Abstract
Post-winter haze events in Delhi, India, comprise great air quality challenges, yet remain poorly understood due to limited measurements of vertical profiles of particulate matter (PM) concentrations. This study employs a drone-mounted PM low-cost sensor (PM-LCS) with an optimized sampling system to capture vertical PM₂․₅ profiles during March 2021. Elevated PM₂․₅ concentrations (160 µg/m³) were observed at an altitude of 100 m, being 60% higher than ground level. Vertical profiles of the PM1/PM₂․₅ ratio under humid conditions (RH > 70%), showed that haze formation is likely driven by hygroscopic inorganic aerosols. Comparison with model simulations showed significant underestimation of PM₂․₅ (-52.6 ± 5.5%) during morning haze episodes, coinciding with a dry bias in modeled RH (-30.1 ± 8.3%). During non-hazy episodes, PM₂․₅ underestimation decreased to 10.8 ± 1.2% with a minimal RH bias. This suggests that the dry bias of the model limits its ability to simulate aerosol hygroscopic growth. Overall, our findings demonstrate that drone-mounted PM-LCS provide a valuable vertical air quality assessment tool.