Emission Characteristics of Greenhouse Gases and Air Pollutants in Northern Hemisphere Cities: Comprehensive Assessment Using Ground‐Based Fourier Transform Spectrometers

Despite the importance of understanding the urban emission characteristics of greenhouse gases (GHGs) and air pollutants, few studies have conducted integrated assessments across diverse urban environments. Herein, we conducted a comprehensive evaluation of the emission characteristics of GHGs and air pollutants in seven cities in the Northern Hemisphere using ground-based Fourier transform spectrometers. Our analysis primarily focused on emission ratios of excess column-averaged dry-air mole fractions of carbon monoxide (CO) to carbon dioxide (CO$_2$ ) (∆XCO:∆XCO$_2$ ) and those of methane (CH$_4$ ) to CO$_2$ (∆XCH$_4$ :∆XCO$_2$ ). We found that the emission ratios varied significantly across cities. Xianghe (China) and Pasadena (USA), known for severe air pollution, showed the highest emission ratios. Notably, Seoul (South Korea) showed lower ∆XCO:∆XCO2 (3.32 ± 0.10 ppb/ppm) but relatively higher ∆XCH$_4$ :∆XCO$_2$ (4.85 ± 0.04 ppb/ppm), which was comparable to the ∆XCH$_4$ :∆XCO$_2$ value of Xianghe (5.15 ± 0.10 ppb/ppm), suggesting that targeted CH$_4$ reduction strategies may be required for climate change mitigation in Seoul.