Spatial distribution pattern and long-term trend of atmospheric methane in the Atlantic-Mediterranean transition region based on TROPOMI and GOSAT measurements.

Methane (CH) spatial distribution and its trends in the Atlantic-Mediterranean transition region of southwestern Europe were studied using TROPOMI and GOSAT observations. TROPOMI XCH provided insights into the distribution across the entire Iberian Peninsula, highlighting the high XCH mixing ratios in the two main valleys and the southern sub-plateau, characterized by agricultural and livestock activities. The marine-continental region in the southwestern Iberian Peninsula was identified as a major CH hot spot. Focusing on this region, the seasonal distribution of CH and its trends were studied. The lowest XCH levels were recorded in a semi-natural landscape in a mountain range with an average mixing ratio of 1871.9 ± 18.5 nmol mol, while the highest trend was recorded at 10.9 ± 0.7 nmol mol year for the period 2019-2023. Conversely, the highest XCH levels were obtained over an inland wetland, averaging 1888.7 ± 15.8 nmol mol; while recording the lowest regional trend, 6.4 ± 0.9 nmol mol year. Aiming to identify potential regional changes in the XCH trend, GOSAT observations were used, revealing a long-term trend of 8.7 ± 0.3 nmol mol year from 2009 to 2023. Focusing on the first and last five years of the series, trends of 6.5 ± 0.9 nmol mol year and 14.9 ± 1.2 nmol mol year were detected, indicating a trend acceleration ratio of 2.3. An analogous pattern was found in the Atlantic Ocean and central Mediterranean at the same latitude, with ratios of 2.7 and 2.4 respectively. Furthermore, these outcomes were corroborated by ground-based observations from the Azores and Lampedusa stations, which belong to the NOAA network. Despite CH increasing globally and its trend accelerating, this work highlights the complexity of its dynamics, with regional variations in its mixing ratios, horizontal distribution, and temporal trends.