## 3 Results and Discussion
### Primary and Secondary VOC Emissions  
#### 3.2.1 From Wood-Based Materials  
The concentrations of the 15 detected VOCs (out of the 24 in the target list), which fell into groups of aromatics, esters, aldehydes, ketones, and aliphatic compounds, were quantified after exposing the wood-based materials to a certain level of ozone, i.e., <10 ppb, 100 ppb, or 300 ppb. Then, the emission factors, \(F_{i}\)s, were determined by Equation (4) for all the detected compounds and are summarized in Figure 5a,b. Full data on emission factors of detected VOCs from wood-based panels can be found in Table S1. Background emissions were removed. Although no clear trends can be directly summarized from Figure 5, a few conclusions can be drawn as follows. In general, more chemical compounds are emitted in larger quantities from wood-based materials when ozone concentrations are increased. However, the emission results for each material varied. For the green-core OSB, when no detectable concentration of ozone was present, formaldehyde was emitted at about 111.7 \(\upmu\)g\(\cdot\)m\({}^{-2}\).h\({}^{-1}\), along with benzene and toluene (both < 10 \(\upmu\)g\(\cdot\)m\({}^{-2}\).h\({}^{-1}\)). By increasing \(y_{\text{ozone,in}}\) to 100 or 300 ppb, the \(F_{i}\)s for formaldehyde also increased to 120.4 and 176.2 \(\upmu\)g\(\cdot\)m\({}^{-2}\).h\({}^{-1}\), respectively. Additional aldehydes were detected at emission factors ranging from 1.2-32.2 \(\upmu\)g\(\cdot\)m\({}^{-2}\).h\({}^{-1}\). For aromatics, esters, and aliphatic compounds, butyl acetate (<10 \(\upmu\)g\(\cdot\)m\({}^{-2}\).h\({}^{-1}\)), ethylbenzene (<10 \(\upmu\)g\(\cdot\)m\({}^{-2}\).h\({}^{-1}\)), and xylene (10.1-26 \(\upmu\)g\(\cdot\)m\({}^{-2}\).h\({}^{-1}\)) were detected when \(y_{\text{ozone,in}}\) was set at 100 ppb. Compared to the compounds emitted at 100 ppb of ozone exposure, additional compounds, such as toluene (93.3 \(\upmu\)g\(\cdot\)m\({}^{-2}\).h\({}^{-1}\)) and benzene, were detected under 300 ppb of ozone. Although the other emissions were small, the emissions of butyl acetate were as high as 22.8 \(\upmu\)g\(\cdot\)m\({}^{-2}\).h\({}^{-1}\). Figure 5: Primary and secondary emissions between ozone and wood-based panels. (**a**) Aromatics, esters, and aliphatic compounds. (**b**) Aldehydes and ketones. As expected, a considerable amount of formaldehyde was detected from the lauan plywood during each round of testing, with emissions ranging from 43.6 to 99.2 \(\mathrm{\SIUnitSymbolMicro m^{-2}.h^{-1}}\). For the other target VOCs, when \(y_{\mathrm{ozone,in}}\) was less than 10 ppb, no other compounds were detected, except for methyl benzaldehyde at 55.7 \(\mathrm{\SIUnitSymbolMicro m^{-2}.h^{-1}}\). When blowing ozone into the chamber at 100 ppb, very small quantities of acetaldehyde (6.5 \(\mathrm{\SIUnitSymbolMicro m^{-2}.h^{-1}}\)) and acetone (1.8 \(\mathrm{\SIUnitSymbolMicro m^{-2}.h^{-1}}\)) were obtained, while xylene was identified at 23.7-33.8 \(\mathrm{\SIUnitSymbolMicro m^{-2}.h^{-1}}\) and styrene at a smaller rate of 9.6 \(\mathrm{\SIUnitSymbolMicro m^{-2}.h^{-1}}\). When the \(y_{\mathrm{ozone,in}}\) concentration was as high as 300 ppb, a diverse group of compounds, including hexanal and methyl benzaldehyde, was observed in the sampled air, and the \(F_{i}\) values were determined at 8.9-41.0 \(\mathrm{\SIUnitSymbolMicro m^{-2}.h^{-1}}\). For the paint-free pine, relatively smaller and stable emissions, ranging from 30.3 to 35.8 \(\mathrm{\SIUnitSymbolMicro m^{-2}.h^{-1}}\) for formaldehyde, were found in all three rounds of experiments. By introducing ozone at 100 ppb or 300 ppb, few additional aldehydes and ketones were observed at high emission rates, i.e., all the detected compounds were close to or less than 10 \(\mathrm{\SIUnitSymbolMicro m^{-2}.h^{-1}}\). For aromatics, esters, and aliphatic compounds, toluene was frequently observed in the samples, albeit at different emission rates (8.5-35.6 \(\mathrm{\SIUnitSymbolMicro m^{-2}.h^{-1}}\)).