Aerosol optical absorption coefficients at a rural site in Northwest China: The great contribution of dust particles

An intensive measurement campaign was conducted at a rural site in Northwest China to investigate aerosol optical absorption properties, using the ground-based mobile facility of the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL). The average mass concentration of PM2.5 was 103 ± 4 μg m−3 during the 20-day campaign in April 2014. Black carbon (BC) only accounted for ∼0.4% of the PM2.5 on average, with the mean concentration of 443 ± 12 ng m−3 measured using a single particle soot photometer (SP2). The aerosol absorption coefficient (σap) was 5.69 ± 0.01 Mm−1 on average, recorded by a multi-angle absorption photometer (MAAP) at the wavelength of 637 nm. It showed a linear relationship with BC mass concentration during non-dust periods, especially at their diurnal peaks of 07:00-09:00 a.m. (local standard time), deriving a bulk mass absorption efficiency (MAE) for BC of 8.5 ± 1.1 m2 g−1. The σap increased sharply during the dust storm, while the BC remained at a lower concentration than other moments, implying that the dust particles had a considerable contribution to light absorption. On average, dust particles accounted for 26.7% of the aerosol absorption and increased to 71.6% during the dust storm. The MAE of dust was calculated to be 0.014 ± 0.00028 m2 g−1, which was comparable to that measured in the downwind regions in East Asia. Based on the Mie theory for spherical particles, the refractive index (m) of natural mineral dust particles was estimated to be 1.50-0.0007i in Northwest China.