The Green Label measurement protocol (GL protocol) was developed in the mid nineties to evaluate the ammonia emission of low emission housing systems and other mitigation measures in the Netherlands. The measurements are used to assign official emission factors to low emission housing systems, used in farm permit procedures. More recently a new multi-site sampling approach has been developed, based on the experiences with the GL protocol and results from a statistical analysis of an extensive ammonia emission database. The objective of this paper is to describe and explain the evolution process from the GL protocol to the multi-site approach. Key features of the GL protocol are: evaluation at one farm location, long measurements during winter and summer periods, continuous high frequency sampling, ranges to stay within for critical management factors. In 2005 a statistical analysis was performed on a large ammonia emission database of pig housing systems that provided estimates of so called between farm(s2b), within farm (s 2 w) and instrumental measurement variances that determine the accuracy with which the mean ammonia emission of a housing system can be measured. The analysis demonstrated that both s2b and s 2 w of housing systems varied in the range of 30 - 40% ( relative standard deviations). The large size of s2b implies that the accuracy of the GL protocol is strongly limited by its single farm approach. Recently a new multi-site sampling approach has been approved based on four farm locations where at each location six independent 24h sampling intervals are distributed over one year. The multi-site approach allows for a less intensive and less costly measuring effort for each location, whereas the accuracy of the mean yearly ammonia emission of a low emission housing system is drastically improved.
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