Indoor surface chemistry variability: microspectroscopic analysis of deposited particles in dwellings across the United States†

Abstract

Dwellings across the United States range dramatically with respect to numerous variables (e.g., size, ventilation, and proximity to outdoor sources), and there are considerable uncertainties regarding the heterogeneity in chemical composition and physical properties of indoor particles and surfaces. Stay-at-home orders early in the COVID-19 pandemic led to significant portions of the population spending high fractions of their time at their primary dwelling. Stay-at-HomeChem leveraged a network of indoor chemistry researchers to study indoor air quality and surface chemistry in their homes (March–April 2020). Within this effort, glass microscope slides were deployed in kitchens and other rooms in dwellings across the country for time periods ranging from as short as three hours up to three weeks. Overall, results from 10 occupied homes (15 distinct rooms) showed that collected material on this time scale was primarily deposited particles, rather than thick films, based on optical microscopy and profilometry. Raman microspectroscopy and optical photothermal infrared (O-PTIR) spectroscopy showed that organic modes were dominant, including ν(C–H), δ(C–H), and ν(CO), with minimal contributions from inorganic ions commonly observed in outdoor particulate matter (sulfate, nitrate, or ammonium). Spectral variability within the C–H stretching and fingerprint regions demonstrate differing compositions of deposited particles, often related to cooking activities (e.g., organic particles from cooking oils). Differences within a single dwelling, highlighted that particles from cooking were key contributors in some other rooms, but not all, reinforcing that sources and ventilation likely led to quite distinct surfaces in different rooms. Overall, these results demonstrate the need for real-world measurements to assess the representativeness of assumptions regarding exposure to organic material indoors.