A. Wiedensohler

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.

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.

Variability of the aerosol number size distribution in Beijing, China: New particle formation, dust storms, and high continental background

Continuous measurements of aerosol number size distributions from 3 nm to 10 μm have been first performed within the city area of Beijing since March 2004. Size distributions of the first 45 measurement days (March 05 to April 18, 2004) were investigated in terms of their high variability. Two dust storm events were observed indicated by high number concentrations greater than 1 μm and mass concentrations around 1 mg m -3 . Continental highly polluted air was observed during 12 days indicated by a number peak in the accumulation mode range, and submicrometer volume concentrations above 150 μm 3 cm -3 were observed. Newly formed particles with more than 100,000 cm -3 were observed on 25 days when the particle surface area concentration drops below a critical value (100-2000 μm 2 cm -3 ) because of clean air from the north. Measurements show only a slight growth (∼1 nm h -1 ) of the particles indicating that they are produced within the city area of Beijing.

Design and calibration of a thermodenuder with an improved heating unit to measure the size-dependent volatile fraction of aerosol particles

Separated measurements of volatile and non-volatile fractions of atmospheric aerosols have become more important due to possible effects of non-volatile particle on respiratory diseases. Measurements of non-volatile particle fractions with considerable size and time resolution are therefore needed. The combination of a thermodenuder (TD) with aerosol size spectrometers can provide size distributions of non-volatile particles. TDs usually consist of two units: a heating and a cooling tube. Unfortunately, available TDs have some disadvantages concerning the heated tube, where volatile material should be desorbed from the particles. The residence time in the heating zone might be too short to completely evaporate the volatile material or the temperature at the exit of the heating tube might be below the condensation temperature of the evaporated species. Here, a new TD was developed with a longer residence time at the requested temperature and a more effective insulation to prevent recondensation at the end of the heating unit. Additionally, the TD was calibrated for size-dependent particle losses at different heater temperatures.

Spectral absorption coefficients and imaginary parts of refractive indices of Saharan dust during SAMUM-1

During the SAMUM-1 experiment, absorption coefficients and imaginary parts of refractive indices of mineral dust particles were investigated in southern Morocco. Main absorbing constituents of airborne samples were identified to be iron oxide and soot. Spectral absorption coefficients were measured using a spectral optical absorption photometer (SOAP) in the wavelength range from 300 to 800 nm with a resolution of 50 nm. A new method that accounts for a loading-dependent correction of fibre filter based absorption photometers, was developed. The imaginary part of the refractive index was determined using Mie calculations from 350 to 800 nm. The spectral absorption coefficient allowed a separation between dust and soot absorption. A correlation analysis showed that the dust absorption coefficient is correlated (R2 up to 0.55) with the particle number concentration for particle diameters larger than 0.5 μm, whereas the coefficient of determination R2 for smaller particles is below 0.1. Refractive indices were derived for both the total aerosol and a dust aerosol that was corrected for soot absorption. Average imaginary parts of refractive indices of the entire aerosol are 7.4 × 10−3, 3.4 × 10−3 and 2.0 × 10−3 at wavelengths of 450, 550 and 650 nm. After a correction for the soot absorption, imaginary parts of refractive indices are 5.1 × 10−3, 1.6 × 10−3 and 4.5 × 10−4.

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.

Identification of extratropical two‐way troposphere‐stratosphere mixing based on CARIBIC measurements of O3, CO, and ultrafine particles

Simultaneous measurements of O3, CO, and ultrafine aerosol particles (UFP), conducted on board of a Boeing 767-ER passenger aircraft flying from Sri Lanka to Germany (project CARIBIC), are used to study two-way cross-tropopause mixing near a subtropical tropopause fold. On the equatorward side of the fold, downward mixing of stratospheric air into the upper troposphere is identified by enhanced concentrations of O3 and 14CO. Very high UFP number concentrations of up to 1.5×104 cm−3 (STP) were encountered inside the poleward half of the fold. This accumulation of small particles is explained by recent extensive aerosol nucleation, most likely triggered by the mixing of stratospheric air with tropospheric air injected into the fold. Further, nine particle formation events were observed outside the fold which are attributed to isolated cells of deep convection and to rising air parcels under cyclonic conditions that mix with surrounding air. In the upper troposphere O3 and CO were found to be correlated with high ΔO3/ΔCO ratios of 0.6 to 1.5. In the fold the correlation was strongly negative with ΔO3/ΔCO; = −3.5; but the high CO mixing ratios of 100 ppb at O3 mixing ratios of 250 ppb point to earlier injection of tropospheric air, in agreement with the UFP measurements.

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.

SUBMICROMETER AEROSOL SIZE DISTRIBUTIONS AND MASS CONCENTRATION OF THE MILLENNIUM FIREWORKS 2000 IN lEIPZIG, GERMANY

In this technical note, we present submicrometer aerosol number and volume size distribution as well as mass concentration during the Millennium fireworks measured at the Institute of Tropospheric Research (IfT) in Leipzig and their correlation with simultaneous measurements.