Aerosol Hygroscopicity Based On Scattering Measurements At A Rural Site Of The North China Plain

The North China Plain is one of the most polluted areas in China,especially in winter.Aerosol hygroscopicity have important effects on visibility and radiative forcing.A humidification system was deployed to measure aerosol hygroscopicity at a rural site of the North China Plain during December2016-February 2018.The aerosol scattering coefficients(PM₁₀/PM₁)under dry[relative humidity?RH?<30%]and wet?RH in the range of 40%–85%?conditions were simultaneously measured at wavelengths of 450,550,and 700 nm.The result shows that the aerosol scattering coefficient,backscattering coefficient,?ngstr?m exponent,and the backscatter ratio for PM₁ and PM₁₀ are 427.8 Mm^-1,56.3 Mm^-1,1.56,0.138;487.2Mm^-1,64.7 Mm^-1,1.37,0.139,respectively at Gucheng station?ngstr?m.Aerosol hygroscopicity is the strongest in summer,stronger in spring and autumn,and the weakest in winter.At relative humidity of80%,the average aerosol scattering enhancement factor during the whole observation period is 1.35?PM₁?and 1.29(PM₁₀),respectively.The scattering enhancement factor of PM₁ is nearly 5%higher than that of PM₁₀.This ratio increased to about 7%in summer,while it was only 2%in winter.When the aerosol scattering coefficient is larger,the scattering enhancement factor is smaller.When the scattering coefficient is smaller,the scattering enhancement factor is larger.The southwest wind and the northeast wind are dominant at Gucheng station.When the northeast wind appears,the scattering enhancement factor is large,while the scattering enhancement factor is smaller in the southwest wind.When the wind speed is small?WS<1.5m/s?,the scattering enhancement factor is larger no regardless of wind direction.During December 17^th-22^th 2016,heavy pollution occurred in North China Plain.Due to the switching valve failure,the measurement was carried out only for PM₁₀.The average of scattering coefficient?550nm?is 1699±292 Mm^-1 and the aerosol scattering coefficient and backscattering coefficient increased by only 29%and 10%,respectively when RH went up from 40%to 80%,while the hemispheric backscatter fraction went down by 14%,implying that the aerosol hygroscopicity represented by the aerosol scattering enhancement factor f?RH?is relatively low and RH exerted little effects on the aerosol light scattering in this case.The scattering enhancement factors do not show significant differences at the three wavelengths,only with an approximate 2%variation,suggesting that the aerosol hygroscopicity is not evident dependent of the wavelength.Aerosol hygroscopicity is highly dependent on the aerosol chemical composition.When there is a large mass fraction of inorganics and a small mass fraction of organic matter,f?RH?reaches a high value.The fraction of NO₃^-was strongly correlated with the aerosol scattering coefficient at RH=80%,which suggests that NO₃^-played an important role in aerosol hygroscopic growth during the heavy pollution period.