A comprehensive evaluation of aerosol extinction apportionment in Beijing using a high-resolution time-of-flight aerosol mass spectrometer

Highlights: • A new method of caculating aerosol extinction coeificent by AOD dividing the mixing layer height was established. • NR-PM1 and black carbon together contributed about 88% to the whole aerosol light extinction. • Contribution of organics to light extinction increased on polluted days. • Aerosol extinction coefficient increased rapidly when the ratio of of NR-PM1 and BC reached 7. An aerosol mass spectrometer (AMS) was used to measure the chemical composition of non-refractory submicron particles (NR-PM₁) in Beijing from 2012 to 2013. The average concentration of NR-PM₁ was 56 μg·m⁻³, with higher value of 106 μg·m⁻³ when Beijing was influenced by air masses from south in winter. Organics was the primary chemical component with a concentration of 26 μg·m⁻³, accounting for 46% of the total NR-PM₁. The ratio of NO₃⁻/SO₄²⁻ was utilized to identify the relative contribution of stationary and traffic related resource to PM pollution. When NR-PM₁ concentration was between 50 and 200 μg·m⁻³, NO₃⁻/SO₄²⁻was larger than 1, indicating traffic resource contributed more than stationary resource during the aerosol growth. A new method was developed to calculate aerosol extinction coefficient (σ) as a function of aerosol optical depth (AOD) and the mixing layer height (MLH). σ derived from the new method showed a statistically significant correlation with that obtained from traditional method, which was calculated using visibility (y = 0.99x + 85 R² = 0.69). Multiple linear regressions in dependence of chemical component were performed to evaluate light extinction apportionment. Under the overall condition, NR-PM₁ contributed about 88% to the whole aerosol light extinction; organics, ammonium chloride, ammonium nitrate, ammonium sulfate, black carbon contributed 30%, 6%, 24%, 26% and 6% of the NR-PM₁ light extinction, respectively. By further comparing the light extinction apportionment under the different dominated air masses, we concluded that the organics and ammonium sulfate contributed more in polluted days (36% and 23%) than that in clean days (21% and 21%). Mass ratio (MR) between NR-PM₁ and black carbon (MR = mass_NR-PM1/mass_BC) was used to identify black carbon aging degree, and the result showed that aerosol mass extinction efficiency increased rapidly after MR reached about 7 in the process of black carbon aging.