4. Conclusions

A prototype glowing wire nanoparticle aerosol generator has been developed. The generator is optimized for the carrier gas flow rate of 1-1.5 L min^-1 and operating voltage of 6-9 V (electrical power: 30-50 W) to give steady and continuous number concentration of the order of 10⁷ cm^-3. Two chemically different metal wires were used in experiments focused on evaluating the performance of generator and specifying the characteristics of the generated aerosols. Measurements carried out using SMPS (> 10 nm) at generator exit showed that the geometric mean size of aerosols coming out from generator is ~ 15 nm with a geometric standard deviation of 1.4. The high concentration nanoparticles generated through this instrument provides controlled and tunable source term for performing some of the aerosol behavioral experiments aimed at understanding processes such as coagulation effects. The generator can further be used for synthesis of coated or uncoated metal nanoparticles for specific use.

References

• Peineke C, Attoui M, Robles R, Reber A C, Khanna S N, Schmidt-Ott A (2009) Production of equal sized clusters by a hot wire . J. Aerosols Sci, 40:423-430



• Schmidt-Ott A, Schurtenberger P, Siegmann H (1980) Enormous yield of photoelectrons from small particles. Physical Review Letters, 45(15):1284–1287.^-1

(Full version of this work can be found in J Nanopart Res (2014) 16:2776, DOI 10.1007/s11051-014-2776-5)

Arshad Khan ,   Manish Joshi ,
P Khandare ,  Amruta Koli ,
S Anand ,   B K Sapra

Radiological Physics and Advisory Division, Health Physics Division, Bhabha Atomic Research Centre, Mumbai – 400085, India

E-Mail: bsapra@barc.gov.in

Charge size distribution of common lab generated aerosols

Introduction

Particle size, shape, concentration and charge are some of the important parameters governing aerosol evolution, transport and deposition. Among these, charge was not an easily measurable parameter because of the tedious indirect measurements and hence ignored until recent times. In the past, charge measurement studies have been conducted for nebulizers and combustion sources (Bruce Forsyth et.al, 1998; Chuen-Jinn Tsai et. al, 2005). Earlier studies used Electrical analyzer or Dual mobility particle sizer to measure the signal in terms of number concentration which was then converted to charge fractions using charge equilibrium theories (Rodrigues et.al, 2006; Sahu et.al, 2012). With the advent of Electrical Low Pressure Impactor (ELPI), charge measurement has become more direct and easy.

Precise knowledge of charge status of particles is key information for sampling, transport, deposition on surfaces or in the human respiratory tract system, and applications like micro-contamination control. Electrical effects of particles are also studied extensively for applications in drug delivery, electrostatic precipitators used in industries, effect on residence time of aerosols in atmosphere etc.

Aerosol particles can be electrically charged by different techniques and the number of charges on a particle depends on the charging mechanisms, which in turn, are sensitive to material properties, temperature, humidity etc. There are several routes for generation of aerosol particles. They can be synthesized from the liquid phase by atomizing a solution of specific composition to form droplets that crystallize to solid particles upon subsequent evaporation of the solvent. They can also be formed from gas-to-particle conversion via nucleation and growth by condensation and coagulation. Gas-to-particle synthesis can be achieved by using furnace, flame, plasma, or laser

reactors, glowing wires and spark discharges. The magnitude and/or polarity of the charge are generally very difficult to predict even if the initial conditions during the generation process are known.

Most commonly utilized laboratory generator for aerosol studies is the nebulizer, which produces aerosols by mechanical dispersal of a solution. Combustion sources, on the other hand, are used to generate aerosols with very high number concentration (at source) of ultrafine size. Electrical heating of metal wires (usage for heating) is another source of aerosols which distinguishably generates high number concentrations in nano-size regime. Although characteristics such as concentration, size, chemical nature etc. have been measured and studied for these generators; very few studies are available related to their charge measurement.

The aim of the present study was to measure the charge size distribution of aerosols generated via three different mechanisms viz., Nebulizer (NaCl), Combustion (Incense stick) and Hot wire generator (Nichrome wire). Electrical low pressure impactor (ELPI) has been employed for charge measurements in this work.


Experiments were carried out in an aerosol chamber equipped with fan for homogenization and particle measurement systems such as ELPI and Condensation Particle Counter (CPC) (fig. 1). Condensation Particle Counter (CPC) was used for integral number concentration measurement and to validate the steadiness of the aerosol concentration in the chamber whereas, ELPI is a particle spectrometer for measuring airborne particle size distribution (number and charge) in