Background: An estimated 99% of the global population lives in environments where PM2.5 levels exceed the WHO air quality guideline of 15 μg/m³. In 2018, air pollution contributed to approximately 4.2 million deaths worldwide. In Mon golia, air pollution—particularly in urban centers like Ulaanbaatar, worsens significantly during the winter season, posing a serious public health and local concern. Therefore, it is compulsory to compare the outdoor air quality in Ulaanbaatar, the capital and Darkhan city. Aim: To assess and compare the outdoor PM2.5 concentrations in Ulaanbaatar and Darkhan during the winter season. Materials and Methods: This study was conducted in Ulaanbaatar and Darkhan from December 10, 2024, to February 19, 2025. A total of 60 PurpleAir Classic+ sensors (30 per city) were installed to assess PM2.5 concentrations at 2-minute intervals. We analyzed collected data using R software. The 24-hour average PM2.5 concentrations were compared with both the Mongolian National Air Quality Standard (MNS4585:2016) and the WHO air quality guidelines (2021). Results: The 24-hour average PM2.5 concentration in Ulaanbaatar was 112.3±62.2 μg/m³, which was significantly higher than that in Darkhan (79.2±25.6 μg/m³; p<0.05). In Ulaanbaatar, the monthly averages were 119.9±67.7 μg/m³ (Decem ber), 113.5±60.8 μg/m³ (January), and 95.0±51.9 μg/m³ (February) respectively (p<0.05). In contrast, Darkhan city’s monthly average PM2.5 remained relatively close across the months: 79.1±22.2 μg/m³ (December), 78.7±28.6 μg/m³ (January), and 84.6±30.0 μg/m³ (February), with no statistical significance (p>0.05). During the study period, the 24-hour average PM2.5 concentrations exceeded the MNS4585:2016 (50 μg/m³) in 69.8% of days in Ulaanbaatar and 64.6% in Darkhan. WHO’s guideline of 15 μg/m³ was exceeded 93.4% of the time in both cities. Conclusion Darkhan city has lower PM2.5 concentrations compared to Ulaanbaatar, both cities significantly exceeded MNS4585:2016 standard and the WHO air quality guidelines (2021) during the winter months.