Droplet Mode Research Articles

A comparison study on airborne particles during haze days and non-haze days in Beijing

Airborne particles in Beijing during haze days and non-haze days were collected by an eleven-stage cascade impactor (MOUDI 110, MSP, USA), and the mass concentrations and water soluble inorganic ions of the size segregated airborne particles were quantitatively analyzed. PM10 concentrations during haze days ranged from 250.5 to 519.4μgm−3 which were about 3–8 times greater than those (ranged from 67.6 to 94.0μgm−3) during non-haze days, and PM1.8 concentrations during haze periods were in the range of 117.6–378.6μgm−3 which were 3–14 times higher than those (27.0 to 36.8μgm−3) during non-haze days. In comparison with non-haze days, all water soluble inorganic ions investigated in the airborne particles greatly enhanced during haze days. NH4+, NO3− and SO42− were found to be the dominant water soluble inorganic ions, accounting for 91–95% of the total inorganic ions in PM1.8 during haze days, and 73–81% during non-haze days. The size distributions of SO42−, NO3−, Cl−, K+ and Na+ exhibited bimodal types, while single mode was found for NH4+, Ca2+ and Mg2+. Only with exception of Ca2+ and Mg2+, all ions were concentrated in fine particles around 0.56–1.0μm of “droplet mode” during haze days, while 0.32–0.56μm of “condensation mode” during non-haze days. The extremely high mole ratio (>2) of [NH4+]/[SO42−] during haze days implied that the main form of ammonium in PM1.8 might be (NH4)2SO4 and NH4NO3. The mass ratio of NO3−/SO42− was >1 in PM1.8 during haze days and ~1 during non-haze days, indicating that NOx from the vehicle exhaust in Beijing is playing more and more important role on fine particle formation.

Size-resolved aerosol ionic composition and secondary formation at Mount Heng in South Central China

To understand the size-resolved aerosol ionic composition and the factors influencing secondary aerosol formation in the upper boundary layer in South Central China, size-segregated aerosol samples were collected using a micro-orifice uniform deposit impactor (MOUDI) in spring 2009 at the summit of Mount Heng (1269 m asl), followed by subsequent laboratory analyses of 13 inorganic and organic water-soluble ions. During non-dust-storm periods, the average PM1.8 concentration was 41.8 μg·m-3, contributing to 55% of the PM10. Sulfates, nitrates, and ammonium, the dominant ions in the fine particles, amounted to 46.8% of the PM1.8. Compared with Mount Tai in the North China Plain, the concentrations of both fine and coarse particles and the ions contained therein were substantially lower. When the air masses from Southeast Asia prevailed, intensive biomass burning there led to elevated concentrations of sulfates, nitrates, ammonium, potassium, and chloride in the fine particles at Mount Heng. The air masses originating from the north Gobi brought heavy dust storms that resulted in the remarkable production of sulfates, ammonium, methane sulfonic acid, and oxalates in the coarse particles. Generally, the sulfates were primarily produced in the form of (NH4)2SO4 in the droplet mode via heterogeneous aqueous reactions. Only approximately one-third of the nitrates were distributed in the fine mode, and high humidity facilitated the secondary formation of fine nitrates. The heterogeneous formation of coarse nitrates and ammonium on dry alkaline dust surfaces was found to be less efficient than that on the coarse particles during non-dust-storm periods.

Size distribution of water-soluble components in particulate matter emitted from biomass burning

Size-resolved measurements of particulate matter (PM) emissions from 10 biomass materials (rice straw, soybean stem, green perilla stem, red pepper stem, pine needles, cherry leaves, cherry stem, maple leaves, gingko leaves and gingko stem) were conducted in a laboratory hood chamber environment using a 10-stage MOUDI. Samples were analyzed to determine the mass, water soluble organic carbon (WSOC), and water soluble inorganic species. This study examines how particle emissions and size distributions of chemical components vary with biomass materials. Mass fractions of water soluble organic mass (WSOM) (=1.6 × WSOC) and ionic species to the PM1.8 emissions varied significantly depending on the biomass type burned. The percent mass of WSOM in PM1.8 emissions ranged from 19.8% (green perilla stem) to 41.9% (red pepper stem) for agricultural crop residues, while the tree category accounted for 9.6% (gingko leaves) to 44.0% (gingko stem) of the PM1.8 emissions. Total ionic species contents in the PM1.8 mass ranged from 7.4% (rice straw) to 26.9% (green perilla stem) for the agricultural waste category, and 5.8% (maple leaves) to 23.5% (gingko stem) for the tree category. The ionic species fraction of the PM1.8 emission was dominated by K+, Cl−, and SO42−, while Ca2+ was important in the coarse mode particles (>3.1 μm). PM1.8 emissions of K+, Cl−, and SO42− were as high as 16.9%, 9.0%, and 5.8%, respectively, and were from the green perilla stem, red pepper stem, and gingko stem emissions.Normalized size distributions of mass, WSOC, K+, Cl−, SO42−, and oxalate in the biomass burning emissions showed a unimodal size distribution, peaking in the size ranges of 0.32–0.55 μm and 0.55–1.0 μm. Size-resolved PM mass fractions of WSOM, K+, Cl−, and SO42− showed fairly consistent distributions for each biomass type, with higher fractions in the ultrafine mode (<0.10 μm) and lower fractions in the accumulation mode of 0.32–1.0 μm. The size distributions of WSOC were strongly correlated (mostly R2 > 0.90) with those of K+ in the particle size range of <0.1 μm and 0.1–1.8 μm and the biomass types. Strong correlations between the concentrations of K+–Cl− and K+−SO42− were observed for the following size ranges; <0.1 μm, 0.1–1.0 μm, 1.0–1.8 μm, and 1.8–3.1 μm for most biomass burning emissions. Regression line slopes for K+/WSOC (i.e., mass of K+/mass of WSOC from biomass burning emissions) were not significantly changed for particle size and biomass type, but slopes for Cl−/K+ and SO42−/K+ varied significantly with the particle size (ultrafine, condensation, droplet, and coarse modes) and biomass type.

Chemical characteristics of size-resolved aerosols from Asian dust and haze episode in Seoul Metropolitan City

We collected aerosol particles in Seoul using a 10 stage Micro-Orifice Uniform Deposit Impactor (MOUDI) to investigate the size distributions of aerosol mass and water-soluble inorganic ions (Na+, NH4+, K+, Mg2+, Ca2+, Cl−, NO3−, and SO42−) for the two high-mass episodes taking place in February and April, 2009. The former was a heavy Asian dust (AD) event and the latter was a haze episode associated with stagnant condition that prevailed over the Yellow Sea region. In AD plume, the mass peak was noticeable at coarse mode between 1.0 and 1.8μm but SO42− and NH4+ were enriched in condensation mode between 0.056 and 0.1μm. There was little chance for the heavy AD plume to pick up SO2 and water vapor, which are in good accordance with its transport paths and the chemical characteristics of aerosols and gaseous species. These results imply that the heterogeneous reaction of SO2 on dust particles would not be substantial in determining sulfate concentrations for this particular type of dust plume, considering the possibility of loss of large soil particles in MOUDI. During the haze episode, both total aerosol mass and water-soluble inorganic ions showed bimodal size distributions with the droplet (0.32–0.56μm) and coarse (1.0–1.8μm) mode peaks. In this haze event, acidic gases tend to be dissolved more efficiently in larger particles, shifting the peaks of SO42− and NO3− to larger droplet particles. For NH4+, however, the mode change was not observed, which was probably due to the depleted source and high solubility of NH3. These results demonstrated that the availability of precursor gases such as SO2, NO2, and NH3, and the water-vapor contents were important factor to determine the formation of droplet-mode particles and their sizes.

Size distributions of aerosol sulfates and nitrates in Beijing during the 2008 Olympic Games: Impacts of pollution control measures and regional transport

For the 2008 Olympic Games, drastic control measures were implemented on industrial and urban emissions of sulfur dioxide (SO2), nitrogen oxides (NOx) and other pollutants to address the issues of poor air quality in Beijing. To investigate the effects of SO2 and NOx reductions on the particulate sulfate and nitrate concentrations as well as their size distributions, size-segregated aerosol samples were collected using micro-orifice uniform deposit impactors (MOUDIs) at urban and downwind rural sites in Beijing before and after full-scale controls. During the sampling period, the mass concentrations of fine particles (PM1.8) at the urban and rural sites were 94.0 and 85.9 μg m−3, respectively. More than 90% of the sulfates and ∼60% of nitrates formed as fine particles. Benefiting from the advantageous meteorological conditions and the source controls, sulfates were observed in rather low concentrations and primarily in condensation mode during the Olympics. The effects of the control measures were separately analyzed for the northerly and the southerly air-mass-dominated days to account for any bias. After the control measures were implemented, PM, sulfates, and nitrates were significantly reduced when the northerly air masses prevailed, with a higher percentage of reduction in larger particles. The droplet mode particles, which dominated the sulfates and nitrates before the controls were implemented, were remarkably reduced in mass concentration after the control measures were implemented. Nevertheless, when the polluted southerly air masses prevailed, the local source control measures in Beijing did not effectively reduce the ambient sulfate concentration due to the enormous regional contribution from the North China Plain.

The secondary formation of inorganic aerosols in the droplet mode through heterogeneous aqueous reactions under haze conditions

Secondary inorganic aerosols play important roles in visibility reduction and in regional haze pollution. To investigate the characteristics of size distributions of secondary sulfates and nitrates as well as their formation mechanisms under hazes, size-resolved aerosols were collected using a Micro-Orifice Uniform Deposit Impactor (MOUDI) at an urban site in Jinan, China, in all four seasons (December 2007–October 2008). In haze episodes, the secondary sulfates and nitrates primarily formed in fine particles, with elevated concentration peaks in the droplet mode (0.56–1.8 μm). The fine sulfates and nitrates were completely neutralized by ammonia and existed in the forms of (NH4)2SO4 and NH4NO3, respectively. The secondary formation of sulfates, nitrates and ammonium (SNA) was found to be related to heterogeneous aqueous reactions and was largely dependent on the ambient humidity. With rising relative humidity, the droplet-mode SNA concentration, the ratio of droplet-mode SNA to the total SNA, the fraction of SNA in droplet-mode particles and the mass median aerodynamic diameter of SNA presented an exponential, logarithmic or linear increase. Two heavily polluted multi-day haze episodes in winter and summer were analyzed in detail. The secondary sulfates were linked to heterogeneous uptake of SO2 followed by the subsequent catalytic oxidation by oxygen together with iron and manganese in winter. The fine nitrate formation was strongly associated with the thermodynamic equilibrium among NH4NO3, gaseous HNO3 and NH3, and showed different temperature-dependences in winter and summer.

Aerosol ionic components at Mt. Heng in central southern China: Abundances, size distribution, and impacts of long-range transport

Water-soluble ions in PM2.5 were continuously measured, along with the measurements of many other species and collection of size-resolved aerosol samples, at the summit of Mt. Heng in the spring of 2009, to understand the sources of aerosols in rural central southern China. The mean concentrations of SO42−, NH4+ and NO3− in PM2.5 were 8.02, 2.94 and 1.47μg/m3, indicating a moderate aerosol pollution level at Mt. Heng. Water-soluble ions composed approximately 40% of the PM2.5 mass on average. PM2.5 was weakly acidic with about 66% of the samples being acidic. SO42−, NO3− and NH4+ exhibited similar diurnal patterns with a broad afternoon maximum. SO42− and NH4+ were mainly present in the fine aerosols with a peak in the droplet mode of 0.56–1μm, suggesting the important role of cloud processing in the formation of aerosol sulfate. NO3− was largely distributed in the coarse particles with a predominant peak in the size-bin of 3.2–5.6μm. Long-distance transport of processed air masses, dust aerosols, and cloud/fog processes were the major factors determining the variations of fine aerosol at Mt. Heng. The results at Mt. Heng were compared with those obtained from our previous study at Mt. Tai in north China. The comparison revealed large differences in the aerosol characteristics and processes between southern and northern China. Backward trajectories indicated extensive transport of anthropogenic pollution from the coastal regions of eastern/northern China and the Pearl River Delta (PRD) to Mt. Heng in spring, highlighting the need for regionally coordinated control measures for the secondary pollutants.

Time resolved imaging of electrohydrodynamic jetting on demand induced by square pulse voltage

Electrohydrodynamic (EHD) printing is receiving much attention due to its high resolution printing capability, as well as its many advantages such as reduced manufacturing steps and non-toxic, flexible processes. However, many issues need be resolved before it can be widely used. In this work, to demonstrate EHD printing on demand, square pulse voltage signals are applied between a hole-type electrode and the nozzle supplied with working fluid from a reservoir at constant pressure. Continuous fine jet, pulsating fine jet, and pulsating droplet modes are observed depending on the voltage signal. To obtain the optimum voltage pulse duration, the change in the jetting start time is measured by varying the voltage configurations and hydrostatic pressure applied to the nozzle. Among the modes, the pulsating droplet mode is studied in detail for various voltage signals. Futhermore, the printing is demonstrated using the optimized voltage signal.

Enhanced trimethylamine-containing particles during fog events detected by single particle aerosol mass spectrometry in urban Guangzhou, China

Trimethylamine (TMA) plays an important role in atmospheric chemistry, yet its pathway towards aerosol is not clear. We report in this study the measurement of TMA in submicron particles using a single particle aerosol mass spectrometer (SPAMS) in urban Guangzhou, China for the period of 30 April through 22 May, 2010. The number fraction of TMA-containing particles relative to total detected particle number dramatically increased within the occurrence of fogs. It changed from an average of ∼7% for clear days to ∼35% for the fog events. For particles with size larger than 0.5 μm, the fraction could even account up to ∼60%. Averaged relative intensity of TMA was correlated well with relative humidity, indicating the important role of aerosol water content in the gas-to-particle partitioning of TMA. During the fog events, number based size distribution of TMA-containing particles shifted towards larger mode, peaking at droplet mode (0.5–1.2 μm), corresponding to the build-up of aerosol mass, suggesting the significant mass transfer of TMA and other semi-volatile species from gas to aerosol. Results also reveal that most of TMA-containing particles also contained signals from ammonium, nitrate and sulfate. A much larger fraction of TMA-containing particles was found to contain nitrate during the fog events than on clear days. During the fog events, nitrate and sulfate were more strongly associated with TMA compared to ammonia, which was oppositely observed on clear days. These results indicate that gas-to-particle partitioning of TMA in urban Guangzhou occurs preferentially during fog processing. The study could help improve our understanding in fog processing and potential roles of TMA-containing particles in urban Guangzhou.

Effect of aqueous-phase processing on aerosol chemistry and size distributions in Fresno, California, during wintertime

Environmental contextAqueous-phase processes in fogs and clouds can significantly alter atmospheric fine particles with consequences for climate and human health. We studied the influence of fog and rain on atmospheric aerosol properties, and show that aqueous-phase reactions contribute to the production of secondary aerosol species and change significantly the composition and microphysical properties of aerosols. In contrast, rains effectively remove aerosols and reduce their concentrations. AbstractSubmicrometre aerosols (PM1) were characterised in situ with a high resolution time-of-flight aerosol mass spectrometer and a scanning mobility particle sizer in Fresno, CA, from 9 to 23 January 2010. Three dense fog events occurred during the first week of the campaign whereas the last week was influenced by frequent rain events. We thus studied the effects of aqueous-phase processing on aerosol properties by examining the temporal variations of submicrometre aerosol composition and size distributions. Rains removed secondary species effectively, leading to low loadings of PM1 dominated by primary organic species. Fog episodes, however, increased the concentrations of secondary aerosol species (sulfate, nitrate, ammonium and oxygenated organic aerosol). The size distributions of these secondary species, which always showed a droplet mode peaking at ~500 nm in the vacuum aerodynamic diameter, increased in mode size during fog episodes as well. In addition, the oxygen-to-carbon ratio of oxygenated organic species increased in foggy days, indicating that fog processing likely enhances the production of secondary organic aerosol as well as its oxidation degree. Overall, our observations show that aqueous-phase processes significantly affect submicrometre aerosol chemistry and microphysics in the Central Valley of California during winter, responsible for the production of secondary inorganic and organic aerosol species and the formation of droplet mode particles, thus altering the climatic and health effects of ambient aerosols in this region.

Shear induced phase inversion of dilute smectic liquid crystal/polymer blends

The rheology and morphology of 8CB (4-octyl-4-biphenylcarbonitrile) and PDMS (polydimethylsiloxane) blends with different concentrations of 8CB were investigated. The blends showed gel-like behaviour when 8CB is in the smectic state during mixing, whose morphologies resemble the foam-like structure of a highly concentrated emulsion with 8CB as the continuous phase. Once formed, such a structure is stable when 8CB is in a smectic and nematic state but unstable when 8CB is in an isotropic state. The stability of the foam-like structure is ascribed to the elasticity of 8CB films, which is justified by the concentration dependence of the interfacial contribution to the plateau modulus, i.e., the yield stress of 8CB instead of the Laplace pressure controls the deformation of the PDMS droplet. A new mechanism of phase inversion is suggested based on the direct observation of the breakup mode of an 8CB droplet. The formation of the foam-like structure was the result of the erosion breakup of the 8CB droplets and the subsequent local phase inversion, where the erosion breakup is ascribed to the shear banding inside the droplet due to the yield stress of 8CB in the smectic state.

Effect of droplet heat content distribution on humping formation in high speed GMAW

1) MOE Key Laboratory for Liquid-Solid Structure Evolution and Materials Processing, Instituteof Materials Joining, Shandong University, Jinan 250061, China2) State Key Laboratory for Advanced Welding & Joining, Harbin Institute of Technology, Harbin 150001,China3) BAM Federal Institute for Materials Research and Testing, Division V.5 Safety of Joined Components,Berlin 12205, GermanyManuscript received 22 March 2011; in revised form 18 June 2011The momentum of strong backward °owing melt jet and the thermal action fromtransferred droplets are two dominating factors afiecting the formation of humpingbead in high speed gas metal arc welding (GMAW). Appropriate describing the in°u-ence of the distribution mode of droplet heat content in the weld pool is essential tounderstand the physical mechanism of humping bead formation. Based on the exper-imental results, four kinds of droplet heat content distribution modes are proposedand employed to calculate the transient evolution of the temperature fleld and weldpool during high speed GMAW process. Through making comparison of predictedand measured weld bead dimensions, a suitable and adaptive distribution mode ofdroplet heat content is found,

Flow Modes and Mode Transitions for Falling Films on Flat Tubes

Flow mode transitions for a liquid film falling over flat tubes are studied for a wide range of conditions. Hydraulic oil, ethylene glycol, ethylene-glycol-water mixtures (at three different volume ratios), and water are used as working fluids in experiments conducted under adiabatic conditions, without an imposed vapor flow. The flow modes are observed over a range of flow rate and tube spacing. The flow modes are more complex but similar to those manifested on round tubes, and include the sheet, sheet-jet, jet, jet-droplet, and droplet modes. However, the transitions at Ga1/4 ≤ 16 (mainly for hydraulic oil, ethylene glycol) are very different from those on round tubes: there is no sheet-jet mode during transition between the sheet and jet flow modes. Moreover, at all Ga, the Re range over which the jet mode occurs is narrower than that for round tubes, and the hysteresis in mode transitions is not as pronounced as for round tubes. The transitional Re increases slowly with tube spacing, also in contrast to observations of round-tube falling films. The flat-tube falling-film flow regimes are described, and new correlations are provided to relate the transitional Re number to Ga and geometry. By analyzing repeated experiments, it is shown that the standard deviation of the measurements from the correlations is less than 5.2%.

Secondary organic aerosol formation in cloud droplets and aqueous particles (aqSOA): a review of laboratory, field and model studies

Abstract. Progress has been made over the past decade in predicting secondary organic aerosol (SOA) mass in the atmosphere using vapor pressure-driven partitioning, which implies that SOA compounds are formed in the gas phase and then partition to an organic phase (gasSOA). However, discrepancies in predicting organic aerosol oxidation state, size and product (molecular mass) distribution, relative humidity (RH) dependence, color, and vertical profile suggest that additional SOA sources and aging processes may be important. The formation of SOA in cloud and aerosol water (aqSOA) is not considered in these models even though water is an abundant medium for atmospheric chemistry and such chemistry can form dicarboxylic acids and "humic-like substances" (oligomers, high-molecular-weight compounds), i.e. compounds that do not have any gas phase sources but comprise a significant fraction of the total SOA mass. There is direct evidence from field observations and laboratory studies that organic aerosol is formed in cloud and aerosol water, contributing substantial mass to the droplet mode. This review summarizes the current knowledge on aqueous phase organic reactions and combines evidence that points to a significant role of aqSOA formation in the atmosphere. Model studies are discussed that explore the importance of aqSOA formation and suggestions for model improvements are made based on the comprehensive set of laboratory data presented here. A first comparison is made between aqSOA and gasSOA yields and mass predictions for selected conditions. These simulations suggest that aqSOA might contribute almost as much mass as gasSOA to the SOA budget, with highest contributions from biogenic emissions of volatile organic compounds (VOC) in the presence of anthropogenic pollutants (i.e. NOx) at high relative humidity and cloudiness. Gaps in the current understanding of aqSOA processes are discussed and further studies (laboratory, field, model) are outlined to complement current data sets.

Characterization of water-soluble organic aerosol in coastal New England: Implications of variations in size distribution

Abstract Size distributions up to 10-micron aerosol diameter (DP) of organic carbon (OC) and water-soluble organic carbon (WSOC) were measured at two sites in coastal New England, slightly inland at Thompson Farm (TF) and offshore at Isles of Shoals (IOS). Significant OC concentrations were measured across the full size distribution at TF and IOS, respectively. The WSOC fraction (WSOC/OC) was largest in the accumulation mode with values of 0.86 and 0.93 and smallest in the coarse mode with values of 0.61 and 0.79 at TF and IOS, respectively. Dicarboxylic acids containing up to five carbon atoms (C5) were concentrated in droplet and accumulation mode aerosol with only minor contributions in the coarse mode. C1–C3 monocarboxylic acids were generally near or below detection limits. Results from proton nuclear magnetic resonance (H+-NMR) spectroscopy analyses showed that the organic functional group characterized by protons in the alpha position to an unsaturated carbon atoms ([H–C–C ]) was the dominant WSOC functionality at both TF and IOS, constituting 34 and 43% of carbon-weighted H+-NMR signal, respectively. Size distributions of each H+-NMR-resolved organic functionality are presented. Source apportionment using H+-NMR fingerprints is also presented, and results indicate that nearly all of the WSOC at TF and IOS spectroscopically resembled secondary organic aerosol, regardless of DP.

Mass size distributions of soluble sulfate, nitrate and ammonium in the Madrid urban aerosol

Abstract This paper analyzes the mass size distribution of some inorganic species present in the atmospheric aerosol from a field campaign carried out in Madrid throughout a complete year (February 2007–February 2008). Samplings were performed by means of a micro-orifice uniform deposit impactor (MOUDI). Ambient air was sampled during consecutive nocturnal and diurnal periods, and diurnal/nocturnal behaviors were compared for the twenty night-day sampling pairs that were gathered. Annual and seasonal averages were obtained, and some case studies under specific atmospheric conditions are discussed in the paper. Results have shown that the sulfate and ammonium mass was concentrated in the accumulation mode, between 0.18 and 0.56 μm, so that gas-phase and condensation processes for secondary aerosol formation prevailed during the sampling periods in this area. An exception to this behavior was found during a fog event when distributions for these two species were centered in the 0.56–1 and 1–1.8 μm size stages, corresponding to the droplet mode. In most of the samples, the ammonium mass measured in these size ranges was enough or almost enough to neutralize inorganic acidity by formation of ammonium sulfate and nitrate. However, a significant sulfate mass not neutralized by ammonium was found in the impactor backup quartz filter (aerodynamic diameter Regarding the nitrate concentration, it was found higher in the coarse mode than in the accumulation mode on an annual basis. The highest concentrations were measured in winter episodic situations. The marked seasonal variability shown in the accumulation mode by this species is related to the volatility of ammonium nitrate. Among all the studied ions, nitrate also presented the highest differences between the diurnal and nocturnal concentrations in all size ranges. This is probably related to the short time scale required for its formation from gaseous precursors.

Adaptive method of lines for multi-component aerosol condensational growth and CCN activation

Abstract. The process of formation of cloud droplets on an ensemble of aerosol particles is modelled by numerous investigators using the method of lines (MOL). The method involves discretisation of the aerosol size spectrum into bins whose positions evolve with time. One of the drawbacks of the method is its poor representation of the aerosol spectrum shape in the region between the unactivated aerosol mode and the activated droplet mode. An adaptive grid refinement procedure is introduced. The procedure splits any overly wide bins into several narrower ones during integration. The number of new bins added is a function of particle concentration in the bin being split. Application of the grid refinement procedure results in suppression of the sensitivity of the computed cloud droplet spectrum characteristics such as droplet number concentration or effective radius to the initial grid choice. A model of droplet formation on multi-component aerosol is formulated for the purpose of the study. Model formulation includes explicit treatment of the droplet temperature evolution. Several examples of the model set-up are used to demonstrate model capabilities. Model results are compared to those without adaptivity. A C++ implementation of the model is available as an electronic supplement of the paper.

Size Distributions of Polycyclic Aromatic Hydrocarbons at Two Receptor Sites in the Pearl River Delta Region, China: Implications of a Dominant Droplet Mode

Size distributions of twelve polycyclic aromatic hydrocarbons (PAHs), elemental carbon (EC), organic carbon (OC) and major inorganic ions were measured at two receptor locations in the Pearl River Delta (PRD) Region in southern China during 2006–2008. The size distributions of three- and four-ring PAHs were characterized with three modes, a condensation mode with a mass median aerodynamic diameter (MMAD) in the range of 0.2–0.3 μm, a droplet mode (MMAD: .7–1.0 μm), and a coarse mode (MMAD: 3–5 μm). The less volatile five- to six-ring PAHs were mainly distributed in the condensation mode and droplet mode, with little presence in the coarse mode. The droplet mode was the most prominent mode for all PAHs at both sites, accounting for more than 70% of the total particle-phase five- and six-ring PAHs and more than 60% of the total particle-phase three- and four-ring PAHs. The dominant droplet mode could be attributed to a result of in-cloud processing of vehicular soot particles and biomass burning particles during the transport of source aerosols to the receptor sites. Presence of sulfate aqueous coating on the droplet-mode particles was hypothesized to make PAHs inaccessible to volatilization from particle phase and to oxidation by gaseous oxidants. Further implications of a dominant droplet-mode PAHs were discussed.

Dominant Modes of Influenza Transmission: An Epidemiologic Perspective

S-29A8-2 Background/Aims: Influenza can be transmitted through respirable (small airborne particles), inspirable (intermediate size), large droplet, and contact modes. How these modes are affected by features of the virus strain (infectivity, survivability, transferability, or shedding profiles), host population (behavior, susceptibility, or shedding profiles), and environment (host density, surface area to volume ratios, or host movement patterns) have only recently come under investigation. There is much disagreement in the literature with regard to what the dominant route of influenza transmission is. Methods: A discrete-event, continuous-time, stochastic transmission model is constructed to analyze the environmental processes through which virus passes from one person to another via different transmission modes and explore which factors increase or decrease different modes of transmission. Using empirical estimates where possible, we vary 18 parameters that are relevant to the respiratory, inspiratory, droplet-spray, and contact-mediated transmission routes. These parameters are features of the agent, host, or environment. Results: With the exception of the inspiratory route, each route on its own can cause high transmission in isolation of other modes depending on the specific context. Mode-specific transmission is highly sensitive to parameter values. For example, droplet and respirable transmission usually required high host density, while the contact route had no such requirement. Conclusion: Depending on the specific context one or more modes may be sufficient to cause high transmission, while in other contexts no transmission may result. Because of this, when making intervention decisions which involve blocking environmental pathways, it is important to take into account the specific features of the system in question. Therefore, it should not be assumed a prior that interventions which target only one of these routes may be optimal in all settings.

Modal characteristics of carbonaceous aerosol size distribution in an urban atmosphere of South China

Size distributions can provide important information about aerosol sources, formation, and growth mechanisms. However, compared to size distributions of inorganic aerosols, size distributions of carbonaceous aerosols have been much less studied and reported in the literature. In this paper, we systematically measured size distributions of elemental carbon (EC), organic carbon (OC), oxalate, polycyclic aromatic hydrocarbons (PAHs), as well as major inorganic ions in urban aerosols in Shenzhen, China. Totally 24 sets of samples were collected using a ten-stage micro orifice uniform deposit impactor (MOUDI) during October 2009 to February 2010. Three lognormal modes contained in the size distributions of species were resolved based on positive matrix factorization (PMF) analysis of the measured dataset, corresponding to the condensation (peak=0.34μm), droplet (peak=0.84μm), and coarse (peak=5.4μm) modes, respectively. The mean concentrations of EC in the condensation, droplet, and coarse modes were 2.20, 1.18, and 0.64μgm−3, respectively, and the modal characteristics of EC indicate that fresher local combustion emissions contributed mostly to aerosol EC in the urban atmosphere of Shenzhen. The mean concentrations of OC in the condensation, droplet, and coarse modes were 2.29, 3.34, and 3.51μgm−3, respectively, and the modal characteristics of OC indicate that its sources were more primary in the condensation mode while more secondary in the droplet mode. The modal characteristics of aerosol oxalate and PAHs suggest that they were predominantly from in-cloud secondary formation and local emissions, respectively.