Real‐time transformation of outdoor aerosol components upon transport indoors measured with aerosol mass spectrometry

Outdoor aerosols are transported indoors, where their component concentrations depend on aerosol size, physiochemical properties, indoor sources and losses, and cross‐environment gradients of temperature and relative humidity. We explored these dependencies by measuring real‐time outdoor and indoor non‐refractory, submicron (PM₁) aerosol component mass concentrations in a mixed‐use laboratory space with an Aerodyne mini‐aerosol mass spectrometer (AMS) and black carbon (BC) with an aethalometer. The median indoor/outdoor (I/O) ratios were 0.60 for sulfate, 0.25 for nitrate, 0.52 for ammonium, 0.73 for organics, and 0.61 for BC. Positive matrix factorization (PMF) on organic aerosol data identified hydrocarbon‐like (HOA), cooking (COA), and oxygenated (OOA) factors. By assuming sulfate was nonvolatile, lost only by mechanical processes, and without indoor sources, the transformations of other components i due to partitioning changes or indoor sources were parameterized by normalizing their I/O ratios by sulfate's I/O ratio, that is, (I/O)_i/SO4. Component‐specific behavior was quantified by regressions of (I/O)_i/SO4 to outdoor‐to‐indoor temperature differences. Nitrate and HOA strongly and OOA weakly showed losses with increasing temperatures indoors vs. outdoors, and HOA likely had an indoor source. To our knowledge, this is the first reported deployment of an AMS to analyze real‐time indoor aerosol composition and outdoor‐to‐indoor transformation.