The particle size distribution of aerosols produced by electronic cigarettes was measured in an undiluted state by a spectral transmission procedure and after high dilution with an electrical mobility analyzer. The undiluted e-cigarette aerosols were found to have particle diameters of average mass in the 250-450 nm range and particle number concentrations in the 10(9) particles/cm(3) range. These measurements are comparable to those observed for tobacco burning cigarette smoke in prior studies and also measured in the current study with the spectral transmission method and with the electrical mobility procedure. Total particulate mass for the e-cigarettes calculated from the size distribution parameters measured by spectral transmission were in good agreement with replicate determinations of total particulate mass by gravimetric filter collection. In contrast, average particle diameters determined for e-cigarettes by the electrical mobility method are in the 50 nm range and total particulate masses calculated based on the suggested diameters are orders of magnitude smaller than those determined gravimetrically. This latter discrepancy, and the very small particle diameters observed, are believed to result from almost complete e-cigarette aerosol particle evaporation at the dilution levels and conditions of the electrical mobility analysis. A much smaller degree, ~20% by mass, of apparent particle evaporation was observed for tobacco burning cigarette smoke. The spectral transmission method is validated in the current study against measurements on tobacco burning cigarette smoke, which has been well characterized in prior studies, and is supported as yielding an accurate characterization of the e-cigarette aerosol particle size distribution.

Particle size distribution and number concentration measurements of mainstream cigarette smoke are reported for commercial cigarettes encompassing a broad range of design parameters. Measurements were made using a Cambustion DMS500 fast particulate spectrometer. Twenty-nine brand styles were evaluated using a 60-mL puff of 2-s duration taken once every 30 s. A subset of cigarettes was evaluated using additional smoking regimens to explore the influence of puff volume and filter ventilation blocking. The DMS500-derived particulate matter mass was compared with filter-collected mass to assess the reliability of the aerosol measurements. Under the 60-mL/2-s puffing condition, all puffs for all products were observed to exhibit count median diameters between 145 nm and 189 nm. Measured particle size was 12-22 nm smaller for a 60-mL puff relative to a 35-mL puff. Partial or complete filter ventilation blocking under the 60-mL/2-s puffing condition had a small effect on particle size. Some trends in particle size as a function of puff number and smoking regimen appear consistent with a tobacco-rod residence time/coagulation hypothesis; however, other observations suggest that smoke formation processes in addition to coagulation influence particle size. The DMS500 underestimates smoke particulate mass relative to gravimetric filter collection, indicating evaporation of cigarette smoke particulate matter within the instrument. Approximately 75% of the evaporated mass can be attributed to particulate phase water. Some data also suggest a possible underestimation of number concentration. This introduces a significant confounding bias in the measurements and limits the information on smoke formation that can be extracted.

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The effect of Brownian coagulation on the particle size distribution of mainstream cigarette smoke subjected to conditions encountered in the mouth during human smoking has been examined experimentally and simulated with a numerical coagulation model. Smoke puffed into an artificial mouth was subjected to variable aging times and exhausted to a fast electrical mobility analyzer for particle size distribution measurement. The experimental results agreed well with the predictions of a sectional-based model of Brownian coagulation that allowed for the modeling of various continuous feed and fixed volume coagulation environments. Due to the steady input of fresh, smaller particles, particle growth during the filling of the mouth with smoke, a process intrinsic to the puffing maneuver, was significant but slower than that during fixed volume, static aging. Mouth hold times and initial smoke mass concentration were found to be strong determinants of the average particle size of smoke exiting the mouth into the respiratory tract during inhalation. The results also suggest that the smallest particles present in fresh smoke, those less than 0.1 µm diameter, are greatly reduced in number during the unavoidable mouth coagulation during puffing and virtually eliminated after 1 s of mouth hold.

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