Optical source profiles of brown carbon in size-resolved particulate matter from typical domestic biofuel burning over Guanzhong Plain, China

In this study, both PM_2.5 and size-resolved source samples were collected from a “heated kang” and an advanced stove to investigate the optical properties of brown carbon (BrC). The light-absorption coefficient (b_abs), the absorption Ångström exponent (AAE), and the mass absorption cross-section (MAC) of both water and methanol-extracted BrC were investigated. The methanol-extracted BrC (BrC_methanol) had higher light absorption than water-extracted BrC (BrC_water). The value of PM_2.5 b_abs of BrC_methanol at 365 nm (b_{abs365,methanol}) dramatically decreased from 64,669.8 Mm^{− 1} for straw burning in the “heated kang” to 1169.2 Mm^{− 1} for maize straw briquettes burning in the advanced stove at the same burning rate. The value of PM_2.5 MAC for BrC_methanol at 365 nm (MAC_365,methanol) decreased from 1.8 m² g^{− 1} in the “heated kang” to 1.3 m² g^{− 1} in the advanced stove. For smoldering burning in the “heated kang”, b_{abs365,methanol}, MAC_365,methanol, and K⁺ showed a unimodal distribution that peaked at sizes < 0.4 μm. However, the b_{abs365,methanol} and MAC_365,methanol size distributions of the briquette burning in the advanced stove showed a bimodal pattern, with a large peak at sizes < 0.4 μm and a minor peak in the size range of 4.7–5.8 μm. The b_{abs365,methanol} value for sizes < 0.4 μm (277.4 Mm^{− 1}) was only 12.3% compared to those obtained from the “heated kang”. The burning rate did not influence the size distribution pattern of either the “heated kang” or the advanced stove. Results from a radiative model show that biomass burning is an important factor for light absorptivity, and the use of an advanced stove can reduce the simple forcing efficiency value by nearly 20% in UV bands compared to the “heated kang”. Our results indicate that changing the combustion style from maize straw smoldering to briquette burning in an advanced stove can effectively reduce BrC emissions during heating seasons in rural areas of Guanzhong Plain.