Transformations of Aerosol Particles from an Outdoor to Indoor Environment

Aerosol particle size and chemical composition during summer and winter were investigated in this study. An automated switching valve allowed for indoor and outdoor environments to be sampled near-simultaneously with the same high temporal-resolution instrumentation. During the study, no known indoor sources were present and the sampled room was unoccupied throughout. Accumulation mode indoor/outdoor (I/O) ratios were substantially lower in winter than in summer. This reduction was attributed to particles of outdoor origin shrinking as they entered the warmer and drier indoor environment. An essential factor in this process appeared to be the difference (gradient) between the temperature and relative humidity of the indoor and outdoor environments during the winter. Online aerosol mass spectrometer measurements recorded a 34-38% decrease in I/O ratios for all nonrefractory species during the winter relative to the summer. A similar change in I/O ratios for all species indicated that physical, rather than chemical, processes were responsible. To assess the relative influence of various physical factors on I/O relationships, Spearman rank statistical tests were carried out. These identified wind speed to be negatively correlated to the indoor concentrations for all species. Wind roses incorporating I/O ratios were applied and showed that the wind speed and direction influenced the changes in the indoor composition. The relative outdoor concentration of different aerosol species, steepness of the I/O temperature gradient, and wind speed variability are concluded to be essential factors in I/O aerosol transformations.