Study On Aerosol Optical Properties And Mass Spectrometry And Absorption Spectrum Of Atmospheric Brown Carbon During Winter In Shanghai

This study adopted PM2.5 filter sampling and offline analysis,and online multiple optical instruments monitoring to investigate the chemical composition and optical properties of atmospheric aerosol in Yangpu district,Shanghai.Two systems being composed of CAPS instruments(no heating and heating)were built up to study the aerosol absorption enhancement and explore the impact of atmospheric brown carbon and lensing effect.Furthermore,the technology of HPLC-DAD-Q-TOF-MS was used to characterize BrC on molecular level.The mass concentration of PM2.5 in Shanghai was 51.65 ± 30.49 ?g/m3 and the secondary soluble ion accounted for 56%of the PM2.5 mass concentration.In addition,NO3-/SO42->1 revealed that the emission of mobile pollution was the main contributor for the air quality in Yangpu district,Shanghai.The average OC/EC ratio of 2.86 and the estimation of SOC indicated that there was SOC pollution in the atmosphere,being caused by the exhaust of vehicles and combustion of coal and biomass.Totally,18 kinds of metal element were analyzed,automobile exhaust and coal burning contributed significantly to metals in atmospheric aerosols.The average absorption coefficient and MAE365 of the methanol extract at 365 nm were 6.28 ±2.69 Mm-1 and 1.35±0.39 m2/g,indicating that biomass burning could be on major factor for brown carbon during winterin Yangpu district,Shanghai.Low level of EC and SSA indicated that there was high absorption efficiency EC in the air,meanwhile,SAE550/700,SAE450/700 and SAE450/550 were all greater than 1,indicating that the mainly extinction contribution were due to fine particles.SAE showed a higher value in the afternoon,relating to the formation of secondary fine particles.The absorption coefficients measured after heating at 450 and 530 nm accounted for 51 and 70%of those before heating.Due to the smaller wavelength dependence of the lens effect,it indicated that there may be absorption caused by brown carbon during the monitoring period.Therefore,through calculation,it was found that on January 2 and 3,the absorption coefficient of brown carbon were larger,indicating that this period may contain a higher concentration of BrC or chromophore with strong light absorption properties.The compounds with the absorption peaks at 365 nm for both positive and negative modes were investigated using HPLC-DAD-Q-TOF-MS,and then most amount of chromophoresobserved during the period were 4-nitrophenol,4-nitrocatechol,3-methyl-4-nitrophenol,2-methyl-4-nitrophenol,et al.,and two organic sulfates.Compare to the absorption of 35 chromophores of brown carbon at the wavelength range of 250-450 nm,it was found that the largest absorption contributor at 365 nm was 4-nitrophenol,which accounted for 16%.Furthermore,the mass spectrometry and standard reagents were used to obtain the quantitativeinformation of 4-nitrophenol,4-nitrocatechol,3-methyl-4-nitrophenol,2-methyl-4-nitrophenol inthe methanol extract and HULIS samples,the concentrations on various days were significantly different.Long-term continuous online monitoring results showed that the scattering coefficient was related to PM2.5,temperature and wind speed.The scattering coefficients presented high values in the northwest and southwest direction.The northwest direction was due to long-distance transport from the areas that adopt central heating supply systems during winter,whereas the southwest direction was due to local transport.