## 3 Results and Discussion
### Gas Concentrations
#### 3.2.2 So\({}_{2}\)  
Recorded concentrations of SO\({}_{2}\) gas shared a similar pattern to NH\({}_{3}\) concentrations (Figure 7), displaying high but variable concentrations during manure agitation and more broadly elevated concentrations during weekend daytimes when agitation did not occur. This similarity was reflected by a strong positive correlation (r = 0.751, \(p\) < 0.001) between the concentrations of NH\({}_{3}\) and SO\({}_{2}\) (Figure 8, Table 3). Although sharing a similar profile, SO\({}_{2}\) concentrations were consistently two orders of magnitude lower than NH\({}_{3}\) concentrations, averaging 307.2 \(\pm\) 17.4 \(\mu\)g m\({}^{-3}\) over the monitoring period. Unlike NH\({}_{3}\), the average concentrations of SO\({}_{2}\) during weekdays (371 \(\pm\) 30.5 \(\mu\)g m\({}^{-3}\)) and nights (321.1 \(\pm\) 23.7 \(\mu\)g m\({}^{-3}\)) were not found to be significantly different (_p_ < 0.05). In addition, unlike NH\({}_{3}\), average weekend daytime SO\({}_{2}\) concentrations (265 \(\pm\) 45.6 \(\mu\)g m\({}^{-3}\)) were lower than both weekday daytime and nighttime concentrations although also not statistically different (_p_ > 0.95). However, average SO\({}_{2}\) concentrations on weekend nights (147.2 \(\pm\) 15.7 \(\mu\)g m\({}^{-3}\)) were found to be significantly lower (_p_ < 0.05) than SO\({}_{2}\) concentrations at all other times. During weekday manure agitation events, SO\({}_{2}\) concentrations shared a similar pattern with NH\({}_{3}\), with three main concentration spikes related to the sequential agitation of the three manure windows. Weekday SO\({}_{2}\) concentrations were consistently measured to be lowest in the period directly after agitation activity ceased. However, again showing similarities to NH\({}_{3}\) concentration patterns, weekend daytime concentrations of SO\({}_{2}\) were elevated for longer durations throughout the day and the lowest concentrations occurred during the night, prior to sunrise.  
These data suggests that both NH\({}_{3}\) and SO\({}_{2}\) were emitted during the manure composting process, and the similarity in the patterns of the recorded concentrations may suggest similar emission mechanisms. SO\({}_{2}\) can result from incomplete oxidation during manure breakdown by the activity of microorganisms, perhaps as a result of intermittently aerobic conditions [17]. However, unlike this study, research by [PERSON] [31] found that indoor SO\({}_{2}\) concentrations in a belt-layer house Beijing related predominantly to the ambient air. While that study found a positive correlation between internal and regional SO\({}_{2}\) concentrations, data from the current study rule out any substantial effect of ambient air on internal SO\({}_{2}\) concentrations at the PMCF (Table 3). Indeed, the strong positive correlation between  
Figure 6: Comparison of average and peak NH\({}_{3}\) concentrations inside the facility (internal) and outside, at 3 m height and ground level (external) adjacent to the manure agitation hall.  
NH\({}_{3}\) and SO\({}_{2}\) concentrations suggests that manure composite is the predominant source of both. Two existing studies both found that SO\({}_{2}\) was emitted during the manure composite process; however, they produced widely different results for sulfur mass loss, ranging from <1% [32] to approximately 20% [17], and the SO\({}_{2}\) emission mechanism is not well understood.