Secondary organic aerosol formation from the oxidation of aromatic hydrocarbons in the presence of dry submicron ammonium sulfate aerosol

A laboratory study was conducted to examine formation of secondary organic aerosols. A smog chamber system was developed for studying gas–aerosol interactions in a dynamic flow reactor. These experiments were conducted to investigate the fate of gas and aerosol phase compounds generated from hydrocarbon–nitrogen oxide (HC/NO_x) mixtures irradiated in the presence of fine (<2.5 μm) particulate matter. The goal was to determine to what extent photochemical oxidation products of aromatic hydrocarbons contribute to secondary organic aerosol formation through uptake on pre-existing inorganic aerosols in the absence of liquid water films. Irradiations were conducted with toluene, p-xylene, and 1,3,5-trimethylbenzene in the presence of NO_x and ammonium sulfate aerosol, with propylene added to enhance the production of radicals in the system. The secondary organic aerosol yields were determined by dividing the mass concentration of organic fraction of the aerosol collected on quartz filters by the mass concentration of the aromatic hydrocarbon removed by reaction. The mass concentration of the organic fraction was obtained by multiplying the measured organic carbon concentration by 2.0, a correction factor that takes into account the presence of hydrogen, nitrogen, and oxygen atoms in the organic species. The mass concentrations of ammonium, nitrate, and sulfate concentrations as well as the total mass of the aerosols were measured. A reasonable mass balance was found for each of the aerosols. The largest secondary organic aerosol yield of 1.59±0.40% was found for toluene at an organic aerosol concentration of 8.2 μm⁻³, followed by 1.09±0.27% for p-xylene at 6.4 μg m⁻³, and 0.41±0.10% for 1,3,5-trimethylbenzene at 2.0 μg m⁻³. In general, these results agree with those reported by Odum et al. and appear to be consistent with the gas–aerosol partitioning theory developed by Pankow. The presence of organic in the aerosol did not affect significantly the hygroscopic properties of the aerosol.     

Determination of aerosol yields from 3-methylcatechol and 4-methylcatechol ozonolysis in a simulation chamber

Secondary Organic Aerosol (SOA) formation during the ozonolysis of 3-methylcatechol (3-methyl-1,2-dihydroxybenzene) and 4-methylcatechol (3-methyl-1,2-dihydroxybenzene) was investigated using a simulation chamber (8 m³) at atmospheric pressure, room temperature (294 ± 2 K) and low relative humidity (5–10%). The initial mixing ratios were as follows (in ppb): 3-methylcatechol (194–1059), 4-methylcatechol (204–1188) and ozone (93–531). The ozone and methylcatechol concentrations were followed by UV photometry and GC–FID (Gas chromatography–Flame ionization detector), respectively and the aerosol production was monitored using a SMPS (Scanning Mobility Particle Sizer). The SOA yields (Y) were determined as the ratio of the suspended aerosol mass corrected for wall losses (M_o) to the total reacted methylcatechol concentrations assuming a particle density of 1.4 g cm^?3. The aerosol formation yield increases as the initial methylcatechol concentration increases, and leads to aerosol yields ranging from 32% to 67% and from 30% to 64% for 3-methylcatechol and 4-methylcatechol, respectively. Y is a strong function of M_o and the organic aerosol formation can be expressed by a one-product gas/particle partitioning absorption model. These data are comparable to those published in a recent study on secondary organic aerosol formation from catechol ozonolysis. To our knowledge, this work represents the first investigation of SOA formation from the ozone reaction with methylcatechols.     

Homogeneous and heterogeneous reactions of phenanthrene with ozone

The reactions of gas-phase phenanthrene and suspended phenanthrene particles with ozone were conducted in a 200l chamber. The secondary organic aerosol formation was observed in the reaction of gas-phase phenanthrene with ozone and simultaneously the size distribution of the secondary organic aerosol was monitored with a scanning mobility particle sizer during the formation process. The particulate ozonation products from both reactions were analyzed with a vacuum ultraviolet photoionization aerosol time-of-flight mass spectrometer. 2,2′-Diformylbiphenyl was identified as the dominant product in both homogeneous and heterogeneous reactions of phenanthrene with ozone. GC/MS analysis of ozonation products of phenanthrene in glacial acetic acid was carried out for assigning time-of-flight mass spectra of reaction products formed in the homogeneous and heterogeneous reactions of phenanthrene with ozone.     

Identification and quantification of ozonation products of anthracene and phenanthrene adsorbed on silica particles

Primary products of the reactions of gas-phase ozone with anthracene and phenanthrene adsorbed on silica model particles have been investigated. Silica was selected as proxy for mineral atmospheric particles. The particles, coated with anthracene or phenanthrene and placed on a filter, were exposed in a reaction cell to a gaseous ozone flow. Ozone concentration was constant ((6.0±0.6)×10¹³ molecule cm⁻³) during the experiments. Anthracene, phenanthrene and their ozonation products were then extracted by focused microwave-assisted extraction or fluid pressurized extraction and analyzed by gas chromatography coupled to mass spectrometry. Anthraquinone and anthrone on the one hand, and 1,1′-biphenyl-2,2′-dicarboxaldehyde on the other hand were identified as the products of anthracene and phenanthrene, respectively and quantified versus time of ozone exposure. This kinetical approach allowed to show that anthraquinone, anthrone and 1,1′-biphenyl-2,2′-dicarboxaldehyde are the primary products of the studied reactions, and to determine their formation yields (respectively, 0.42±0.04, 0.056±0.005 and 1.0±0.4).     

Low molecular weight carboxylic acids in an urban atmosphere: Winter measurements in São Paulo City,Brazil

Atmospheric gas and particle-phase carboxylic acids were measured during July 1996, Winter, in an urban area of São Paulo, a highly polluted Latin American city. Ion chromatography and capillary electrophoresis techniques were used to determine the species. As oxalic (36.2±21.4%), pyruvic (15.0±7.9%), β-hydroxy-butyric (9.15±9.00%) and glycolic (3.55±2.26%) acids were determined in aerosol particles, formic and acetic acids were determined both in the gaseous (4.36±2.70 and 3.66±2.63 ppbv, respectively) and particulate phases (17.8±12.4 and 18.2±9.8%, respectively). Approximately 98% of the total acetic and formic acids were in the gas-phase and the gas–aerosol equilibrium was influenced by high levels of relative humidity. Gaseous formic-to-acetic ratios fell in the 0.94–1.85 range. Photochemical production appeared to be a very likely source of the gaseous acetic and formic acid levels found in this investigation. Direct emissions, mainly motor exhaust of vehicles also contributed to their presence in air.The observed amounts of formic and acetic acids in the particle phase were comparable with those observed in other urban sites. Results from aerosol particles indicated lower concentrations of the carboxylic acids at night, but their diurnal and nocturnal variation were similar.Using a correlation matrix, it was possible to suggest some sources for the carboxylic acids in the particulate phase. During daytime, vehicular emission appeared to be the primary source of acetic acid, whereas formic and pyruvic acids should be formed photochemically. Moreover, emissions from biogenic primary sources appeared to be an important contribution to atmospheric concentrations of formic and glycolic acids. Presumably, the photooxidation of pyruvic and glycolic acids gave rise to the oxalic acid.No source for acetic and pyruvic acids at nighttime was possible to suggest. However, direct vehicular and biogenic emissions might be major sources of TOC in nocturnal measurements. Oxalic acid might result from vehicular emission, glycolic acid from biogenic emission and formic acid from both sources.     

Light-induced heterogeneous ozone processing on organic coated particles: Kinetics and condensed-phase products

For the first time we investigated the effect of solar irradiation upon the heterogeneous ozonation of adsorbed 3,4,5-trimethoxybenzaldehyde on solid surface. Light-induced heterogeneous reactions between gas-phase ozone and 3,4,5-trimethoxybenzaldehyde adsorbed on silica particles were performed and the consecutive reaction products were identified. At an ozone mixing ratio of 250 ppb, the loss of 3,4,5-trimethoxybenzaldehyde ranged from 1.0 · 10^?6 s^?1 in the dark to 2.9 · 10^?5 s^?1 under light irradiation. Such large enhancement of 29 times clearly shows the importance of light (λ > 300 nm) during the heterogeneous ozonolysis on organic coated particles.The reaction products identified in this study (3,4,5-trimethoxybenzoic acid, syringic acid, methyl 3,4,5-trimethoxybenzoate) absorb light in the spectral window (λ > 300 nm) which implies that light-induced heterogeneous ozone processing can have an influence on the aerosol surfaces by changing their physico-chemical properties.The main identified product of the heterogeneous reactions between gas-phase ozone and 3,4,5-trimethoxybenzaldehyde under dark conditions and in presence of light was 3,4,5-trimethoxybenzoic acid. For this reason we estimated the carbon yield of 3,4,5-trimethoxybenzoic acid. Carbon yields of 3,4,5-trimethoxybenzoic acid decreased with increasing ozone mixing ratio; from 40% at 250 ppb to 15% at ≥2.5 ppm under dark conditions. At ozone mixing ratio (250 ppb–1 ppm), carbon yields of 3,4,5-trimethoxybenzaldehyde are relatively higher in the experiment under dark condition than under simulated solar light.     

Heterogeneous loss of HO2 by KCl,synthetic sea salt,and natural seawater aerosol particles

The HO₂ uptake to aerosol particles is potentially significant sink for the HO₂ radical in the marine atmosphere. To assess the heterogeneous loss of HO₂ on marine aerosol particles, we have investigated the uptake coefficients (γ) of HO₂ for submicron aerosol particles of KCl, synthetic sea salt, and natural seawater under ambient conditions (760 Torr and 296 ± 2 K) using an aerosol flow tube (AFT) coupled with a chemical conversion/laser-induced fluorescence (CC/LIF) technique. γ values determined for dry and wet aerosols of KCl were 0.02 ± 0.01 and 0.07 ± 0.03 at 66% and 75% RH, respectively, while γ values for those doped with CuSO₄ was 0.55 ± 0.19 at 75% RH. γ values determined for synthetic sea-salt particles were 0.07 ± 0.03, 0.12 ± 0.04 and 0.13 ± 0.04 at 35%, 50%, 75% RH, respectively, while γ values for natural seawater particles were 0.10 ± 0.03, 0.11 ± 0.02 and 0.10 ± 0.03 at 35%, 50%, 75% RH, respectively. We recommend a HO₂ uptake coefficient in marine areas of 0.1 for modeling and estimated the contribution of heterogeneous loss of HO₂ by sea-salt aerosol particles in marine areas using a box model. Our box-model simulations suggested that daytime maximum HO₂ concentrations decreased to 87–94% of the values without heterogeneous loss.     

Experimental evidence for a significant contribution of cellulose to indoor aerosol mass concentration

An apartment bedroom located in a residential area of Aveiro (Portugal) was selected with the aim of characterizing the cellulose content of indoor aerosol particles. Two sets of samples were taken: (1) PM₁₀ collected simultaneously in indoor and outdoor air; (2) PM₁₀ and PM_2.5 collected simultaneously in indoor air. The aerosol particles were concentrated on quartz fibre filters with low-volume samplers equipped with size selective inlets. The filters were weighed and then extracted for cellulose analysis by an enzymatic method. The average indoor cellulose concentration was 1.01 ± 0.24 μg m^?3, whereas the average outdoor cellulose concentration was 0.078 ± 0.047 μg m^?3, accounting for 4.0% and 0.4%, respectively, of the PM₁₀ mass. The corresponding average ratio between indoor and outdoor cellulose concentrations was 11.1 ± 4.9, indicating that cellulose particles were generated indoors, most likely due to the handling of cotton-made textiles as a result of routine daily activities in the bedroom. Indoor cellulose concentrations averaged 1.22 ± 0.53 μg m^?3 in the aerosol coarse fraction (determined from the difference between PM₁₀ and PM_2.5 concentrations) and averaged 0.38 ± 0.13 μg m^?3 in the aerosol fine fraction. The average ratio between the coarse and fine fractions of cellulose concentrations in the indoor air was 3.6 ± 2.1. This ratio is in line with the primary origin of this biopolymer. Results from this study provide the first experimental evidence in support of a significant contribution of cellulose to the mass of suspended particles in indoor air.     

Online investigations on ozonation products of pyrene and benz[a]anthracene particles with a vacuum ultraviolet photoionization aerosol time-of-flight mass spectrometer

The reaction products of ozone with pyrene and benz[a]anthracene absorbed on azelaic acid particles under the pseudo-first-order reaction conditions have been investigated with a vacuum ultraviolet photoionization aerosol time-of-flight mass spectrometer (VUV-ATOFMS). The pyrene and benz[a]anthracene particles with the initial concentrations of ～1 mg m^?3 are respectively exposed to ～22 ppm ozone in a reaction chamber with a volume of ～180 L. The time-of-flight mass spectra of the particulate ozonides are obtained. The assignments of the mass spectra reveal that 4-carboxy-5-phenanthrene-carboxyaldehyde (71%) and hydroxypyrene (23%) are the main solid state ozonides of pyrene, while 2-(2-formyl)phenyl-3-naphthoic acid (35%), hydroxybenz[a]anthrone (30%), and benz[a]anthracene-7,12-dione (18%) are the main solid state ozonides of benz[a]anthracene. The pathways of the ozonations are proposed in the paper.     

Chemical characterization and source apportionment of fine and coarse particulate matter in Lahore,Pakistan

Lahore, Pakistan is an emerging megacity that is heavily polluted with high levels of particle air pollution. In this study, respirable particulate matter (PM_2.5 and PM₁₀) were collected every sixth day in Lahore from 12 January 2007 to 19 January 2008. Ambient aerosol was characterized using well-established chemical methods for mass, organic carbon (OC), elemental carbon (EC), ionic species (sulfate, nitrate, chloride, ammonium, sodium, calcium, and potassium), and organic species. The annual average concentration (±one standard deviation) of PM_2.5 was 194 ± 94 μg m^?3 and PM₁₀ was 336 ± 135 μg m^?3. Coarse aerosol (PM_10?2.5) was dominated by crustal sources like dust (74 ± 16%, annual average ± one standard deviation), whereas fine particles were dominated by carbonaceous aerosol (organic matter and elemental carbon, 61 ± 17%). Organic tracer species were used to identify sources of PM_2.5 OC and chemical mass balance (CMB) modeling was used to estimate relative source contributions. On an annual basis, non-catalyzed motor vehicles accounted for more than half of primary OC (53 ± 19%). Lesser sources included biomass burning (10 ± 5%) and the combined source of diesel engines and residual fuel oil combustion (6 ± 2%). Secondary organic aerosol (SOA) was an important contributor to ambient OC, particularly during the winter when secondary processing of aerosol species during fog episodes was expected. Coal combustion alone contributed a small percentage of organic aerosol (1.9 ± 0.3%), but showed strong linear correlation with unidentified sources of OC that contributed more significantly (27 ± 16%). Brick kilns, where coal and other low quality fuels are burned together, are suggested as the most probable origins of unapportioned OC. The chemical profiling of emissions from brick kilns and other sources unique to Lahore would contribute to a better understanding of OC sources in this megacity.     

Heterogeneous reactions of ozone with pyrene, 1-hydroxypyrene and 1-nitropyrene adsorbed on particles

This work deals with the kinetic study of the reactions of ozone with pyrene, 1-hydroxypyrene and 1-nitropyrene, adsorbed on model particles. Experiments were performed at room temperature and atmospheric pressure, using a quasi-static flow reactor in the absence of light. Compounds were extracted from particles using pressurized 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 O₃ concentrations from which second order rate constants were calculated. The following rate constant values were obtained at 293 K: k(O₃ + Pyrene) = (3.2 ± 0.7) × 10^?16 cm³ molecule^?1 s^?1; k(O₃ + 1OHP) = (7.7 ± 1.4) ×10 ^?16 cm³ molecule^?1 s^?1; and k(O₃ + 1NP) = (2.2 ± 0.5) × 10^?17 cm³ molecule^?1 s^?1, for pyrene, 1-hydroxypyrene and 1-nitropyrene adsorbed on silica particles. The variation in the rate constants demonstrates the strong influence of the substituent (OH or NO₂) on the heterogeneous reactivity of pyrene. The pyrene particulate concentration was also varied in order to check how this parameter may influence the experiments. Finally, oxidation products were investigated for all reactions and some were detected and identified for the first time for ozone heterogeneous reaction with pyrene adsorbed on particles.     

Atmospheric phosphorus deposition at a montane site: Size distribution,effects of wildfire,and ecological implications

The dry deposition of atmospheric particulate matter can be a significant source of phosphorus (P) to oligotrophic aquatic ecosystems, including high-elevation lakes. In this study, measurements of the mass concentration and size distribution of aerosol particles and associated particulate P are reported for the southern Sierra Nevada, California, for the period July–October, 2008. Coarse and fine particle samples were collected with Stacked Filter Units and analyzed for Total P (TP) and inorganic P (IP) using a digestion-extraction procedure, with organic P (OP) calculated by difference. Particle size-resolved mass and TP distributions were determined concurrently using a MOUDI cascade impactor. Aerosol mass concentrations were significantly elevated at the study site, primarily due to transport from offsite and emissions from local and regional wildfires. Atmospheric TP concentrations ranged from 11 to 75 ng m^?3 (mean = 37 ± 16 ng m^?3), and were typically dominated by IP. Phosphorus was concentrated in the coarse (>1 μm diameter) particle fraction and was particularly enriched in the 1.0–3.2 μm size range, which accounted for 30–60% of the atmospheric TP load. Wildfire emissions varied widely in P content, and may be related to fire intensity. The estimated dry depositional flux of TP for each daily sampling period ranged between 7 and 118 μg m^?2 d^?1, with a mean value of 40 ± 27 μg m^?2 d^?1. Relative rates of dry deposition of N and P in the Sierra Nevada are consistent with increasing incidence of N limitation of phytoplankton growth and previously observed long-term eutrophication of lakes.     

Phase transition behaviour of sodium oleate aerosol particles

Field measurements have shown that organic surfactants are significant components of atmospheric aerosols. While fatty acids, among other surfactants, are prevalent in the atmosphere, the influence of these species on the chemical and physical properties of atmospheric aerosols remains not fully characterized. In order to assess the phase in which particles may exist, a detailed study of the deliquescence of a model surfactant aerosol has been carried out. Sodium oleate was chosen as a surfactant proxy relevant in atmospheric aerosol. Sodium oleate micelle aerosol particles were generated nebulizing a sodium oleate aqueous solution. In this study, the water uptake and phase transition of sodium oleate aerosol particles have been studied in a room temperature aerosol flow tube system (AFT) using Fourier transform infrared (FTIR) spectroscopy. Aerosol morphology and elemental composition were also analysed using scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDX) techniques. The particles are homogeneously distributed as ellipsoidal-shape aggregates of micelles particles with an average size of ∼1.1 μm. The deliquescence by the sodium oleate aerosol particles was monitored by infrared extinction spectroscopy, where the dried aerosol particles were exposed to increasing relative humidity as they passed through the AFT. Observations of the infrared absorption features of condensed phase liquid water enable to determine the sodium oleate deliquescence phase transition at 88±2%.     

Size-resolved,real-time measurement of water-insoluble aerosols in metropolitan Atlanta during the summer of 2004

During the month of August 2004, the size-resolved number concentration of water-insoluble aerosols (WIA) from 0.25 to 2.0 μm was measured in real-time in the urban center of Atlanta, GA. Simultaneous measurements were performed for the total aerosol size distribution from 0.1 to 2.0 μm, the elemental and organic carbon mass concentration, the aerosol absorption coefficient, and the aerosol scattering coefficient at a dry (RH=30%) humidity. The mean aerosol number concentration in the size range 0.1–2.0 μm was found to be 360±175 cm⁻³, but this quantity fluctuated significantly on time scales of less than one hour and ranged from 25 to 1400 cm⁻³ during the sample period. The mean WIA concentration (0.25–2.0 μm) was 13±7 cm⁻³ and ranged from 1 to 60 cm⁻³. The average insoluble fraction in the size range 0.25–2.0 μm was found to be 4±2.5% with a range of 0.3–38%. The WIA population was found to follow a consistent diurnal pattern throughout the month with concentration maxima concurring with peaks in vehicular traffic flow. WIA concentration also responded to changes in meteorological conditions such as boundary layer depth and precipitation events. The temporal variability of the absorption coefficient followed an identical pattern to that of WIA and ranged from below the detection limit to 55 Mm⁻¹ with a mean of 8±6 Mm⁻¹. The WIA concentration was highly correlated with both the absorption coefficient and the elemental carbon mass concentration, suggesting that WIA measurements are dominated by fresh emissions of elemental carbon. For both the total aerosol and the WIA size distributions, the maximum number concentration was observed at the smallest sizes; however the WIA size distribution also exhibited a peak at 0.45 μm which was not observed in the total population. Over 60% of the particles greater than 1.0 μm were observed to be insoluble in the water sampling stream used by this instrumentation. Due to the refractive properties of black carbon, it is highly unlikely that these particles could be composed of elemental carbon, suggesting a crustal source for super-micron WIA.     

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.     

Interpretation of Meteorological Pollutant Roses in Areas Subject to Nocturnal Drainage Winds

The composition of aerosol particle products formed from the photochemical reaction of terpenes with NO_x and the chemical reaction of terpenes with ozone was determined using direct insertion probe/high resolution mass spectrometry. Samples of the aerosol particles generated from these gas phase reactions were collected on stainless steel disks using a specially-designed impactor. The samples were analyzed using computer-controlled high resolution mass spectrometry. The photochemical reaction of limonene with NO_x produced more than 30 reaction products in the aerosol phase. The major products identified included aldehydes, alcohols, acids, peroxides, and nitrate esters of alcohols, acids, and peroxides. In addition, there was evidence of dimeric and possibly trimeric reaction products. The composition of aerosol particle products formed from the dark reaction of ozone with limonene was determined and found similar to those products generated in the photochemical reaction, excluding the nitrated species. Aerosol concentrations were monitored using nephelometry which indicated a conversion of terpene to aerosol of 50% or greater for both the limonene and terpinolene reaction systems. The results show that direct insertion probe high resolution mass spectrometric technique has the capability for determining the composition of very polar and high molecular weight materials in aerosol particles. The composition of terpene aerosol particle products and the mass spectral data obtained from their analysis can be used in further studies to determine the importance of terpene aerosol particle formation in ambient air.     

Nitro-PAH formation studied by interacting artificially PAH-coated soot aerosol with NO2 in the temperature range of 295–523 K

Diesel particulate matter poses a threat to human health, and in particular nitrated polycyclic aromatic hydrocarbons (NPAHs) found within and on the surface of these particles. Although diesel particulate filters (DPFs) have been designed and implemented to reduce these and other harmful diesel emissions, the particle loaded filters may act as a reaction chamber for the enhanced production of NPAHs from the nitration of PAHs with NO₂.Focus is on the investigation of the heterogeneous reactions that occur on soot particles by exposing laboratory produced pyrene- or benzo(a)pyrene-coated spark discharge soot particles to varying concentrations of NO₂ and temperatures while following the formation of products over time. The sole nitration product that was observed throughout the experiments with pyrene-coated soot was 1-nitropyrene (1-NPYR), which increased linearly with reaction time for all NO₂ concentrations chosen (0.11, 1.0, 2.0, 4.0 ppm, m m^?1). Resulting 1-NPYR formation rate increased exponentially with [NO₂]. Throughout the 3-h experiments less than 10% of pyrene has been converted to 1-NPYR and the partial reaction order with regard to [NO₂] was estimated to 1.52. Benzo(a)pyrene (BaP) was more reactive than pyrene. After 3 h reaction time almost 80% of the BaP has been converted to 6-NBaP.Highest 1-NPYR concentrations on particles were detected at 373 K, and at higher temperatures a considerable decrease in particulate 1-NPYR was observed. A similar trend was observed in a DPF simulation system (PM-Kat^®-like) with BaP-coated soot. In this case, highest 6-NBaP concentration on particles was detected at 423 K. Backed by corroborating results from separate gas/solid-phase partition experiments with 1-NPYR and 6-NBaP, it is likely that the newly formed 1-NPYR and 6-NBaP became transferred from particle to gas phase at higher temperatures. Results from this study confirm the presence of 1-NPYR and 6-NBaP in particulate and gas phase under conditions encountered in DPFs, especially when operated at low temperature situations of the aftertreatment system.     

Model evidence for a significant source of secondary organic aerosol from isoprene

We investigate how a recently suggested pathway for production of secondary organic aerosol (SOA) affects the consistency of simulated organic aerosol (OA) mass in a global three-dimensional model of oxidant-aerosol chemistry (GEOS-Chem) versus surface measurements from the interagency monitoring of protected visual environments (IMPROVE) network. Simulations in which isoprene oxidation products contribute to SOA formation, with a yield of 2.0% by mass reduce a model bias versus measured OA surface mass concentrations. The resultant increase in simulated OA mass concentrations during summer of 0.6–1.0 μg m⁻³ in the southeastern United States reduces the regional RMSE to 0.88 μg m⁻³ from 1.26 μg m⁻³. Spring and fall biases are also reduced, with little change in winter when isoprene emissions are negligible.     

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.     

Products and mechanisms of the gas-phase reactions of OH radicals and O3 with 2-methyl-3-buten-2-ol

Products of the gas-phase reactions of OH radicals (in the presence of NO) and O₃ with the biogenic organic compound 2-methyl-3-buten-2-ol have been investigated using gas chromatography with flame ionization detection (GC-FID), combined gas chromatography–mass spectrometry (GC-MS), gas chromatography with Fourier transform infrared detection (GC-FTIR), in situ FT-IR spectroscopy and in situ atmospheric pressure ionization tandem mass spectrometry (API-MS/MS). Formaldehyde, 2-hydroxy-2-methylpropanal and acetone were identified from both the OH radical and O₃ reactions, glycolaldehyde and organic nitrate (s) were also observed from the OH radical reaction, and the OH radical formation yield from the O₃ reaction was measured. The formaldehyde, 2-hydroxy-2-methylpropanal, glycolaldehyde, acetone and organic nitrate yields from the OH radical reaction were 0.29±0.03, 0.19±0.07, 0.61±0.09, 0.58±0.04 and 0.05±0.02, respectively, and the formaldehyde, 2-hydroxy-2-methylpropanal and OH radical formation yields from the O₃ reaction were 0.29±0.03, 0.30±0.06 (0.47 from FT-IR measurements) and 0.19 (uncertain to a factor of 1.5), respectively. Acetone was also observed from the O₃ reaction, but appeared to be formed from secondary reactions. Reaction mechanisms are presented and discussed.