvinyl ketone have been investigated at (301±3) K in 1013 mbar of (N₂+O₂) bath gas at varying proportions. In 1013 mbar of synthetic air the following rate coefficients have been obtained (in units of cm³ molecule⁻¹ s⁻¹): acrolein (3.21±0.11)×10⁻¹²; methacrolein (2.33±0.08)×10⁻¹¹; methyl vinyl ketone (1.87±0.06)×10⁻¹¹. This study represents the first determination of the rate coefficients for these compounds. As for other unsaturated hydrocarbons the rate coefficient with Br was found to increase with increasing partial pressure of O₂. From the product studies of the reactions it has been established that addition of Br radicals to the terminal C-atom is the major pathway in all three cases. However, for acrolein H atom abstraction from the -CO–H group is also significant. Mechanisms are proposed to explain the observed products, mainly β-brominated carbonyl compounds.     

FT-IR product study on the photo-oxidation of dimethyl sulphide in the presence of NOx—temperature dependence

The products of the OH radical-initiated oxidation of dimethyl sulphide (DMS) have been investigated as a function of temperature (284, 295, and 306 K) and different initial NO_x (NO+NO₂) concentrations: initial NO was varied between 434 and 2821 ppb and NO₂ between 135 and 739 ppb. The experiments were performed at 1000 mbar total pressure in synthetic air using the photolysis of H₂O₂ as the OH-radical source and FT-IR spectroscopy to monitor reactants and products. The major sulphur-containing products identified were SO₂, dimethyl sulphoxide (DMSO), dimethyl sulphone (DMSO₂), methane sulphonic acid (MSA), methane sulphonyl peroxynitrate (MSPN) and OCS. The variation of the product yields with temperature and NO_x concentration are consistent with the occurrence of both addition and abstraction channels in OH radical-initiated oxidation of DMS. Distinct trends in the yields of the various products have been observed as a function of temperature, initial NO_x conditions and also reaction time as NO is consumed in the system. Increasing the initial NO concentration was found to depress the DMSO, SO₂ and OCS formation yields and enhance those of DMSO₂, MSA and MSPN. The yield–time behaviour of DMSO₂ is supportive of a formation mechanism involving addition of O₂ to a (CH₃)₂SOH adduct, formed via the addition channel, followed by sequential reactions with NO and O₂. The mechanisms controlling the concentration–time profiles of the individual products under the present experimental conditions are discussed in detail and consideration is given to possible implications for the photo-oxidation of DMS under ambient conditions.     

A new measurement method for nitrogen oxides in the air using an annular diffusion scrubber coated with titanium dioxide

A new convenient measurement method of nitrogen oxides (NO_x) in the ambient air was developed. The collection of NO_x is performed by an annular diffusion scrubber coated with a mixture of titanium dioxide (TiO₂) and hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) and the analysis is carried out by ion chromatography with conductivity detection. Under ultraviolet light (UV) illumination, TiO₂ produces reactive oxygen species such as super oxide (O₂⁻), hydroxyl radical (OH·) and peroxyhydroxyl radical (HO₂·), by which nitric oxide (NO) is oxidized to nitrogen dioxide (NO₂), and is further oxidized to nitric acid (HNO₃). The yielded HNO₃ and NO₂ are effectively adsorbed on the surface of TiO₂ and hydroxyapatite. The collection efficiencies of NO and NO₂ by the annular diffusion scrubber coated with the catalysts under UV illumination are higher than 98%, respectively, at the air flow rate of 0.2–1.0 l min⁻¹. After the collection of NO_x, by feeding deionized water into the annular diffusion scrubber, HNO₃ and NO₂ which adsorbed on the catalysts are extracted as forms of nitrite ion (NO₂⁻) and nitrate ion (NO₃⁻). The extraction efficiencies of NO and NO₂ are almost 100%. The activity of the washed catalysts can be completely recovered by drying with the purified air. Further, a simultaneous separated measurement of NO and NO₂ can be performed by utilizing the UV illumination dependence. This method was applied to the measurement of NO_x in the ambient air. The NO_x concentration measured by this method was in good agreement with that obtained using the chemiluminescence NO_x analyzer.     

Nitrogen oxide fluxes between corn (Zea mays L.) leaves and the atmosphere

In the United States, fertilized corn fields, which make up approximately 5% of the total land area, account for approximately 45% of total soil NO_x emissions. Leaf chamber measurements were conducted of NO and NO₂ fluxes between individual corn leaves and the atmosphere in (1) field-grown plants near Champaign, IL (USA) in order to assess the potential role of corn canopies in mitigating soil–NO_x emissions to the atmosphere, and (2) greenhouse-grown plants in order to study the influence of various environmental variables and physiological factors on the dynamics of NO₂ flux. In field-grown plants, fluxes of NO were small and inconsistent from plant to plant. At ambient NO concentrations between 0.1 and 0.3 ppbv, average fluxes were zero. At ambient NO concentrations above 1 ppbv, NO uptake occurred, but fluxes were so small (14.3±0.0 pmol m⁻² s⁻¹) as to be insignificant in the NO_x inventory for this site. In field-grown plants, NO₂ was emitted to the atmosphere at ambient NO₂ concentrations below 0.9 ppbv (the NO₂ compensation point), with the highest rate of emission being 50 pmol m⁻² s⁻¹ at 0.2 ppbv. NO₂ was assimilated by corn leaves at ambient NO₂ concentrations above 0.9 ppbv, with the maximum observed uptake rate being 643 pmol m⁻² s⁻¹ at 6 ppbv. When fluxes above 0.9 ppbv are standardized for ambient NO₂ concentration, the resultant deposition velocity was 1.2±0.1 mm s⁻¹. When scaled to the entire corn canopy, NO₂ uptake rates can be estimated to be as much as 27% of the soil-emitted NO_x. In greenhouse-grown and field-grown leaves, NO₂ deposition velocity was dependent on incident photosynthetic photon flux density (PPFD; 400–700 nm), whether measured above or below the NO₂ compensation point. The shape of the PPFD dependence, and its response to ambient humidity in an experiment with greenhouse-grown plants, led to the conclusion that stomatal conductance is a primary determinant of the PPFD response. However, in field-grown leaves, measured NO₂ deposition velocities were always lower than those predicted by a model solely dependent on stomatal conductance. It is concluded that NO₂ uptake rate is highest when N availability is highest, not when the leaf deficit for N is highest. It is also concluded that the primary limitations to leaf-level NO₂ uptake concern both stomatal and mesophyll components.     

Air quality measurements in urban green areas – a case study

The influence of traffic-induced pollutants (e.g. CO, NO, NO₂ and O₃) on the air quality of urban areas was investigated in the city of Essen, North Rhine-Westphalia (NRW), Germany. Twelve air hygiene profile measuring trips were made to analyse the trace gas distribution in the urban area with high spatial resolution and to compare the air hygiene situation of urban green areas with the overall situation of urban pollution. Seventeen measurements were made to determine the diurnal concentration courses within urban parks (summer conditions: 13 measurements, 530 30 min mean values, winter conditions: 4 measurements, 128 30 min mean values). The measurements were carried out during mainly calm wind and cloudless conditions between February 1995 and March 1996. It was possible to establish highly differentiated spatial concentration patterns within the urban area. These patterns were correlated with five general types of land use (motorway, main road, secondary road, residential area, green area) which were influenced to varying degrees by traffic emissions. Urban parks downwind from the main emission sources show the following typical temporal concentration courses: In summer rush-hour-dependent CO, NO and NO₂ maxima only occurred in the morning. A high NO₂/NO ratio was established during weather conditions with high global radiation intensities (K>800 W m⁻²), which may result in a high O₃ formation potential. Some of the values measured found in one of the parks investigated (Gruga Park, Essen, area: 0.7 km²), which were as high as 275 μg m⁻³ O₃ (30-min mean value) were significantly higher than the German air quality standard of 120 μg m⁻³ (30-min mean value, VDI Guideline 2310, 1996) which currently applies in Germany and about 20% above the maximum values measured on the same day by the network of the North Rhine–Westphalian State Environment Agency. In winter high CO and NO concentrations occur in the morning and during the afternoon rush-hour. The highest concentrations (CO=4.3 mg m⁻³, NO=368 μg m⁻³, 30-min mean values) coincide with the increase in the evening inversion. The maximum measured values for CO, NO and NO₂ do not, however, exceed the German air quality standards in winter and summer.     

Ozone response to precursor controls: comparison of data analysis methods with the predictions of photochemical air quality simulation models

Comparisons were made between the predictions of six photochemical air quality simulation models (PAQSMs) and three indicators of ozone response to emission reductions: the ratios of O₃/NO_z and O₃/NO_y and the extent of reaction. The values of the two indicator ratios and the extent of reaction were computed from the model-predicted mixing ratios of ozone and oxidized nitrogen species and were compared to the changes in peak 1 and 8 h ozone mixing ratios predicted by the PAQSMs. The ozone changes were determined from the ozone levels predicted for base-case emission levels and for reduced emissions of volatile organic compounds (VOCs) and oxides of nitrogen (NO_x). For all simulations, the model-predicted responses of peak 1 and 8 h ozone mixing ratios to VOC or NO_x emission reductions were correlated with the base-case extent of reaction and ratios of O₃/NO_z and O₃/NO_y. Peak ozone values increased following NO_x control in 95% (median over all simulations) of the high-ozone (>80 ppbv hourly mixing ratio in the base-case) grid cells having mean afternoon O₃/NO_z ratios less than 5 : 1, O₃/NO_y less than 4 : 1, or extent less than 0.6. Peak ozone levels decreased in response to NO_x reductions in 95% (median over all simulations) of the grid cells having peak hourly ozone mixing ratios greater than 80 ppbv and where mean afternoon O₃/NO_z exceeded 10 : 1, O₃/NO_y was greater than 8 : 1, or extent exceeded 0.8. Ozone responses varied in grid cells where O₃/NO_z was between 5 : 1 and 10 : 1, O₃/NO_y was between 4 : 1 and 8 : 1, or extent was between 0.6 and 0.8. The responses in such grid cells were affected by ozone responses in upwind grid cells and by the changes in ozone levels along the upwind boundaries of the modeling domains.     

Personal exposures to NO2 in the EXPOLIS-study: relation to residential indoor,outdoor and workplace concentrations in Basel,Helsinki and Prague

Personal exposures, residential indoor, outdoor and workplace levels of nitrogen dioxide (NO₂) were measured for 262 urban adult (25–55 years) participants in three EXPOLIS centres (Basel; Switzerland, Helsinki; Finland, and Prague; Czech Republic) using passive samplers for 48-h sampling periods during 1996–1997. The average residential outdoor and indoor NO₂ levels were lowest in Helsinki (24±12 and 18±11 μg m⁻³, respectively), highest in Prague (61±20 and 43±23 μg m⁻³), with Basel in between (36±13 and 27±13 μg m⁻³). Average workplace NO₂ levels, however, were highest in Basel (36±24 μg m⁻³), lowest in Helsinki (27±15 μg m⁻³), with Prague in between (30±18 μg m⁻³). A time-weighted microenvironmental exposure model explained 74% of the personal NO₂ exposure variation in all centres and in average 88% of the exposures. Log-linear regression models, using residential outdoor measurements (fixed site monitoring) combined with residential and work characteristics (i.e. work location, using gas appliances and keeping windows open), explained 48% (37%) of the personal NO₂ exposure variation. Regression models based on ambient fixed site concentrations alone explained only 11–19% of personal NO₂ exposure variation. Thus, ambient fixed site monitoring alone was a poor predictor for personal NO₂ exposure variation, but adding personal questionnaire information can significantly improve the predicting power.     

A new detailed chemical model for indoor air pollution

A detailed chemical box model has been constructed based on a comprehensive chemical mechanism (the Master Chemical Mechanism) to investigate indoor air chemistry in a typical urban residence in the UK. Unlike previous modelling studies of indoor air chemistry, the mechanism adopted contains no simplifications such as lumping or the use of surrogate species, allowing more insight into indoor air chemistry than previously possible. The chemical mechanism, which has been modified to include the degradation reactions of key indoor air pollutants, contains around 15,400 reactions and 4700 species. The results show a predicted indoor OH radical concentration up to 4.0×10⁵ molecule cm⁻³, only a factor of 10–20 less than typically observed outdoors and sufficient for significant chemical cycling to take place. Concentrations of PAN-type species and organic nitrates are found to be important indoors, reaching concentrations of a few ppb. Sensitivity tests highlight that the most crucial parameters for modelling the concentration of OH are the light-intensity levels and the air exchange rate. Outdoor concentrations of O₃ and NO_X are also important in determining radical concentrations indoors. The reactions of ozone with alkenes and monoterpenes play a major role in producing new radicals, unlike outdoors where photolysis reactions are pivotal radical initiators. In terms of radical propagation, the reaction of HO₂ with NO has the most profound influence on OH concentrations indoors. Cycling between OH and RO₂ is dominated by reaction with the monoterpene species, whilst alcohols play a major role in converting OH to HO₂. Surprisingly, the absolute reaction rates are similar to those observed outdoors in a suburban environment in the UK during the summer. The results from this study highlight the importance of tailoring a model for its particular location and the need for future indoor air measurements of radical species, nitrated species such as PANs and organic nitrates, photolysis rates of key species over the range of wavelengths observed indoors and concurrent measurements of outdoor air pollutant concentrations.