A simplified chemical reaction scheme for the oxidation of VOCs is shown in Fig. 1. Direct (primary) emissions of VOCs were observed using a proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS) during two mopping periods and are shown in Figs. 2A and 3A. Monoterpene concentrations are expressed as a sum of the signals detected at mass/charge ratio (m/z) 81 and m/z 137, which represent a known fragment (C₆H₉)⁺ and the protonated mass (C₁₀H₁₆)H⁺, respectively, and can be from a variety of monoterpenes (α- and β-pinene, limonene, camphene, myrcene, and 3-carene, among others) (31). Analysis by gas chromatography–electron ionization mass spectrometry (GC-MS) revealed that the liquid cleaning solution is composed of a mixture of various monoterpenes, including limonene, α-pinene, β-pinene, and camphene (table S1). C₁₀H₁₆O, possibly citral, is shown as m/z 153 in Fig. 2A. Citral, an aldehyde with a citrus odor, has been previously found as a component of other commercial cleaning products (28). C₁₀H₁₆O, along with C₁₀H₁₈O, can also be C10 alcohols, like α-terpineol, isoborneol, and myrcenol (isomers with molecular formula C₁₀H₁₈O) because C10 alcohols were also listed as ingredients in the commercial product and were detected in the GC-MS analysis. Mixing ratios of these C10 compounds are shown in Fig. 3A and fig. S6A.

During the first and second mopping periods, monoterpene mixing ratios peaked at 280 and 380 ppb, respectively (Fig. 2A); this is 140 to 190 times more than the peak outdoor monoterpene mixing ratios observed outside the research building for this day (~2 ppb). While indoor mixing ratios depend on a variety of factors, including the AER, cleaning solution concentrations, and cleaning surface area, the limonene mixing ratios observed in this study were approximately 1.3 to 2.2 times more than a lemon-scented furniture polish (wax) application to a coffee table inside a 25-m³ chamber (15), about 1.8 to 2.4 times more than the peak limonene concentration from a household product used in a 50-m³ chamber (11), approximately 20 times more than the peak indoor mixing ratios previously observed in an Australian classroom (~17 ppb) (13), 3.7 to 5 times more than monoterpene emissions from botanical disinfectants sprayed onto a glass kitchen countertop (32), and about 56 to 76 times more than the peak mixing ratios observed during the HOMEChem field campaign (~5 ppb) (33), where similar mopping experiments using a monoterpene-based cleaner were done. The volume of the HOMEChem test house was larger (250 m³) as compared to the room used in this study (~50 m³); thus, the emissions from the mopping events dispersed into a larger volume, resulting in lower concentrations during the mopping episode. Concomitantly, indoor O₃ decreased to less than 1 ppb during the mopping event, from initial background concentrations of about 5 and 10 ppb for the first and second mopping events, respectively (Fig. 2B). After the steep decline, indoor O₃ concentrations slowly increased as a result of outdoor air introduction via mechanical ventilation. This contrasts with chamber SOA studies where O₃ is usually in excess; these short bursts or “pulses” introduced by the mopping events are similar to a rapid injection of VOCs in chamber studies but within a more realistic indoor scenario, i.e., a real room with mechanical ventilation in an occupied building.

Other compounds known in the ambient environment that can be oxidized leading to SOA formation, such as isoprene and sesquiterpenes, were also detected. Background isoprene (m/z 69) and a lower limit to sesquiterpene (m/z 205, parent ion only) mixing ratios in the room were about 7 and 0.2 ppb, respectively. During the first mopping event, they increased to 13 and 0.8 ppb, respectively; for the second mopping event, they increased to 19 and 1 ppb, respectively. Isoprene likely originated from the exhaled breath of the person mopping (34) or from the ambient forest environment, where isoprene was previously observed to be on the order of 1 to 4 ppb (35). On the other hand, the sesquiterpenes were likely emitted directly from the cleaning product, which contained pine oil (36). While these compounds are also known to be SOA precursors, indoor oxidants (O₃ and OH) are most likely to react with limonene and other monoterpenes and form peroxy radicals and particles because of their much higher abundance relative to isoprene and sesquiterpenes.

Figure 2C shows the subsequent increase in the HO₂ and RO₂ radical concentrations after the mopping event, peaking near 2 × 10⁹ to 3 × 10⁹ molecules cm⁻³, higher than the maximum total HO₂ + RO₂ concentration of approximately 1 × 10⁹ molecules cm⁻³ previously measured in the outdoor forested area of the research building where the test room was located (37). In addition, the observed HO₂ concentration was about two orders of magnitude higher than that measured in an unoccupied classroom in Marseille, France, where the HO₂ concentration was reported to be 0.6 × 10⁷ to 3.7 × 10⁷ molecules cm⁻³ (29). While this is not the first time that these radicals have been measured indoors, it is the first time that concentrations of HO₂ and RO₂ have been measured as a result of O₃/terpene chemistry using a commercial cleaner in an indoor setting. The increase in radical concentrations did not correlate with changes in the amount of direct sunlight to the room, as represented by the measurements of the NO₂ photolysis frequency, J_NO2 (fig. S2). Rather, HO₂ and RO₂ concentrations during the mopping events increased as the monoterpene concentrations increased and the O₃ concentration decreased, suggesting that the radical concentrations were not derived from photolysis but produced solely from monoterpene ozonolysis. While the measured concentration of HO₂ radicals was similar during the two mopping events, the concentration of RO₂ radicals was greater during the second mopping event, consistent with the higher concentration of both monoterpenes and O₃ during this event. During the mopping events, 5 to 10 ppb of O₃ were sufficient to induce indoor SOA formation from ozonolysis (Fig. 2, E and F), notably less O₃ than previously reported to induce SOA either in a reaction chamber or under indoor-relevant conditions (4, 7, 8, 12, 13, 17). Similar results were observed by Pagonis et al. (27) who found that a comparable level of O₃ (approximately 5 ppb) was enough to induce SOA formation from the ozonolysis of limonene emitted by peeling a navel orange inside a university art museum.

Gas-phase oxidation products were also observed immediately after the mopping period, and select products are shown in Figs. 2D and 3. Previous work, such as that of Leungsakul et al. (4), used a combination of modeling and chamber measurements to outline a semiexplicit mechanism of limonene and have identified limonaldehyde and limononic acid as major products of gas-phase ozonolysis. Similarly, Hammes et al. (5) used a high-resolution time-of-flight chemical ionization mass spectrometer with a filter inlet for gases and aerosols to determine that carboxylic acids dominate the gas-phase products from limonene ozonolysis. In the current work, Fig. 2D shows the real-time increase in C₁₀H₁₆O₂ (possibly limonaldehyde, from limonene oxidation; pinonaldehyde, from α-pinene oxidation; or a mixture of both; see table S2, labels 3A and 3B), C₁₀H₁₆O₄ (possibly a highly oxidized carboxylic acid; see table S2, label 1A), and C₉H₁₄O₄ [possibly limonic acid, ketolimononic acid, or another highly oxygenated organic molecule (HOM); see table S2, labels 5A to 5D] as detected by PTR-ToF-MS. Figure 3 (B to D) includes other highly oxidized, next-generation oxidation products (see ozonolysis mechanisms in figs. S3 and S4). A delay relative to the monoterpene trace in the increase and maxima of these oxidation products was observed, emphasizing their later emergence in the gas phase as secondary oxidation products resulting from monoterpene ozonolysis. On the other hand, while the decrease in the concentration of these oxidation products may be attributed to physical processes, such as air exchange, deposition, or gas-particle partitioning, some fraction of these gas-phase oxidation products may have undergone next-generation oxidation (Fig. 3, B to D) to yield HOMs that ultimately led to the observed indoor NPF events.

Indoor NPF was observed immediately after the formation of radicals and gas-phase oxidation products, as evidenced by the slight delay in the increase of particle number concentrations (Fig. 2, C to F). Here, we report the first direct measurements of the nucleation of sub–3-nm nanocluster aerosol (NCA) due to indoor monoterpene ozonolysis initiated using a terpene-based cleaner. Indoor NCA number concentrations increased rapidly to ~10⁵ cm⁻³ (Fig. 2F) and dominated the particle number size distributions during the first few minutes of the NPF event (Fig. 2E). The observed NCA number concentrations were generally similar to, or greater than, those reported during atmospheric NPF events in field and chamber studies (38).

The peak number concentration of the newly formed sub–3-nm particles was 0.91 × 10⁵ and 5.76 × 10⁵ cm⁻³ during the first and second mopping episodes, high enough to grow rapidly into the nucleation and Aitken modes via condensation and coagulation, approaching modal diameters of approximately 30 nm in about 10 min after the start of the mopping period. This suggests particle growth rates of approximately 200 nm hour⁻¹, which is one to two orders of magnitude higher than those reported for typical atmospheric NPF events under different conditions but similar to selected observations reported in coastal areas (39). Sub–100-nm ultrafine particle number concentrations were sustained at around 10⁵ cm⁻³ for the duration of the mopping events, which is similar to observations made during indoor combustion activities, such as cooking on a gas stove or lighting a candle (2), and outdoors in a traffic-impacted area (40). Peak number concentrations were higher by approximately an order of magnitude than previously observed in a similar indoor mopping experiment by Morawska et al. (13), possibly owing to a smaller room (140 m³ versus 50 m³ in this study) and measurement of particles down to 1.2 nm. The ozonolysis of skin oil has also been observed to initiate the formation of NCA in indoor environments (41). As one human volunteer was present during the cleaning episode, skin oil ozonolysis may have contributed to the measured NCA number concentrations. However, the contribution is expected to be small during cleaning-initiated monoterpene ozonolysis as the NCA number concentrations observed by Yang et al. (41) during skin oil ozonolysis in the presence of four human volunteers (~1 × 10² to 5 × 10² cm⁻³ for 35 to 40 ppb of O₃ at an AER of 3.2 hour⁻¹) are much lower than those measured here (~10⁵ cm⁻³).

The estimated aerosol mass fractions (AMFs) ranged from 0.31 to 0.88 for limonene and 0.17 to 0.24 for α-pinene (see the “Materials and Methods: Single-zone mass balance model for predicting SOA mass concentrations” section), which are within the range reported in chamber studies for terpene ozonolysis (42). Moreover, peak mass concentrations (for the 1.2 to 500 nm size range) were very close to the U.S. Environmental Protection Agency 24-hour guideline value for a particle pollution of 35 μg m⁻³. The newly formed sub–100-nm particles that contribute little to particle mass, but dominate particle number, may be more health relevant because of their high efficiency of deposition in all regions of the respiratory system (fig. S8) and their propensity to penetrate to the deepest regions of the lung (see the “Human exposure implications of monoterpene ozonolysis during mopping” section).

It should be noted that in the current study, the AER was 4.5 hour⁻¹, approximately four times higher than in the study of Morawska et al. (13), showing that aerosol formation is rapid enough that even a high AER in a small room is not sufficient to flush out secondary aerosols and their precursors or outweigh the rate of in situ particle nucleation and growth via condensation and coagulation. The current study room, while three times less in volume than the previously studied classroom, is similar to a typical office (43). The fourfold increase in AER was not able to compensate for the much faster rate of aerosol formation. This illustrates the tremendous aerosol formation capacity of high concentrations of monoterpenes and monoterpenoids in mechanically ventilated indoor environments during floor cleaning activities (Fig. 2, E and F).

Figure 3 illustrates other gas-phase oxidation products detected using the PTR-ToF-MS. Notably, these highly oxygenated products were present in the few parts per billion range (i.e., 0.1 to 1 ppb). Possible structures of these compounds are detailed in table S2. This list is not exhaustive; it only shows possible structures for oxygenated species that might have come from the oxidation of α-pinene, β-pinene, and limonene and that were detected by the PTR-ToF-MS.

During nonmopping periods, background monoterpene concentrations were approximately 10 ppb, generally higher than that usually measured outdoors either in forested (44) or urban (45) areas. The exact identities of these monoterpenes are unknown, but previous studies have shown α- and β-pinene to be dominant monoterpenes in the surrounding forest (35) and thus may have influenced the indoor VOC composition via mechanical ventilation of outdoor air. It is also possible that there were indoor monoterpene sources within the building, such as wood waxes or polishes, or surfaces (wooden or otherwise) that off-gas into the recirculated air in the building, resulting in a background concentration.

O₃ concentrations continuously increased throughout the morning, starting from 5 ppb and increasing to 10 ppb around 13:00 (Fig. 2B). This range of O₃ concentrations is not unusual in enclosed indoor environments with natural ventilation (7, 17, 33). There were no known indoor O₃ sources inside the test room at the time of mopping, and thus, it is believed to predominantly have come from outdoor air via mechanical ventilation. The increasing upward diurnal trend is likely mirroring the diurnal trend in the outdoor O₃ concentration measured previously at this site [approximately 10 ppb at 09:00 and 45 ppb at 13:00 (35)]. The measured mixing ratios of O₃ and NO_x were consistent with background measurements in other indoor environments (33).

Significant background concentrations of HO₂ and RO₂ radicals (~10⁸ molecules cm⁻³) were measured, similar to the ambient concentrations observed previously in the surrounding forest (35). They were most likely produced in situ, given that the background concentrations of 1 to 2 ppb of NO (fig. S2) result in peroxy radical chemical lifetimes of less than 5 s, and thus less likely introduced to the room via the mechanical ventilation system. Production of HO₂ and RO₂ may have come from background monoterpene concentrations oxidized by background O₃, subsequently producing both RO₂ and OH radicals. The latter can also oxidize VOCs as shown in Fig. 1 and thus produce more RO₂ and, subsequently, HO₂.

Nonzero background particle concentrations were most likely due to the transport of outdoor particles to the indoor environment via mechanical ventilation. The air handling unit (AHU) of the ventilation system included a minimum efficiency reporting value 8 filter, which generally has a low particle removal efficiency for sub–1000-nm particles (46). Thus, a significant fraction of outdoor particles entrained into the AHU were likely delivered to the test room during the experiments. To account for this entrainment of outdoor particles and other non-SOA–related sources, we introduced a source term S_g into the SOA model (see Materials and Methods).

Although in situ SOA formation from background O₃ and monoterpene concentrations may be possible, there was a low background concentration of sub–10-nm particles. The number concentrations of 1.2- to 10-nm particles were approximately 10² cm⁻³ during background periods. In addition, because the initial concentration of background monoterpenes was low, the rate of ozonolysis was slower, and thus, the production of low-volatility oxygenated products was slower, favoring condensation onto preexisting particles rather than nucleation. Moreover, because the rate of the ozonolysis reaction was slower, losses of low-volatility compounds to indoor surfaces could become more important. This has been previously observed in chamber studies (42). While higher than that measured in other indoor environments (7, 17, 33), it is unlikely that the background concentrations of monoterpenes in this study affected the radical production and NPF observed during the mopping events.