P. Villani

High frequency new particle formation in the Himalayas

Rising air pollution levels in South Asia will have worldwide environmental consequences. Transport of pollutants from the densely populated regions of India, Pakistan, China, and Nepal to the Himalayas may lead to substantial radiative forcing in South Asia with potential effects on the monsoon circulation and, hence, on regional climate and hydrological cycles, as well as to dramatic impacts on glacier retreat. An improved description of particulate sources is needed to constrain the simulation of future regional climate changes. Here, the first evidence of very frequent new particle formation events occurring up to high altitudes is presented. A 16-month record of aerosol size distribution from the Nepal Climate Observatory at Pyramid (Nepal, 5,079 m above sea level), the highest atmospheric research station, is shown. Aerosol concentrations are driven by intense ultrafine particle events occurring on >35% of the days at the interface between clean tropospheric air and the more polluted air rising from the valleys. During a pilot study, we observed a significant increase of ion cluster concentrations with the onset of new particle formation events. The ion clusters rapidly grew to a 10-nm size within a few hours, confirming, thus, that in situ nucleation takes place up to high altitudes. The initiation of the new particle events coincides with the shift from free tropospheric downslope winds to thermal upslope winds from the valley in the morning hours. The new particle formation events represent a very significant additional source of particles possibly injected into the free troposphere by thermal winds.

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 calculation of the minimum length of heating tube needed to completely evaporate the aerosol particles at high temperature with sufficient residence time, as well as to cool the aerosol sample down to ambient temperature. Next, the aerosol heating rate and aerosol deposition losses within the flow tube were estimated, and re-condensation of volatilized compounds evaluated. Then the VTDMA was calibrated and tested in the laboratory to determine the transfer efficiency, and finally, atmospheric aerosols were analyzed, with the first results presented here. This work emphasizes the need for better standardization of thermo-desorbing units for atmospheric aerosol studies. *Now at: Laboratoire de Chimie et Environnement, Université de Provence.

Design and Validation of a Volatility Hygroscopic Tandem Differential Mobility Analyzer (VH-TDMA) to Characterize the Relationships Between the Thermal and Hygroscopic Properties of Atmospheric Aerosol Particles

The nature of atmospheric aerosols is extremely complex and often requires advanced analytical tools for the determination of its physical and chemical properties. In particular, the interaction of particles with atmospheric water is a complex function of both particle size and composition. The ability of a particle to grow in a humid environment can be measured by humidity tandem differential mobility analyzing techniques (H-TDMA). In this article, we present a new development combining thermo-desorption and humidification aerosol conditioning in series that allows to measure changes in the hygroscopic behavior of aerosol at 90% relative humidity (RH) after conditioning of the particle by thermo-desorption to a temperature between 25°C and 300°C. The main feature of this system, named Volatility Hygroscopic—Tandem Differential Mobility Analyzer (VH-TDMA), is to allow for rapid (10 minutes) series of scans to control particle response to 1-thermal conditioning, 2- RH increase to 90% and 3—a combination of both thermal and RH conditioning. The VH-TDMA is, therefore, suited to investigate particle ageing through a simple coupling of H-TDMA and V-TDMA performances. The aim of the present article is to describe the instrument design and to validate its performances by focusing on the measurement of hygroscopic behavior of pure inorganic particles such as sodium chloride or ammonium sulfate, as well as internally mixed organic-inorganic particles. Based on laboratory experiments and applications to natural aerosols, we show that the VH-TDMA system can be used to investigate the hygroscopic properties of the non-volatile fraction of ambient sub-micrometer aerosols in the range of 20 to 150 nm and the influence of the more volatile fraction of the particle on hygroscopic growth.

Role of the Volatile Fraction of Marine Aerosol on its Hygroscopic Properties

The hygroscopic growth factor (HGF) of 85 and 20 nm particles marine aerosols was measured during January 2006 for a three-week period within the frame of the MAP (Marine Aerosol Production) winter campaign, using the TDMA technique. The results were compared to aerosol produced in a simulation tank by bubbling air through sea water sampled at the coastal site of Mace Head during the campaign, and through synthetic sea water (exempt of organic substances). This simulation was assimilated to primary production. The 85 nm HGF observed in the atmosphere during clean marine sectors were lower than the ones measured from the bubbling processes: the sea salt HGF mode is slightly lower and there is an additional 1.5 HGF mode on the atmospheric aerosol. This would indicate that either the sea water sampled near Mace Head was not as rich in hydrophobic matter as further up wind or that secondary processes have occurred during transport. The role of the volatile fraction of the aerosols was then studied by gently heating the particles at 90°C (without particle size change) and measuring the subsequent HGF change, with a combination of Volatility and Hygroscopicity TDMA (i.e., the VHTDMA). We observed that the volatilization of less than 10% by diameter of the particles lead to an inhibition of the 1.5 GF mode for 85 nm particles but not for 20 nm particles. This result would indicate that secondary condensing processes, implying volatile substances, would have influenced the 85 nm particles. These results only apply to low biological activity periods.