An uncertainty methodology for solar occultation flux measurements: ammonia emissions from agriculture

Abstract. Ammonia (NH3) emissions can negatively affect ecosystems and human health, so they should be monitored and mitigated. This study introduces a novel methodology for evaluating uncertainties in NH3 emissions measurements using the Solar Occultation Flux (SOF) method. The reactive nature of NH3 makes its measurement challenging, but SOF offers a reliable open-path passive method, utilizing solar spectrum data, thereby avoiding gas adsorption within the instrument. To compute NH3 gas fluxes, horizontal and vertical wind speed profiles, as well as plume height estimates and spatially resolved column measurements are integrated. A unique aspect of this work is the first-time description of plume height estimations derived from ground and column NH3 concentration measurements aimed at uncertainty reduction. Initial validation tests indicated measurement errors between −31 % and +14 % on average, slightly larger than the estimated expanded uncertainty ranging from ±12 % to ±17 %. Application of the methodology to assess emission rates from farms of various sizes showed uncertainties between ±21 % and ±37 %, generally influenced by systematic wind uncertainties and random errors. The method demonstrates the capacity to measure NH3 emissions from both small (~1 kg h−1) and large (~100 kg h−1) sources in high-density farming areas. Generally, the SOF method provided an expanded uncertainty below 30 % in measuring NH3 emissions from livestock production, which could be further improved by adhering to best application practices. The findings also have implications when using SOF to measure other gaseous species and in other applications.