D. R. Worsnop

University of Helsinki, Helsinki, Uusimaa, Finland

Are you D. R. Worsnop?

Claim your profile

Publications (163)356.15 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Atmospheric amines may play a crucial role in formation of new aerosol particles via nucleation with sulfuric acid. Recent studies have revealed that concentrations below 1 pptV can significantly promote nucleation of sulfuric acid particles. While sulfuric acid detection is relatively straightforward, no amine measurements to date have been able to reach the critical sub-pptV concentration range and atmospheric amine concentrations are in general poorly characterized. In this work we present a proof-of-concept of an instrument capable of detecting dimethyl amine (DMA) with concentrations even down to 70 ppqV (parts per quadrillion, 0.07 pptV) for a 15 min integration time. Detection of ammonia and amines other than dimethyl amine is discussed. We also report results from the first ambient measurements performed in spring 2013 at a boreal forest site. While minute signals above the signal-to-noise ratio that could be attributed to trimethyl or propyl amine were observed, DMA concentration never exceeded the detection threshold of ambient measurements (150 ppqV), thereby questioning, though not excluding, the role of DMA in nucleation at this location.
    Atmospheric Measurement Techniques 10/2015; 8(10):4001-4011. DOI:10.5194/amt-8-4001-2015 · 2.93 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Sulfuric acid is an important gas influencing atmospheric new particle formation (NPF). Both the binary (H2SO4–H2O) system and the ternary system involving ammonia (H2SO4–H2O–NH3) may be important in the free troposphere. An essential step in the nucleation of aerosol particles from gas-phase precursors is the formation of a dimer, so an understanding of the thermodynamics of dimer formation over a wide range of atmospheric conditions is essential to describe NPF. We have used the CLOUD chamber to conduct nucleation experiments for these systems at temperatures from 208 to 248 K. Neutral monomer and dimer concentrations of sulfuric acid were measured using a chemical ionization mass spectrometer (CIMS). From these measurements, dimer evaporation rates in the binary system were derived for temperatures of 208 and 223 K. We compare these results to literature data from a previous study that was conducted at higher temperatures but is in good agreement with the present study. For the ternary system the formation of H2SO4·NH3 is very likely an essential step in the formation of sulfuric acid dimers, which were measured at 210, 223, and 248 K. We estimate the thermodynamic properties (dH and dS) of the H2SO4·NH3 cluster using a simple heuristic model and the measured data. Furthermore, we report the first measurements of large neutral sulfuric acid clusters containing as many as 10 sulfuric acid molecules for the binary system using chemical ionization–atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometry.
    Atmospheric Chemistry and Physics 09/2015; 15(18):10701-10721. DOI:10.5194/acp-15-10701-2015 · 5.05 Impact Factor
  • Source
    Y. L. Sun · Z. F. Wang · W. Du · Q. Zhang · Q. Q. Wang · P. Q. Fu · X. L. Pan · J. Li · J. Jayne · D. R. Worsnop
    [Show abstract] [Hide abstract]
    ABSTRACT: High concentrations of fine particles (PM2.5) are frequently observed during all seasons in Beijing, China, leading to severe air pollution and human health problems in this megacity. In this study, we conducted real-time measurements of non-refractory submicron aerosol (NR-PM1) species (sulfate, nitrate, ammonium, chloride, and organics) in Beijing using an Aerodyne Aerosol Chemical Speciation Monitor for 1 year, from July 2011 to June 2012. This is the first long-term, highly time-resolved (~ 15 min) measurement of fine particle composition in China. The seasonal average (± 1σ) mass concentration of NR-PM1 ranged from 52 (± 49) μg m−3 in the spring season to 62 (± 49) μg m−3 in the summer season, with organics being the major fraction (40–51%), followed by nitrate (17–25%) and sulfate (12–17%). Organics and chloride showed pronounced seasonal variations, with much higher concentrations in winter than in the other seasons, due to enhanced coal combustion emissions. Although the seasonal variations of secondary inorganic aerosol (SIA = sulfate + nitrate + ammonium) concentrations were not significant, higher contributions of SIA were observed in summer (57–61%) than in winter (43–46%), indicating that secondary aerosol production is a more important process than primary emissions in summer. Organics presented pronounced diurnal cycles that were similar among all seasons, whereas the diurnal variations of nitrate were mainly due to the competition between photochemical production and gas–particle partitioning. Our data also indicate that high concentrations of NR-PM1 (> 60 μg m−3) are usually associated with high ambient relative humidity (RH) (> 50%) and that severe particulate pollution is characterized by different aerosol composition in different seasons. All NR-PM1 species showed evident concentration gradients as a function of wind direction, generally with higher values associated with wind from the south, southeast or east. This was consistent with their higher potential as source areas, as determined by potential source contribution function analysis. A common high potential source area, located to the southwest of Beijing along the Taihang Mountains, was observed during all seasons except winter, when smaller source areas were found. These results demonstrate a high potential impact of regional transport from surrounding regions on the formation of severe haze pollution in Beijing.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 09/2015; 15(17):10149-10165. DOI:10.5194/acp-15-10149-2015 · 5.05 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report comprehensive, demonstrably contaminant-free measurements of binary particle formation rates by sulfuric acid and water for neutral and ion-induced pathways conducted in the CERN CLOUD chamber. The recently developed Atmospheric Pressure interface-Time Of Flight-Mass Spectrometer was used to detect contaminants in charged clusters and to identify runs free of any contaminants. Four parameters were varied to cover ambient conditions: sulfuric acid concentration (105 to 109 molecules cm−3), relative humidity (11% to 58%), temperature (207K to 299K), and total ion concentration (0 to 6800 ions cm−3). Formation rates were directly measured with novel instruments at sizes close to the critical cluster size (mobility size of 1.3 nm to 3.2 nm). We compare our results with predictions from Classical Nucleation Theory normalized by Quantum Chemical calculation (QC-normalized CNT), which is described in a companion paper. The formation rates predicted by the QC-normalized CNT were extended from critical cluster sizes to measured sizes using the UHMA2 sectional particle microphysics model. Our results show, for the first time, good agreement between predicted and measured particle formation rates for the binary (neutral and ion-induced) sulfuric acid- water system. Formation rates increase with RH, sulfuric acid and ion concentrations and decrease with temperature at fixed RH and sulfuric acid concentration. Under atmospheric conditions neutral particle formation dominates at low temperatures, while ion-induced particle formation dominates at higher temperatures. The good agreement between the theory and our comprehensive data set gives confidence in using the QC-normalized CNT as a powerful tool to study neutral and ion-induced binary particle formation in atmospheric modeling.
    Journal of Geophysical Research Atmospheres 09/2015; DOI:10.1002/2015JD023539 · 3.43 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The mitigation of air pollution in megacities remains a great challenge because of the complex sources and formation mechanisms of aerosol particles. The 2014 Asia- Pacific Economic Cooperation (APEC) summit in Beijing serves as a unique experiment to study the impacts of emission controls on aerosol composition, size distributions, and oxidative properties. Herein, a high-resolution time-of-flight aerosol mass spectrometer was deployed in urban Beijing for real-time measurements of size-resolved non-refractory submicron aerosol (NR-PM1) species from 14 October to 12 November 2014, along with a range of collocated measurements. The average (±σ) PM1 was 41.6 (±38.9) μg m−3 during APEC, which was decreased by 53 % compared with that before APEC. The aerosol composition showed substantial changes owing to emission controls during APEC. Secondary inorganic aerosols (SIA = sulfate + nitrate + ammonium) showed significant reductions of 62–69 %, whereas organics presented much smaller decreases (35 %). The results from the positive matrix factorization of organic aerosols (OA) indicated that highly oxidized secondary OA (SOA) showed decreases similar to those of SIA during APEC. However, primary OA (POA) from cooking, traffic, and biomass burning sources were comparable to those before APEC, indicating the presence of strong local source emissions. The oxidation properties showed corresponding changes in response to OA composition. The average oxygen-to-carbon level during APEC was 0.36 (±0.10), which is lower than the 0.43 (±0.13) measured before APEC, demonstrating a decrease in the OA oxidation degree. The changes in size distributions of primary and secondary species varied during APEC. SIA and SOA showed significant reductions in large accumulation modes with peak diameters shifting from ~ 650 to 400 nm during APEC, whereas those of POA remained relatively unchanged. The changes in aerosol composition, size distributions, and oxidation degrees during the aging processes were further illustrated in a case study of a severe haze episode. Our results elucidated a complex response of aerosol chemistry to emission controls, which has significant implications that emission controls over regional scales can substantially reduce secondary particulates. However, stricter emission controls for local source emissions are needed for further mitigating air pollution in the megacity of Beijing.
    Atmospheric Chemistry and Physics 08/2015; 15(16):23407-23455. DOI:10.5194/acpd-15-23407-2015 · 4.88 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The composition of PM1 (particulate matter with diameter less than 1 μm) in the greater London area was characterized during the Clean Air for London (ClearfLo) project in winter 2012. Two High-Resolution Time-of-Flight Aerosol Mass Spectrometers (HR-ToF-AMS) were deployed at a rural site (Detling, Kent) and an urban site (North Kensington, London). The simultaneous and high-temporal resolution measurements at the two sites provide a unique opportunity to investigate the spatial distribution of PM1. We find that the organic aerosol (OA) concentration is comparable between the rural and urban sites, but the sources of OA are distinctly different. The concentration of solid fuel OA at the urban site is about twice as high as at the rural site, due to elevated domestic heating in the urban area. While the concentrations of oxygenated OA (OOA) are well-correlated between the two sites, the OOA concentration at the rural site is almost twice that of the urban site. At the rural site, more than 70 % of the carbon in OOA is estimated to be non-fossil, which suggests that OOA is likely related to aged biomass burning considering the small amount of biogenic SOA in winter. Thus, it is possible that the biomass burning OA contributes a larger fraction of ambient OA in wintertime than what previous field studies have suggested. A suite of instruments was deployed downstream of a thermal denuder (TD) to investigate the volatility of PM1 species at the rural Detling site. After heating at 250 °C in the TD, 40 % of the residual mass is OA, indicating the presence of non-volatile organics in the aerosol. Although the OA associated with refractory black carbon (rBC, measured by a soot-particle aerosol mass spectrometer) only accounts for < 10 % of the total OA (measured by a HR-ToF-AMS) at 250 °C, the two measurements are well-correlated, suggesting that the non-volatile organics have similar sources or have undergone similar chemical processing as rBC in the atmosphere. Finally, we discuss the relationship between the OA volatility and atomic O : C and find that particles with a wide range of O : C could have similar mass fraction remaining after heating. This analysis emphasizes the importance of understanding the distribution of volatility and O : C in bulk OA.
    Atmospheric Chemistry and Physics 08/2015; 15(16):23173-23229. DOI:10.5194/acpd-15-23173-2015 · 4.88 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The megacity of Beijing has experienced frequent severe fine particle pollution during the last decade. Although the sources and formation mechanisms of aerosol particles have been extensively investigated on the basis of ground measurements, real-time characterization of aerosol particle composition and sources above the urban canopy in Beijing is rare. In this study, we conducted real-time measurements of non-refractory submicron aerosol (NR-PM1) composition at 260 m at the 325 m Beijing Meteorological Tower (BMT) from 10 October to 12 November 2014, by using an aerosol chemical speciation monitor (ACSM) along with synchronous measurements of size-resolved NR-PM1 composition at near ground level using a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR–ToF–AMS). The NR-PM1 composition above the urban canopy was dominated by organics (46 %), followed by nitrate (27 %) and sulfate (13 %). The high contribution of nitrate and high NO3−/SO42− mass ratios illustrate an important role of nitrate in particulate matter (PM) pollution during the study period. The organic aerosol (OA) was mainly composed by secondary OA (SOA), accounting for 61 % on an average. Different from that measured at the ground site, primary OA (POA) correlated moderately with SOA, likely suggesting a high contribution from regional transport above the urban canopy. The Asia–Pacific Economic Cooperation (APEC) summit with strict emission controls provides a unique opportunity to study the impacts of emission controls on aerosol chemistry. All aerosol species were shown to have significant decreases of 40–80 % during APEC from those measured before APEC, suggesting that emission controls over regional scales substantially reduced PM levels. However, the bulk aerosol composition was relatively similar before and during APEC as a result of synergetic controls of aerosol precursors such as SO2, NOx, and volatile organic compounds (VOCs). In addition to emission controls, the routine circulations of mountain–valley breezes were also found to play an important role in alleviating PM levels and achieving the "APEC blue" effect. The evolution of vertical differences between 260 m and the ground level was also investigated. Our results show complex vertical differences during the formation and evolution of severe haze episodes that are closely related to aerosol sources and boundary layer dynamics.
    Atmospheric Chemistry and Physics 08/2015; 15(16):22889-22934. DOI:10.5194/acpd-15-22889-2015 · 4.88 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Understanding organic composition of gases and particles is essential to identifying sources and atmospheric processing leading to organic aerosols (OA), but atmospheric chemical complexity and the analytical techniques available often limit such analysis. Here we present speciated measurements of semivolatile and intermediate volatility organic compounds (S/IVOCs) using a novel dual-use instrument (SVTAG-AMS) deployed at Manitou Forest, CO during the Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics & Nitrogen – Rocky Mountain Biogenic Aerosol Study (BEACHON-RoMBAS) 2011 campaign. This instrument provides on-line speciation of ambient organic compounds with 2 h time resolution. The species in this volatility range are complex in composition, but their chemical identities reveal potential sources. Observed compounds of biogenic origin include sesquiterpenes with molecular formula C15H24 (e.g. β-caryophyllene and longifolene), which were most abundant at night. A variety of other biogenic compounds were observed, including sesquiterpernoids with molecular formula C15H22, abietatriene and other terpenoid compounds. Many of these compounds have been identified in essential oils and branch enclosure studies but were observed in ambient air for the first time in our study. Semivolatile polycyclic aromatic hydrocarbons (PAHs) and alkanes were observed with highest concentrations during the day and the dependence on temperature suggests the role of an evaporative source. Using statistical analysis by positive matrix factorization (PMF), we classify observed S/IVOCs by their likely sources and processes, and characterize them based on chemical composition. The total mass concentration of elutable S/IVOCs was estimated to be on the order of 0.7 μg m−3 and their volatility distributions are estimated for modeling aerosol formation chemistry.
    Atmospheric Chemistry and Physics 08/2015; 15(16):22331-22377. DOI:10.5194/acpd-15-22331-2015 · 4.88 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Sulfuric acid is an important gas influencing atmospheric new particle formation (NPF). Both the binary (H2SO4-H2O) system, and the ternary system involving ammonia (H2SO4-H2O-NH3) may be important in the free troposphere. An essential step in the nucleation of aerosol particles from gas-phase precursors is the formation of a dimer, so an understanding of the thermodynamics of dimer formation over a wide range of atmospheric conditions is essential to describe NPF. We have used the CLOUD chamber to conduct nucleation experiments for these systems at temperatures from 208 to 248 K. Neutral monomer and dimer concentrations of sulfuric acid were measured using a Chemical Ionization Mass Spectrometer (CIMS). From these measurements dimer evaporation rates in the binary system were derived for temperatures of 208 and 223 K. We compare these results to literature data from a previous study that was conducted at higher temperatures but is in good agreement with the present study. For the ternary system the formation of H2SO4 • NH3 is very likely an essential step in the formation of sulfuric acid dimers, which were measured at 210, 223, and 248 K. We estimate the thermodynamic properties (dH and dS) of the H2SO4 • NH3 cluster using a simple heuristic model and the measured data. Furthermore, we report the first measurements of large neutral sulfuric acid clusters containing as many as 10 sulfuric acid molecules for the binary system using Chemical Ionization-Atmospheric Pressure interface-Time Of Flight (CI-APi-TOF) mass spectrometry.
    Atmospheric Chemistry and Physics 05/2015; 15(10):13957-14006. DOI:10.5194/acpd-15-13957-2015 · 4.88 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The composition of secondary organic aerosols (SOAs) formed by β-pinene ozonolysis was experimentally investigated in the Juelich aerosol chamber. Partitioning of oxidation products between gas and particles was measured through concurrent concentration measurements in both phases. Partitioning coefficients (Kp) of 2.23 × 10−5 ± 3.20 × 10−6 m3 μg−1 for nopinone, 4.86 × 10−4 ± 1.80 × 10−4 m3 μg−1 for apoverbenone, 6.84 × 10−4 ± 1.52 × 10−4 m3 μg−1 for oxonopinone and 2.00 × 10−3 ± 1.13 × 10−3 m3 μg−1 for hydroxynopinone were derived, showing higher values for more oxygenated species. The observed Kp values were compared with values predicted using two different semi-empirical approaches. Both methods led to an underestimation of the partitioning coefficients with systematic differences between the methods. Assuming that the deviation between the experiment and the model is due to non-ideality of the mixed solution in particles, activity coefficients of 4.82 × 10−2 for nopinone, 2.17 × 10−3 for apoverbenone, 3.09 × 10−1 for oxonopinone and 7.74 × 10−1 for hydroxynopinone would result using the vapour pressure estimation technique that leads to higher Kp. We discuss that such large non-ideality for nopinone could arise due to particle phase processes lowering the effective nopinone vapour pressure such as diol- or dimer formation. The observed high partitioning coefficients compared to modelled results imply an underestimation of SOA mass by applying equilibrium conditions.
    Physical Chemistry Chemical Physics 05/2015; 17(22). DOI:10.1039/C5CP01608H · 4.49 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Isoprene produced in forest ecosystems is the most abundantly emitted non-methane volatile organic compound in the Earth’s atmosphere. Recent laboratory and field studies suggest that photooxidation of isoprene forms appreciable amounts of secondary organic aerosol (SOA), which likely affects climate by scattering solar radiation and by acting as cloud condensation nuclei (CCN). As these climate effects depend on both aerosol size and number concentration, it is crucial to understand how SOA partitioning influences particle growth kinetics. Recent studies indicate that biogenic SOA becomes increasingly viscous as relative humidity decreases, with the theoretical implication that its reduced bulk diffusivity slows down further growth via condensation of semivolatile organic compounds. We present here evidences from chamber experiments and the CARES field campaign for such bulk diffusion-limited growth of isoprene SOA formed in the presence of pre-existing Aitken and accumulation mode aerosols and relative humidity ≤ 50%. Kinetic modeling of the aerosol size distribution evolution suggests that the condensing isoprene photooxidation products are semivolatile and the bulk diffusivity is on the order 10^-15 cm2 s-1. The model also successfully reproduces the evaporation kinetics of size-selected chamber particles using similar estimates of bulk diffusivity and volatility for isoprene SOA. As the bulk diffusion timescale decreases a hundred-fold with a ten-fold decrease in size, the hindered growth of viscous accumulation mode particles effectively promotes the growth of the Aitken mode particles that are competing to absorb the semivolatile vapors. Isoprene SOA formation over forests thus represents a Gaia-like self-regulating system that produces CCN more efficiently under progressively drier conditions.
    ASR Science Team Meeting, Vienna (VA); 03/2015
  • S. Xiao · M. Y. Wang · L. Yao · M. Kulmala · B. Zhou · X. Yang · J. M. Chen · D. F. Wang · Q. Y. Fu · D. R. Worsnop · L. Wang
    [Show abstract] [Hide abstract]
    ABSTRACT: Particle size distributions in the range of 1.34-615 nm were recorded from 25 November 2013 to 25 January 2014 in urban Shanghai, using a combination of one nano condensation nucleus counter system, one nano scanning mobility particle sizer (SMPS), and one long-SMPS. Measurements of sulfur dioxide by an SO2 analyzer with pulsed UV fluorescence technique allowed calculation of sulfuric acid proxy. In addition, concentrations of ammonia were recorded with a differential optical absorption spectroscopy. During this 62-day campaign, 13 new particle formation (NPF) events were identified with strong bursts of sub-3 nm particles and subsequent fast growth of newly formed particles. The observed nucleation rate (J(1.34)), formation rate of 3 nm particles (J(3)), and condensation sink were 112.4-271.0 cm(-3) s(-1), 2.3-19.2 cm(-3) s(-1), and 0.030-0.10 s(-1), respectively. Subsequent cluster/nanoparticle growth (GR) showed a clear size dependence, with average values of GR(1.35 similar to 1.39), GR(1.39 similar to 1.46), GR(1.46 similar to 1.70), GR(1.70 similar to 2.39), GR(2.39 similar to 7), and GR(7 similar to 20) being 1.6 +/- 1.0, 1.4 +/- 2.2, 7.2 +/- 7.1, 9.0 +/- 11.4, 10.9 +/- 9.8, and 11.4 +/- 9.7 nm h(-1), respectively. Correlation between nucleation rate (J(1.34)) and sulfuric acid proxy indicates that nucleation rate J(1.34) was proportional to a 0.65 +/- 0.28 power of sulfuric acid proxy, indicating that the nucleation of particles can be explained by the activation theory. Correlation between nucleation rate (J(1.34)) and gas-phase ammonia suggests that ammonia was associated with NPF events. The calculated sulfuric acid proxy was sufficient to explain the subsequent growth of 1.34-3 nm particles, but its contribution became smaller as the particle size grew. Qualitatively, NPF events in urban Shanghai likely occur on days with low levels of aerosol surface area, meaning the sulfuric acid proxy is only a valid predictor when aerosol surface area is low.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 02/2015; 15(4):1769-1781. DOI:10.5194/acp-15-1769-2015 · 5.05 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Elemental compositions of organic aerosol (OA) particles provide useful constraints on OA sources, chemical evolution, and effects. The Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) is widely used to measure OA elemental composition. This study evaluates AMS measurements of atomic oxygen-to-carbon (O : C), hydrogen-to-carbon (H : C), and organic mass-to-organic carbon (OM : OC) ratios, and of carbon oxidation state ([bar over OS][subscript C]) for a vastly expanded laboratory data set of multifunctional oxidized OA standards. For the expanded standard data set, the method introduced by Aiken et al. (2008), which uses experimentally measured ion intensities at all ions to determine elemental ratios (referred to here as "Aiken-Explicit"), reproduces known O : C and H : C ratio values within 20% (average absolute value of relative errors) and 12%, respectively. The more commonly used method, which uses empirically estimated H[subscript 2]O[superscript +] and CO[superscript +] ion intensities to avoid gas phase air interferences at these ions (referred to here as "Aiken-Ambient"), reproduces O : C and H : C of multifunctional oxidized species within 28 and 14% of known values. The values from the latter method are systematically biased low, however, with larger biases observed for alcohols and simple diacids. A detailed examination of the H[subscript 2]O[superscript +], CO[superscript +], and CO[subscript 2][superscript +] fragments in the high-resolution mass spectra of the standard compounds indicates that the Aiken-Ambient method underestimates the CO[superscript +] and especially H[subscript 2]O[superscript +] produced from many oxidized species. Combined AMS–vacuum ultraviolet (VUV) ionization measurements indicate that these ions are produced by dehydration and decarboxylation on the AMS vaporizer (usually operated at 600 °C). Thermal decomposition is observed to be efficient at vaporizer temperatures down to 200 °C. These results are used together to develop an "Improved-Ambient" elemental analysis method for AMS spectra measured in air. The Improved-Ambient method uses specific ion fragments as markers to correct for molecular functionality-dependent systematic biases and reproduces known O : C (H : C) ratios of individual oxidized standards within 28% (13%) of the known molecular values. The error in Improved-Ambient O : C (H : C) values is smaller for theoretical standard mixtures of the oxidized organic standards, which are more representative of the complex mix of species present in ambient OA. For ambient OA, the Improved-Ambient method produces O : C (H : C) values that are 27% (11%) larger than previously published Aiken-Ambient values; a corresponding increase of 9% is observed for OM : OC values. These results imply that ambient OA has a higher relative oxygen content than previously estimated. The [bar over OS][subscript C] values calculated for ambient OA by the two methods agree well, however (average relative difference of 0.06 [bar over OS][subscript C] units). This indicates that [bar over OS][subscript C] is a more robust metric of oxidation than O : C, likely since [bar over OS][subscript C] is not affected by hydration or dehydration, either in the atmosphere or during analysis.
    Atmospheric Chemistry and Physics 02/2015; 15(1):253-272. DOI:10.5194/acp-15-253-2015 · 5.05 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The formation of particles from precursor vapors is an important source of atmospheric aerosol. Research at the Cosmics Leaving OUtdoor Droplets (CLOUD) facility at CERN tries to elucidate which vapors are responsible for this new-particle formation, and how in detail it proceeds. Initial measurement campaigns at the CLOUD stainless-steel aerosol chamber focused on investigating particle formation from ammonia (NH3) and sulfuric acid (H2SO4). Experiments were conducted in the presence of water, ozone and sulfur dioxide. Contaminant trace gases were suppressed at the technological limit. For this study, we mapped out the compositions of small NH3-H2SO4 clusters over a wide range of atmospherically relevant environmental conditions. We covered [NH3] in the range from <2 to 1400 pptv, [H2SO4] from 3.3 x 10(6) to 1.4 x 10(9) cm(-3) (0.1 to 56 pptv), and a temperature range from -25 to +20 degrees C. Negatively and positively charged clusters were directly measured by an atmospheric pressure interface time-of-flight (APi-TOF) mass spectrometer, as they initially formed from gas-phase NH3 and H2SO4, and then grew to larger clusters containing more than 50 molecules of NH3 and H2SO4, corresponding to mobility-equivalent diameters greater than 2 nm. Water molecules evaporate from these clusters during sampling and are not observed. We found that the composition of the NH3-H2SO4 clusters is primarily determined by the ratio of gas-phase concentrations [NH3] / [H2SO4], as well as by temperature. Pure binary H2O-H2SO4 clusters (observed as clusters of only H2SO4 / only form at [NH3] / [H2SO4] < 0.1 to 1. For larger values of [NH3] / [H2SO4], the composition of NH3-H2SO4 clusters was characterized by the number of NH3 molecules m added for each added H2SO4 molecule n (Delta m/Delta n), where n is in the range 4-18 (negatively charged clusters) or 1-17 (positively charged clusters). For negatively charged clusters, Delta m/Delta n saturated between 1 and 1.4 for [NH3] / [H2SO4] > 10. Positively charged clusters grew on average by Delta m/Delta n = 1.05 and were only observed at sufficiently high [NH3] / [H2SO4]. The H2SO4 molecules of these clusters are partially neutralized by NH3, in close resemblance to the acid-base bindings of ammonium bisulfate. Supported by model simulations, we substantiate previous evidence for acid-base reactions being the essential mechanism behind the formation of these clusters under atmospheric conditions and up to sizes of at least 2 nm. Our results also suggest that electrically neutral NH3-H2SO4 clusters, unobservable in this study, have generally the same composition as ionic clusters for [NH3] / [H2SO4] > 10. We expect that NH3-H2SO4 clusters form and grow also mostly by Delta m/Delta n > 1 in the atmosphere's boundary layer, as [NH3] / [H2SO4] is mostly larger than 10. We compared our results from CLOUD with APi-TOF measurements of NH3-H2SO4 anion clusters during new-particle formation in the Finnish boreal forest. However, the exact role of NH3-H2SO4 clusters in boundary layer particle formation remains to be resolved.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2015; 15(1-1):55-78. DOI:10.5194/acp-15-55-2015 · 5.05 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Recent developments in high-resolution time-of-flight chemical ionization mass spectrometry (HR-ToF-CIMS) have made it possible to directly detect atmospheric organic compounds in real time with high sensitivity and with little or no fragmentation, including low-volatility, highly oxygenated organic vapors that are precursors to secondary organic aerosol formation. Here, using ions identified by high-resolution spectra from an HR-ToF-CIMS with acetate reagent ion chemistry, we develop an algorithm to estimate the vapor pressures of measured organic acids. The algorithm uses identified ion formulas and calculated double bond equivalencies, information unavailable in quadrupole CIMS technology, as constraints for the number of possible oxygen-containing functional groups. The algorithm is tested with acetate chemical ionization mass spectrometry (acetate-CIMS) spectra of O-3 and OH oxidation products of alpha-pinene and naphthalene formed in a flow reactor with integrated OH exposures ranged from 1.2 x 10(11) to 9.7 x 10(11) molec scm(-3), corresponding to approximately 1.0 to 7.5 days of equivalent atmospheric oxidation. Measured gas-phase organic acids are similar to those previously observed in environmental chamber studies. For both precursors, we find that acetate-CIMS spectra capture both functionalization (oxygen addition) and fragmentation (carbon loss) as a function of OH exposure. The level of fragmentation is observed to increase with increased oxidation. The predicted condensed-phase secondary organic aerosol (SOA) average acid yields and O / C and H / C ratios agree within uncertainties with previous chamber and flow reactor measurements and ambient CIMS results. While acetate reagent ion chemistry is used to selectively measure organic acids, in principle this method can be applied to additional reagent ion chemistries depending on the application.
    Atmospheric Measurement Techniques 01/2015; 8(1):1-18. DOI:10.5194/amt-8-1-2015 · 2.93 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The soot-particle aerosol mass spectrometer (SP-AMS) uses an intra-cavity infrared laser to vaporize refractory black carbon (rBC) containing particles, making the particle beam–laser beam overlap critical in determining the collection efficiency (CE) for rBC and associated non-refractory particulate matter (NR-PM). This work evaluates the ability of the SP-AMS to quantify rBC and NR-PM mass in internally mixed particles with different thicknesses of organic coating. Using apparent relative ionization efficiencies for uncoated and thickly coated rBC particles, we report measurements of SP-AMS sensitivity to NR-PM and rBC, for Regal Black, the recommended particulate calibration material. Beam width probe (BWP) measurements are used to illustrate an increase in sensitivity for highly coated particles due to narrowing of the particle beam, which enhances the CE of the SP-AMS by increasing the laser beam–particle beam overlap. Assuming complete overlap for thick coatings, we estimate CE for bare Regal Black particles of 0.6 ± 0.1, which suggests that previously measured SP-AMS sensitivities to Regal Black were underestimated by up to a factor of 2. The efficacy of the BWP measurements is highlighted by studies at a busy road in downtown Toronto and at a non-roadside location, which show particle beam widths similar to, but greater than that of bare Regal Black and coated Regal Black, respectively. Further BWP measurements at field locations will help to constrain the range of CE for fresh and aged rBC-containing particles. The ability of the SP-AMS to quantitatively assess the composition of internally mixed particles is validated through measurements of laboratory-generated organic coated particles, which demonstrate that the SP-AMS can quantify rBC and NR-PM over a wide range of particle compositions and rBC core sizes.
    Atmospheric Measurement Techniques 12/2014; 7(12):4507. DOI:10.5194/amt-7-4507-2014 · 2.93 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Ternary aerosol nucleation experiments were conducted in the CLOUD chamber at CERN in order to investigate the influence of ions on new particle formation. Neutral and ion-induced nucleation experiments, i.e. without and with the presence of ions, respectively, were carried out under precisely controlled conditions. The sulfuric acid concentration was measured with a chemical ionisation mass spectrometer (CIMS) during the new particle formation experiments. The added ternary trace gases were ammonia (NH3), dimethylamine (DMA, C2H7N) or oxidised products of pinanediol (PD, C10H18O2). When pinanediol was introduced into the chamber, an increase in the mass spectrometric signal used to determine the sulfuric acid concentration (m/z 97, i.e. HSO4-) was observed due to ions from the CLOUD chamber. The enhancement was only observed during ion-induced nucleation measurements by using either galactic cosmic rays (GCRs) or the proton synchrotron (PS) pion beam for the ion generation, respectively. The ion effect typically involved an increase in the apparent sulfuric acid concentration by a factor of similar to 2 to 3 and was qualitatively verified by the ion measurements with an atmospheric-pressure interface-time of flight (APi-TOF) mass spectrometer. By applying a high-voltage (HV) clearing field inside the CLOUD chamber, the ion effect on the CIMS measurement was completely eliminated since, under these conditions, small ions are swept from the chamber in about 1 s. In order to exclude the ion effect and to provide corrected sulfuric acid concentrations during the GCR and PS beam nucleation experiments, a parameterisation was derived that utilises the trace gas concentrations and the UV light intensity as input parameters. Atmospheric sulfuric acid measurements with a CIMS showed an insignificant ion effect.
    11/2014; 7(7):6595-6624. DOI:10.5194/amtd-7-6595-2014
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Southern Africa is a significant source region of atmospheric pollution. A recently established atmospheric measurement station in South Africa, Welgegund, is strategically situated to capture regional background concentrations, as well as emissions from the major source regions in the interior of South Africa. We measured non-refractive submicron aerosols (NR-PM1) and black carbon over a one year period in Welgegund, and investigated the seasonal and diurnal patterns of aerosol concentration levels, chemical composition, acidity and oxidation level. Based on air mass back trajectories, four distinct source regions were determined for NR-PM1. Supporting data included particle number size distributions, aerosol absorption, trace gas concentrations, and meteorological variables. The dominant submicron aerosol constituent during the dry season was organic aerosol, reflecting high contribution from savannah fires and other combustion sources. Organic aerosol concentrations were lower during the wet season, presumably due to wet deposition as well as reduced emissions from combustion sources. Sulphate concentrations were usually high and exceeded organic aerosol concentrations when air-masses were transported over regions containing major emission point sources. Sulphate and nitrate concentrations peaked when air masses passed over the industrial Highveld (iHV) area. In contrast, concentrations were much lower when air masses passed over the cleaner background (BG) areas. Air masses associated with the anti-cyclonic recirculation (ACBIC) source region contained largely aged OA. Positive Matrix Factorization (PMF) analysis of aerosol mass spectra was used to characterise the organic aerosol (OA) properties. The factors identified were: oxidised organic aerosols (OOA) and biomass burning organic aerosols (BBOA) in the dry season and low-volatile (LV-OOA) and semi-volatile (SV-OOA) organic aerosols in the wet season. The results highlight the importance of primary BBOA in the dry season, which represented 33% of the total OA. Aerosol acidity and its potential impact on the evolution of OOA are also discussed.
    National Association for Clean Air (NACA) annual 2014 Conference, Umhlanga, KZN, South Africa; 10/2014
  • [Show abstract] [Hide abstract]
    ABSTRACT: The most important parameters describing the atmospheric new particle formation process are the particle formation and growth rates. These together determine the amount of cloud condensation nuclei attributed to secondary particle formation. Due to difficulties in detecting small neutral particles, it has previously not been possible to derive these directly from measurements in the size range below about 3 nm. The Airmodus Particle Size Magnifier has been used at the SMEAR II station in Hyytiälä, southern Finland, and during nucleation experiments in the CLOUD chamber at CERN for measuring particles as small as about 1 nm in mobility diameter. We developed several methods to determine the particle size distribution and growth rates in the size range of 1–3 nm from these data sets. Here we introduce the appearance-time method for calculating initial growth rates. The validity of the method was tested by simulations with the Ion-UHMA aerosol dynamic model.
    Boreal Environment Research 09/2014; 19:215-236. · 1.48 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The performance of the miniature Versatile Aerosol Concentration Enrichment System (m-VACES; Geller et al., 2005) was investigated in laboratory and field studies using online instruments. Laboratory tests focused on the behavior of monodisperse ammonium sulfate (AS) or dioctyl sebacate (DOS) particles in the m-VACES measured with the aerodynamic particle sizer (APS) and scanning mobility particle sizer (SMPS). The ambient measurements were conducted at an urban site in Helsinki, Finland, where the operation of the m-VACES was explored in conjunction with a Soot Particle Aerosol Mass Spectrometer (SP-AMS) in addition to the SMPS. In laboratory tests, the growth of particles in water vapor produced a stable droplet size distribution independent of the original particle size. However, when the droplets were dried with the goal of measuring the original size distribution, a shift to larger particles was observed for small particle sizes (up to similar to 200 nm in mobility diameter). That growth was probably caused by watersoluble organic compounds absorbed on the water droplets from the gas phase, but not evaporated in the drying phase. In ambient measurements, a similar enrichment was observed for nitrate and sulfate in the m-VACES whereas the presence of acidic ambient particles affected the enrichment of ammonium. Gaseous ammonia was likely to be absorbed on acidic particles in the m-VACES, neutralizing the aerosol. For organics, the enrichment efficiency was comparable with sulfate and nitrate but a small positive artifact for hydrocarbons and nitrogen-containing organic compounds was noticed. Ambient and concentrated organic aerosol (OA) was analyzed further with positive matrix factorization (PMF). A three-factor solution was chosen for both of the data sets but the factors were slightly different for the ambient and concentrated OA, however, the data set used for the PMF analysis was limited in size (3 days) and therefore had substantial uncertainty. Overall, the operation of the m-VACES was not found to lead to any severe sampling artifacts. The effect of acidity could be an issue in locations where the aerosol is acidic, however, in those cases the use of a denuder (which was not used in this study) is recommended. Further ambient tests are needed for the characterization of the m-VACES as the time period for the ambient measurements was only 5 days in this study. Especially for OA additional tests are important as the chemical properties of organics can differ widely depending on time and location.
    Atmospheric Measurement Techniques 07/2014; 7(7):2121-2135. DOI:10.5194/amt-7-2121-2014 · 2.93 Impact Factor

Publication Stats

852 Citations
356.15 Total Impact Points


  • 2010–2015
    • University of Helsinki
      • Department of Physics
      Helsinki, Uusimaa, Finland
    • Finnish Meteorological Institute
      • Air Quality Research
      Helsinki, Province of Southern Finland, Finland
  • 2004–2015
    • Aerodyne Research, Inc.
      Биллерика, Massachusetts, United States
    • Chestnut Hill College
      Boston, Massachusetts, United States
  • 2013
    • University of Eastern Finland
      • Department of Applied Physics
      Kuopio, Eastern Finland Province, Finland
  • 2011
    • IST Austria
      Klosterneuberg, Lower Austria, Austria