Condensation Particle Research Articles

Особенности и эффективность наносекундной лазерной абляции высокоуглеродистой стали сканирующим пучком импульсного излучения Yb:YAG лазера

The experiments results on the ablation of high-carbon (> 97 % Fe, 1.3 % C) and low-carbon (> 97 % Fe, 0.3 % C) steel targets by a nanosecond pulse laser radiation scanning beam are presented. The dependence of the depth and energy efficiency of ablation on the power density in the range q = 4·108 – 1010 W/cm2 has been determined. It has been established that the maximum efficiency of material removal is achieved at q = 4·109 W/cm2 for a high-carbon steel target and in the range q = 7·108 – 5·109 W/cm2 for a low-carbon steel target. The size distribution of ejected microparticles was determined. It has been established that a back flow of particles occurs upon irradiation of high-carbon steel and the flow origin mechanism is associated with nanosized condensate particles. Based on the reflectivity measurements and the electron microscopy microstructure study of the irradiated surface, it has been suggested that the mechanism for the higher efficiency of ablation of high-carbon steel compared to low-carbon steel is the condensation process of supersaturated carbon vapors on the target surface that increases the irradiated target surface absorptivity and, consequently, the material removal efficiency of the subsequent scanning pass is increased.

Analysis on Feasibility of Applying Low-Temperature Emission to Coal-fired Power Plant in Cold Weather

After the ultra-low emission(ULE) transformation, the emissions of ULE coal-fired power plants in China have been reduced greatly. However, considering the total amount of emissions, coal-fired power plants are still among the largest sources. And there may still have great potential in the removal of colored plume and some unconventional pollutants such as condensable particles matter (CPM) and sulfur trioxide (sulfuric acid mist). Though in cold days, hazes occur frequently. Besides, low-temperature condensation is conducive to the condensation and removal of vapor, CPM and sulfur trioxide in the flue gas. In this paper, a new method of low-temperature flue gas emission is proposed for coal-fired power plants in severe cold areas. The flue gas is cooled by ambient air, and discharged without reheating, and remarkable effects can be achieved in removing the pollutants. The technical feasibility and environmental compliance of applying this method to a power plant in cold weather have been analyzed. The results show that low-temperature emission of flue gas for coal-fired power plants in winter is effective in water-saving, emission reduction of pollutants and elimination of wet plume.

International comparison CCQM-P189: particle number concentration (100 to 20 000 cm-3) and particle charge concentration (0.15 to 3 fC cm-3)

Main textThis report presents the results of CCQM-P189, a pilot comparison between 10 laboratories which tested the participants’ capability to measure particle number concentration (in the range of 100 to 20 000 cm-3) using condensation particle counters (CPCs), and particle charge concentration (in the range of 0.15 to 3 fC cm-3) using aerosol electrometers (AEs).Measurements of aerosol particle number concentration are needed to demonstrate compliance to vehicle emission legislation and are becoming increasingly important in other areas such as ambient air and workplace monitoring. The measurements are typically carried out using condensation particle counters, which are calibrated using either reference CPCs or reference AEs.An analogous report is available for the CCQM-K150 comparison. CCQM-K150 was identical to, and used the same experimental data as CCQM-P189 with one exception: data from TROPOS, which is not a National Measurement Institute (NMI) or Designated Institute (DI), were only included in CCQM-P189.CCQM-P189 was an amount-of-substance Track C comparison.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/.The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

High resolution characterization of a sheathed axisymmetric variable supersaturation condensation particle sizer

Diffusive condensation particle counters (CPCs) injecting the aerosol near the symmetry axis within a larger flow of a vapor-containing sheath gas have previously exhibited a size resolving power considerably lower than theoretically expected. We hypothesize that reexamining these instruments with particles of improved uniformity in composition, size and shape should reveal a much better performance, well suited for high resolution particle sizing and basic heterogeneous nucleation studies. Accordingly, we investigate the performance of the Variable Supersaturation Condensation Particle Sizer (VSCPS) of Gallar et al. (2006) with an aerosol of uniform singly charged polyethylene glycol particles (3–9 nm diameter), produced by a bipolar electrospray, and size-selected by a differential mobility analyzer of high resolution. The condensation size resolution obtained with n-butanol vapor almost triples that previously demonstrated for this instrument, exceeds that established for any other CPC, and can apparently be further increased. Flow and temperature field calculations of the maximal supersaturation in the VSCPS would enable basic heterogeneous nucleation studies, as well as high resolution sizing of neutral or charged particles.

CAMP: an instrumented platform for balloon-borne aerosol particle studies in the lower atmosphere

Abstract. Airborne observations of vertical aerosol particle distributions are crucial for detailed process studies and model improvements. Tethered balloon systems represent a less expensive alternative to aircraft to probe shallow atmospheric boundary layers (ABLs). This study presents the newly developed cubic aerosol measurement platform (CAMP) for balloon-borne observations of aerosol particle microphysical properties. With an edge length of 35 cm and a weight of 9 kg, the cube is an environmentally robust instrument platform intended for measurements at low temperatures, with a particular focus on applications in cloudy Arctic ABLs. The aerosol instrumentation on board CAMP comprises two condensation particle counters with different lower detection limits, one optical particle size spectrometer, and a miniaturized absorption photometer. Comprehensive calibrations and characterizations of the instruments were performed in laboratory experiments. The first field study with a tethered balloon system took place at the Leibniz Institute for Tropospheric Research (TROPOS) station in Melpitz, Germany, in the winter of 2019. At ambient temperatures between −8 and 15 ∘C, the platform was operated up to a 1.5 km height on 14 flights under both clear-sky and cloudy conditions. The continuous aerosol observations at the ground station served as a reference for evaluating the CAMP measurements. Exemplary profiles are discussed to elucidate the performance of the system and possible process studies. Based on the laboratory instrument characterizations and the observations during the field campaign, CAMP demonstrated the capability to provide comprehensive aerosol particle measurements in cold and cloudy ABLs.

Excitation Spectrum in an Ensemble of Hubbard Bosons

An approach that makes it possible to correctly derive equations describing the Bose–Einstein condensation and the spectrum of elementary excitations in the ensemble of Hubbard bosons in the strong correlation regime (U gg {text{|}}{{t}_{{fm}}}{text{|}}) has been developed in the atomic representation using the Dyson method with the introduced indefinite metric. The kinematic Dyson interaction caused by the properties of the commutation relations of dynamic variables plays an important role in such a system. An effective Hamiltonian has been obtained using the operator form of perturbation theory at finite U values. It has been shown that the properties of the ensemble of Hubbard bosons have been determined by the kinematic interaction, correlated hopping, and the attraction between Hubbard bosons. Numerical calculations have demonstrated the effect of these interactions on the characteristics of the energy spectrum of excitations of the ensemble of Hubbard bosons and on the dependence of the density of condensate particles on the density of bosons in the system.

Experimental study of moisture effects on spontaneous combustion of Baiyinhua lignite from individual particles to stockpile

Evaporation and condensation of moisture affect dramatically the spontaneous combustion of lignite with high moisture content at low temperatures. Moisture migration between the lignite and the external environment of coal stockpile encompasses the scales of individual particles and stockpile. The cyclic moisture evaporation/condensation test was applied to investigate moisture migration of individual lignite particles at different particle sizes and temperatures. The wire-mesh basket test was selected primarily to analyze the heating process and critical self-ignition temperature of lignite stockpile. Results of the cyclic moisture evaporation/condensation test reveal that the moisture content of re-saturated lignite is much lower than that of the original one. Moisture content versus time showed a tendency to overlap in repeated moisture evaporation processes, and so did the repeated moisture evaporation processes. Therefore, a model of moisture migration of individual particles was built to describe the repeated moisture evaporation and condensation processes. The time factor determines the speed of moisture evaporation and condensation of individual lignite particles, being inversely proportional to ambient temperature and proportional to particle size, respectively. In the wire-mesh basket test, the heating process of the lignite stockpile can be divided into four stages, but the influence of moisture was more pronounced in stage II. Critical self-ignition temperature is proportional to particle size and increases linearly with moisture content. According to Frank-Kamenetskii theory, an increase in either particle size or moisture content can restrain the tendency of spontaneous combustion of Baiyinhua lignite stockpile.

Self-interacting superfluid dark matter droplets

ABSTRACT We assume dark matter to be a cosmological self-gravitating Bose–Einstein condensate of non-relativistic ultralight scalar particles with competing gravitational and repulsive contact interactions and investigate the observational implications of such model. The system is unstable to the formation of stationary self-bound structures that minimize the energy functional. These cosmological superfluid droplets, which are the smallest possible gravitationally bound dark matter structures, exhibit a universal mass profile and a corresponding universal rotation curve. Assuming a hierarchical structure formation scenario where granular dark matter haloes grow around these primordial stationary droplets, the model predicts cored haloes with rotation curves that obey a single universal equation in the inner region ($r\, \lesssim \, 1$ kpc). A simultaneous fit to a selection of galaxies from the SPARC data base chosen with the sole criterion of being strongly dark matter dominated even within the innermost region, indicates that the observational data are consistent with the presence of a Bose–Einstein condensate of ultralight scalar particles of mass m ≃ 2.2 × 10−22 eV c−2 and repulsive self-interactions characterized by a scattering length as ≃ 7.8 × 10−77 m. Such small self-interactions have profound consequences on cosmological scales. They induce a natural minimum scale length for the size of dark matter structures that makes all cores similar in length (∼1 kpc) and contributes to lower their central densities.

Utility of outdoor central site monitoring in assessing exposure of school children to ultrafine particles

Epidemiological studies investigating the association between daily particle exposure and health effects are frequently based on a single monitoring site located in an urban background. Using a central site in epidemiological time-series studies has been established based on the premises of low spatial variability of particles within the areas of interest and hence the adequacy of the central sites to monitor the exposure. This is true to a large extent in relation to larger particles (PM2.5, PM10) that are typically monitored and regulated. However, the distribution of ultrafine particles (UFP), which in cities predominantly originate from traffic, is heterogeneous. With increasing pressure to improve the epidemiology of UFP, an important question to ask is, whether central site monitoring is representative of community exposure to this size fraction of particulate matter; addressing this question is the aim of this paper. To achieve this aim, we measured personal exposure to UFP, expressed as particle number concentration (PNC), using Philips Aerasense Nanotracers (NT) carried by the participants of the study, and condensation particle counters (CPC) or scanning mobility particle sizers (SMPS) at central fixed-site monitoring stations. The measurements were conducted at three locations in Brisbane (Australia), Cassino (Italy) and Accra (Ghana). We then used paired t-tests to compare the average personal and average fixed-site PNC measured over the same 24-h, and hourly, periods. We found that, at all three locations, the 24-h average fixed-site PNC was no different to the personal PNC, when averaged over the study period and all the participants. However, the corresponding hourly averages were significantly different at certain times of the day. These were generally times spent commuting and during cooking and eating at home. Our analysis of the data obtained in Brisbane, showed that maximum personal exposure occurred in the home microenvironment during morning breakfast and evening dinner time. The main source of PNC for personal exposure was from the home-microenvironment. We conclude that the 24-h average PNC from the central-site can be used to estimate the 24-h average personal exposure for a community. However, the hourly average PNC from the central site cannot consistently be used to estimate hourly average personal exposure, mainly because they are affected by very different sources.

Mechanical Properties and Coagulation Characteristics of Flue Gas Desulfurization Gypsum-Based Polymer Materials.

To resolve problems caused by the accumulation of flue gas desulfurization gypsum (FGDG) in the environment, a polymer material was prepared using FGDG, granulated blast furnace slag (GBFS), fly ash (FA), and solid sodium silicate (SSS). The compressive strength of these polymer specimens cured for 3, 28, and 60 d was regularly measured, and their condensation behavior was analyzed. Both the formation behavior of mineral crystals and microstructure characteristics were analyzed further using X-ray diffraction and scanning electron microscopy. The compressive strength of pure FGDG polymer specimen (whose strength is generated by particle condensation crystallization) is insufficient and the condensation is slow. The addition of appropriate amounts of GBFS, FA, and SSS can continuously and considerably improve the compressive strength and shorten the setting time. The optimal proportions of FGDG, GBFS, and FA are 50%, 20%, and 30%, respectively, with the SSS addition amount of 20 g. The incorporation of GBFS, FA, and SSS can promote the polymerization of calcium, silicon, and aluminum in FGDG to form silicate and aluminosilicate minerals. Their formation is the main reason for the increased compressive strength and accelerated coagulation.

Effect of COVID-19 lockdown on the characterization and mixing state of carbonaceous particles in the urban atmosphere of Liaocheng, the North China Plain

To investigate the effect of COVID-19 control measures on aerosol chemistry, the chemical compositions, mixing states, and formation mechanisms of carbonaceous particles in the urban atmosphere of Liaocheng in the North China Plain (NCP) were compared before and during the pandemic using a single particle aerosol mass spectrometry (SPAMS). The results showed that the concentrations of five air pollutants including PM2.5, PM10, SO2, NO2, and CO decreased by 41.2%–71.5% during the pandemic compared to those before the pandemic, whereas O3 increased by 1.3 times during the pandemic because of the depressed titration of O3 and more favorable meteorological conditions. The count and percentage contribution of carbonaceous particles in the total detected particles were lower during the pandemic than those before the pandemic. The carbonaceous particles were dominated by elemental and organic carbon (ECOC, 35.9%), followed by elemental carbon-aged (EC-aged, 19.6%) and organic carbon-fresh (OC-fresh, 13.5%) before the pandemic, while EC-aged (25.3%), ECOC (17.9%), and secondary ions-rich (SEC, 17.8%) became the predominant species during the pandemic. The carbonaceous particle sizes during the pandemic showed a broader distribution than that before the pandemic, due to the condensation and coagulation of carbonaceous particles in the aging processes. The relative aerosol acidity (Rra) was smaller before the pandemic than that during the pandemic, indicating the more acidic particle aerosol during the pandemic closely related to the secondary species and relative humidity (RH). More than 95.0% and 86.0% of carbonaceous particles in the whole period were internally mixed with nitrate and sulfate, implying that most of the carbonaceous particles were associated with secondary oxidation during their formation processes. The diurnal variations of oxalate particles and correlation analyses suggested that oxalate particles before the pandemic were derived from aqueous oxidation driven by RH and liquid water content (LWC), while oxalate particles during the pandemic were originated from O3-dominated photochemical oxidation.

Bose–Einstein condensation for particles with repulsive short-range pair interactions in a Poisson random external potential in

AbstractWe study Bose gases in$d \ge 2$dimensions with short-range repulsive pair interactions at positive temperature, in the canonical ensemble and in the thermodynamic limit. We assume the presence of hard Poissonian obstacles and focus on the non-percolation regime. For sufficiently strong interparticle interactions, we show that almost surely there cannot be Bose–Einstein condensation into a sufficiently localized, normalized one-particle state. The results apply to the canonical eigenstates of the underlying one-particle Hamiltonian.

Comparison of Particle Sizers and Counters with Soot-like, Salt, and Silver Particles

Vehicle emission regulations in Europe and many Asian countries include a particle number limit. The number concentration is measured, typically, with condensation particle counters (CPCs). For research purposes, the size distributions provide useful information. Scanning mobility particle sizers (SMPSs) accurately provide the size distribution but are not suitable for transient aerosol. Engine (fast) exhaust particle sizers (EEPSs) cover this gap, but with less accuracy. Fast size distribution instruments are commonly used in the research and development of engines. In the last few years, instrument algorithms have been improved, but studies assessing the improvements are limited, in particular in their lower size range, around 10–20 nm, and for soot-like aerosol. In this paper, we compared the three instruments using salt, silver, diffusion flame soot (CAST), and spark discharge graphite particles. Moreover, vehicle exhaust number concentration measurements with EEPSs over a seven-year period were presented. In terms of particle concentration, EEPS overestimated, on average, 25% compared to CPC, in agreement with previous studies. Its accuracy for mean particle size determination was better than 5 nm compared to SMPS. The agreement between the instruments was satisfactory but the uncertainty increased at low concentrations and larger particle sizes, showing that there is still room for further improvements. Experimental challenges, such as low concentration levels of modern vehicles, losses in the diluters, use of photometric mode by the CPCs, and the material impact, are also discussed.

Improvement and Observation of Condensation Particle Counter in Atmospheric Research Aircraft NARA for Condensation Particle Research in Korea

Improvement and Observation of Condensation Particle Counter in Atmospheric Research Aircraft NARA for Condensation Particle Research in Korea

Volatility of aircraft exhaust ultrafine particles inferred from field measurements at Narita International Airport

We conducted field measurements of aerosols near a runway at Narita International Airport (NRT) in February 2018 to characterize the physical and chemical properties of aircraft exhaust ultrafine particles (UFPs; diameter <100 nm). An ultrafine condensation particle counter (UCPC; d50 = 2.5 nm), a condensation particle counter (CPC; d50 = 10 nm), and an engine exhaust particle sizer (EEPS; 6.0–523 nm) were used to measure the particle number concentrations and size distributions in ambient air. The sample flow for the UCPC and CPC was passed through an evaporation tube for heated measurements (150 °C, 250 °C, or 350 °C) or through a bypass tube for unheated measurements. The evaporation tube temperature (Ttube) of 350 °C was used as a threshold point for the separation of volatile and non-volatile particles. The removal efficiency of the volatile particles at intermediate temperatures (<350 °C) was evaluated in the laboratory using triacontane (C30) and tetracontane (C40) as test compounds. The critical temperature at which the laboratory-generated 30-nm C30 and C40 particles were almost entirely removed was ∼130 °C and ∼250 °C, respectively. The particle number concentrations measured by the UCPC, CPC, and EEPS at NRT were used to investigate the volatility of aircraft exhaust UFPs under real-world operating conditions. The number fraction of the non-volatile particles strongly depended on the geometric mean diameter (GMD) of the unheated particle number size distributions, whereas the vaporization temperature of the volatile particles did not exhibit significant dependency on GMD. The particle number fraction remaining, calculated for the selected evaporation tube temperature (Ttube), indicates that ∼50% and ≳ 90% of the volatile particles were removed at Ttube = 150 °C and 250 °C, respectively, on a number basis. These results suggest that a large fraction (more than half by number) of the volatile UFPs emitted from aircraft was equally or less volatile than the 30 nm pure C30 particles and only a small fraction of the volatile UFPs was equally or less volatile than the 30 nm pure C40 particles.

Turbulent Flux Measurements of the Near‐Surface and Residual‐Layer Small Particle Events

AbstractAccording to recent field studies, almost half of the New Particle Formation (NPF) events occur aloft, in a residual layer, near the top of the boundary layer. Therefore, measurements of the meteorological parameters, precursor gas concentrations, and aerosol loadings conducted at the ground level are often not representative of the conditions where the NPFs take place. This paper presents new measurements obtained during the Turbulent Flux Measurements of the Residual Layer Nucleation Particles, conducted at the Southern Great Plains research site. Vertical turbulent fluxes of 3–10 nm‐sized particles were measured using a sonic anemometer and two condensation particle counters with nominal cutoff diameters of 3 nm and 10 nm mounted at the top of the 10‐m telescoping tower. Aerosol number size distribution (5–300 nm) was determined through the ground‐based Scanning Mobility Particle Sizers. The size selected (15–50 nm) particle hygroscopicity was derived with the Humidified Tandem Differential Mobility Analyzer. The ground level observations were supplemented by vertically‐resolved measurements of horizontal and vertical wind speeds and aerosol backscatter. The data analysis suggests that (a) turbulent flux measurements of 3–10 nm particles can distinguish between near‐surface and residual‐layer small particle events; (b) sub‐50 nm particles had a hygroscopicity value of 0.2, suggesting that organic compounds dominate atmospheric nanoparticle chemical composition at the site; and (c) current methodologies are inadequate for estimating the dry deposition velocity of sub‐10 nm particles because it is not feasible to measure particle concentration very near the surface, in the diffusion sublayer.

Slow thermal decomposition of lignocelluloses compared to numerical model: Fine particle emission, gaseous products analysis

This paper is focused on aerosol fine particle release during the thermal decomposition of beech wood samples with identical mass under an oxidizing and inert atmosphere. This thermal degradation of wood is a significant part of the biomass combustion and pyrolytic process. This contribution focuses on nanoparticles emission with specifying sizes and concentrations in the range of 18–545 nm during thermal degradation processes and determining correlations between individual thermal degradation processes of a wood sample with the same weights but with different heating rates. The flue gases' chemical composition was simultaneously analyzed using a quadrupole mass spectrometer during the thermal degradation process. The weight loss of the sample and its derivation were monitored using a thermogravimeter. This combination made it possible to identify the ranges with the highest particle emission concerning the wood sample and the sample's heating rate. The fine particle identification apparatus uses a Scanning Mobility Particle Sizer and a condensation particle counter. This experimentally obtained data were then compared with the numerical model CPD-Bio. In this case, the comparison was focused on the prediction of the temperatures of the individual pyrolysis stages, at which gaseous products and especially tars are formed.

Characterisation of Particles Emitted during Laser Cutting of Various Metal Sheets and an Exposure Assessment for the Laser Operators

Laser cutting is used in many industrial settings to achieve precise cuts of metal sheets. Laser operators may be exposed to particles formed during cutting when opening the cabinet or when metal sheets are exchanged. To characterise the potential exposure, particles formed during laser cutting were studied with scanning electron microscopy equipped with an energy dispersive X-ray detector and an energy backscatter diffraction detector. The total concentration of particles (11–615 nm) was determined online with a scanning mobility particle sizer. The chemical composition of the particles formed during the cutting of the different metal sheets was determined by inductively coupled plasma mass spectrometry (ICP-MS). X-ray diffraction was applied to determine the phase composition. The occupational exposure was assessed gravimetrically and by ICP-MS for five laser operators handling different laser cutters, and materials and were found to be low. Agglomerates and aggregates of condensation particles were formed during laser cutting, independent of the sheet type. Iron, present as both magnetite and α-Fe, was the main element found in the particles formed when cutting steel sheets. The size of the particles generated was mainly below 300 nm. Open laser cutters may lead to higher metal exposures, which is especially relevant when cutting metal sheets containing heavy metals.

Secondary aerosol formation in marine Arctic environments: a model measurement comparison at Ny-Ålesund

Abstract. In this study, we modeled the aerosol particle formation along air mass trajectories arriving at the remote Arctic research stations Gruvebadet (67 m a.s.l.) and Zeppelin (474 m a.s.l.), Ny-Ålesund, during May 2018. The aim of this study was to improve our understanding of processes governing secondary aerosol formation in remote Arctic marine environments. We run the Lagrangian chemistry transport model ADCHEM, along air mass trajectories generated with FLEXPART v10.4. The air masses arriving at Ny-Ålesund spent most of their time over the open ice-free ocean. In order to capture the secondary aerosol formation from the DMS emitted by phytoplankton from the ocean surface, we implemented a recently developed comprehensive DMS and halogen multi-phase oxidation chemistry scheme, coupled with the widely used Master Chemical Mechanism (MCM). The modeled median particle number size distributions are in close agreement with the observations in the marine-influenced boundary layer near-sea-surface Gruvebadet site. However, while the model reproduces the accumulation mode particle number concentrations at Zeppelin, it overestimates the Aitken mode particle number concentrations by a factor of ∼5.5. We attribute this to the deficiency of the model to capture the complex orographic effects on the boundary layer dynamics at Ny-Ålesund. However, the model reproduces the average vertical particle number concentration profiles within the boundary layer (0–600 m a.s.l.) above Gruvebadet, as measured with condensation particle counters (CPCs) on board an unmanned aircraft system (UAS). The model successfully reproduces the observed Hoppel minima, often seen in particle number size distributions at Ny-Ålesund. The model also supports the previous experimental findings that ion-mediated H2SO4–NH3 nucleation can explain the observed new particle formation in the marine Arctic boundary layer in the vicinity of Ny-Ålesund. Precursors resulting from gas- and aqueous-phase DMS chemistry contribute to the subsequent growth of the secondary aerosols. The growth of particles is primarily driven via H2SO4 condensation and formation of methane sulfonic acid (MSA) through the aqueous-phase ozonolysis of methane sulfinic acid (MSIA) in cloud and deliquescent droplets.

The effect of environmental parameters on squalene-ozone particle formation

Particles formed from the squalene-ozone (SqOz) reaction have potential health concerns. Squalene, a component of skin oil, is ubiquitously found indoors on skin, surfaces, and clothing. Since people spend the majority of their time indoors, it is important to understand how indoor relative humidity (RH), ozone concentrations, and squalene loadings affect SqOz particle formation. Particle formation from the SqOz reaction within a reaction chamber was monitored using a condensation particle counter (CPC) while varying environmental conditions. The particle production curves over time were similar as RH was varied from 15% to 45%, but particles were not formed at RH 55% and 65%, probably due to a shift in primary formation pathway towards volatile products and/or potential losses of volatile compounds from particles produced as secondary reaction products. As ozone concentrations were increased from 25 ppb to 200 ppb, the time to maximum particle number concentration decreased, and the maximum particle number concentration and initial rate of particle generation increased non-linearly. This suggests that particle formation was dependent upon the ozone concentration but was limited by the amount of squalene remaining on the surface. As surface loadings were raised from 1.28 μg/cm2 to 270 μg/cm2 the maximum particle number concentration increased linearly. Further increases in squalene loading did not cause higher levels of particle production. This pattern is consistent with a shift from a monolayer to bi-layer formation or non-reactive products coating the squalene at the higher loadings. Our results indicate that SqOz particle formation is a function of environmental conditions.