Ammonia emissions from different pig production scales and their temporal variations in the North China Plain

Pig production systems in China are shifting from small to industrial scale. Significant variation in housing ammonia (NH₃) emissions can exist due to differences in diet, housing design, and management practices. However, there is a knowledge gap regarding the impacts of farm-scale in China, which may be critical in identifying hotspots and mitigation targets. Here, continuous in-situ NH₃ concentration measurements were made at pig farms of different scales for sows and fattening pigs over periods of 3-6 days during two different seasons (summer vs. winter). For the sow farms, NH₃ emission rates were greater at the small farm (summer: 0.52 g pig^-1 hr^-1; winter: 0.21 g pig^-1 hr^-1) than at the large farm (summer: 0.34 g pig^-1 hr^-1; winter: 0.12 g pig^-1 hr^-1). For the fattening pig farms, NH₃ emission rates were greater at the large farm (summer: 0.22 g pig^-1 hr^-1; winter: 0.16 g pig^-1 hr^-1) than at the small farm (summer: 0.19 g pig^-1 hr^-1; winter: 0.07 g pig^-1 hr^-1). Regardless of farm scale, the NH₃ emission rates measured in summer were greater than those in winter; the NH₃ emission rates were greater in the daytime than at the nighttime; a positive relationship (R² = 0.06-0.68) was established between temperature and NH₃ emission rate, whereas a negative relationship (R² = 0.10-0.47) was found between relative humidity and NH₃ emission rate. The effect of farm-scale on indoor NH₃ concentration could mostly be explained by the differences in ventilation rates between farms. The diurnal variation in NH₃ concentration could be partly explained by ventilation rate (R² = 0.48-0.78) in the small traditional farms and by emission rate (R² = 0.26-0.85) in the large industrial farms, except for the large fattening pig farm in summer. Overall, mitigation of NH₃ emissions from sow farms should be a top priority in the North China Plain. Implications: The present study firstly examined the farm-scale effect of ammonia emissions in the North China Plain. Of all farms, the sow farm was identified as the greatest source of ammonia emission. Regardless of farm scale, ammonia emission rates were observed to be higher in summer. Ammonia concentrations were mostly higher in the large industrial farms partly due to lower ventilation rates than in the small traditional farms.