Figure 2: Typical ZnO particle size distribution during ALI submerged (SUB) exposure conditions, respectively. When comparing the particle size distributions during ALI and submerged exposures, one has to consider that different sizing instruments were used. As discussed in the text, both SMPS (ALI) and DLSS (submerged) measure the particle mobility diameter. Furthermore, the volume distribution (normalized to the maximum volume level) is approximately equal to the (normalized) light intensity distribution in the size range between about 315 to 1250 nm (highlighted by the grey shaded area), which encompasses most of the size regime of interest here. The ALI size distributions showed volume-weighted median diameters of about  nm and a width of (geometric standard deviations). For the submerged conditions (SUB), dynamic light scattering measurements (DLSS) showed ZnO aggregates of about 900 nm (mobility diameter) with a less pronounced (~20%) secondary mode near 350 nm. Thus it is evident that the average ZnO agglomerates were considerably larger during submerged than during ALI exposure condition. For comparison with other studies, the number-weighted size distribution of the ZnO particles during ALI exposure had a count median diameter of about 140 nm (data not shown).