Enhanced mixing state of black carbon with nitrate in single particles during haze periods in Zhengzhou, China

Black carbon (BC) plays an important role in air quality and climate change,which is closely associated with its mixing state and chemical compositions.In this work the mixing state of BC-containing single particles was investigated to explore the evolution process of ambi-ent BC particles using a single particle aerosol mass spectrometer (SPAMS) in March 2018 in Zhengzhou,China.The BC-containing particles accounted for 61.4％ of total detected am-bient single particles and were classified into five types including BC-nitrate (BC-N,52.3％)as the most abundant species,followed by BC-nitrate-sulfate (BC-NS,22.4％),BCOC (16.8％),BC-fresh (BC-F,4.5％) and BC-sulfate particles (BC-S,4.0％).With enhancement of the ambi-ent nitrate concentration,the relative peak area (RPA) of nitrate in BC-N and BC-NS particles both increased,yet only the number fraction (Nf) of BC-N particles increased while the Nf of BC-NS particles decreased,suggesting that the enhanced mixing state of BC with nitrate was mainly due to the increase in the ambient nitrate mass concentration.In addition,the Nf of BC-N decreased from 65.3％ to 28.4％ as the absorbing (A)ngstr(o)m exponents (AAE) of eBC increased from 0.75 to 1.45,which indicated the reduction of light absorption ability of aged BC particles with the enhanced formation of BC-N particles.The results of this work indicated a change in the mixing state of BC particles due to the dominance of nitrate in PM2.5,which also influenced the optical properties of aged BC particles.