Size segregated water uptake of the urban submicrometer aerosol in Beijing

Physical and chemical properties of submicrometer aerosol particles were measured in summer 2004 (June/July) and winter 2005 (January/February) in Beijing, Peoples Republic of China, using a Twin-Differential Mobility Particle Sizer (T-DMPS), a Hygroscopicity-Tandem Differential Mobility Analyzer (H-TDMA), and a Micro Orifice Uniform Deposit Impactor (MOUDI). Particle number–size distributions were measured in the diameter range Dp = 3–800 nm and hygroscopic properties were determined at initial dry particle diameters of Dp_j (j = 30, 50, 80, 150, 250, and 350 nm) at a relative humidity (RH) of 90%. Hygroscopic properties were compared with chemical analyses of aerosol samples taken with the MOUDI. Based on the hygroscopicity data, the total hygroscopic particle volume was modeled, including dependence on dry particle size, season and level of pollution using a simple approach.Overall, the chemical analysis showed ammonium sulfate to be the major inorganic component of the urban submicrometer aerosol in Beijing along with relatively high fractions of elemental carbon (10–25%) and organic matter (15–60%) depending on particle size and season.The hygroscopic growth distributions (H-TDMA) subdivided the aerosol population into three different groups of particles with varying growth factors depending on dry particle size, namely nearly hydrophobic (growth factor = 0.96–1.07), less hygroscopic (1.06–1.29) and more hygroscopic (1.26–1.62).Hydrophobic particle fractions indicating freshly emitted soot/carbonaceous particles varied between 10 and 32% depending on dry particle size and season. During heavily polluted times, a decreasing number of hydrophobic particle fractions indicated that the urban submicrometer aerosol in Beijing was highly influenced by more aged aerosol transported from the industrial regions around Beijing containing sulfate as a major component.Based on model calculations, the urban submicrometer aerosol in Beijing showed strong compositional variations. The calculated total hygroscopic volume fractions varied between 16 and 65% depending on size, level of pollution and season.     

Efflorescence relative humidity of airborne sodium chloride particles: A theoretical investigation

The previously developed theoretical model [Gao, Y., Chen, S.B., Yu, L.E., 2006. Efflorescence relative humidity for ammonium sulfate particles. Journal of Physical Chemistry A, 110, 7602–7608], which has successfully predicted the efflorescence relative humidity (ERH) of ammonium sulfate ((NH₄)₂SO₄) particles at room temperature, is employed to estimate the ERH of sodium chloride (NaCl) particles in sizes ranging from 6 nm to 20 μm. The theoretical predictions well agree with the reported experimental data in literatures. When the NaCl particles are larger than 70 nm, the ERH decreases with decreasing dry particle sizes, and reach a minimum around 44% RH, otherwise the ERH increases with decreasing dry particle sizes (<70 nm) because of the Kelvin effect. Compared with (NH₄)₂SO₄ particles, the Kelvin effect on ERH is stronger for NaCl particles smaller than 30 nm, while the dry particle size exerts weaker influence on NaCl particles larger than 70 nm.     

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.     

The impact of biomass burning on the environmental aerosol concentration in Gaborone,Botswana

Biomass burning, in the form of savanna fires and firewood for cooking and warmth, is widespread during the dry winter months in Southern Africa. This study was carried out to investigate its impact on the environment in Gaborone, Botswana, which is a small-sized city with very little pollution from industrial sources. Measurements of aerosol size and number concentrations were carried out at the University of Botswana campus in Gaborone from September 1999 to July 2000 using two automatic laser scattering particle counters. Particles were monitored in eight size ranges from 0.1 to 5.0 μm. The mean daily particle concentrations were found to vary from about 200 cm⁻³ on clear visibility days during the summer to a high of over 9000 cm⁻³ on cold winter evenings, when there was a significant smoke haze over the city. Particle concentrations were noticeably higher during the winter than in the summer. During a typical winter day, the total particle concentration peaked between 18 and 23 h, often showing an increase of over four-fold from mid-morning minimum values. The aerosol number size distributions under various conditions were investigated and the corresponding surface area and volume distributions were derived. In general, both the surface and volume distributions were bimodal with peaks close to 0.2 μm and at 5.0 μm or greater. A hand-held counter with a minimum detectable particle size of 0.3 μm was used to monitor the size and number concentrations of aerosols across the city. The results indicate a consistent pattern of maximum concentration in the highly populated areas close to the city centre, falling significantly in the sparsely populated outlying areas by up to an order of magnitude during peak biomass burning, suggesting that much of the smoke particles in the city are removed by wind.     

Aromatic hydrocarbons at urban,sub-urban,rural (8°52′N; 67°19′W) and remote sites in Venezuela

Using the novel on-line proton transfer reaction mass spectrometry (PTR-MS) technique, atmospheric concentrations of benzene, toluene, xylenes, and C₉-benzenes were measured in Caracas (urban), Altos de Pipe (sub-urban), Calabozo (rural) and Parupa (remote), during various campaigns in 1999 and 2000.Average daytime mixing ratios measured in Caracas are 1.1, 3.2, 3.7, and 2.7 nmol/mol for benzene, toluene, xylenes, and C₉-benzenes. At the sub-urban site, located only few km from Caracas, relatively low levels (∼20% of the levels measured in Caracas) of these aromatic hydrocarbons were observed.At the rural site during the dry season, higher concentrations of benzene (0.15 nmol/mol) were recorded, whereas those of toluene (0.08 nmol/mol) were lower during that time. The aromatic hydrocarbon ratios in the wet season (benzene: 0.08 nmol/mol; toluene: 0.09 nmol/mol) are consistent with an aged urban plume, whereas biomass burning emissions dominate during the dry season. From rural and urban [benzene]/[toluene] ratios a mean HO concentration of 2.6×10⁶ molecules/cm³ was estimated during the wet season. This value must be considered an overestimate because it does not account for background concentrations which are likely for benzene and toluene.At the remote “La Gran Sabana” region (Parupa) very low mixing ratios (0.031 and 0.015 nmol/mol for benzene and toluene) are showing the pristine region to be unaffected by local sources. From the [benzene]/[toluene] ratio we deduced, that “urban” air arriving from the coastline (350 km) is likely mixed with air containing some background of benzene and toluene.Urban emissions (automobiles) should be the major source of aromatic compounds, however, during the dry season biomass burning seems to make an important contribution.     

Tropical tropospheric ozone observed in Thailand

The mixing ratios of surface ozone at two rural/remote sites in Thailand, Inthanon and Srinakarin, have been measured continuously for the first time. Almost identical seasonal variations of O₃ with dry season maximum and a wet season minimum with a large seasonal amplitude are observed at both sites during 1996–1998. At Inthanon, the monthly averaged O₃ mixing ratios range 9–55 ppb, with the annual average of 27 ppb. The ozone mixing ratios at Srinakarin are in the similar range, 9–45 ppb with annual average of 28 ppb. Based on trajectory analysis of O₃ data at Inthanon, the long-range transport of O₃ under Asian monsoon regime could primarily explain the low O₃ mixing ratios of 13 ppb in clean marine air mass from Indian Ocean during wet season but only partly explain the relatively low O₃ mixing ratios, 26 ppb or less, in continental air mass from northeast Asia either in wet or dry season. The highest O₃ mixing ratios are found in air masses transported within southeast Asia, averaged 46 ppb in dry season. The high O₃ mixing ratios during the dry season are suggested to be significantly due to the local/sub-regional scale O₃ production triggered by biomass burning in southeast Asia rather than long-range transport effect.     

The chemical composition of tropospheric aerosols and their contributing sources to a continental background site in northern Zimbabwe from 1994 to 2000

Atmospheric aerosols were collected in separate coarse (2–10 μm diameter) and fine (diameter less than 2 μm) size fractions at Rukomechi Research Station (16.1°S, 29.4°E), Zimbabwe, in the central part of southern Africa, from September 1994 to January 2000. The samples were analysed for the particulate mass (PM), black carbon, and 47 elements. The overall data set and the separate wet and dry season data sets were examined with absolute principal component analysis (APCA). Natural and anthropogenic aerosol sources were identified in both seasons, but the sources and their contributions to the total PM were found to vary between seasons and between size fractions. Crustal matter, sea salt (SS), a mixed biogenic (BIO) emission/biomass burning (BB) component, and a copper component were identified for the coarse aerosols during the wet season. APCA attributed 29% of the total wet season coarse PM to the mixed BIO/BB component, and 32% to SS. The copper component is likely due to the copper smelters in the Zambian Copperbelt. The dry season coarse PM originated from crustal matter, BB, BIO, and SS sources, with the major contribution (32%) coming from BB. Four components (crustal matter, BB, non-ferrous smelters, and SS) were identified for the fine particles for both the wet and dry seasons. The BB component provided the major contribution to the total fine PM, accounting for 44% and 79% in the wet and dry seasons, respectively. The relative contributions to the total PM (both fine and coarse) for all sources were greater in the dry season than the wet season, except for SS.     

Dust emission factors for environment of Northern China

This paper gives the dust emission inventory in the Northern China where the climate is very dry and large desert areas exist. Before calculating the distribution of fugitive dust emission factors (emission rates of particles smaller than 0.05 mm in diameter) from natural surfaces with a US EPA formula, Chinese data of pedology and climatology were processed so as to suit requirements of the formula. The computed dust emission factors for this environment of Northern China are shown by contours, their distribution and seasonal variations are briefly discussed. The dust emission rate in the area increases from east to west by five orders. Also, the total amount of the dust emitted from natural surfaces of Northern China into the atmosphere is found to be some 43 million t yr^-1, with half of the emissions concentrated in the spring season (March–May).     

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.     

Particle volatility in the vicinity of a freeway with heavy-duty diesel traffic

During February–March 2006, a major field sampling campaign was conducted adjacent to the Interstate 710 (I-710) freeway in Los Angeles, CA. I-710 has high traffic volumes (ca. 11,000 vehicles h⁻¹) and a high percentage (17–18%) of heavy-duty diesel vehicle (HDDV) traffic. The volatility of ambient particles of 20, 40, 80 and 120 nm in diameter was investigated using a Tandem Differential Mobility Analyzer (TDMA) at two locations—close to the freeway (10 m) and approximately 150 m downwind. The smallest particles (20 nm) are largely volatile at both locations. Larger particles, e.g., ⩾40 nm) showed evidence of external mixing, with the non-volatile fraction increasing with particle size. Particle volatility increased with decreasing ambient temperature. The HDDVs contribute to relatively larger non-volatile particle number and volume fractions and greater external mixing than earlier observations at a pure light-duty gasoline vehicle freeway [Kuhn et al., 2005c. Atmospheric Environment 39, 7154–7166]. Finally, the fraction of externally mixed soot particles decreased as the downwind distance increased from the I-710, due to atmospheric processes such as vapor adsorption and condensation as well as particle coagulation.     

Fine particle counting with aerodynamic particle focusing and corona charging

The conceptual design and evaluation of a fine particle sizing and counting instrument are introduced in this paper. A corresponding laboratory prototype was developed by coupling aerodynamic particle focusing with corona charging techniques that could detect particle sizes down to 25 nm in diameter. Comparison between the prototype and a condensation particle counter (CPC) using identical monodisperse particles showed that the measurements agreed well for the particle sizes in the range of 60–300 nm.     

Particulate size distributions and mass measured at a motorway during the BAB II campaign

From 1 May to 25 May 2001, the BAB II campaign was carried out at the motorway BAB (656) near Heidelberg. Atmospheric concentrations of particulate matter and gases were measured together with the meteorological conditions. This paper is focused on the particulate matter measured upwind and downwind from the motorway at ground level. In order to determine the source contribution from the motorway traffic, it was necessary to measure upwind and downwind simultaneously due to variations in background concentrations. The particle number contribution from the motorway was found to be 35,000 particles cm⁻³ for particles with diameters close to 20 nm and 5000 particles cm⁻³ for particles with diameters close to 70 nm. Bimodal size distributions were observed on the downwind side, whereas the upwind side showed unimodal size distributions. For particulate mass, it can be estimated that the contribution from the motorway to the PM₁ concentrations is in a range 0.6–1.3 μg m⁻³ for the chosen measurement sites approximately 60 m from the road at a height of 6 m. The soot measurements showed diurnal variation; however, the upwind downwind difference was not measured. Correlation factors showed good correlation between total particle number and number of particles with diameters below 80 nm, CO and NO. There was no correlation between particle number and PM₁₀, which is due to the observation that particle number was dominated by the 20 nm particles.     

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.     

Biomass and combustion characteristics of secondary mixed deciduous forests in Eastern Ghats of India

Biomass quantities at three different sites in tropical moist mixed secondary deciduous forests before and after burning have been quantified in the forest patches cleared for shifting cultivation purposes. The main objective of the experiments was to study the spatial variability in the amount of biomass burnt and the contribution of different components viz., bole, branch, and mixed leaf litter in the burning process affecting the combustion factors. Species composition at the three sites varied with respect to each other. The total density of species before burning at the three sites for more than 10 cm diameter were found to be 3192 (site 1), 1194 (site 2) and 1444 (site 3) stems/area, respectively. Analysis of the results from girth-class and density relationships suggests that nearly 80% of the stems occurred in the range from 10–40 cm girth for site one, 64.2% in 10–55 cm girth class for the site two and more than 80% of stems in 10–40 cm girth class for the site three, indicating very poor and secondary nature of the forest. The fire intensity is found to be high for site one with 60 847 kJ s⁻¹ m⁻¹, when compared to 31 086 and 42 789 kJ s⁻¹ m⁻¹ for second and third sites, respectively. The values are comparatively higher than the mean value of 2566 kJ s⁻¹ m⁻¹ reported for savanna fires. The individual combustion completeness suggested that among the different components of biomass, branch material with less than 10 cm and upto 5 cm dbh contributed to more than 60% of combustion. Mixed leaf litter contributed to about more than 50% of individual combustion completeness at all the sites, with third site having the highest (84%). The contribution of dry biomass material having more than 70 cm diameter is found to be very low indicating that most of the trunks were burnt superficially. The overall combustion completeness suggested that mixed leaf litter and branch material contributed to most of the combustion. Of all the three sites, site three had been found to be having highest combustion completeness of about 30.04% when compared to 20.18% and 16.1% for first and second sites, respectively. In the study, comparison of combustion factors for different vegetation types has also been made.     

Effect of biomass burning and regional background aerosols on CCN activity derived from airborne in-situ measurements

Airborne in-situ measurements were analyzed to investigate the effects of biomass burning and regional background aerosols on cloud condensation nuclei (CCN) activity in the Pacific Dust Experiment (PACDEX) during April and May 2007. Airmass trajectories with both horizontal and vertical motions were provided to identify the aerosol sources. In the biomass burning cases, the elevated aerosol layers were clearly observed at dry conditions because of the convection of airmass in the source region. The relative aging of aerosols was supported by the ratios of BC to particles with size ranging from 0.1 to 1.0 μm (N_0.1–1.0) and BC to carbon monoxide. Compared to aerosols in the precedent plume of biomass burning, aged particles in the latter plume were more activated to CCN at 0.4% (CCN_0.4%) than 0.1% supersaturation (CCN_0.1%) due to aerosols chemical modification during the aging process. On the other hand, significant difference of CCN_0.4% and CCN_0.1% at regional background aerosols over the Pacific Ocean was due to the activated particles below 1 μm in diameter. Although higher concentrations of aged particles were observed over the eastern Pacific Ocean, activated aerosols to cloud droplet was comparatively similar in the western Pacific Ocean because of the similar concentrations of N_0.1–1.0 in both cases.     

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.     

Dilution and aerosol dynamics within a diesel car exhaust plume—CFD simulations of on-road measurement conditions

Vehicle particle emissions are studied extensively because of their health effects, contribution to ambient PM levels and possible impact on climate. The aim of this work was to obtain a better understanding of secondary particle formation and growth in a diluting vehicle exhaust plume using 3-d information of simulations together with measurements. Detailed coupled computational fluid dynamics (CFD) and aerosol dynamics simulations have been conducted for H₂SO₄–H₂O and soot particles based on measurements within a vehicle exhaust plume under real conditions on public roads.Turbulent diffusion of soot and nucleation particles is responsible for the measured decrease of number concentrations within the diesel car exhaust plume and decreases coagulation rates. Particle size distribution measurements at 0.45 and 0.9 m distance to the tailpipe indicate a consistent soot mode (particle diameter D_p∼50 nm) at variable operating conditions. Soot mode number concentrations reached up to 10¹³ m⁻³ depending on operating conditions and mixing.For nucleation particles the simulations showed a strong sensitivity to the spatial dilution pattern, related cooling and exhaust H₂SO_4(g). The highest simulated nucleation rates were about 0.05–0.1 m from the axis of the plume. The simulated particle number concentration pattern is in approximate accordance with measured concentrations, along the jet centreline and 0.45 and 0.9 m from the tailpipe. Although the test car was run with ultralow sulphur fuel, high nucleation particle (D_p⩽15 nm) concentrations (>10¹³ m⁻³) were measured under driving conditions of strong acceleration or the combination of high vehicle speed (>140 km h⁻¹) and high engine rotational speed (>3800 revolutions per minute (rpm)).Strong mixing and cooling caused rapid nucleation immediately behind the tailpipe, so that the highest particle number concentrations were recorded at a distance, x=0.45 m behind the tailpipe. The simulated growth of H₂SO₄–H₂O nucleation particles was unrealistically low compared with measurements. The possible role of low and semi-volatile organic components on the growth processes is discussed. Simulations for simplified H₂SO₄–H₂O–octane–gasoil aerosol resulted in sufficient growth of nucleation particles.     

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.     

Large-scale aircraft observations of ultra-fine and fine particle concentrations in the remote Siberian troposphere: New particle formation studies

Ultra-fine particle number concentrations were measured over Siberia during two large-scale airborne measurement campaigns in April and September 2006. During both campaigns, an aircraft flew between Novosibirsk and Yakutsk, collecting every 200 km vertical profiles up to 7 km. This dataset was completed by 5 years of monthly profiles above Novosibirsk. Particle number concentration was measured in the size ranges 3–70 and 70–200 nm, along with other tracers. Free troposphere (FT) particle concentrations (N_3–200) varied between 60 and 460 cm^?3, inferior to boundary layer concentrations (100–7000 cm^?3). In April, high concentrations of ～500 cm^?3 were observed in a polluted air mass recently uplifted at 5–6 km altitude over eastern Siberia, with no sign of significant new particle formation. In September, particle concentrations decreased with altitude, but with a steeper gradient in N_70–200 compared to N_3–70, the latter accounting for 90% of the total particle concentration in the free troposphere at 6–7 km altitude. Because ultra-fine particles presumably have short lifetimes, these observed particles could have been formed in situ in the clean Siberian atmosphere. Two cases of possible nucleation with high concentration and N_3–70/N_70–200 ratios are reported for the September campaign, in the upper troposphere and in cloud outflow in the mid-troposphere. In the seasonal analysis, a FT N_3–70 maximum is found in July–August between 6 and 7 km altitude, with N_3–70 accounting for ～90% of N_3–200 supporting the hypothesis of in situ formation in the FT. A secondary FT maximum of N_3–70 was identified later in autumn. In the boundary layer, seasonally maximum N_3–70 concentrations were found over Novosibirsk in May and September, but not in summer, possibly due to scavenging by precipitations and a large condensational sink from biomass burning aerosols. Our dataset has a limited size resolution and no speciation capability; more investigation is thus required to understand the conditions leading to in situ nucleation processes in the Siberian air shed.     

Chemical composition of aerosols during a major biomass burning episode over northern Europe in spring 2006: Experimental and modelling assessments

The long-range transported smokes emitted by biomass burning had a strong impact on the PM_2.5 mass concentrations in Helsinki over the 12 days period in April and May 2006. To characterize aerosols during this period, the real-time measurements were done for PM_2.5, PM_2.5–10, common ions and black carbon. Moreover, the 24-h PM₁ filter samples were analysed for organic and elemental carbon (OC and EC), water-soluble organic carbon (WSOC), ions and levoglucosan. The Finnish emergency and air quality modelling system SILAM was used for the forecast of the PM_2.5 concentration generated by biomass burning. According to the real-time PM_2.5 data, the investigated period was divided into four types of PM situations: episode 1 (EPI-1; 25–29 April), episode 2 (EPI-2; 1–5 May), episode 3 (EPI-3; 5–6 May) and a reference period (REF; 24 March–24 April). EPI-3 included a local warehouse fire and therefore it is discussed separately. The PM₁ mass concentrations of biomass burning tracers—levoglucosan, potassium and oxalate—increased during the two long-range transport episodes (EPI-1 and EPI-2). The most substantial difference between the episodes was exhibited by the sulphate concentration, which was 4.9 (±1.4) μg m⁻³ in EPI-2 but only 2.4 (±0.31) μg m⁻³ in EPI-1 being close to that of REF (1.8±0.54 μg m⁻³). The concentration of particulate organic matter in PM₁ was clearly higher during EPI-1 (11±3.3 μg m⁻³) and EPI-2 (9.7±4.0 μg m⁻³) than REF (1.3±0.45 μg m⁻³). The long-range transported smoke had only a minor impact on the WSOC-to-OC ratio. According to the model simulations, MODIS detected the fires that caused the first set of concentration peaks (EPI-1) and the local warehouse fire (EPI-3), but missed the second one (EPI-2) probably due to dense frontal clouds.