Hygroscopic Properties Of Urban Aerosol In Nanjing During Wintertime

The hygroscopicity of aerosol particles can significantly affect their sizes,optical properties,cloud condensation nuclei(CCN)activity and chemical reactivity,thereby having an important impact on the atmospheric visibility,radiation forcing,acid deposition and hunman health directly or indirectly.A Hygroscopic Tandem Differential Mobility Analyzer(H-TDMA)was deployed to measure the hygroscopic growth factors(GF)with chemical particulate composition monitor(MARGA)and related meteorological during wintertime in urban region of Nanjing.Comparing the difference of the hygroscopicity of aerosol particles due to different particle sizes and pollution conditions as well as chemical compositions observations.The results are as follows:(1)the probability distribution of growth factor(GF-PDF)is bimodal,the peaks of which are GF=1.0±0.010(the less-hygroscopic group)and GF=1.4±0.035(the more-hygroscopic group)during wintertime in urban region of Nanjing.At relative humidity(RH)of 85%,the number fraction of the less-hygroscopic particles(NFLH)decreases slightly from 40%to 20%as those particles' size increased(i.e.,40 nm,80 nm,110 nm,150 nm,and 200 nm),while the number fraction of more-hygroscopic particles(NFMH)increase from 60%to 80%.The dispersion(?LH)of GF-PDF for NFLH-group particles varies in the range 0.04-0.05,but the ?MH of NFLH-group particles presents larger values(more than 0.1),suggesting variously complex chemical compounds of NFMH-group particles being much more external mixing.Comparing with size-dependent analysis of the diurnal cycles,the mean hygroscopic growth factors(GFmean)and NFMH presents same bimodal peaks at around 7:00 and 17:00,respectively.Overall,the GFmean and NFMH are larger during nighttime than that during daytime,which might be due to the influence of lower atmospheric boundary layer height and heterogenous reaction process that promotes the formation of the more-hygroscopic particles under the nighttime conditions.(2)According to the study of persistent heavy pollution events in the winter of 2015.It is found that the change of the hygroscopicity of aerosol particles was affected by the internal emission and the external transmission.Comparing with size-dependent analysis of NHMH and GFmean the NHMH is 68%,76%,79%,83%,85%and the GFmean is 1.26,1.31,1.33,1.37,1.4 in different particles' size range during polluted episodes.However,NHMH increase from 60%to 80%,GFMH increase froml.23tol.35 in air clean.GFmeam and NFMH in all particles,size range show higher values during polluted episodes rather than that in air clean.(3)According to the study of the soluble chemical composition of PM2.5 during heavy pollution events during February 2016 in urban region of Nanjing.The mass concentration of sulphate,nitrate,and ammonium is(17.57±9.07,26.16±11.39,13.61 ±6.68)?g/m3,which is twice as much as non haze(9.62±3.58,12.12±7.51,5.78±3.59)?g/m3.The soluble chemical composition is in turn No3->SO42->NH4+>Cl>K+>Ca2+>Na+>Mg2+.The contribution of nitrate is the greatest,accounting for 29%,sulphate accounts for 14%,ammonium accounts for 8%.The other water-soluble components(Cl-,K+,Ca2+,Na+,Mg2+)accounts for 5.9%approximately.There is no obvious diurnal change in mass concentration,which is kept at a high level.(4)The GF-PDF is bimodal during the observation.The peaks of less-hygroscopic particles are obvious than more-hygroscopic particles at 40nm.The GF of the more-hygroscopic and less-hygroscopic particles is 1.335±0.03,1.054±0.008 at 40nm.The GF is 1.348±0.03,1.053±0.011 at 80nm.The hygroscopicity of more-hygroscopic particles increases with the increase of size,the hygroscopicity of less-hygroscopic particles decreases.The GF of particles is 1.265±0.05 at 40nm,which is 1.345±0.03 at 80nm during the observation.The GF of particles is 1.307±0.08,1.413±0.07 during the haze,and is 1.230±0.03,1.300±0.03 during the non haze.There is no obvious change of the GF-PDF of less-hygroscopic particles in the cold air,as the black carbon is less affected by cold air.The GF-PDF of more-hygroscopic particles is towards the left,the particles is cleared by the cold air,the mass concentration of old particles decreases,the hygroscopicity of particles decreases.(5)The sulphate and nitrate have similar trend of change with the relative humidity.It indicates that relative humidity plays an important role in the formation of haze.The mass concentration of PM2.5 is 87.56±25.87?g/m3,69.31±28.75?g/m3 during the haze and observation.The mass concentration of water-soluble components increases twice than the non haze.The GF of particles decreases from 1.325±0.03 to 1.301±0.07.The GF of more-hyroscopic particles at 110nm decreases obviously during the spring festival.The mass concentration of sulphate,nitrate,ammonium decreases obviously too,the change shows the good consistence.The surrounding factories and catering enterprises stop work and rest,resulting in a larger proportion of traffic emissions.Automobile exhaust mainly emits black carbon and some organic matter,the hygroscopicity in the less hygroscopic group increased,The hygroscopicity of the accumulating mode decreases in more hygroscopic group.