## 3 Results and Discussion
### Particulate Matter Concentrations  
Average external PM\({}_{2.5}\) concentrations during the monitoring period were 23.7 \(\pm\) 2 ug m\({}^{-3}\) (Table 2), and unlike internal NH\({}_{3}\) concentrations, there was no significant variability between day-night or weekday-weekend periods. The hourly external PM\({}_{2.5}\) concentrations measured at the PMCF closely matched regional PM\({}_{2.5}\) concentrations (Figure 10), with which they had a strong positive correlation (r = 0.67, \(p\) < 0.001). There were two exceptions, on the 25 and 26 June 2020, respectively, when external PMCF PM\({}_{2.5}\) concentrations greatly diverged from regional concentrations (Figure 10). However, internal NH\({}_{3}\), SO\({}_{2}\), and NOx concentration data for these two dates did not appear to show any large anomalies that might have corresponded with the high PM\({}_{2.5}\)  
Figure 10: Comparison of PM\({}_{2.5}\) results between beta-ray measurements and data from Paju weather station. Figure 9: Negative correlation of relative humidity with NO\({}_{2}\) concentrations. concentrations. This suggests that the elevated PM\({}_{2.5}\) concentrations measured outside the facility on the 25 and 26 June did not originate from the manure composite function of the facility and were from a different source. Together with the high correlation with regional PM\({}_{2.5}\) concentrations, these data suggests that the PMCF is unlikely to be a significant local or regional source of PM\({}_{2.5}\). Average internal PM\({}_{2.5}\) concentrations were 36.9 \(\pm\) 2.6 \(\upmu\)g m\({}^{-3}\). Similar PM\({}_{2.5}\) concentrations (39-56 \(\upmu\)g m\({}^{-3}\)) were found during monitoring of slatted-floor hog houses in China [35], and these were are slightly lower than PM\({}_{2.5}\) concentrations (40-80 \(\upmu\)g m\({}^{-3}\)) at a commercial laying hen house in Canada [36]. However, they were much lower than the PM\({}_{2.5}\) concentration (100-140 \(\upmu\)g m\({}^{-3}\)) measured by [PERSON] in Chinese manure-belt laying hen houses [31], and in a range of swine (75-177 \(\upmu\)g m\({}^{-3}\)) and poultry houses (233-389 \(\upmu\)g m\({}^{-3}\)) in the mid-western USA [37]. Elevated periods of PM\({}_{2.5}\) concentrations appear to have coincided with manure agitation (Figure 11) and could have been a result of mechanical lofting of fine-fraction dust particles and/or secondary aerosol formation from elevated concentrations of precursor gas. Secondary aerosol formation can occur when NH\({}_{3}\) gas reacts with acidic species SO\({}_{2}\) and NO\({}_{x}\) to form NH\({}_{4}\)\({}^{+}\) salts like (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\) and NH\({}_{4}\)NO\({}_{3}\)[9, 21]. Reaction between NH\({}_{3}\) and sulfuric acids occurs preferentially over nitric acids under normal atmospheric conditions [21], and the reaction rate between NH\({}_{3}\) and sulfuric acids is faster than for nitric acids [22]. Peak PM\({}_{2.5}\) concentrations were recorded at 889 \(\upmu\)g m\({}^{-3}\) and 1479 \(\upmu\)g m\({}^{-3}\) on the 14 and 15 July 2020, respectively (Figure 11), showing that very high internal PM\({}_{2.5}\) concentrations occasionally occurred for brief durations coincident with agitation activity. However, PM\({}_{2.5}\) concentrations rarely rose above 200 \(\upmu\)g m\({}^{-3}\) and it is likely that the high-volume ventilation system at the PMCF was an important factor in the relatively low internal PM\({}_{2.5}\) concentrations, removing fine-fraction dust particles and precursor gases before PM\({}_{2.5}\) formation was able to occur. Due to the shorter duration (approx. 10 days) over which internal PM\({}_{2.5}\) was measured, it was not possible to assess statistical differences between weekend and weekday concentrations. Although internal concentrations of PM\({}_{2.5}\) were observed to reach much higher levels on weekdays following manure agitation, concentrations appeared to show comparatively little variability at weekends.