We calculated particle volume in the 8.5 -700 nm size range (electrical mobility diameter) from the DMPS particle size distribution assuming spherical particles. To estimate particle mass we assumed a density of 1.5 g cm , corresponding approximately to dilute sulfuric acid droplets at the temperature (~10 -15 °C) and relative humidity (15 -20%) conditions observed here (sulfuric acid weight fraction ~ 60%) . We denominate the particle mass calculated this way with PM0.7DMPS, and assign to it a relative uncertainty of 30% (k = 2) considering uncertainties in the particle density and the DMPS collection efficiency.