In this editorial, we have highlighted key research opportunities and challenges in four topical themes for indoor chemistry: human occupants as agents influencing indoor chemistry; oxidative chemistry; surface phenomena; and semivolatile organic compounds. In each case, enough prior work has been done to demonstrate the importance of the theme and to create a foundation for future studies. Extensive achievements and ongoing progress in (outdoor) atmospheric chemistry—both in the analytical methods developed and in the scientific knowledge created—also contribute to a strong foundation from which to achieve rapid research progress in this exciting new domain.

Building ventilation rates and indoor airflow conditions influence occupants' exposure to indoor air pollutants. By making time- and space-resolved measurement of 3 inert tracers steadily released in a single-family house in California for 8 weeks in summer and 5 weeks in winter, this study quantifies the air change rate of the living zone with 2-hour time resolution; estimates airflow rates between the living zone, attic, and crawlspace; and characterizes mixing of air in the split-level living space. Occupant behaviors altered the air change rates, primarily through opening windows and secondarily through operating the heating system. The air change rate correlated with the number of window openings, accounting for 57% of the variability measured across 2 seasons. There were substantial upward interzonal airflows between the crawlspace, living zone, and attic; downward airflows were negligible by comparison. More than 70% of the airflow entering the living zone in the winter and at night during summer came through the crawlspace, rather than directly from outdoors. The airflow from the living zone to the attic increased with the attic-outdoor temperature difference, indicating that buoyancy associated with solar heating of the attic induced airflow from the living zone, increasing the air change rate.

Airborne flux measurements of isoprene were performed over the Californian oak belts surrounding the Central Valley. The authors demonstrate for the first time 1) the feasibility of airborne eddy covariance measurements of reactive biogenic volatile organic compounds; 2) the effect of chemistry on the vertical transport of reactive species, such as isoprene; and 3) the applicability of wavelet analysis to estimate regional fluxes of biogenic volatile organic compounds. These flux measurements demonstrate that instrumentation operating at slower response times (e.g., 1-5 s) can still be used to determine eddy covariance fluxes in the mixed layer above land, where typical length scales of 0.5-3km were observed. Flux divergence of isoprene measured in the planetary boundary layer (PBL) is indicative of OH densities in the range of 4-7 3 106 molecules per cubic centimeter and allows extrapolation of airborne fluxes to the surface with Damköhler numbers (ratio between the mixing time scale and the chemical time scale) in the range of 0.3-0.9. Most of the isoprene is oxidized in the PBL with entrainment fluxes of about 10% compared to the corresponding surface fluxes. Entrainment velocities of 1-10 cm s21 were measured. The authors present implications for parameterizing PBL schemes of reactive species in regional and global models. © 2013 American Meteorological Society.

Semivolatile organic compounds (SVOCs) represent a diverse and dynamic class of chemicals found indoors with poorly understood temporal behavior. Past analytical work has incorporated time-averaged sampling procedures, limiting the scope of inquiries about SVOCs in indoor air. Thermal desorption aerosol gas chromatography (SV-TAG) is an established analytical technique for measuring SVOCs in the gas- and particle-phases with hourly time resolution. In this study, SV-TAG was used to measure the indoor distribution of SVOCs in an occupied California home over a two-month period. Data reveal dynamic and persistent ranges of individual chemicals and chemical classes, including phthalates. The effects of physical parameters and occupant behavior on airborne phthalate concentrations and other SVOCs of interest were explored. Phthalate levels were roughly five times higher indoors than outdoors. Occupant behavior had no clear effect on total phthalate emissions, which were dominated by the contents of the house and strongly modulated by temperature.

Residences represent an important site for bioaerosol exposure. We studied bioaerosol concentrations, emissions, and exposures in a single-family residence in northern California with 2 occupants using real-time instrumentation during 2 monitoring campaigns (8 weeks during August-October 2016 and 5 weeks during January-March 2017). Time- and size-resolved fluorescent biological aerosol particles (FBAP) and total airborne particles were measured in real time in the kitchen using an ultraviolet aerodynamic particle sizer (UVAPS). Time-resolved occupancy status, household activity data, air-change rates, and spatial distribution of size-resolved particles were also determined throughout the house. Occupant activities strongly influenced indoor FBAP levels. Indoor FBAP concentrations were an order of magnitude higher when the house was occupied than when the house was vacant. Applying an integral material-balance approach, geometric mean of total FBAP emissions from human activities observed to perturb indoor levels were in the range of 10-50 million particles per event. During the summer and winter campaigns, occupants spent an average of 10 and 8.5 hours per day, respectively, awake and at home. During these hours, the geometric mean daily-averaged FBAP exposure concentration (1-10 μm diameter) was similar for each subject at 40 particles/L for summer and 29 particles/L for winter.

The present study provides detailed investigation of source characteristics and emission dynamics of volatile organic compounds (VOCs) in a single-family house in California, utilizing time- and space-resolved VOC measurements for eight weeks in summer and five in winter. In total, about 300 VOCs were measured. Spatially resolved VOC measurements, in conjunction with airflow investigation using multi-tracer technique, reveal that VOCs in the living space were mainly emitted directly into the living space, with minor contributions from coupled interior spaces (crawlspace and attic) and from outdoors. Time series of most indoor VOCs were characterized by elevated baseline levels with episodic enhancements. The high baseline level highlights the importance of continuous emissions into the house from building materials and contents. Associated emissions rates were quantified with 2-h resolution. Influences of environmental parameters on emissions were explored. Episodic enhancements were associated with occupants and their activities, such as cooking and personal-care-product use.

Residences are important sites of daily bioaerosol exposure; yet the relative contributions of outdoor and indoor sources are not well-understood. Biological particles can be detected using laser-induced fluorescence. Applying real-time instrumentation, this work investigated the contributions of indoor and outdoor sources to fluorescent aerosol particle exposures in a northern California residence with three human residents and a dog. Time- and size-resolved total and fluorescent particle concentrations were measured indoors and outdoors, using an ultraviolet aerodynamic particle sizer (UVAPS) and a switching valve. Time-resolved occupancy status and household activities, such as cooking and cleaning, were obtained using occupant-maintained logs and wireless electronic sensors. Our observations show the indoor to outdoor ratio of fluorescent particles is strongly influenced by occupancy, and that indoor sources contribute a considerable fraction of total daily fluorescent particle exposures at home. This work will improve understanding of the relationship between human activities and bioaerosol exposure in residences.

Biogenic isoprene fluxes were measured onboard the CIRPAS Twin Otter aircraft as part of the California Airborne Biogenic volatile organic compound (BVOC) Emission Research in Natural Ecosystem Transects (CABERNET) campaign during June 2011. The airborne virtual disjunct eddy covariance (AvDEC) approach used measurements from a proton transfer reaction mass spectrometer (PTR-MS) and a wind radome probe to directly determine fluxes of isoprene over 7400 km of flight paths focusing on areas of California predicted to have the largest emissions. The fast Fourier transform (FFT) approach was used to calculate fluxes of isoprene over long transects of more than 15 km, most commonly between 50 and 150 km. The continuous wavelet transformation (CWT) approach was used over the same transects to also calculate instantaneous isoprene fluxes with localization of both frequency and time independent of non-stationarities. Fluxes were generally measured by flying consistently at 400 m ± 50 m (a.g.l.) altitude, and extrapolated to the surface according to the determined flux divergence determined in the racetrack-stacked profiles. The wavelet-derived surface fluxes of isoprene averaged to 2 km spatial resolution showed good correspondence to basal emission factor (BEF) land-cover data sets used to drive BVOC emission models. The surface flux of isoprene was close to zero over Central Valley crops and desert shrublands, but was very high (up to 15 mg '2 h−1) above oak woodlands, with clear dependence of emissions on temperature and oak density. Isoprene concentrations of up to 8 ppb were observed at aircraft height on the hottest days and over the dominant source regions. Even though the isoprene emissions from agricultural crop regions, shrublands, and coniferous forests were extremely low, observations at the Walnut Grove tower south of Sacramento demonstrate that isoprene oxidation products from the high emitting regions in the surrounding oak woodlands accumulate at night in the residual layer above the valley and mix down into the valley in the morning. Thus, the isoprene emissions surrounding the valley have relevance for the regional photochemistry that is not immediately apparent solely from the direct emission flux distribution. This paper reports the first regional observations of fluxes from specific sources by eddy covariance from an aircraft which can finally constrain statewide isoprene emission inventories used for ozone simulations by state agencies. While previously there was no available means to constrain the biogenic models, our results provide a good understanding of what the major sources of isoprene are in California, their magnitude, and how they are distributed.

This data set on isoprene fluxes will be particularly useful for evaluating potential model alternatives which will be dealt with in a separate paper to assess isoprene emission models and their driving variable data sets.