Analysis of Methane Loss Rate in the Atmosphere over Iran: A Regional Computational Study Using CAMS and ERA5 Data (Mar 2024 – Feb 2025)

Methane is the second most potent anthropogenic greenhouse gas, making the accurate quantification of its atmospheric sink vital for regional climate modeling. This study presents a high-resolution, time-dependent computational analysis of the primary CH ₄ ​ chemical sink (reaction with the hydroxyl radical, OH·) over the Iranian plateau and surrounding territories from March 2024 to February 2025. The analysis utilizes 3-hourly, 0.4°×0.4° spatial resolution data for CH ₄ ​ column-averaged molar fraction, temperature (t2m), and humidity (d2m) derived from the Copernicus Atmosphere Monitoring Service (CAMS) and ERA5 reanalysis datasets. The CH ₄ ​ loss rate is calculated using the standard kinetic rate law with a temperature-dependent rate coefficient, while OH· concentration is estimated empirically based on relative humidity (RH). The results show a mean annual CH ₄ ​ loss rate of 1.5502×10 ⁵ molecule⋅cm ^− 3 ⋅s ^− 1 across Iran. Spatially, the highest loss rates are consistently found over the humid Caspian Sea and Persian Gulf coastlines. Temporally, the loss rate exhibits a strong seasonal cycle, with the peak occurring in spring and autumn (November/March), which is counter-intuitive to peak temperature seasons. This temporal pattern is attributed to the governing influence of Relative Humidity (RH) on the empirically estimated OH· concentration, which dominates the Arrhenius temperature dependence. The study provides critical, highly resolved regional data for improving national CH ₄ ​ budget models.