W. Birmili

Rapid aerosol particle growth and increase of cloud condensation nucleus activity by secondary aerosol formation and condensation: A case study for regional air pollution in northeastern China

[1] This study was part of the international field measurement Campaigns of Air Quality Research in Beijing and Surrounding Region 2006 (CAREBeijing-2006). We investigated a new particle formation event in a highly polluted air mass at a regional site south of the megacity Beijing and its impact on the abundance and properties of cloud condensation nuclei (CCN). During the 1-month observation, particle nucleation followed by significant particle growth on a regional scale was observed frequently (~30%), and we chose 23 August 2006 as a representative case study. Secondary aerosol mass was produced continuously, with sulfate, ammonium, and organics as major components. The aerosol mass growth rate was on average 19 μg m -3 h -1 during the late hours of the day. This growth rate was observed several times during the 1-month intensive measurements. The nucleation mode grew very quickly into the size range of CCN, and the CCN size distribution was dominated by the growing nucleation mode (up to 80% of the total CCN number concentration) and not as usual by the accumulation mode. At water vapor supersaturations of 0.07-0.86%, the CCN number concentrations reached maximum values of 4000-19,000 cm -3 only 6-14 h after the nucleation event. During particle formation and growth, the effective hygroscopicity parameter κ increased from about 0.1-0.3 to 0.35-0.5 for particles with diameters of 40-90 nm, but it remained nearly constant at ~0.45 for particles with diameters of ~190 nm. This result is consistent with aerosol chemical composition data, showing a pronounced increase of sulfate.

Particle number size distributions in a street canyon and their transformation into the urban-air background: measurements and a simple model study

Abstract Car traffic is one of the main anthropogenic aerosol sources in modern cities. The characterization of these emissions is important for describing the quality of urban air. Measurements in a street canyon in a German urban area were made. Maximum number concentrations occurred during morning hours from Monday to Friday when the traffic density is highest. The maximum of the number size distribution measured during rush hour near a busy city street was at a particle diameter of 15 nm . This differs significantly from size distributions directly measured in vehicle exhaust (vehicles placed on chassis dynamometers used for vehicle emissions certification), typically about 50 nm . The size distributions measured in the urban area depended on the distance to the nearest road. With increasing distance, the maximum of the size distribution increased, and the total number concentration decreased. This seems to be a result of particle growth due to processes such as coagulation and condensation, and dilution with the surrounding air. To clarify the transformation of the particle number size distributions measured in a street canyon into the urban-air background, a sectional aerosol model was used to calculate the evolution of the number size distribution, and included the effect of condensation, coagulation, dilution, and continuous entrainment of freshly emitted particles yielding good agreement with measurements.

New particle formation in the continental boundary layer: Meteorological and gas phase parameter influence

New particle formation in the polluted continental boundary layer was studied, based on 1.5-year observations of the particle size distribution, meteorological and gas phase parameters. Events of new particle formation involving significant ultrafine particle number concentrations (>104 cm−3 in the size range 3–11 nm) were observed on 20% of all days, pointing out that a frequent particle production from gaseous precursors can occur despite the relatively high pre-existing particle surface area in the area of investigation. The maximum in the observed particle size distributions was mostly above 3 nm, suggesting the actual particle nucleation to take place upwind of the measurement site. A particle growth analysis yielded 2.3±1.4 h as an upper limit of the time for the particles to grow from the critical cluster size till the observation of the peak in ultrafine number concentration. On 80% of the significant events of new particle formation (though not on all), SO2 concentrations increased considerably (by an average factor of 7), most likely by entrainment from aloft. Particle surface area was, on average, higher on event days compared to non-event days, indicating only a weak competition between condensation onto the pre-existing particle surface area and the new particle formation process. The highest statistical correlation was found between the events of new particle formation and solar radiation, indicating a high degree of meteorological control.

Atmospheric particle number size distribution in central Europe: Statistical relations to air masses and meteorology

Atmospheric particle number size distributions determined over 1.5 years at a central European site were statistically analyzed in terms of their relation to time of day, season, meteorology, and synoptic-scale air masses. All size distributions were decomposed into lognormal particle modes corresponding to the accumulation, Aitken, aged nucleation, and nucleation modes. The concentration of nucleation mode particles ( 30 nm) lacked such diurnal behavior, and proved to be indicative of different synoptic-scale air mass types. Over 70% of the time, air masses of Atlantic origin and maritime character prevailed, showing obvious signs of anthropogenic influence most of the time (accumulation mode: 500 cm−3; Aitken mode: 2300 cm−3). During a limited period of time (10%), however, continentally aged air with significantly enhanced concentrations of aerosol was observed (accumulation mode: 1200 cm−3; Aitken mode: 3300 cm−3). These air masses were advected from source regions in Russia, and eastern, southeastern, and central Europe, mainly under anticyclonic and high-pressure influence. The analysis provides a refined picture of the behavior of the particle number size distribution and provides parameterizations that are representative for a variety of air masses in Europe and thus suitable for future climate modeling applications.

A closure study of sub-micrometer aerosol particle hygroscopic behaviour

Abstract The hygroscopic properties of sub-micrometer aerosol particles were studied in connection with a ground-based cloud experiment at Great Dun Fell, in northern England in 1995. Hygroscopic diameter growth factors were measured with a Tandem Differential Mobility Analyser (TDMA) for dry particle diameters between 35 and 265 nm at one of the sites upwind of the orographic cloud. An external mixture consisting of three groups of particles, each with different hygroscopic properties, was observed. These particle groups were denoted less-hygroscopic, more-hygroscopic and sea spray particles and had average diameter growth factors of 1.11–1.15, 1.38–1.69 and 2.08–2.21 respectively when taken from a dry state to a relative humidity of 90%. Average growth factors increased with dry particle size. A bimodal hygroscopic behaviour was observed for 74–87% of the cases depending on particle size. Parallel measurements of dry sub-micrometer particle number size distributions were performed with a Differential Mobility Particle Sizer (DMPS). The inorganic ion aerosol composition was determined by means of ion chromatography analysis of samples collected with Berner-type low pressure cascade impactors at ambient conditions. The number of ions collected on each impactor stage was predicted from the size distribution and hygroscopic growth data by means of a model of hygroscopic behaviour assuming that only the inorganic substances interacted with the ambient water vapour. The predicted ion number concentration was compared with the actual number of all positive and negative ions collected on the various impactor stages. For the impactor stage which collected particles with aerodynamic diameters between 0.17–0.53 μm at ambient relative humidity, and for which all pertinent data was available for the hygroscopic closure study, the predicted ion concentrations agreed with the measured values within the combined measurement and model uncertainties for all cases but one. For this impactor sampling occasion, the predicted ion concentration was significantly higher than the measured. The air mass in which this sample was taken had undergone extensive photochemical activity which had probably produced hygroscopically active material other than inorganic ions, such as organic oxygenated substances.

Determination of Differential Mobility Analyzer Transfer Functions Using Identical Instruments in Series

The differential mobility analyzer (DMA) is an important tool for determining particle size distributions. The physical performance of a DMA is quantified by the concept of the transfer function. Therefore, knowledge of the transfer function is important to interpret the mobility distributions recorded by a DMA. During a calibration workshop in preparation for the Aerosol Characterization Experiment 1 (ACE1) field campaign, the transfer functions of five types of different mobility analyzers (Vienna-type DMA short, medium, and long, CIT radial, and TSI long; CIT = California Institute of Technology, Pasadena, CA; TST = TSI Inc., St. Paul, MN) were experimentally characterized by height, width, and area of their transfer functions. Different particles size ranges between 3 and 200 nm were investigated. The transfer function was determined by scanning a DMA across the mobility distribution produced by another, identical DMA. Subsequently, the data were processed by a deconvolution algorithm assuming a triangular shape for the transfer function. For all DMA types, the area of the transfer function decreased with particle size, especially for ultrafine particles (d_p < 20 nm). The gradient with which this area decreases with particle size, however, is different for each of the DMA types investigated. The calibration provides an improved description of the performance of each DMA, particularly in the ultrafine size range.

The contribution of sulfuric acid and non‐volatile compounds on the growth of freshly formed atmospheric aerosols

[1] The formation of atmospheric aerosol particles (homogeneous nucleation, forming of stable clusters ∼1 nm in size), their subsequent growth to detectable sizes (>3 nm), and to the size of cloud condensation nuclei, remains one of the least understood atmospheric processes upon which global climate change critically depends. However, a quantitative model explanation for the growth of freshly formed aerosols has been missing. In this study, we present observations explaining the nucleation mode (3-25 nm) growth. Aerosol particles typically grow from 3 nm to 60-70 nm during a day, while their non-volatile cores grow by 10-20 nm as well. The total particle growth rate is 2-8 nm/h while the non-volatile core material can explain 20-40%. According to our results, sulfuric acid can explain the remainder of the growth, until the particle diameter is around 10-20 nm. After that secondary organic compounds significantly take part in growth process.

Evolution of newly formed aerosol particles in the continental boundary layer : A case study including OH and H2SO4 measurements

An event of new particle formation is presented, based on simultaneous measurements of aerosol number size distributions, relevant gaseous components including H2SO4 and OH, and meteorological parameters. Measurements were conducted at Hohenpeissenberg, a rural continental mountain site in southern Germany. The event was observed under intense solar radiation, with total particle number concentrations increasing from 6000 to 25000 cm−3 within one hour, and ultrafine particles (3–11 nm) accounting for more than 50% of total number. Observed OH and H2SO4 concentrations reached maximum levels around 107 cm−3. A lower limit of the particle nucleation rate was estimated to be 3 cm−3·s−1, consistent with present models of ternary nucleation involving the H2SO4-H2O-NH3 system. Roughly 80% of the subsequent drop in ultrafine mode particle number concentration could be explained by coagulation. The observed particle growth rate of 2.1±0.1 nm/h was largely attributed to the condensation of measured H2SO4, assuming neutralization by ammonia.

Structure, variability and persistence of the submicrometre marine aerosol

Submicrometre dry number size distributions from four marine and one continental aerosol experiment were evaluatedjointly in the present study. In the marine experiments only data with back trajectories of at least 120 h without landcontact were used to minimize continental contamination. Log-normal functions were fitted to the size distributions.Basic statistics of the marine aerosol indicate a closed character of the size distribution at the lower size limit as opposedto an open character for corresponding continental data. Together with the infrequent occurrences of marine particlesbelow20 nmthis finding supports hypotheses and model results suggesting lowprobabilities of homogeneous nucleationin the marine boundary layer. The variability of submicrometre marine number concentrations was parametrized witha bimodal log-normal function that quantifies the probability of finding different number concentrations about a givenmedian value. Together with a four-modal log-normal approximation of the submicrometre marine size distributionitself, this model allows a statistical representation of the marine aerosol that facilitates comparison of experiments andvalidation of aerosol models. Autocorrelation at the one fixed marine site with a minimum of interruptions in timesseriesrevealed a strong size dependency of persistence in particle number concentration with the shortest persistenceat the smallest sizes. Interestingly, in the marine aerosol (at Cape Grim) persistence exhibits a size dependency thatlargely matches the modes in dg0, i.e. near the most frequent geometric mean diameters number concentrations aremost persistent. Over the continent, persistence of particle numbers is strongly constrained by diurnal meteorologicalprocesses and aerosol dynamics. Thus, no strong modal structure appears in the size-dependent persistence at Melpitz.As with the aerosol variability, marine aerosol processes in models of aerosol dynamics can be tested with these findings.

Size-segregated chemical, gravimetric and number distribution-derived mass closure of the aerosol in Sagres, Portugal during ACE-2

During the ACE-2 field campaign in the summer of 1997 an intensive, ground-based physical and chemical characterisation of the clean marine and continentally polluted aerosol was performed at Sagres, Portugal. Number size distributions of the dry aerosol in the size range 3−10 000 nm were continuously measured using DMPS and APS systems. Impactor samples were regularly taken at 60% relative humidity (RH) to obtain mass size distributions by weighing the impactor foils, and to derive a chemical mass balance by ion and carbon analysis. Hygroscopic growth factors of the metastable aerosol at 60% RH were determined to estimate the number size distribution at a relative humidity of 60%. A size segregated 3-way mass closure study was performed in this investigation for the first time. Mass size distributions at 60% RH derived from number size distribution measurements and impactors samples (weighing and chemical analysis) are compared. A good agreement was found for the comparison of total gravimetrically-determined mass with both number distribution-derived (slope=1.23/1.09; R2>0.97; depending on the parameters humidity growth and density) and chemical mass concentration (slope=1.02; R2=0.79) for particles smaller than 3 μm in diameter. Except for the smallest impactor size range relatively good correlations (slope=0.86−1.42) with small deviations (R2=0.76−0.98) for the different size fractions were found. Since uncertainties in each of the 3 methods are about 20% the observed differences in the size-segregated mass fractions can be explained by the measurement uncertainties. However, the number distribution-derived mass is mostly higher than the chemically and gravimetrically determined mass, which can be explained by sampling losses of the impactor, but as well with measurement uncertainties as, e.g., the sizing of the DMPS/APS.