## 4 Conclusions  
Strong diurnal variations of NH\({}_{3}\) concentrations and emissions in summer were identified for three typical laying hen houses (CC, AV, and NM) in NCP. The impact of environmental factors on NH\({}_{3}\) emissions was also analyzed. The following findings were summarized:  
1. Timely manure removal by the belt system and high VR of the houses in summer could help maintain relatively stable and low NH\({}_{3}\) concentrations in CC, AV, and NM, which also could efficiently reduce NH\({}_{3}\) emissions. NH\({}_{3}\) mitigation efforts should focus on manure accumulation time inside and storage methods outside in the future.
2. House-level average daily NH\({}_{3}\) emissions were highest in the NM at 27.16 \(\pm\) 13.12, followed by the AV at 4.08 \(\pm\) 3.23 and the CC at 3.43 \(\pm\) 1.46 mg/h-hen in summer. A clear diurnal pattern of NH\({}_{3}\) emission was observed.
3. Manure accumulation significantly affected NH\({}_{3}\) emission rates, and it was strongly recommended to store manure on the belt for less than 4 days in NM.
4. Laying hen houses with similar operational information and environmental variables had similar NH\({}_{3}\) emission rates. A positive impact of temperature and VR, and a negative impact of RH on NH\({}_{3}\) emissions in AV and NM were indicated, while the environmental factors did not significantly affect the NH\({}_{3}\) emissions in CC.  
It should be mentioned that the data collected for analysis were only in warm weather periods (June, July, and September). It is expected the emission would be varied due to the change of weather and, consequently, ventilation airflow rate for indoor climate control. Therefore, studies on possible seasonal effects should be conducted in the future.  
Conceptualization, C.W.; methodology, C.W., G.Z., and L.R.; formal analysis, Y.L., Z.L., and S.W.; writing-original draft preparation, Y.L.; writing--review and editing, C.W., G.Z., and L.R.; project administration, C.W. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Key Research and Development Program of China (grant number: 2017 YFD0701602).  
The authors declare no conflict of interest.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Day** & **CC** & **AV** & **NM** \\ \hline
1 st day & 1.57 \(\pm\) 0.69  & 9.03 \(\pm\) 3.27 [FOOTNOTE:]Footnote : The data after manure removal was analyzed on the first day. 2 Different capital letters show there are significant differences in the same row at 0.05 level; different lowercase letters show there are significant differences in the same line at 0.05 level.[ENDFOOTNOTE] \\
2 nd day & 3.66 \(\pm\) 0.86  & 11.59 \(\pm\) 7.58 [FOOTNOTE:]Footnote : The data after manure removal was analyzed on the first day. 2 Different capital letters show there are significant differences in the same row at 0.05 level; different lowercase letters show there are significant differences in the same line at 0.05 level.[ENDFOOTNOTE] \\ \hline \hline \end{tabular}
\end{table}
Table 4: Average NH\({}_{3}\) emission rates on the first and second day of manure accumulation in CC, AV, and NM (mg/h-hen) 1.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Factors** & **CC** & **AV** & **NM** \\ \hline Temperature outside the house & 0.27 & 0.86 ** 1  & 0.92 ** \\ RH outside the house & \(-\)0.27 & \(-\)0.67 ** & \(-\)0.93 ** \\ Temperature inside the house & 0.20 & 0.77 ** & 0.87 ** \\ RH inside the house & \(-\)0.17 & \(-\)0.55 ** & \(-\)0.65 ** \\ VR & 0.16 & 0.97 ** & 0.88 ** \\ \hline \hline \end{tabular} 1  
\end{table}
Table 3: Correlations between NH\({}_{3}\) emission rates and environmental factors.