Temporal Characteristics and Vertical Profiles of Atmospheric CH4 at the Northern Foot of the Qinling Mountains in China

A two-year (March 2021 to February 2023) continuous in-situ atmospheric CH4 measurements and periodic vertical CH4 measurements (< 2000 m) in July and November 2021 were conducted at the northern foot of the Qinling Mountains in Xi’an, China, aiming to study the temporal and vertical variations in atmospheric CH4 and the influence of air mass transport. The two-year average CH4 concentration at this site was 2119.6 ± 108.8 ppb. Seasonal CH4 concentrations were the highest in winter (2177.6 ± 121.5 ppb) and lower in summer (2079.1 ± 77.6 ppb) and spring (2073.9 ± 68.4 ppb). Diurnal CH4 peaked at 10:00 ∼ 11:00. Atmospheric CH4 there was mainly from local source emissions from Xi’an and short distance transport from the southern Qinling Mountains through the valleys. On the July sampling days, vertical CH4 concentrations increased in the morning (08:00) and at mid-night (23:30), while they decreased in the early afternoon (13:00) and late afternoon (18:00) from near the surface (20 m) to 200 m, increased at 200 ∼ 500 m, and then decreased at 500 ∼ 1000 m. During the heating period in November, vertical CH4 concentrations increased by 20.0 ∼ 86.8 ppb at 20 ∼ 200 m due to horizontal transport and poor atmospheric dispersion conditions, and then decreased up to 2000 m, with the fastest decrease rate (-44.4 ± 51.4 ∼ -17.6 ± 19.3 ppb/100 m) at 500 ∼ 1000 m. Vertical CH4 concentrations increased at all heights especially below 500 m in serve haze, mainly attributed to horizontal transport from the northwestern and southeastern polluted regions. Vertical observations further confirmed the important influence of transport on CH4 levels at this site.