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The evolution of physical, chemical and optical properties of urban aerosolparticles was characterized during an extreme haze episode in Beijing, PRC,from 24 through 31 January 2013 based on in situ measurements. Theaverage mass concentrations of PM1, PM2.5 and PM10 were99 ± 67 μg m−3 (average ± SD),188 ± 128 μg m−3 and 265 ± 157 μg m−3, respectively. A significant increase in PM1-2.5fraction was observed during the most heavily polluted period. The averagescattering coefficient at 550 nm was 877 ± 624 Mm−1. Anincreasing relative amount of coarse particles can be deduced from thevariations of backscattering ratios, asymmetry parameter and scatteringÅngström exponent. Particle number-size distributions between 14 and 2500 nm diameter showed high number concentrations, particularly in thenucleation mode and accumulation mode. Size-resolved chemical composition ofsubmicron aerosol from a high-resolution time-of-flight aerosol mass spectrometershowed that the mass concentrations of organic, sulfate, nitrate, ammoniumand chlorine mainly resided on particles between 500 and 800 nm (vacuum diameter),and nitrate and ammonium contributed greatly to particle growth during theheavily polluted day (28 January).Increasing relative humidity and stable synoptic conditions on 28 Januarycombined with heavy pollution on 28 January, leading to enhanced water uptakeby the hygroscopic submicron particles and formation of secondary aerosol,which might be the main reasons for the severity of the haze episode.Light-scattering apportionment showed that organic, sulfate, ammonium nitrate andammonium chloride compounds contributed to light-scattering fractions of54, 24, 12 and 10%, respectively. This study indicated thatthe organic component in submicron aerosol played an important role invisibility degradation during the haze episode in Beijing

Aerosol physicochemical properties and implications for visibility during an intense haze episode during winter in Beijing