Photochemical Kinetics Of Nitro Polycyclic Aromatic Hydrocarbons In Snow

Snow is an important constituent of the ecosystems in polar and high-altitude regions.During the process of snowfall,atmospheric pollutants can enter the snow by dry and wet deposition.They can also be adsorbed in the snow after deposition,which makes snow an important store of contaminants.From the end of the 20 th century,researchers have detected a wide range of persistent organic pollutants?POPs?in polar and high altitude region.POPs are persistent,semi-volatile and highly toxic,through long-term migration they may be harmful to human health and may also threaten environmental safety.It is investigated that POPs in snow can volatilize into atmosphere,can be transformed by photochemical reaction,and can be transformed into more persistent,more toxic products by photochemistry.Nitro polycyclic aromatic hydrocarbons?NPAHs?are a kind of pollutants with direct mutagenic and carcinogenic activities,and their mutagenic and carcinogenic effect was higher than their parent PAHs,severely threatening the environmental safety and human health,so it is of great interest from widespread researchers in recent years.NPAHs containing four benzene rings have the highest mutagenicity,whereas NPAHs containing two nitro groups have higher activity than NPAHs with one nitro group.The most abundant NPAHs in vehicle exhaust is 1-nitro-pyrene?1-NP?,1,8-dinitrobyrene?1,8-DNP?is the most highly mutagenic compound that has ever been reported.In the low temperature of snow and ice,the conventional chemical reaction is slow,while photochemistry is an important way for NPAHs to migration and transformation in the environment.Researchers have studied the photochemistry of NPAHs in the atmosphere,solid surface and organic solvent,but little research has been conducted on the photochemistry of NPAHs in water and snow.In this paper,the quantitative analysis method of NPAHs was established,and the photoconversion laws of 1-NP and 1,8-DNP were studied in water and snow by indoor simulation experiments,and also established the photoconversion kinetic equation.The effects of H₂O₂,NO₂^-,NO₃^-,Fenton,Black Carbon?BC?,p H and methanol on NPAHs were investigated under the UV light.The main conclusions are as follows:1.Quantitative analysis of trace NPAHs by LC-MS/MS in waterThe contents of trace NPAHs in water was determined by LC-MS/MS with APCI ion source,by optimizing the mass spectrometry and chromatographic conditions.As a result,1-NP and 1,8-DNP displayed excellent linearity in the concentration range of 0.72¹2 ?g/L and 0.48⁴8 ?g/L,the correlation coefficients?R2?of the calibration curve was 0.9990 and 0.9992.The average recoveries ranged from 97.67%¹04.44% and 93.33¹03.13%.The RSD were below 5%.Besides,for 1-NP and 1,8-DNP,the limit of detection?LOD?of this method was 0.21 ?g/L and 0.15 ?g/L?S/N=3?,and the limit of quantitation?LOQ?was 0.72 and 0.26 ?g/L?S/N=10?respectively.The method gets the advantages of no pretreatment,simple operation and high sensitivity,and is superior to other liquid chromatographic and gas chromatographic methods.2.Photolysis of NPAHs in water under UV light?1?Under the irradiation of UV light,1-NP and 1,8-DNP can undergo light conversion reaction in water,and obeyed to the first-order kinetics equation;?2?The photodegradation rate of NPAHs in water decreased with the increase of the initial concentration of NPAHs.?3?The addition of photosensitizer?H₂O₂,NO₂^-and NO₃^-?in water promoted the photolysis of NPAHs,and the photodegradation rate increased with the increase of photosensitizer concentration;?4?The effect of Fenton on the photolysis of NPAHs in water was inhibition at first and then promotion,and the optimum concentration ratio of Fe2+ and H₂O₂ was 1:3;?5?1,8-DNP was BC removed from the water mainly due to the adsorption,the contribution of photolysis is almost negligible;?6?Acid condition and alkaline conditions promoted the photoconversion of NPAHs in water,and the promotion effect was stronger under acidic conditions;?7?Methanol promoted the photoconversion of NPAHs in water,and the higher the concentration of methanol,the faster the photolysis of NPAHs.3.Photochemical of NPAHs in snow under UV light?1?Under the irradiation of UV light,1-NP and 1,8-DNP can undergo photoconversion reaction in snow,and obeyed to the first-order kinetics equation.Low concentration of H₂O₂?50 ?mol/L?has little effect on the photoconversion of 1,8-DNP in snow,high concentration of H₂O₂?200 ?mol/L?promoted the photolysis of 1-NP in snow;?2?With the increase of the initial concentration of1,8-DNP,the photodegradation rate of 1,8-DNP in snow became faster.?3?High concentration of H₂O₂?200 ?mol/L?promoted the photolysis of 1-NP in snow;while low concentration of H₂O₂?50 ?mol/L?had little effect on the photolysis of1,8-DNP in snow;?4?NO₂^-and NO₃^-promoted the photoconversion of NPAHs in snow;?5?Fenton had little effect on the photolysis of 1-NP and 1,8-DNP in snow;?6?1,8-DNP was BC removed from snow mainly due to the adsorption,the contribution of photolysis is almost negligible;?7?the co-existence of BC and photosensitizer promoted the photoconversion of 1,8-DNP in snow,and the promotion effect of BC+NO₂^-on 1,8-DNP photolysis was more obvious than BC+H₂O₂ and BC+NO₃^-in snow;?8?Both acid and alkaline conditions promoted the photoconversion of 1-NP in snow;while acid conditions promoted the photoconversion of 1,8-DNP in snow,and alkaline conditions inhibited the photoconversion of 1,8-DNP in snow.