Photochemical Activity Study Of Water-Soluble Organic Compounds In Motor Vehicle Exhaust Particulate Matter

Water-soluble organic compounds（WSOC）is one of the main organic components constituting atmospheric particulate matter（PM）,and photochemical studies of the aqueous environment show that WSOC such as humic acid and fulvic acid in water can photoinduce the generation of reactive oxygen species（ROS）such as WSOC excited triplet state（³WSOC^*）,singlet state oxygen（¹O₂）,hydroxyl（·OH）and superoxide radical（O₂·ˉ）with high oxidative activity.Mobile sources are one of the important sources of PM in China’s cities,and PM from different sources has some variability in component composition,and its photochemical behavior in the atmospheric microaqueous environment is closely related to human health.In this paper,we focus on the photogenerated ROS capacity of WSOC（D-WSOC）in PM from diesel vehicles and WSOC（G-WSOC）in PM from automobiles in the atmospheric microaqueous environment.Due to the complex composition of the atmospheric environment,this paper also focuses on the effects of the presence of common salt components（NH₄NO₃,（NH₄）₂SO₄ and Na Cl）and metal ions（Fe（III）and Cu（II））,which can be important drivers of the atmospheric water cycle through redox reactions,on the photogenerated ROS capacity of WSOC in the atmospheric microaqueous environment,and conclude:（1）The quantum yield coefficients f _TMP for ³WSOC^*,andΦ_1O2 for ¹O₂,showed a decreasing trend in values with increasing aging time,and the changes in values from stock solution to aging for 48 h were,respectively,f _TMP:D-WSOC（997.17～91.63）,G-WSOC（232.21～150.13）;Φ_1O2:D-WSOC（18.12～4.31）,G-WSOC（4.27～3.93）.In addition,the yields of·OH and O₂·ˉshowed a tendency to decrease with increasing aging time.The numerical comparison results showed that the above studied systems mainly produced ³WSOC^*,followed by ¹O₂,and less·OH and O₂·ˉ,and the ability of photogenerated ROS was greater in the D-WSOC system than in the G-WSOC system.（2）A linear fit of the values of f _TMP andΦ_1O2 for D-WSOC-primary,G-WSOC-primary and aged solutions was performed.Linear fits were also performed for the values of f _TMP andΦ_1O2 with the values of spectral parameters(E₂:E₃,E₄:E₆,SUVA₂₅₄)and electron donating capacity（EDC）,respectively.The results showed that there was a positive correlation between f _TMP andΦ_1O2.f _TMP andΦ_1O2 showed negative correlation with E₂:E₃ and positive correlation with E₄:E₆,SUVA₂₅₄ and EDC,respectively.（3）Effect of the presence of different solutes NH₄NO₃,（NH₄）₂SO₄,Na Cl and Fe（III）and Cu（II）,respectively,on the degradation of phenolics representing 4-hydroxy-3-methoxypropiophenone（GA）present in the atmospheric microaqueous environment in the WSOC system.The ratio k’_GA,_absolute/k’_GA,0 of GA degradation rates in the presence and absence of solute was:addition of 500μM NH₄NO₃/no NH₄NO₃:D-WSOC（39.72）,G-WSOC（29.72）;addition of 2 M（NH₄）₂SO₄/no（NH₄）₂SO₄:D-WSOC（6.5）,G-WSOC（1.74）;addition of 2 M of Na Cl/no Na Cl:D-WSOC（1.07）,G-WSOC（1.02）;addition of 20μM Fe（III）/no Fe（III）:D-WSOC（1.16）,G-WSOC（1.12）;addition of 10μM Cu（II）/no Cu（II）:D-WSOC（0.23）,G-WSOC（0.28）.The presence of NH₄NO₃ had the greatest effect on GA degradation,indicating the greatest promotion of photogenerated ROS in WSOC;Cu（II）appeared to inhibit GA degradation,indicating that Cu（II）had an inhibitory effect on the photogenerated ROS capacity of WSOC.The remaining solutes all had different degrees of promotion on the ability of WSOC to photogenerate ROS.