Identification,abundance and seasonal variation of anthropogenic organic aerosols from a mega-city in China

PM_2.5 aerosols were collected in Nanjing, a typical mega-city in China, during summer and winter 2004 and were characterized for aromatic and cyclic compounds using a GC/MS technique to understand the air pollution problem. They include polycyclic aromatic hydrocarbons (PAHs), hopanes, phthalates and hydroxy-PAHs (OH-PAHs). PAHs, hopanes and OH-PAHs presented higher concentrations in winter (26–178, 3.0–18, and 0.013–0.421 ng m⁻³, respectively) than in summer (12–96, 1.6–11, and 0.029–0.171 ng m⁻³, respectively) due to an enhanced coal burning for house heating and atmospheric inversion layers developed in the cold season. In contrast, phthalates are more abundant in summer (109–368 ng m⁻³, average 230 ng m⁻³) than in winter (33–390 ng m⁻³, average 170 ng m⁻³) due to an enhanced evaporation from plastics during the hot season and the subsequent deposition on the pre-existing particles. Generally, all the identified compounds showed higher concentrations in nighttime than in daytime due to inversion layers and increased emissions from heavy-duty trucks at night. PAHs, hopanes and phthalates in Nanjing aerosols are 5–100 times more abundant than those in Los Angeles, USA, indicating a serious air pollution problem in the city. Concentrations of OH-PAHs are 1–3 orders of magnitude less than their parent PAHs and comparable to those reported from other international cities. Source identification using diagnostic ratios of the organic tracers suggests that PAHs in Nanjing urban area are mainly derived from coal burning, whereas hopanes are more attributable to traffic emissions.     

Roadside measurements of fine and ultrafine particles at a major road north of Gothenburg

Particle measurements were conducted at a road site 15 km north of the city of Gothenburg for 3 weeks in June 2000. The size distribution between 10 and 368 nm was measured continuously by using a differential mobility particle sizer (DMPS) system. PM_2.5 was sampled on a daily basis with subsequent elemental analysis using EDXRF-spectroscopy. The road is a straight four-lane road with a speed limit of 90 kph. The road passing the site is flat with no elevations where the vehicles run on a steady workload and with constant speed. The traffic intensity is about 20,000 cars per workday and 13,000 vehicles per day during weekends. The diesel fuel used in Sweden is low in sulphur content (<10 ppm) and therefore the diesel vehicles passing the site contribute less to particle emissions in comparison with other studies. A correlation between PM_2.5 and accumulation mode particles (100–368 nm) was observed. However, no significant correlation was found between number concentrations of ultrafine particles (10–100 nm) and PM_2.5 or the accumulation mode number concentration. The particle distribution between 10 and 368 nm showed great dependency on wind speed and wind direction, where the wind speed was the dominant factor for ultrafine (10–100 nm) particle concentrations. The difference in traffic intensity between workday and weekend together with wind data made it possible to single out the traffic contribution to particle emissions and measure the size distribution. The results presented in combination with previous studies show that both PM_2.5 and the mass of accumulation mode particles are bad estimates for ultrafine particles.     

Nanoparticle emissions on Minnesota highways

The objective of this project was to characterize on-road aerosol on highways surrounding the Minneapolis area. Data were collected under varying on-road traffic conditions and in residential areas to determine the impact of highway traffic on air quality. The study was focused on determining on-road nanoparticle concentrations, and estimating fuel-specific and particle emissions km⁻¹.On-road aerosol number concentrations ranged from 10⁴ to 10⁶ particles cm⁻³. The highest nanoparticle concentrations were associated with high-speed traffic. At high vehicular speeds engine load, exhaust temperature, and exhaust flow all increase resulting in higher emissions. Less variation was observed in particle volume, a surrogate measure of particle mass. Most of the particles added by the on-road fleet were below 50 nm in diameter. Particles in this size range may dominate particle number, but contribute little to particle volume or mass. Furthermore, particle number is strongly influenced by nucleation and coagulation, which have little or no effect on particle volume. Measurements made in heavy traffic, speeds<32 km h⁻¹, produced lower number concentrations and larger particles.Number concentrations measured in residential areas, 10–20 m from the highway, were considerably lower than on-road concentrations, but the size distributions were similar to on-road aerosol with high concentrations of very small (<20 nm) particles. Much lower number concentrations and larger particles were observed in residential areas located 500–700 m from the highway.Estimated emissions of total particle number larger than 3 nm ranged from 1.9 to 9.9×10¹⁴ particles km⁻¹ and 2.2–11×10¹⁵ particles (kg fuel)⁻¹ for a gasoline-dominated vehicle fleet.     

Biogenic VOC emissions from fresh leaf mulch and wood chips of Grevillea robusta (Australian Silky Oak)

The emissions of VOC from freshly cut and shredded Grevillea robusta (Australian Silky Oak) leaves and wood have been measured. The VOC emissions from fresh leaf mulch and wood chips lasted typically for 30 and 20 h respectively, and consisted primarily of ethanol, (E)-2-hexenal, (Z)-3-hexen-1-ol and acetaldehyde. The integrated emissions of the VOCs were 0.38±0.04 g kg⁻¹ from leaf mulch, and 0.022±0.003 g kg⁻¹ from wood chips. These emissions represent a source of VOCs in urban and rural air that has previously been unquantified and is currently unaccounted for. These VOCs from leaf mulch and wood chips will contribute to both urban photochemistry and secondary organic aerosol formation. Any CH₄ emissions from leaf mulch and wood chips were <1×10⁻¹¹ g g dry mass⁻¹ s⁻¹.     

Anhydrosugar and sugar alcohol organic markers associated with carboxylic acids in particulate matter from incense burning

Aerosol from the burning two types of sandalwood-based incense, Hsing Shan and Lao Shan, was analyzed to characterize the chemical profile of total particulate matter emitted. The total particulate matter (PM) mass emission factors were 46.3 ± 2.68 mg g^?1 of Hsing Shan incense and 43.7 ± 1.08 mg g^?1 of Lao Shan incense. Chemical analysis of emissions from the two types of incense revealed that of the 25 components in four groups characterized, anhydrosugars formed the major group, at 46.7–52.2% w/w of the identified particulate and 1078.3–1169.8 μg g^?1 of incense, followed by inorganic salts at 30.4–31.8% w/w of identified particulate and 681.6–734.0 μg g^?1 of incense, carboxylic acids at 12.0–17.1% w/w of the identified particulate and 268.6–392.8 μg g^?1 of incense, and sugar alcohols at 4.44–5.38% w/w of the identified particulate and 102.3–120.6 μg g^?1 of incense. More anhydrosugars and sugar alcohols were emitted from Lao Shan incense than from Hsing Shan incense whereas more carboxylic acids and organic salts were emitted from Hsing Shan than from Lao Shan. These differences were due to structural and functional differences in the young sandalwood used to make Hsing Shan and the aged sandalwood used to make Lao Shan. The anhydrosugar levoglucosan, used as a marker of biomass burning, was always the most abundant species in emitted PM for both incenses (Lao Shan 21.7 mg g^?1 of PM and Hsing Shan 18.7 mg g^?1). K⁺ and Cl^? were the second most abundant components (K⁺ and Cl^? were summed), accounting for 10.6 mg g^?1 of Hsing Shan PM and 9.85 mg g^?1 of Lao Shan PM. The most abundant carboxylic acids in the emissions were formic, acetic, succinic, glutaric and phthalic acid. The latter is a fragrance ingredient and a potential health hazard and was twice as prevalent in Lao Shan emissions. Xylitol was the most prevalent of the sugar alcohols at 35.7–36.6% w/w of total identified sugar alcohols. These abundant species are potential markers for incense burning. K⁺, levoglucosan, mannosan and xylitol are already reported in discriminator ratios for wood burning and it is proposed here that these can and should also apply to incense burning. The calculated discriminator ratios for two types of incense burning reported here are 0.229–0.288 for K/Levo, 12.5–13.5 for Levo/Manno, and 21.5–23.7 for the novel discriminator ratio Levo/Xylitol.     

A mobile pollutant measurement laboratory—measuring gas phase and aerosol ambient concentrations with high spatial and temporal resolution

A mobile pollutant measurement laboratory was designed and built at the Paul Scherrer Institute (Switzerland) for the measurement of on-road ambient concentrations of a large set of trace gases and aerosol parameters with high time resolution (<15 s for most instruments), along with geographical and meteorological information. This approach allowed for pollutant level measurements both near traffic (e.g. in urban areas or on freeways/main roads) and at rural locations far away from traffic, within short periods of time and at different times of day and year. Such measurements were performed on a regular base during the project year of gas phase and aerosol measurements (YOGAM). This paper presents data measured in the Zürich (Switzerland) area on a late autumn day (6 November) in 2001. The local urban particle background easily reached 50 000 cm⁻³, with additional peak particle number concentrations of up to 400 000 cm⁻³. The regional background of the total particle number concentration was not found to significantly correlate with the distance to traffic and anthropogenic emissions of carbon monoxide and nitrogen oxides. On the other hand, this correlation was significant for the number concentration of particles in the size range 50–150 nm, indicating that the particle number concentration in this size range is a better traffic indicator than the total number concentration. Particle number size distribution measurements showed that daytime urban ambient air is dominated by high number concentrations of ultrafine particles (nanoparticles) with diameters <50 nm, which are immediately formed by traffic exhaust and thus belong to the primary emissions. However, significant variation of the nanoparticle mode was also observed in number size distributions measured in rural areas both at daytime and nighttime, suggesting that nanoparticles are not exclusively formed by primary traffic emissions. While urban daytime total number concentrations were increased by a factor of 10 compared to the nighttime background, corresponding factors for total surface area and total volume concentrations were 2 and 1.5, respectively.     

A novel field measurement method for determining fine particle and gas emissions from residential wood combustion

Emission data from residential wood combustion are usually obtained on test stands in the laboratory but these measurements do not correspond to the operational conditions in the field because of the technological boundary conditions (e.g. testing protocol, environmental and draught conditions). The field measurements take into account the habitual practice of the operators and provide the more reliable results needed for emission inventories. In this study, a workable and compact method for measuring emissions from residential wood combustion in winter conditions was developed. The emissions for fine particle, gaseous and PAH compounds as well as particle composition in real operational conditions were measured from seven different appliances. The measurement technique worked well and was evidently suitable for winter conditions. It was easy and fast to use, and no construction scaffold was needed. The dilution of the sample with the combination of a porous tube diluter and an ejector diluter was well suited to field measurement. The results indicate that the emissions of total volatile organic carbon (TVOC) (17 g kg⁻¹ (of dry wood burned)), carbon monoxide (CO) (120 g kg⁻¹) and fine particle mass (PM₁) (2.7 g kg⁻¹) from the sauna stove were higher than in the other measured appliances. In the masonry heaters, baking oven and stove, the emissions were 2.9–9 g kg⁻¹ TVOC, 28–68 g kg⁻¹ CO and 0.6–1.6 g kg⁻¹ PM₁. The emission of 12 PAHs (PAH₁₂) from the sauna stove was 164 mg kg⁻¹ and consisted mainly of PAHs with four benzene rings in their structure. PAH₁₂ emission from other appliances was, on average, 21 mg kg⁻¹ and was dominated by 2-ring PAHs. These results indicate that despite the non-optimal operational practices in the field, the emissions did not differ markedly from the laboratory measurements.     

Distribution characteristics of polycyclic aromatic hydrocarbons with particle size in urban aerosols at the roadside in Ho Chi Minh City,Vietnam

The purpose of this study was to characterize size distributions of atmospheric polycyclic aromatic hydrocarbons (PAHs) with 4–6 rings at the roadside in Ho Chi Minh City, Vietnam. Ten PAHs (fluoranthene, pyrene, triphenylene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, benzo[ghi]perylene and indeno[1,2,3-cd]pyrene) in atmospheric particulate matters (PM) at the roadside were measured in the dry and rainy seasons in 2005 at Ho Chi Minh City, using a low-pressure cascade impactor. The PM were separated into nine fractions by their aerodynamic diameter, i.e. >9.0, 9.0–5.8, 5.8–4.7, 4.7–3.3, 3.3–2.1, 2.1–1.1, 1.1–0.7, 0.7–0.4 and <0.4 μm (a final filter). PAHs were analyzed by high-performance liquid chromatography with fluorescence detection. Total PAHs measured were higher in the rainy season than in the dry season. The mass of coarse particles occupied a higher fraction than that of fine particles in both seasons. Total PAHs were mainly concentrated in particles with aerodynamic diameter smaller than 0.4 μm. The particle size distributions of PAHs investigated were bi-modal with a peak in fine particle mode (<2.1 μm) and another peak in coarse particle mode (>2.1 μm). Generally, 5,6-ring PAHs associated mainly with fine particles and 4-ring PAHs spread out in both fine and coarse particles.     

Effects of soil dust episodes and mixed fuel sources on source apportionment of PM10 particles in Kuopio,Finland

A receptor modeling study was carried out in Kuopio, Finland, between January and April 1994. Near the center of town, the daily mean concentrations were measured for PM10, sulphur dioxide, carbon monoxide and Black Smoke. Elemental concentrations of PM10 samples for 38 days were analyzed by ICP-MS. The main sources and their contributions to the measured concentrations of PM10 particles were solved by receptor modeling using a factor analysis-multiple linear regression (FA-MLR) model. Because a dust episode was very strong during two sampling days, the FA analysis was strongly influenced by this episode and did not give main factors. The factor analysis, when the two episode days were omitted, gave credible factors related to the sources in the study area. The four major sources and their estimated contributions to the average PM10 concentration of 27.2 μg m^-3 were: soil and street dust 46–48%, heavy fuel oil burning 12–18%, traffic exhaust 10–14%, wood burning ca. 11% and unidentified sources 15–25%. However, during spring dust episode days, with maximum PM10 concentration of 150 μg m^-3, the main source of PM10 was soil.     

Inventory of aerosol and sulphur dioxide emissions from India. Part II—biomass combustion

A spatially resolved biomass burning data set, and related emissions of sulphur dioxide and aerosol chemical constituents was constructed for India, for 1996–1997 and extrapolated to the INDOEX period (1998–1999). Sources include biofuels (wood, crop waste and dung-cake) and forest fires (accidental, shifting cultivation and controlled burning). Particulate matter (PM) emission factors were compiled from studies of Indian cooking stoves and from literature for open burning. Black carbon (BC) and organic matter (OM) emissions were estimated from these, accounting for combustion temperatures in cooking stoves. Sulphur dioxide emission factors were based on fuel sulphur content and reported literature measurements. Biofuels accounted 93% of total biomass consumption (577 MT yr⁻¹), with forest fires contributing only 7%. The national average biofuel mix was 56 : 21 : 23% of fuelwood, crop waste and dung-cake, respectively. Compared to fossil fuels, biomass combustion was a minor source of SO₂ (7% of total), with higher emissions from dung-cake because of its higher sulphur content. PM_2.5 emissions of 2.04 Tg yr⁻¹ with an “inorganic fraction” of 0.86 Tg yr⁻¹ were estimated. Biomass combustion was the major source of carbonaceous aerosols, accounting 0.25 Tg yr⁻¹ of BC (72% of total) and 0.94 Tg yr⁻¹ of OM (76% of total). Among biomass, fuelwood and crop waste were primary contributors to BC emissions, while dung-cake and forest fires were primary contributors to OM emissions. Northern and the east-coast India had high densities of biomass consumption and related emissions. Measurements of emission factors of SO₂, size resolved aerosols and their chemical constituents for Indian cooking stoves are needed to refine the present estimates.     

Atmospheric polycyclic aromatic hydrocarbons observed over the North Pacific Ocean and the Arctic area: Spatial distribution and source identification

During the 2003 Chinese Arctic Research Expedition from the Bohai Sea to the high Arctic (37–80°N) aboard the icebreaker Xuelong (Snow Dragon), air samples were collected using a modified high-volume sampler that pulls air through a quartz filter and a polyurethane foam plug (PUF). These filters and PUFs were analyzed for particulate phase and gas phase polycyclic aromatic hydrocarbons (PAHs), respectively, in the North Pacific Ocean and adjacent Arctic region. The ∑PAHs (where ∑=15 compounds) ranged from undetectable level to 4380 pg m⁻³ in the particulate phase and 928–92 600 pg m⁻³ in the gas phase, respectively. A decreasing latitudinal trend was observed for gas-phase PAHs, probably resulting from temperature effects, dilution and decomposition processes; particulate-phase PAHs, however, showed poor latitudinal trends, because the effects of temperature, dilution and photochemistry played different roles in different regions from middle-latitude source areas to the high latitudes. The ratios of PAH isomer pairs, either conservative or sensitive to degradation during long-range transport, were employed to interpret sources and chemical aging of PAHs in ocean air. In this present study the fluoranthene/pyrene and indeno[123-cd]pyrene/benzo[ghi]pyrene isomer pairs, whose ratios are conservative to photo-degradation, implies that biomass or coal burning might be the major sources of PAHs observed over the North Pacific Ocean and the Arctic region in the summer. The isomer ratios of 1,7/(1,7+2,6)-DMP (dimethylphenanthrene) and anthracene/phenanthrene, which are sensitive to aging of air masses, not only imply chemical evolving of PAHs over the North Pacific Ocean were different from those over the Arctic, but reveal that PAHs over the Arctic were mainly related to coal burning, and biomass burning might have a larger contribution to the PAHs over the North pacific ocean.     

Urban scale modeling of particle number concentration in Stockholm

A three-dimensional dispersion model has been implemented over the urban area of Stockholm (35×35 km) to assess the spatial distribution of number concentrations of particles in the diameter range 3–400 nm. Typical number concentrations in the urban background of Stockholm is 10 000 cm⁻³, while they are three times higher close to a major highway outside the city and seven times higher within a densely trafficked street canyon site in the city center. The model, which includes an aerosol module for calculating the particle number losses due to coagulation and dry deposition, has been run for a 10-day period. Model results compare well with measured data, both in levels and in temporal variability. Coagulation was found to be of little importance in terms of time averaged concentrations, contributing to losses of only a few percent as compared to inert particles, while dry deposition yield particle number losses of up to 25% in certain locations. Episodic losses of up to 10% due to coagulation and 50% due to deposition, are found some kilometers downwind of major roads, rising in connection with low wind speed and suppressed turbulent mixing. Removal due to coagulation and deposition will thus be more significant for the simulation of extreme particle number concentrations during peak episodes.The study shows that dispersion models with proper aerosol dynamics included may be used to assess particle number concentrations in Stockholm, where ultrafine particles principally originate from traffic emissions. Emission factors may be determined from roadside measurements, but ambient temperature must be considered, as it has a strong influence on particle number emissions from vehicles.     

Sources and concentration of nanoparticles (<10 nm diameter) in the urban atmosphere

Whilst limited information on particle size distributions and number concentrations in cities is available, very few data on the very smallest of particles, nanoparticles, have been recorded. Measurements in this study show that road traffic and stationary combustion sources generate a significant number of nanoparticles of diameter <10 nm. Measurements at the roadside (4 m from the kerb) and downwind from the traffic (more than 25 m from the kerb) show that nanoparticles (<10 nm diameter) accounted for more than 36–44% of the total particle number concentrations. Measurements designed to sample the plume of individual vehicles showed that both a diesel- and a petrol-fuelled vehicle generated nanoparticles (<10 nm diameter). The fraction of nanoparticles was even greater in a plume 350 m downwind of a stationary combustion source. On a few occasions, a temporal association between nanoparticles in the size range 3–7 nm and solar radiation was observed in urban background air at times when no other local sources were influential, which suggests that homogeneous nucleation can also be an important source of particles in the urban atmosphere.     

Reactions between ozone and building products: Impact on primary and secondary emissions

Reactions of ozone on common building products were studied in a dedicated emission test chamber system. Fourteen new and unused products were exposed to 100–160 ppb of ozone at 23 °C and 50% RH during 48 h experiments. Ozone deposition velocities calculated at steady state were between 0.003 cm s⁻¹ (alkyd paint on polyester film) and 0.108 cm s⁻¹ (pine wood board). All tested product showed modified emissions when exposed to ozone and secondary emissions of several aldehydes were identified. Carpets and wall coverings emitted mainly C₅–C₁₀ n-aldehydes, typical by-products of surface reactions. Linoleum, polystyrene tiles and pine wood boards also showed increased emissions of formaldehyde, benzaldehyde and hexanal associated with reduced emissions of unsaturated compounds suggesting the occurrence of gas-phase reactions. The ozone removal on the different tested products was primarily associated with surface reactions. The relative contribution of gas-phase reactions to the total ozone removal was estimated to be between 5% and 30% for pine wood boards depending on relative humidity (RH) and on the incoming ozone concentration and 2% for polystyrene tiles. On pine wood board, decreasing ozone deposition velocities were measured with increasing ozone concentrations and with RH increasing in the range 30–50%.     

Mobile sources of atmospheric polycyclic aromatic hydrocarbons in a roadway tunnel

Amounts of polycyclic aromatic hydrocarbons (PAHs) and oxygenated polycyclic aromatic hydrocarbons (oxy-PAHs) in samples collected from the air, from the dust on a guardrail, and from the soils on a tunnel roadway at five sampling sites in a regular roadway tunnel were chemically analyzed in order to determine their sources. Among the 23 PAHs found in the air samples, pyrene was found in the highest concentration (43±7.2 ng/m³), followed by fluoranthene (26±4.3 ng/m³). Among 20 oxy-PAHs found in the air samples, anthraquinone was found in the greatest amount (56±3.9 ng/m³). The average concentration of the major PAHs found in the guardrail dust samples were 6.9±0.77 μg/g for pyrene, 5.5±0.76 μg/g for fluoranthene, and 2.6±0.30 μg/g for phenanthrene. The average concentration of the major oxy-PAHs found in the guardrail dust samples were 9.2±3.5 μg/g for anthraquinone and 1.4±0.50 μg/g for 2-methylanthraquinone. The average concentration of the major PAHs found in the soil samples were 1.1±0.31 μg/g for fluoranthene, 0.92±0.21 μg/g for pyrene, and 0.72±0.16 μg/g for phenanthrene. The average concentration of the major oxy-PAHs found in the soil samples were 1.2±0.88 μg/g for anthraquinone, 0.18±0.04 μg/g for 4-biphenylcarboxaldehyde, and 0.13±0.08 μg/g for 2-methylanthraquinone. The BeP ratios calculated from the results suggest that most PAHs found in the samples collected from the roadway tunnel were from automobile exhaust gases.     

Ultrafine particles near a major roadway in Raleigh,North Carolina: Downwind attenuation and correlation with traffic-related pollutants

Ultrafine particles (UFPs, diameter < 100 nm) and co-emitted pollutants from traffic are a potential health threat to nearby populations. During summertime in Raleigh, North Carolina, UFPs were simultaneously measured upwind and downwind of a major roadway using a spatial matrix of five portable industrial hygiene samplers (measuring total counts of 20–1000 nm particles). While the upper sampling range of the portable samplers extends past the defined “ultrafine” upper limit (100 nm), the 20–1000 nm number counts had high correlation (Pearson R = 0.7–0.9) with UFPs (10–70 nm) measured by a co-located research-grade analyzer and thus appear to be driven by the ultrafine range. Highest UFP concentrations were observed during weekday morning work commutes, with levels at 20 m downwind from the road nearly fivefold higher than at an upwind station. A strong downwind spatial gradient was observed, linearly approximated over the first 100 m as an 8% drop in UFP counts per 10 m distance. This result agreed well with UFP spatial gradients estimated from past studies (ranging 5–12% drop per 10 m). Linear regression of other vehicle-related air pollutants measured in near real-time (10-min averages) against UFPs yielded moderate to high correlation with benzene (R² = 0.76), toluene (R² = 0.49), carbon monoxide (R² = 0.74), nitric oxide (R² = 0.80), and black carbon (R² = 0.65). Overall, these results support the notion that near-road levels of UFPs are heavily influenced by traffic emissions and correlate with other vehicle-produced pollutants, including certain air toxics.     

Development of particle number size distribution near a major road in Helsinki during an episodic inversion situation

The influence of traffic on urban air quality is highest at low wind speeds and the presence of a temperature inversion. By relying on detailed aerosol measurements conducted simultaneously at two distances close to a major road, we studied one such episode encountered in Helsinki, Finland, during the wintertime. The observed episode was characterized by exceptionally weak dilution of traffic emissions, with particle number concentration decreasing by no more than 10–30% between 9 and 65 m distances from the road. During the nighttime with relatively minor traffic flow, dilution and particle growth by vapor condensation were found to be the dominant processes in this road-to-ambient evolution stage. The latter process shifted a significant fraction of nucleation mode particles to sizes >30 nm diameter, modifying thereby the shape of the particle number size distribution. During the rush hours in the morning, particle number concentrations were elevated by approximately an order of magnitude compared with nighttime, such that also the self-coagulation of nucleation mode particles became important. Our study demonstrates that under suitable meteorological conditions (low wind speeds coupled with temperature inversions), traffic emissions are able to affect submicron particle number concentrations over large areas around major roads and may be a dominant source of ultrafine particles in the urban atmosphere. Under conditions characterized by exceptionally slow mixing, simultaneous processing of ultrafine (nucleation and Aitken mode) particles by dilution, self- and inter-modal coagulation, as well as by condensation and evaporation seriously questions the applicability of particle number emission factors, derived from the measurements at few tens of meters from the roadside.     

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.     

Vertical profiles of ultrafine to supermicron particles measured by aircraft over Osaka metropolitan area in Japan

Intensive aircraft- and ground-based measurements of ultrafine to supermicron particles in the Osaka metropolitan area, Japan, were carried out on 17–19 March 2003, in order to investigate vertical profiles of size-resolved particles in the urban atmosphere. Differently sized particles were observed at different altitudes on 19 March. Relatively higher concentrations of ultrafine particles (31 nm) and submicron particles (0.3–0.5 μm) were measured (100–200 cm⁻³) at altitudes of 300 and 600 m, whereas supermicron particles (2–5 μm) were present (300–600 cm⁻³) at higher altitudes (1300 m in the morning and 2200 m in the afternoon). The chemical composition analysis showed that supermicron particles evidently comprised mainly soil particles mixed internally with anthropogenic species such as carbonaceous components and sulfate. Numerical simulation using the Chemical weather FORecasting System (CFORS) suggested the long-range transport of soil dust and black carbon from the Asian continent. Total number concentrations of particles sized 10–875 nm ranged from 4.8×10³ to 3.0×10⁴ cm⁻³ at an altitude of 300 m and from 7.3×10² to 4.8×10³ cm⁻³ at an altitude of 1300 m. Total number concentrations of particles sized 10–875 nm correlated very well with NO_X concentrations, and, therefore, ultrafine and submicron particles were likely emitted from urban activities such as car traffic and vertically transported. Number size distributions at lower altitudes obtained by aircraft measurements were similar to those obtained by ground measurements, with modal diameters of 20–30 nm on 18 March and about 50 nm on 19 March.