Europa

condition is 1.2_×1013_.

The rationale for the above tests was an attempt to achieve better agreement be- tween measured and modelled data by trying to make the residence time of particles in the charger closer to uniform, as modelled. To have perfectly uniform flow however is an unrealistic expectation for “real world” conditions. Therefore, in order to improve the charging model further consideration would have to be given to modelling the residence times of particles in the charging zone, as well as trying to improve the uniformity of the flow. Possible strategies for further improving the flow uniformity could include flow screens with a still higher blockage, baffles up stream of the flow screen, and also possibly a sheath flow of clean air surrounding the aerosol flow in the charging zone so that the particles do not flow in the boundary layers near the walls where residence times will be longer,

8.5

Additional Charger Data

This section details some additional data, and points of interest relating to the charger that were discovered during these tests.

8.5.1 The Charge Concentration

TheNitselected for use in the model was chosen on the basis that it was the value that gave us the best results, rather than any measure ofNiin the charging zone. Measuring

Ni in the charging zone would be quite challenging, however the current flowing from the corona wires can be measured. The results of this measurement is presented in table 8.1.

Wire Voltage Measured Current Elementary charges per Second

3.5kV 1µA 6.25_×1012_#/s 4kV 4.3µA 2.69_×1013_#/s 4.5kV 18µA 1.13_×1014_#/s 5.0kV 36µA 2.25_×1014_#/s 5.5kV 59.4µA 3.72_×1014_#/s 6.0kV 84.5µA 5.28_×1014_#/s

Table 8.1: Measured corona currents at various voltages.

The charger is operating with a sample flow of 3lpm, and a residence time for particles of about 0.5 sec. Therefore for the values ofNi (#/m3) in the charging zone,

that have been used in the model to be correct. Only a small fraction of the ions produced must make it to the charging zone without being precipitated on the walls of the charger or the screen through which the ions must travel.

The table also shows an increase in the ion production of approximately 4 times between 3.5 and 4kV, and approximately 8 times between 4 and 5kV. If the values used in the model are correct then the increase in the charge concentration in the charging zone is 2.4 times between 3.5 and 4kV , and 2.5 times between 4 and 5kV. This would suggest that the ion production efficiency (number of ions that reach the charging zone/number of ions produced) decreases with increasing voltage. This very same trend in ion production efficiency is seen in our tests on the Sonic Jet Charger in chapter 6.

8.5.2 The Image Force Parameter

The charging model contains an expression for the image force (equations 5.6 and 5.7), this is used to model Van der Waals attraction between ions and particles with low, or neutral charge. To model the image force we must specify the parameter given in equation 8.1.

(₋1)

(+ 1) (8.1)

Whereis the dielectric constant of the particle material.

This parameter only has a significant effect on the prediction of charge distribution on smaller particles. Choosing a value is difficult, as the Nanoparticle Spectrometer is not designed to be able to differentiate particle species. The same charging model used here was tested experimentally [Biskos et al, 2005] for a diffusion charger of similar design. The results in this paper also show good agreement between experimental and predicted results, and the modelled data presented was generated with the image force parameter, set to 1 (i.e. assuming electrically conducting particles).

During the course of our experiments it was also found, through comparison between predicted and measured results, that the best agreement was achieved with the image force parameter set to 1

8.5.3 Accidental Field Charging

The experimental data presented below was obtained during a different set of tests, which are presented more fully in the next Chapter, (section 9.2). However this par- ticular data is presented here as it is most relevant to the charger. (More detailed information about the apparatus and procedure can be found in section 9.2.)

8.5. ADDITIONAL CHARGER DATA 147 If the Nanoparticle Spectrometer is set up to run with the electrometer filters re- moved it is possible to divert some of the sample coming out of one of the channels and analyse it in the SMPS. Figure 8.14 Shows the SMPS measured output from channel 3 in the Nanoparticle Spectrometer.

100 101 102 103 0 200 400 600 800 1000 1200 1400 dp (nm) dn/dlogdp

Figure 8.14: SMPS analysis of channel 3 output, with charger sheath flowin equal to sheath flow out

It is known that only particles of a certain electrical mobility can land on channel 3. It is also known how much charge a particle with a given diameter must have to be of high enough mobility to land in channel 3. The 10nm spike in fig 8.14 is expected. This corresponds to singly charged particles of this size. The second small broad peak is not expected, particles of this size would need a lot of charge to be found in channel 3 in many cases in excess of 50 elementary charges are required.

The origin of these particles could be leakage of uncharged atmospheric particles into the instrument. The other possibility is that they are particles that have passed through the screen in the charger that separates the corona wires from the charging zone. If the particles pass into this region they will be subject to field charging, which can result in higher charge levels than diffusion charging.

In order to prevent particles from passing through this screen and becoming field charged, it was decided to adjust the flow rates in the charger, so that the charger sheath flow in was slightly higher than the sheath flow out. This was intended to create a slight flow of sheath air up through the screen and into the charging zone. Figure 8.15 shows another sample taken from channel 3 after these new flow conditions had been implemented. In this plot no evidence of larger particles can be seen.

tests. 100 101 102 103 0 100 200 300 400 500 600 700 800 dp (nm) dn/dlogdp

SMPS data Channel 3, Charger sheath in > out

Figure 8.15: SMPS analysis of channel 3 output, with charger sheath flow in greater than sheath flowout

Chapter 9

The Nanoparticle Spectrometer:

Testing and Results

9.1

Introduction

This chapter details the testing carried out on the instrument, and presents the results of these tests. The tests presented here were designed to establish the following.

• Where do particles of a given mobility land within the classifier, and does this agree with the theoretical model?

• What is the output signal from the instrument’s electrometers for a given aerosol sample, and does this agree with the theoretical model?

• Can the output signal be used by the look-up program to identify a size distri- bution for the sample, that is in agreement with that identified by a “standard” SMPS.

In this chapter we seek to present some results, and the analysis of these results with some basic discussion and conclusions. A more detailed discussion on the accuracy and meaning of the results, as well as ideas for future work, is presented in the discussion in chapter 10.