Volatility Tandem Differential Mobility Analyzer Research Articles

Influence of soot mixing state on aerosol light absorption and single scattering albedo during air mass aging at a polluted regional site in northeastern China

An aerosol optical closure study was performed using the observed high time‐ and size‐resolved soot mixing states determined by a Volatility Tandem Differential Mobility Analyzer (VTDMA) at a polluted regional site, Yufa, in the south of Beijing during the summer of 2006. Good agreement was found between the simulated and measured aerosol absorption (σap, R = 0.9) and scattering (σsp, R ≥ 0.95). The soot mixing state at Yufa can be generally determined by VTDMA, in terms of properly predicting the σap using a simple optical model combined with spherical homogeneous and core‐shell coated Mie codes. The possible uncertainties in the modeled σap were discussed. Rapid soot aging was observed, which led to large variations in the fractional contributions to σap by externally mixed and coated soot. On average, about 37% of the σap (∼10–60%) arose by the coated soot. The coating enhancement in σap and σsp of the coated soot can reach up to a factor of 8–10 within several hours owing to the secondary processing during daytime. It was contributed not only by the increased thickness of coating shell, but also the transition of soot from externally mixed to coated one. Hence, assuming constant soot mixing state for the regional climate model is not realistic and may lead to uncertainties. In the highly polluted region in northeastern China, the aerosol single scattering albedo may increase very fast owing to the rapid secondary particle formation and condensation (up to 0.90–0.95). This increase took place although the concurrent coating processing enhanced the light absorption capability of soot.

Dependence of size‐resolved CCN spectra on the mixing state of nonvolatile cores observed in Tokyo

Size‐resolved cloud condensation nuclei (CCN) spectra and volatility tandem differential mobility analyzer (VTDMA) measurements were made simultaneously in Tokyo in wintertime 2007 to characterize CCN activity near the urban center. Ambient particles with mobility diameters of 30–200 nm were investigated at supersaturations (SSs) of 0.32, 0.60, and 0.88%. The size distributions of the nonvolatile cores of size‐selected particles measured by the VTDMA were bimodal; one mode showed relatively small changes (<10%) in peak diameter by volatilization, and the other mode showed significant changes in diameter (>10% in peak diameter). The former mode is referred to as less volatile (LV), and the latter mode is called more volatile (MV). The main component of nonvolatile cores in Tokyo is known to be black carbon (BC). Therefore, it is likely that LV particles correspond to soot particles. The size‐resolved CCN spectra were broader than those of (NH4)2SO4, indicating that the observed particles were not uniformly mixed. In addition, CCN/CN (CN: condensation nuclei) ratios were smaller than unity after the stepwise increase. A CCN‐LV correlation analysis shows that for small (<80 nm) particles, the slopes of the correlations were smaller than unity, although the correlations were significant. This indicates that CCN‐inactive fractions are explained by LV particles and particles coemitted with LV particles (likely primary organic aerosol particles). For larger particles (>100 nm), the CCN‐inactive fractions were close to the LV particle fractions, suggesting that CCN‐inactive particles were composed of fresh soot.

Application of the Volatility-TDMA Technique to Determine the Number Size Distribution and Mass Concentration of Less Volatile Particles

A Volatility-Tandem-Differential-Mobility-Analyzer (VTDMA) and a Differential Mobility Particle Sizer (DMPS) were used to determine the number and mass concentration of externally mixed aerosol particles in urban background air. In the VTDMA the less-volatile (LV) particle fraction was measured at 300°C for particles in the size range 20–250 nm. The LV particle fraction was converted to the number concentration of LV particles (NLV) and the mass concentration (MLV). MLV was compared with the mass concentration of black carbon (MBC) measured by a Multi-Angle Absorption Photometer (MAAP). The DMPS and VTDMA data were used for calculating scattering and absorption coefficients (σSP and σ AP) with a Mie model and compared with σ SP and σ AP measured with a TSI nephelometer and the MAAP. The model was run by assuming external and internal mixing of absorbing and scattering aerosol. The best fit of measured and modeled σ SP and σ AP was sought by varying the refractive index. During periods dominated by local e...

Design and Validation of a 6-Volatility Tandem Differential Mobility Analyzer (VTDMA)

A volatility tandem differential mobility analyzer (VTDMA) was developed to allow fast field measurement of the volatile fraction of atmospheric aerosol particles in the particle size range 20–500 nm. In this VTDMA the volatile compounds are evaporated by heating the aerosol to a temperature between 25°C and 300°C. The heating unit is equipped with six symmetric columns kept at different temperatures that allow the heating temperature to be rapidly changed so that a higher temporal resolution can be achieved compared to a regular VTDMA. This work first focuses on the design and calibration of the heating units for the conditioning of a selected aerosol sample while minimizing sample losses due to thermophoresis and diffusion. The design was based on the modeling of the profiles of temperature and velocity and the behavior of a monodisperse aerosol in the heating units, using Computational Fluid Dynamics (CFD) Flow Modeling Software. This allowed for initial estimations of the heater dimensions and also ca...

Dependence of CCN activity of less volatile particles on the amount of coating observed in Tokyo

A novel observation system has been developed to measure cloud condensation nuclei (CCN) activity simultaneously with volatility of aerosol particles at 400°C. CCN/CN ratios were measured using a CCN counter and a condensation particle counter, and volatility was measured with a volatility tandem differential mobility analyzer (VTDMA). This system was connected to a hygroscopicity tandem differential mobility analyzer in tandem to measure these properties of hygroscopicity‐selected particles. These instruments were deployed in Tokyo in November 2004. In Tokyo, the dominant fraction of nonvolatile cores at 400°C has been found to be black carbon (BC). Therefore the number size distribution measured by VTDMA represents the mixing state of black carbon particles. The VTDMA measurement of hygroscopicity‐selected particles revealed that most of the less hygroscopic particles were less volatile, while more hygroscopic particles were more volatile. CCN/CN ratios of less volatile particles [(CCN/CN)LV] are estimated from CCN activity and volatility measurements of less hygroscopic particles. (CCN/CN)LV at SS = 0.5% is correlated with the decrease of peak diameter due to volatilization (Δdm), and (CCN/CN)LV is nearly equal to unity at Δdm = 10 nm. This result suggests that the CCN activity of BC particles significantly depends on the amount of coating.

Characterization of aging wood chip combustion aerosol in an environmental chamber

Aging of aerosol from wood chip combustion in a stoker burner was monitored in an outdoor environmental chamber for 19–27 h in order to study the size, volatility and organic carbon (OC) content of the combustion aerosol particles during aging. A scanning mobility particle sizer, a volatility tandem differential mobility analyzer (VTDMA), and a thermal–optical carbon analyzer were utilized. The VTDMA and carbon analyses were performed at the beginning, after 17–24 h of aging and at one intermediate point. The size decrease of freshly emitted particles was 6–10% when heated to 360 ∘ C , and was found to depend on the experiment start time. For particles aged for 24 h, a 74–86% decrease in particle size at 360 ∘ C was observed. The more volatile OC fraction and the total OC fraction in the particles increased and the less volatile OC fraction decreased with aging. This suggests that during aging more volatile compounds condense on or heavier compounds photodegrade into lighter ones in the particles. Occasionally, new particle formation and growth were observed in the following day. The new particles were found to be composed mainly of volatile material.

Volatility measurements of photochemically and nebulizer-generated organic aerosol particles

A volatility-tandem differential mobility analyzer (VTDMA) was designed to monitor the non-volatile fraction of reaction chamber-generated organic aerosol particles. Using a coiled-tube heater for particle volatilization, an analysis of the carrier gas temperatures was made for wall temperatures of 100, 150, and 200 ∘ C . Oligomerization in organic aerosol particles generated from irradiations of α -pinene/ NO x / H 2 O /pure air in a reaction chamber shows a strong linear relationship with time and has an oligomerization rate of 1.3 % h - 1 for the 100 ∘ C wall temperature case. This rate is almost three times slower than the previously published rate for irradiations of 1 , 3 , 5 -trimethylbenzene/ NO x / H 2 O /pure air. In addition, experiments using the VTDMA demonstrated that oligomerization of nebulizer-generated methylglyoxal particles can occur in the absence of NO x , propene, and irradiation. The oligomerization of organic aerosol particles has implications which are highly relevant to the atmosphere, such as influencing hygroscopic growth and cloud condensation nuclei activation.

Variability of non-volatile fractions of atmospheric aerosol particles with traffic influence

Emissions from vehicular traffic are considered to be a major source of anthropogenic submicrometer particles in the urban environment. In this study, volatile and non-volatile number and volume fractions were distinguished in particles sampled for three consecutive days (Friday, Saturday, and Sunday) close to a highway (HW) (A4) in Germany and for one day in the urban area of Aachen, Germany. A volatility tandem differential mobility analyzer (VTDMA) and a thermodenuder (TD) combined with two scanning mobility particle sizers (SMPS) were used to get insight into a size-resolved mixing state of volatile and non-volatile particle fractions. Operating the VTDMA at 280 °C, the number size distribution of non-volatile particle cores was determined for initial particle sizes of 30, 50, 80, and 150 nm. The number size distributions from 10 to 400 nm of ambient and non-volatile aerosol particles were measured by using parallel a SMPS and TD/SMPS combination, respectively. Number size distributions measured near the HW showed a bimodal size distribution with a maximum number concentration at particle sizes between 10 and 20 nm. The TD/SMPS results for the HW site revealed that the nucleation mode disappeared after heating (completely volatile) and in total 10–20% by number of traffic-related particles were non-volatile. The VTDMA results revealed that only 22% of the 30 nm particles had a non-volatile core above the instrument detection limit of 10 nm. With increasing particle diameter, this non-volatile number fraction increased to ca. 60% for 50 nm particles and to approximately 100% for 80 and 150 nm particles. These findings mean that each particle in the upper Aitken and lower accumulation mode range contains a non-volatile core. With increasing traffic influence the number fraction of less-volatile particles representing mainly primary soot emissions increases to 62% for 50 nm particles and 71% for 80 nm particles.

Measurements of non-volatile fractions of pollution aerosols with an eight-tube volatility tandem differential mobility analyzer (VTDMA-8)

A volatility tandem differential mobility analyzer (VTDMA) was developed that offers high size resolution with high temporal resolution: the eight heating columns of the VTDMA-8 allow rapid measurement of the non-volatile component of aerosol particles within a narrow particle size range in about 15 min . The non-volatile composition of these quasi-monodisperse particles is described in terms of number fraction Φ N and volume fraction Φ V of the non-volatile material. Laboratory studies were made to determine the transfer efficiency and the volatile behavior of sulfate particles in the VTDMA-8. Its applicability to atmospheric aerosols was confirmed during three field measurements. Since the distribution of carbonaceous material depends on the distance to combustion sources and varies with region (i.e., urban or rural) and with time, we made continuous surface measurements in air masses polluted to different levels: (a) in Falkenberg, a moderately polluted, rural region in north-eastern Germany, (b) in the city of Leipzig, a strongly polluted environment, and (c) directly at a combustion source in the exhaust of a diesel-fueled vehicle. The results show that the refractory material contained in 150 nm atmospheric particles (at Falkenberg and Leipzig) ranged between 8% and 25% in volume and represents an external mixture of particles containing various amounts of non-volatile material. However, the vehicle-emitted 50 nm particles at a single combustion source were internally mixed, and that non-volatile fraction ranged between 75% and 98% in volume, depending on the operating conditions of the vehicle (speed, engine revolution). The results show that throughout all measurements the total number of particles remained unchanged after evaporation of the volatile material. Our results demonstrate that the VTDMA-8 can rapidly measure the non-volatile fractions of size-segregated pollution aerosols and that it is applicable to study the mixing state of aerosols and the effect of carbonaceous material on aerosol aging processes.

A New Volatility Tandem Differential Mobility Analyzer to Measure the Volatile Sulfuric Acid Aerosol Fraction

Abstract A volatility tandem differential mobility analyzer (VTDMA) was developed with the intention to measure the fraction of sulfuric acid in marine fine aerosols (Dp < 150 nm). This work focused on the design and calibration of an aerosol conditioner for the standard tandem differential mobility analyzer that heated a selected aerosol sample in a controlled manner while it minimized sample losses due to thermophoresis and diffusion. In designing an aerosol heater, the behavior of a monodisperse aerosol was modeled in a heated flow tube with a temperature-controlled wall under laminar flow conditions. This allowed for initial estimations of aerosol heating rates, heater dimensions, and aerosol deposition losses within the flow tube. The evaporation rate of sulfuric acid particles was also predicted and used to determine the minimum length of the aerosol heater needed to completely evaporate a sulfuric acid aerosol. Finally, the VTDMA was calibrated and tested for its ability to detect a sulfuric acid m...