proteins-a coat protein, a maturation protein, a replicate subunit (or RNA replicase 0 chain), and

a lysis protein and 180 copies of the capsid protein (Fiers et al. 1976). The MS2 virus stock

was prepared by suspending freeze-dried MS2 (ATCC 15597-B1TM), which contains a small

amount of milk proteins and organic molecules for virus preservation, with filtered deionized

(DI) water to a concentration of 109-1010 plaque-forming units (PFU)/mL and stored at 4 OC.

Experimental design

 The experimental set-up to investigate the infectious and non-infectious viruses as a

function of particle size is shown in Figure 4-1. The aerosols containing viruses were produced

by a Collison nebulizer and dried in the dilution dryer to remove water content. The resultant

aerosol had a polydisperse particle size distribution (PSD), which was characterized by using the

scanning mobility particle sizer (SMPS), a device that operates as the combination of an

electrostatic classifier with a long differential mobility analyzer (DMA) and a condensation

particle counter (CPC), as shown in Figure 4-1 (A). Since the change of PSD over the entire

generation is important, the PSD was monitored for 35 min, which was the time needed to

conduct the experiment.

 The voltage applied to the differential mobility analyzer (DMA) can be tuned to allow

only aerosols of a specific size to exit the electrostatic classifier. The size-classified aerosols

were subsequently collected in a BioSampler (SKC Inc., Eighty Four, PA, USA) for 5 mins

with a flow rate of 4.5 Lpm as shown in Figure 4-1 (B). The reason for using a flow rate lower

than the standard one (i.e., 12.5 Lpm) is to avoid significant reaerosolization from the impinger

at the higher flow rate. Because Riemenschneider et al. (2009) reported insignificant

reaerosolization (<1 %) over short sampling periods, 5 min of sampling time was selected. The

samples in the BioSampler were then analyzed with a plaque assay method (Lee et al. 2009) by