Development and application of measurement techniques for quantifying methane and ammonia emissions from livestock production

Agricultural production is set to grow substantially in the coming years, so reducing its environmental impacts is vital. Agriculture's contribution to ammonia (NH₃) and methane (CH₄) gaseous emissions is significant, especially from livestock production. Investigating these emission sources, using atmospheric measurement techniques, helps gather knowledge about emission rates, the factors driving them and the efficiency of mitigation strategies. However, measuring these sources is still limited to intrusive methods and small-scale experiments or restricted to the type of animal production (e.g., mechanically ventilated barns). Consequently, there is a need to provide alternative methods, especially ones that are easier to apply and can provide accurate emission levels.

Thus, the overall aim of this PhD is to develop and apply methods for quantifying NH₃ and CH₄ emissions from livestock production. CH₄ emissions were measured via the tracer gas dispersion method (TDM) and the indirect flux method (IFM). Emissions from cattle production in Denmark and the USA were studied, while pig farms were investigated only from Denmark. Furthermore, NH₃ emissions were measured from dairy production facilities in the USA. The methods were primarily designed to assess emissions from the entire farm, so a stationary approach to the TDM method was refined, by sampling concentrations downwind from the source at fixed points placed in a field, decreasing dependency on driving along roads.

In Denmark, CH₄ emission rates at nine cattle farms ranged from 0.7 to 28 kg/h. CH₄ rates were then normalised per livestock unit (1 LU = 500 kg of body weight), averaging 23 ± 9 g/LU/h. The farms with the most significant emission factors (EFs) were those with deep litter animal houses with long eating spaces. Moreover, CH₄ emission rates from seven pig farms also ranged across a similar scale (0.2 to 20 kg/h). After converting rates to EFs, farms with no manure treatment averaged between 13 ± 6 and 18 ± 9 g/LU/h, whilst farms storing digested manure averaged an EF of 8 ± 7 g/LU/h. In addition, in-house manure acidification was used at two other farms, with average EFs of 2 ± 1 g/LU/h. Measurements were distributed over the year, and emission fluctuations over the months were more significant at pig than at cattle farms, although emission patterns were the same. Furthermore, measured emissions were higher for all farms than the modelled inventory rates, albeit, for cattle farms, inventory estimates and measured rates were within uncertainty levels at most of the studied sites. Conversely, inventory quantifications greatly underestimated pig farms' CH₄ rates.

Furthermore, CH₄ emission rates from ten manure tanks were quantified using mobile and stationary TDMs by running a minimum of six measurement campaigns over the year. Emissions from pig manure were highest, while digested manure had the lowest EFs. The rates varied from 0.01 to 14.3 kg/h. Average annual rates, normalised by manure stored, for all tanks ranged from 0.2 g/m³/h to 2.7 g/m³/h, with pig manure storage showing significant variability among the different tanks. Additionally, covered tanks had higher emissions than uncovered tanks.

Moreover, NH₃ and CH₄ emissions were quantified at 14 dairy concentrated animal feeding operations (CAFOs) in California, using SOF and MeFTIR, respectively. An error budget analysis showed that this techniques had an averaged uncertainty of 37% and 53%, for NH₃ and CH₄ quantifications, respectively. Emission rates ranged from 155 to 874 kg/h for CH₄ and 32 to 191 kg/h for NH₃. Average EFs for the dairies were 40 ± 18 g/LU/h for CH₄ and 9 ± 3 g/LU/h for NH₃. Measurements were only performed in May and October, and variations between the measurements were minimal, as the farms operated in similar temperature conditions. NH₃ measurements were limited to the daytime, albeit emissions for this gas still demonstrated diurnal variations over this measurement time. Similarly, we observed NH₃-to-CH₄ ratio diurnal variability. Modelled NH₃ emissions were similar to those measured when a diurnal emissions model was used. For CH4, modelled emissions were lower than measured emissions; however, a lack of knowledge about manure management at the specific farms influenced the comparison. Finally, the methods employed herein were valuable tools for investigating livestock CH4 and NH3 emissions. They captured emission fluctuations over a complete year, as well as the efficiency of two different CH₄ mitigation system, and helped identify factors affecting CH₄ manure emissions.