pH
The relative viability of MS2 in droplets at pH values of 4.0, 7.0, and 10.0 (i.e., acidic, pH-neutral, and basic) was generally similar at any one RH level (Fig 3A and 3C). At 20% RH, MS2 survived better, although not significantly, in pH-neutral droplets than in more acidic or more basic droplets. There were no significant differences in viability across pH at the other two RHs. Regardless of pH, viability was significantly lower at 50% RH compared to the other RHs.
Fig 3  Concentration of bacteriophages (A) MS2 and (B) Φ6 in droplets with different initial pH values before (dark bars) and after (light bars) 1 h exposure to low, intermediate, and high RH (mean ± s.d. of triplicates). Relative viability of (C) MS2 and (D) Φ6 after 1 h exposure (lines show the mean of triplicates). The number of virions in droplets at the start of the exposure experiments was 105−106 PFU. The dark gray dashed line indicates the detection limit (104 PFU/mL) of plaque assay. ND indicates no viable virus was detected. The relative viability of Φ6 differed significantly by pH, while the patterns in viability were similar across all three RHs (Fig 3B and 3D). At a pH of 4.0, no viable Φ6 was detected in either the control solution or the evaporating droplets after 1 h, suggesting a strong inactivation effect of acidic conditions on Φ6. At a pH of 10.0, the virus decayed by ~1–3 log10 units depending on RH, while the virus survived best in pH-neutral droplets (7.0), in which it decayed by ~1–2 log10 units depending on RH. At both these pHs, relative viability was greater at 20% RH compared to the two higher RHs.