Profiling of CH<sub>4</sub> background mixing ratio in the lower troposphere with Raman lidar: a feasibility experiment

Abstract

Abstract. We present the results of methane profiling in the lower troposphere using LILAS Raman lidar from the Lille University observatory platform (France). The lidar is based on a frequency-tripled Nd:YAG laser, and nighttime profiling up to 4000 with 100 m height resolution is possible for methane. Agreement between the measured photon-counting rate in the CH4 Raman channel in the free troposphere and numerical simulations for a typical CH4 background mixing ratio (2 ppm) confirms that CH4 Raman scattering is detected. The mixing ratio is calculated from the ratio of methane (395.7 nm) and nitrogen (386.7 nm) Raman backscatters, and within the planetary boundary layer, an increase of the CH4 mixing ratio, up to a factor of 2, is observed. Different possible interfering factors, such as leakage of the elastic signal and aerosol fluorescence, have been taken into consideration. Tests using backscattering from clouds confirmed that the filters in the Raman channel provide sufficient rejection of elastic scattering. The measured methane profiles do not correlate with aerosol backscattering, which corroborates the hypothesis that, in the planetary boundary layer, not aerosol fluorescence but CH4 is observed. However, the fluorescence contribution cannot be completely excluded and, for future measurements, we plan to install an additional control channel close to 393 nm, where no strong Raman lines exist and only fluorescence can be observed.

Methane detection in the lower troposphere related to the burning of biomass and leakage in a petrochemical pole, using raman lidar technique

“…For a clean atmosphere (355 = 0.05 km -1 ) uncertainty in the measurements below 10% are possible up to 4 km, while for (355 = 0.2 km -1 ), the interval decreases to 3 km away. The simulation results confirm the need for long-term accumulation signal (several hours) in methane measurements using the Raman lidar (VESELOVSKII, 2019). In order to correlate optical data, we decided to use Cavity ring-down laser spectroscopy (CRDS), which is one of the techniques developed to monitor fugitive gases in the atmosphere.…”

Potential of Raman Lidar for Profiling of Methane Mixing Ratio in the Lower Troposphere

“…During second session in December 2018, it was replaced by the Merion MW 9-100 laser with a 100 Hz repetition rate, and pulse energy of 200 mJ at 355 nm. The results of first session were recently published in [4].…”

Implementação de um sistema lidar Raman para detecção de metano em baixa troposfera, na região metropolitana de São Paulo

“…A linha vibracional Raman para metano, 2914 cm -1 , é bem isolada e possui uma seção transversal de dispersão 8 vezes maior que o nitrogênio, a razão pela qual há necessidade de acúmulo de sinal e um sistema Raman robusto, porque a concentração basal de metano é baixa, cerca de 1800 ppb (Weber, 1979). Ainda com base no acúmulo de sinal, Veselovskii (Veselovskii et al, 2019) apresenta incerteza estatística para perfis verticais, de três valores do coeficiente de extinção de aerossóis: 0,05; 0,1; 0,2 km -1 e um tempo médio de sinal de 4 horas. A taxa de contagem de fótons no canal Raman do metano 4 = 4 ℎ 2 ∆ onde c é a velocidade da luz.…”

“…Para uma atmosfera limpa ( 355 = 0,05 km -1 ), a incerteza nas medições abaixo de 10% é possível até 4 km, enquanto que para ( 355 = 0,2 km -1 ), diminui o intervalo correspondente a 3 km. Os resultados da simulação confirmam a necessidade de um sinal de acumulação de longo prazo (várias horas) nas medições de metano usando o Raman lidar (Veselovskii et al, 2019). Encerrando, o sétimo capítulo apresenta uma conclusão dos resultados obtidos neste doutorado e as perspectivas deste trabalho.…”

“…Medidas com alta resolução temporal e espacial podem ser realizadas com o equipamento denominado LIDAR (LIgth Detection And Ranging). As quais tem atraído muito as atenções nos últimos anos (Veselovskii, 2019;Landulfo, 2018;Da Costa, 2018).…”