Radiative response of biomass-burning aerosols over an urban atmosphere in northern peninsular Southeast Asia

A large concentration of finer particulate matter (PM_2.5), the primary air-quality concern in northern peninsular Southeast Asia (PSEA), is believed to be closely related to large amounts of biomass burning (BB) particularly in the dry season. In order to quantitatively estimate the contributions of BB to aerosol radiative effects, we thoroughly investigated the physical, chemical, and optical properties of BB aerosols through the integration of ground-based measurements, satellite retrievals, and modelling tools during the Seven South East Asian Studies/Biomass-burning Aerosols & Stratocumulus Environment: Lifecycles & Interactions Experiment (7-SEAS/BASELInE) campaign in 2014. Clusters were made on the basis of measured BB tracers (Levoglucosan, nss-K⁺, and NO₃ ⁻) to classify the degree of influence from BB over an urban atmosphere, viz., Chiang Mai (18.795°N, 98.957°E, 354 m.s.l.), Thailand in northern PSEA. Cluster-wise contributions of BB to PM_2.5, organic carbon, and elemental carbon were found to be 54–79%, 42–79%, and 39–77%, respectively. Moreover, the cluster-wise aerosol optical index (aerosol optical depth at 500 nm ≈ 0.98–2.45), absorption (single scattering albedo ≈0.87–0.85; absorption aerosol optical depth ≈0.15–0.38 at 440 nm; absorption Ångström exponent ≈1.43–1.57), and radiative impacts (atmospheric heating rate ≈1.4–3.6 K d⁻¹) displayed consistency with the degree of BB. PM_2.5 during Extreme BB (EBB) was ≈4 times higher than during Low BB (LBB), whereas this factor was ≈2.5 for the magnitude of radiative effects. Severe haze (visibility ≈ 4 km) due to substantial BB loadings (BB to PM_2.5 ≈ 79%) with favorable meteorology can significantly impact the local-to-regional air quality and the, daily life of local inhabitants as well as become a respiratory health threat. Additionally, such enhancements in atmospheric heating could potentially influence the regional hydrological cycle and crop productivity over Chiang Mai in northern PSEA.