Size-segregated Aerosol Research Articles

Fourier transform infrared spectroscopy of size-segregated aerosol deposits on foil substrates

A method based on Fourier transform infrared (FTIR) double-pass transmittance spectroscopy was developed for determining functional group loading in size-segregated ambient aerosol deposits. The impactor employed for sample collection utilized rotating stages, which produced uniform particulate matter (PM) deposits on standard Al foil substrates. Each sample was analyzed without extraction using an FTIR spectrometer equipped with a reflectometer accessory. The use of the reflectometer obviated the need for infrared window materials as substrates. (NH(4))(2)SO(4) aerosol generated under laboratory conditions were used to calibrate deposit mass to the band strength of the relatively isolated nu(4) bending mode of SO(2-)(4) centered near 620 cm(-1). Atmospheric PM was sampled during the summer of 2004 in Huntsville, Ala. Sulfate concentrations determined in this initial study correlated well with measurements made by collocated EPA air samplers.

Carbonaceous materials in size-segregated atmospheric aerosols from urban and coastal-rural areas at the Western European Coast

A high-volume cascade impactor, equipped with a PM10 inlet, was used to collect size-segregated aerosol samples during the summer of 2004 at two Portuguese locations: a coastal-rural area (Moitinhos) and an urban area (Oporto). Concentrations of airborne particulate matter (PM), total carbon (TC), organic carbon (OC), elemental carbon (EC), and water-soluble organic carbon (WSOC) were determined for the following particle size ranges: < 0.49, 0.49–0.95, 0.95–3.0, and 3.0–10 µm. The total PM mass concentrations at the urban and coastal-rural sites ranged from 22.8 to 79.6 μg m − 3 and 19.9 to 28.2 μg m − 3 , respectively, and more than 56% of the total aerosol mass was found in the fractions below 3.0 μm. At both locations the highest concentrations of OC and EC were found in the submicrometer size range. The regional variability for the OC and EC concentrations, with the highest concentrations being found in the urban area, was related to the contribution of local primary sources (mostly traffic emissions). It was also verified an enrichment of the small size particles in WSOC, representing on average 37.3(± 12.4)% and 59.7(± 18.0)% of OC in the very fine aerosol at the coastal-rural and urban areas, respectively. The amount of secondary OC calculated by the minimum OC/EC ratio method indicates that secondary organic aerosol formation was important throughout the study at both sites. The obtained results suggest that long-range transport and favourable summer conditions for photochemical oxidation are key factors determining secondary OC formation in the coastal-rural and urban areas. The ultraviolet absorption properties of the chromophoric constituents of the WSOC fractions were also different among the different particle size ranges and also between the two sampling locations, thus suggesting the strong impact of the diverse emission sources into the composition of the size-segregated organic aerosol.

Mass size distributions of water-soluble inorganic and organic ions in size-segregated aerosols over metropolitan Newark in the US east coast

To characterize the mass size distributions of water-soluble inorganic and organic ions associated with urban particulate matter, a total of 15 sets of size-segregated aerosol samples were collected by a 10-stage Micro-Orifice Uniform Deposit Impactor (MOUDI) in the urban area of Newark in New Jersey from July to December 2006. The mass concentrations of PM1.8 accounted for ∼68% of the mass concentrations of PM10. The mass concentrations of the total water-soluble ions in PM1.8 accounted for 31–81% of the mass concentrations of PM1.8. Sulfate was the dominant ion in fine particles, accounting for 31% of the PM1.8 mass with its dominant mode at 0.32–0.56μm throughout all the samples. Nitrate size distributions were bi-modal, peaking at 0.32–0.56 and 3.2–5.6μm, and the shift of the nitrate dominant fraction between fine and coarse modes was affected by temperature. The ratios of nitrate to PM1.8 varied significantly, 0.5–27%. The C2–C4 dicarboxylic acids accounted for 1.9±0.9% of PM1.8 mass, with oxalate being the dominant ion. The size distributions of oxalate exhibited two to four modes with the dominant one at 0.32–0.56μm. Chloride existed in both coarse and fine modes, suggesting the influence of sea-salt aerosol and anthropogenic emissions. A crucial formation mechanism for the mass size distributions of these ions observed at this location is likely to be a combination of the gas-to-particle conversion and in-cloud/fog processing.

여름 및 겨울철 광주지역 대기 에어로졸 입자의 유기탄소 특성

To characterize organic and elemental carbon (OC and EC), and water-soluble organic carbon (WSOC) contents, daily measurements were performed in August 2006 (summer) and Jan 12 2007 (winter) at an urban site of Gwangju. Daily size-segregated aerosol samples were also collected for WSOC analysis. No clear seasonal variations in EC and WSOC concentrations were observed, while seasonal differences in OC concentration, and OC/EC and WSOC/EC ratios were shown. The WSOC/OC ratio showed higher value in summer (0.56) than in winter (0.40), reflecting the greater enhancement of secondary WSOC formation at the site in summer. Secondary WSOC concentrations estimated using EC tracer method were in the range (average ) and , respectively, accounting for (average 16.8%) and (average 13.1 %) of the measured WSOC concentrations in summer and winter. Sometimes higher WSOC/OC ratio in winter than that in summer could be attributed to two reasons. One is that the stable atmospheric condition often appears in winter, and the prolonged residence time would strengthen atmospheric oxidation of volatile organic compounds. The other is that decrease of ambient temperature in winter would enhance the condensation of volatile secondary WSOC on pre-existing aerosols. In summertime, atmospheric aerosols and WSOC concentrations showed bimodal size distributions, peaking at the size ranges (condensation mode) and (coarse mode), respectively. During the wintertime, atmospheric aerosols showed a bimodal character, while WSOC concentrations showed a unimodal pattern. Size distributions of atmospheric aerosols and WSOC with a peak in the size range were observed for most of the measurement periods. On January 17, however, atmospheric aerosols and WOSC exhibited size distributions with modal peaks in the size range , suggesting that the aerosol particles collected on that day could be expected to be more aged, i.e, longer residence time, than the aerosols at other sampling periods.

Chemical Characterization and Source Apportionment of Size-Segregated Aerosol Collected at an Urban Site in Sicily

Aerosol samples were collected at Catania (Italy), from 16 March to 13 June 2005. The sampling was performed using a low pressure five-stage Berner cascade impactor. The samples were analysed for total aerosol mass, Water Soluble Organic Carbon (WSOC), Total Carbon (TC) and main inorganic ionic species. The Water-Insoluble Carbon (WINC) was derived by the difference: TC-WSOC. The samples share some common features: ammonium sulphate and carbon-containing species (both soluble and insoluble) are the largest contributors of fine particle mass, while coarse particles essentially consist of sea-salt, sodium nitrate and unaccounted PM (probably crustal material). The WINC/WSOC ratio decreases from the smallest size range to the large accumulation mode range (0.42–1.2 μm), while the $${\text{nssSO}}^{ = }_{4} $$ and $$ {\text{NH}}^{ + }_{4} $$ contribution rises. The water-insoluble carbonaceous matter is the dominant component in the smallest particles (0.05–0.14 μm). We identified four different aerosol types, corresponding to different sources, contributing to the total particles load of the investigated urban environment: vehicular traffic, producing primary carbonaceous insoluble particles, secondary aerosols, dominating the composition of accumulation mode particles, and marine particles and mineral dust (both important components of the coarse aerosol fraction).

Organic composition of PM 2.5 and size-segregated aerosols and their sources during the 2002 Bay Regional Atmospheric Chemistry Experiment (BRACE), Florida, USA

PM 2.5 and size-segregated aerosols were collected in May 2002 as part of the Bay Regional Atmospheric Chemistry Experiment (BRACE), Florida, USA. Aerosol organic composition was used to estimate sources of a series of alkanes and polycyclic aromatic hydrocarbons (PAHs) using chemical indices, hierarchical cluster analysis (HCA) and a chemical mass balance receptor model (CMB). Aerosols were collected on quartz fiber filters (QFF) using a PM 2.5 high volume sampler and on aluminum foil discs using a Micro-Orifice Uniform Deposit Impactor (MOUDI, 50% aerodynamic cut diameters were 18, 10, 5.6, 3.2, 1.8, 1.0, 0.56, 0.315 and 0.171 μm). Target compounds included alkanes and PAHs and were solvent extracted using a mixture of dichloromethane, acetone and hexane, concentrated and then analyzed using a gas chromatograph/mass spectrometer (GC/MS). The target compounds in PM 2.5 were dominated by six sources during the study period: mobile sources (39±5%), coal burning (33±5%), biogenic primary emission (20±2%), oil combustion (5±2%), biomass burning (1.0±0.3%) and an unidentified source (3±2%). Results obtained from the chemical indices, HCA and CMB were in very good agreement with each other. PAH size distributions are presented for days dominated by a same source. Seventy-five percent and 50% of the PAH were found below 1.8 and 0.56 μm, respectively (monthly PAH geometric diameters averaged 0.43 μm). Coarse size PAHs were observed on 1 day (15 May) and were correlated with nitrate and sodium size distribution. It is hypothesized that the PAHs, sodium and nitrate were internally mixed and that the PAHs deposited onto a pre-existing marine aerosol. This transfer process has significant implications for PAH deposition and lifetime and warrants further study.

Iron, manganese and copper emitted by cargo and passenger trains in Zürich (Switzerland): Size-segregated mass concentrations in ambient air

Particle emissions caused by railway traffic have hardly been investigated in the past, due to their obviously minor influence on air quality compared to automotive traffic. In this study, emissions related to particle abrasion from wheels and tracks were investigated next to a busy railway line in Zürich (Switzerland), where trains run nearly exclusively with electrical locomotives. Hourly size-segregated aerosol samples (0.1–1, 1–2.5 and 2.5–10 μm) were collected with a rotating drum impactor (RDI) and subsequently analyzed by synchrotron radiation X-ray fluorescence spectrometry (SR-XRF). In this way, hourly elemental mass concentrations were obtained for chromium, manganese, iron and copper, which are the elements most relevant for railway abrasion. Additionally, daily aerosol filters were collected at the same site as well as at a background site for subsequent analysis by gravimetry and wavelength dispersive XRF (WD-XRF). Railway related ambient air concentrations of iron and manganese were calculated for the coarse (2.5–10 μm) and fine (<2.5 μm) particle fraction by means of a Mn/Fe ratio investigation. The comparison to train type and frequency data showed that 75% and 60% of the iron and manganese mass concentrations related to cargo and passenger trains, respectively, were found in the coarse mode. The railway related iron mass concentration normalized by the train frequency ranges between 10 and 100 ng m −3 h iron in 10 m distance to the tracks, depending on train type. It is estimated that the personal exposure next to a busy railway line above ground is more than a magnitude lower than inside a subway station.

Polycyclic aromatic hydrocarbons in bulk PM2.5 and size-segregated aerosol particle samples measured in an urban environment

To analyze polycyclic aromatic hydrocarbons (PAHs) at an urban site in Seoul, South Korea, 24-hr ambient air PM2.5 samples were collected during five intensive sampling periods between November 1998 and December 1999. To determine the PAH size distribution, 3-day size-segregated aerosol samples were also collected in December 1999. Concentrations of the 16 PAHs in the PM2.5 particles ranged from 3.9 to 119.9 ng m(-3) with a mean of 24.3 ng m(-3). An exceptionally high concentration of PAHs( approximately 120 ng m(-3)) observed during a haze event in December 1999 was likely influenced more by diesel vehicle exhaust than by gasoline exhaust, as well as air stagnation, as evidenced by the low carbon monoxide/elemental carbon (CO/EC) ratio of 205 found in this study and results reported by previous studies. The total PAHs associated with the size-segregated particles showed unimodal distributions. Compared to the unimodal size distributions of PAHs with modal peaks at < 0.12 microm measured in highway tunnels in Los Angeles (Venkataraman and Friedlander, 1994), four- to six-ring PAHs in our study had unimodal size distributions, peaking at the larger size range of 0.28-0.53 microm, suggesting the coagulation of freshly emitted ultrafine particles during transport to the sampling site. Further, the fraction of PAHs associated with coarse particles(> 1.8 microm) increased as the molecular weight of the PAHs decreased due to volatilization of fine particles followed by condensation onto coarse particles.

Size Distribution Characteristics of Elemental Carbon Emitted from Chinese Vehicles: Results of a Tunnel Study and Atmospheric Implications

The size distribution characteristics of elemental carbon (EC) emissions from Chinese vehicles have not been previously described. In this study, we collected size-segregated aerosol samples using a 10-stage MOUDI sampler (0.056-18 microm) in the Zhujiang tunnel, a roadway tunnel in the urban area of Guangzhou, China. The samples were analyzed for EC, organic carbon (OC), and inorganic ions. Fine particles had an OC/EC ratio of 0.57, indicating a dominant contribution of EC from diesel vehicles. Both EC and OC showed a dominant accumulation mode with a mass median aerodynamic diameter (MMAD) of 0.42 microm. In comparison, studies available in the literature typically reported a much lower MMAD for EC (approximately 0.1 microm) in vehicular emissions in North America. A theoretical analysis indicated that the larger EC particles observed in this study could not have resulted from after-emission growth processes (i.e., water accretion, coagulation, and vapor condensation). This leaves operating conditions such as high engine loads and low combustion efficiencies, which are more prevalent in diesel-fueled Chinese vehicles, as a more plausible inherent reason for producing the larger EC agglomerates. While fresh 0.1 microm EC particles are unlikely to act as cloud condensation nuclei (CCN), calculations showed that EC particles as large as 0.42 microm are effective CCN at atmospherically relevant critical supersaturation values of less than 1%. As a result, fresh EC particles from Chinese vehicle emissions could readily undergo cloud processing and form internal mixtures with sulfate in the residue droplet mode particles. This prediction is consistent with observations that EC frequently showed a dominant droplet mode in urban atmospheres in this region. The internal mixing of EC with highly hygroscopic sulfate would facilitate its removal by wet deposition and shorten its lifetime in the atmosphere. In addition, the light-absorbing capabilities of EC particles could also be enhanced due to their internal mixing with sulfate. Numerical aerosol models need to take these factors into consideration for better predictions of the behaviors and effects of urban aerosols in China.

Size-segregated aerosol chemical composition at a boreal site in southern Finland, during the QUEST project

Abstract. Size-segregated aerosol samples were collected during the QUEST field campaign at Hyytiälä, a boreal forest site in Southern Finland, during spring 2003. Aerosol samples were selectively collected during both particle formation events and periods in which no particle formation occurred. A comprehensive characterisation of the aerosol chemical properties (water-soluble inorganic and organic fraction) and an analysis of the relevant meteorological parameters revealed how aerosol chemistry and meteorology combine to determine a favorable "environment" for new particle formation. The results indicated that all events, typically favored during northerly air mass advection, were background aerosols (total mass concentrations range between 1.97 and 4.31 µg m-3), with an increasingly pronounced marine character as the northerly air flow arrived progressively from the west and, in contrast, with a moderate SO2-pollution influence as the air arrived from more easterly directions. Conversely, the non-event aerosol, transported from the south, exhibited the chemical features of European continental sites, with a marked increase in the concentrations of all major anthropogenic aerosol constituents. The higher non-event mass concentration (total mass concentrations range between 6.88 and 16.30 µg m-3) and, thus, a larger surface area, tended to suppress new particle formation, more efficiently depleting potential gaseous precursors for nucleation. The analysis of water-soluble organic compounds showed that clean nucleation episodes were dominated by aliphatic biogenic species, while non-events were characterised by a large abundance of anthropogenic oxygenated species. Interestingly, a significant content of α-pinene photo-oxidation products was observed in the events aerosol, accounting for, on average, 72% of their WSOC; while only moderate amounts of these species were found in the non-event aerosol. If the organic vapors condensing onto accumulation mode particles are responsible also for the growth of newly formed thermodynamically stable clusters, our finding allows one to postulate that, at the site, α-pinene photo-oxidation products (and probably also photo-oxidation products from other terpenes) are the most likely species to contribute to the growth of nanometer-sized particles.

Determination of sterols, estrogens and inorganic ions in waste water and size-segregated aerosol particles emitted from waste water treatment

Concentrations of steroids and inorganic ions were measured in waste water of an aerated sand trap as well as in aerosol particles emitted from this tank at the waste water treatment plant (WWTP) of Bayreuth, Germany, in January and February 2003. The investigations comprised seven sterols, two estrogens, and several inorganic ions. Since an appropriate method for the determination of sterols in waste water was not available, a new method based on solid phase extraction was developed. The concentrations of the sterols coprostanol and cholesterol amounted to 30–180 μg l −1 in waste water and to 400–5000 pg m −3 in aerosol particles. All other sterols were present in markedly lower concentrations. The mean concentrations of the two estrogens estrone and 17β-estradiol were about 165 pg m −3 in aerosol particles. The steroid concentrations in both waste water and aerosol particles varied greatly over time, however with the exception of coprostanol, no clear correlation was detected between concentrations in waste water and aerosol particles.

Chemical composition and hygroscopic properties of size-segregated aerosol particles collected at the Adriatic coast of Slovenia

The chemical composition as well as the water uptake characteristics of aerosols was determined in size-segregated samples collected during November 2002 on the Slovenian coast. Major ions, water-soluble organic compounds (WSOC), short-chain carboxylic acids and trace elements were determined in the water-soluble fraction of the aerosol. Total aerosol black carbon (BC) was measured from filter samples. Our results showed that the origin of air masses is an important factor that controls the variation in the size distribution of the main components. Very high concentrations of WSOC as well as higher concentrations of BC were found under mostly continental influence. Besides the main ionic species ( SO 4 2 - , NH 4 + , K +) in the finest size fraction (0.17–0.53 μm), the concentration of NO 3 - was also high. The difference between the two different air mass origins is particularly expressed for Cl −, Na +, Mg 2+ and Ca 2+ determined in particles larger than 1.6 μm. As expected, a very good correlation was found between Na + and Cl −. A good correlation was found between sea salt elements and elements of crustal origin (Na +, Cl −, Mg 2+, Ca 2+, Sr). A good relationship between typical anthropogenic tracers (K, V and Pb) was also observed. The mass growth factors, for all size fractions of aerosols collected under continental influence were very low (maximum 2.23 at 94%, 1.6–5.1 μm), while under marine influence the mass growth factors increased significantly with the particle size. At 97% humidity, the mass growth factors were 6.95 for the size fraction 0.53–1.6 μm and 9.78 for larger particles (1.6–5.1 μm).

The seasonal variabilities in the concentration of atmospheric aerosols over Qingdao, China

Mass concentrations of Total Suspended Particles (TSP) and size-segregated particles were obtained from July 2001 to June 2002 in Qingdao to characterize the seasonal variations of atmospheric aerosols and to show the impact of dust events on the air quality in Qingdao. Data on size-segregated aerosols show that 73.74% of the TSP mass concentration is contributed by particles with diameters less than 11µm. Particles with diameters less than 1.1µm have a higher concentration during the winter. In spring, larger particles tend to have higher mass concentrations. Bimodal particle size distributions have been observed, with maxima around 4.7–7µm and 0.43–0.65µm in the winter season, and 7–11µm and 0.65–1.lµm in the autumn season. Measurements made during the dust events in March 2002 show high concentrations of particles in the size range 2.1–7µm.

Analysis of Sulfur in Deposited Aerosols by Thermal Decomposition and Sulfur Dioxide Analyzer

A thermal decomposition method that measures aerosol sulfur at the nanogram level directly from the collection substrate is described. A thermal decomposition apparatus was designed. A stainless steel strip was used as the aerosol collection substrate. A 0.1 mol/L MnCl2 solution was added as the thermal decomposition catalyst. Currents were passed through the strip where aerosol particles had been deposited. In this way, the strip was heated at 780 +/- 10 degrees C, and particulate sulfur was evaporated. A sulfur dioxide analyzer (SDA) with flame photometric detector (FPD) was used to detect gaseous sulfur. High sulfur recoveries from (NH4)2SO4 and other inorganic sulfates, such as NH4HSO4, K2SO4, MgSO4, and CaSO4, were obtained. From the sulfur blank and the calibration, a lower limited detection of 0.2 ng of sulfur and the determination range of 3.3-167 ng of sulfur were estimated. The method is effective for measuring the sulfate size distributions of urban aerosols in a small sample air volume of 50-60 L. The method is applicable to measuring the sulfur in aqueous extracts of size-segregated urban aerosols collected by impactor and comparing the results with the sulfate data measured by ion chromatography.

Source estimation of anthropogenic aerosols collected by a DRUM sampler during spring of 2002 at Gosan, Korea

Abstract Continuous measurements of trace elements in size-segregated aerosols were performed by X-ray fluorescence analysis in order to assess the transport of natural and anthropogenic aerosol sources to Gosan site in springtime 2002. Various aerosol elemental components (such as Al, Ca, Si, K, Ti, and Mn) had low enrichment factors close to one and exhibited a similar temporal variation in coarse particle range (2.5–12 μm) indicating the dominant influence of soil dust. On the other hand, in the fine particle size range (0.09–2.5 μm), several anthropogenic elements in ambient particles (such as As, Se, Zn, Pb, and Cu) revealed high enrichment (100 Factor analysis was conducted with the size- and time-resolved aerosol composition data in order to identify anthropogenic sources of aerosols during non-Asian dust (NAD) period. As a result, it was found that the coarse particles were mainly influenced by soil, seasalt, coal combustion, and metallurgical emission and fine particles were significantly attributed to several anthropogenic sources, such as oil combustion, nonferrous metal source, and municipal incineration. Especially, submicron particles (0.09–0.56 μm) were further impacted by motor vehicles and field burning emissions in addition to those sources.

Size-segregated characterization of atmospheric aerosols in Taipei during Asian outflow episodes

To investigate the size resolved physico-chemical properties of aerosols transported from the eastern Asian Continent following the winter monsoons, an intensive field experiment was conducted in Taipei, Taiwan in January 2003. Mass concentration and the abundance of major functional groups in size-segregated aerosol samples were measured. The size distribution parameters of the aerosol mass and the associated functional groups were analyzed. The results of the infrared spectroscopic analysis showed that the contents of sulfate and ammonium in Taipei's sub-micron aerosols were significantly increased due to continental pollution. In addition, the Asian outflow aerosols contained higher levels of silicate and nitrate in the super-micron size range. The size-resolved functionalities of the aerosol samples collected during Asian outflow episodes were compared with those of local pollution samples. This comparison revealed that ammonium and sulfate generally coexisted in sub-micron aerosols, suggesting that there are substantial emissions of ammonia to neutralize the acidity of sulfate in the region from eastern China to northern Taiwan. The nitrate had a bimodal distribution over the size range between 0.1 and 10 μm. A significant enhancement was found in coarse nitrate during the Asian outflow episodes, implying heterogeneous reactions of nitric acid on the surface of dust and sea salt particles during the transport processes. High concentrations of sub-micron ammonium nitrate aerosols were observed prior to the frontal passage and served as an important indicator of local photochemical pollution. The aliphatic and carbonyl carbon in the aerosols were suggested as being mostly due to local pollution in Taipei. Nevertheless, our analysis found that the aerosols of Asian outflows had a higher infrared absorbance ratio of carbonyl to aliphatic carbon than those of the local pollution. The enrichment of carbonyl carbon in the Asian outflow aerosols is most likely due to the emissions of biomass burning aerosols in the eastern China.

The water-soluble organic component of size-segregated aerosol, cloud water and wet depositions from Jeju Island during ACE-Asia

Abstract Water-soluble organic compounds (WSOC) in size-segregated aerosol samples, cloud water and wet depositions collected at Jeju Island during the ACE-Asia experiment were characterized in terms of main functional groups and chemical classes by means of liquid chromatography coupled with total organic carbon (TOC) analysis, and by nuclear magnetic resonance (NMR) spectroscopy. The surface-active character of the WSOC was also deduced by measuring the surface tension (ST) of cloud water, wet depositions and aerosol water-extracts as a function of WSOC concentration. WSOC in the size-segregated aerosol samples at the Kosan coastal site show the occurrence of oxidized species, enriched in the accumulation mode, and a functional group composition characteristic of continental industrialized areas, with a small impact from biomass burning sources. The chemical classes identified and quantified by liquid chromatography accounted for 88% of the water-soluble organic carbon on average. The relatively high proportion of polycarboxylic acids (PA, 33–40% of total characterized WSOC) observed throughout the campaign indicates the persistence of WSOC from continental pollution sources in air masses which spent up to 5 days in the marine boundary layer (MBL) of the Yellow Sea. The analysis of cloud water and wet depositions collected at a mountain site in Jeju shows substantial differences in the organic composition between the cloud/rainwater solutes and the water-soluble fraction of the aerosol at the coastal site, with PA acids contributing to cloud water WSOC in far smaller proportions (6–11%) than in the fine MBL aerosols. It was proposed that such differences resulted from the entrainment of aerosol particles from the lifted layers into the stratiform clouds. Following this hypothesis, the saturated aliphatic compounds determined by NMR analysis in the rainwater samples, which are also responsible for the observed significant ST lowering, were scavenged above the boundary layer and had possibly been transported from the continent in the lifted layers. The hypothesized features of the vertical profiles in the inorganic and organic aerosol chemical composition are in agreement with the results of aircraft-based measurements performed during the same experiment.

A box model study on photochemical interactions between VOCs and reactive halogen species in the marine boundary layer

Abstract. A new chemical scheme is developed for the multiphase photochemical box model SEAMAC (size-SEgregated Aerosol model for Marine Air Chemistry) to investigate photochemical interactions between volatile organic compounds (VOCs) and reactive halogen species in the marine boundary layer (MBL). Based primarily on critically evaluated kinetic and photochemical rate parameters as well as a protocol for chemical mechanism development, the new scheme has achieved a near-explicit description of oxidative degradation of up to C3-hydrocarbons (CH4, C2H6, C3H8, C2H4, C3H6, and C2H2) initiated by reactions with OH radicals, Cl- and Br-atoms, and O3. Rate constants and product yields for reactions involving halogen species are taken from the literature where available, but the majority of them need to be estimated. In particular, addition reactions of halogen atoms with alkenes will result in forming halogenated organic intermediates, whose photochemical loss rates are carefully evaluated in the present work. Model calculations with the new chemical scheme reveal that the oceanic emissions of acetaldehyde (CH3CHO) and alkenes (especially C3H6) are important factors for regulating reactive halogen chemistry in the MBL by promoting the conversion of Br atoms into HBr or more stable brominated intermediates in the organic form. The latter include brominated hydroperoxides, bromoacetaldehyde, and bromoacetone, which sequester bromine from a reactive inorganic pool. The total mixing ratio of brominated organic species thus produced is likely to reach 10-20% or more of that of inorganic gaseous bromine species over wide regions over the ocean. The reaction between Br atoms and C2H2 is shown to be unimportant for determining the degree of bromine activation in the remote MBL. These results imply that reactive halogen chemistry can mediate a link between the oceanic emissions of VOCs and the behaviors of compounds that are sensitive to halogen chemistry such as dimethyl sulfide, NOx, and O3 in the MBL.

Size-segregated aerosol mass closure and chemical composition in Monte Cimone (I) during MINATROC

Abstract. Physical and chemical characterizations of the atmospheric aerosol were carried out at Mt. Cimone (Italy) during the 4 June-4 July 2000 period. Particle size distributions in the size range 6nm-10µm were measured with a differential mobility analyzer (DMA) and an optical particle counter (OPC). Size-segregated aerosol was sampled using a 6-stage low pressure impactor. Aerosol samples were submitted to gravimetric and chemical analyses. Ionic, carbonaceous and refractory components of the aerosol were quantified. We compared the sub- and superµm aerosol mass concentrations determined by gravimetric measurements (mGM), chemical analyses (mmCA), and by converting particle size distribution to aerosol mass concentrations (mmSD). Mean random uncertainties associated with the determination of mmGM, mmCA, and mmSD were assessed. The three estimates of the sub-µm aerosol mass concentration agreed, which shows that within experimental uncertainty, the sub-µm aerosol was composed of the quantified components. The three estimates of the super-µm aerosol mass concentration did not agree, which indicates that random uncertainties and/or possible systematic errors in aerosol sampling, sizing or analyses were not adequately accounted for. Aerosol chemical composition in air masses from different origins showed differences, which were significant in regard to experimental uncertainties. During the Saharan dust advection period, coarse dust and fine anthropogenic particles were externally mixed. No anthropogenic sulfate could be found in the super-µm dust particles. In contrast, nitrate was shifted towards the aerosol super-µm fraction in presence of desert dust.

Multicomponent aerosol dynamics model UHMA: model development and validation

Abstract. A size-segregated aerosol dynamics model UHMA (University of Helsinki Multicomponent Aerosol model) was developed for studies of multicomponent tropospheric aerosol particles. The model includes major aerosol microphysical processes in the atmosphere with a focus on new particle formation and growth; thus it incorporates particle coagulation and multicomponent condensation, applying a revised treatment of condensation flux onto free molecular regime particles and the activation of nanosized clusters by organic vapours (Nano-Köhler theory), as well as recent parameterizations for binary H2SO4-H2O and ternary H2SO4-NH3-H2O homogeneous nucleation and dry deposition. The representation of particle size distribution can be chosen from three sectional methods: the hybrid method, the moving center method, and the retracking method in which moving sections are retracked to a fixed grid after a certain time interval. All these methods can treat particle emissions and atmospheric transport consistently, and are therefore suitable for use in large scale atmospheric models. In a test simulation against an accurate high resolution solution, all the methods showed reasonable treatment of new particle formation with 20 size sections although the hybrid and the retracking methods suffered from artificial widening of the distribution. The moving center approach, on the other hand, showed extra dents in the particle size distribution and failed to predict the onset of detectable particle formation. In a separate test simulation of an observed nucleation event, the model captured the key qualitative behaviour of the system well. Furthermore, its prediction of the organic volume fraction in newly formed particles, suggesting values as high as 0.5 for 3–4 nm particles and approximately 0.8 for 10 nm particles, agrees with recent indirect composition measurements.