The Light Absorption Property And Influencing Factors Of Water-soluble Organic Matter In PM_2.5 In Shanghai

The organics in the atmospheric particles can efficiently scatter sunlight,but some organic matter has strong absorption effect on near-violent～visible light,which has an important impact on atmospheric visibility and global radiation balance.Most of the light-absorbing organics is in form of water-soluble organic matter（WSOM）.Research on the light-absorbing characteristics of WSOM is thus of great significance for more comprehensive understanding of the physicochemical properties of atmospheric particulate matter and the mechanism of haze formation,and it can provide basic information for the scientific development of air pollution prevention strategies in Shanghai and the Yangtze River Delta area.In this study,PM_2.5 samples were collected simultaneously at one suburban site（Qingpu,QP）and one urban site（Pudong,PD）in Shanghai from January 2017 to December 2018.The concentration of water-soluble organic carbon（WSOC）and the light absorption characteristics of the water-soluble organics in PM_2.5 were measured.The time and spatial distribution,the influencing factors and possible sources of WSOC and its light absorption capacity were investigated.The following main results were got:（1）The average concentration of WSOC at QP and PD was 2.09μg·m^-3 in 2017 and 2.11μg·m^-3 in 2018,no significant interannual difference found.The suburban site（QP）had higher concentration of WSOC(average at 2.21μg·m^-3 during the entire sampling period)than that at PD(1.98μg·m^-3).But the concentration difference between the two sites was only about 10%,indicating that the distribution of WSOC in PM_2.5 in Shanghai had a good spatial homogeneity. Significant seasonal variation was found for the concentration of WSOC,and the average winter concentration(2.77μg·m^-3 at QP and 2.52μg·m^-3 at PD)was higher than that in summer(1.80 and 1.91μg·m^-3)（P<0.01）.The proportion of WSOC in OC varied from 46%to 61%,and similar WSOC/OC ratio was found in different seasons.The WSOC/OC ratio in 2018（60%）was slightly higher than that in 2017（56%）,indicating that the composition of OC in 2018 was different from that in 2017 to certain degree.（2）The correlation analysis between WSOC and other chemical species,principal component analysis,and tracer yield estimation results showed that WSOC in Shanghai PM_2.5 mainly came from the secondary organic aerosols,and a certain part from biomass burning,which had higher contribution of about 20%in winter than other seasons.The cluster analysis of the 48 hours backward trajectories of air parcels arriving in Shanghai suggested that local emission was the important source of WSOC in PM_2.5 in Shanghai,but it was also affected by the pollutants form the surrounding area and the long-distance transported pollutants from the inland areas of northern China.Stronger impact of anthropogenic pollutants could be found in winter.（3）In this study,analysis of the light absorption coefficient at 365 nm(Abs₃₆₅),mass absorption efficiency MAE₃₆₅ and absorption angstrom exponent AAE showed that the light-absorbing capacity of WSOC in 2017(average Abs₃₆₅ and MAE₃₆₅ of 1.38 Mm^-1 and 0.68 m²·g^-1)was stronger than that in 2018(1.10 Mm^-1 and 0.56 m²·g^-1).There was a significant seasonal variation in the absorbance of WSOC.The Abs₃₆₅ in winter(2.73 Mm^-1 and 1.73 Mm^-1 in 2017 and 2018 at QP respectively;2.15 Mm^-1 and 1.23 Mm^-1 at PD)were about 2.2 times of those in summer(1.04 Mm^-1 and 0.80 Mm^-1 at QP;1.11 Mm^-1 and 0.55 Mm^-1 at PD).The MAE₃₆₅ value in winter(0.96 m²·g^-1 and 0.65 m²·g^-1 in 2017 and 2018 at QP; 0.76 m²·g^-1 and 0.62 m²·g^-1 at PD)were also significantly higher than those in summer(0.64 m²·g^-1 and 0.37 m²·g^-1 at QP;0.51 m²·g^-1 and 0.39 m²·g^-1 at PD), indicating that the light-absorbing ability of WSOM in winter was stronger in Shanghai.（4）The light absorbing characteristics at different wavelengths（365 nm,400 nm and 500 nm）was studied and the results showed that the light absorption coefficient of the WSOM in PM_2.5 in Shanghai at 400～500 nm could reach about 10 to 64% of that at 365 nm,which indicated that WSOM could absorb visible light significantly.Based on the reported absorbance coefficient of the PM_2.5 in winter of 2019 at 370 nm,the WSOM could contribute about 15.3%and 9.3%to the total light absorbance of the PM_2.5 particles in winter of 2017 and 2018.（5）The light absorption wavelength dependent index AAE of WSOM at QP and PD showed no regular season trend,but the AAE value in 2018（4.86）was greater than in 2017（4.21）,indicating that WSOM in 2018 may be affected more strongly by the"light bleach"effect caused by the aging process of SOA. Negative correlation between AAE and Abs_λand MAE_λwas observed,but the correlation was not strong,suggesting the complex influencing factors on AAE.（6）Analysis of the correlation between Abs₃₆₅ and other chemical components and the PMF results showed that SOA and biomass burning were the main contributors of the light-absorbing WSOM in PM_2.5 in Shanghai,accounting for more than 85%of the absorbing coefficient.Increased emission from biomass burning and the nitrogen-containing organic matter possibly formed by the secondary reactions of ammonium/NH₃ enhanced the absorbing ability of the WSOM in winter,while the WSOM in summer was mainly from secondary formation.