Complex Effects Of Environmental Factors On Cyclohexene Photooxidation And Secondary Organic Aerosol Formation

In recent years,atmospheric aerosols have become the main pollutants of urban atmospheric environment in China.Secondary organic aerosol(SOA)is an important component of atmospheric aerosols.SOA could account for 10%to 40%of the total mass of PM2.5,and even up to 80%under extremely polluted conditions.SOA can not only affect the atmospheric environment,human health,and even affect the global climate through the absorption and scattering of light.The SOA formation mechanism is one of the most hotspots of atmospheric research.But as yet,the formation mechanism of SOA,resulting from the atmospheric photochemical reactions,has not been absolutely understood.At the same time,there is a big gap between the simulated and observed values of the atmospheric aerosol model for SOA.The formation process of SOA is greatly influenced by environmental factors,but how the environmental factors impact the SOA formation is still uncertain,especially in a heavily polluted environment.This uncertainty of SOA formation mechanism has hindered the government from formulating policies to effectively mitigate haze pollution.SO2 and sulfate are ubiquitous in the atmosphere,and they are very important to affect SOA formation.Sulfate can change the aerosols acidity and promote the formation of SOA through acid catalytic reaction.However,high sulfate concentration during haze events in China occurred with concurrently elevated RH,NOx,and NH3.The oxidation of SO2 by NO2 in the liquid phase with the presence of NH3 is a very important pathway to promote sulfate formation and increase aerosol acidity.Anyway,it is a challenge to explore the interaction of the multiple environmental factors that contribute to SOA formation.The common effects of SO2,NOx,relative humidity(RH)and oxidation state on the formation of SOA were discussed in this study.The chemical components of SOA were analyzed by ESI-HR-MS,and the mechanism of SOA formation was discussed here.An indoor chamber was established.Stainless steel covered the chamber interior walls to maximize and homogenize the interior light intensity.The Teflon FEP reactor with a volume of 3 m3 is built in the chamber.32 black light lamps(GE F40BLB),with emission bands centered at 365 nm,are used to simulate the spectrum of the UV band in solar irradiation,and the average effective light intensity is determined to be 0.177 min-1.16 TUV lamps are used for photooxidation of hydrogen peroxide to produce OH·.The temperature in the chamber is constant.The measured wall loss rate constant for O3,NOx,and SO2 is 5.1 × 10-6,7.0 × 10-6 and 6.4 × 10-6 s-1,respectively.The average value of the particle wall loss rate constant is 7.2 × 10-5 s-1.The results of this study are as follows:(1)The SOA formation from a cyclohexene/NOx system with various SO2 concentrations under black lights was investigated in the chamber.A clear decrease at first and then an increase in the SOA yield was found with increasing SO2 concentrations.The lowest SOA yield was obtained when the initial SO2 concentration was in the range of 30?40 ppb,while higher SOA yield compared to that without SO2 could not be obtained until the initial SO2 concentration was higher than 85 ppb.The decreasing SOA yield might be due to the fact that the promoting effect of acid-catalysed reactions on SOA formation was less important than the inhibiting effect of decreasing OH· concentration at low initial SO2 concentrations,caused by the competition reactions of OH· with SO2 and cyclohexene.The competitive reaction was an important factor for SOA yield and it should not be neglected in photooxidation reactions.The composition of organic compounds in SOA was measured using several complementary techniques including Fourier transform infrared(FTIR)spectroscopy,ion chromatography(IC),and Exactive Plus Orbitrap mass spectrometer equipped with electrospray interface(ESI).We presented new evidence that organosulfates were produced from the photooxidation of cyclohexene in the presence of SO2.(2)The cyclohexene SOA formation was studied with different initial NOx and SO2 concentrations at low and high relative humidity(RH).Both SOA yield and number concentration first increase drastically and then,decreased when the[VOC]0/[NOx]o ratios changed from 30 to 10 and from 10 to 3.Though the presence of SO2 could increase the SOA number concentration,the SOA yield could only increase under[VOC]o/[NOx]o=10 and high RH,and[VOC]0/[NOx]0=3 and low RH experimental conditions,while decreasing under[VOC]0/[NOx]0=10 and low RH conditions.In the presence of SO2,the high RH and high NOx conditions were keys to efficient sulfate formation and could promote the SOA formation.The chemical composition of SOA was characterized,and few organosulfates were identified.A visible enhancement of organosulfates and the formation of high molecular weight organic compounds were observed at high RH conditions,and this seemed to be the reason for the SOA yield increase at high RH.(3)The role of relative humidity(RH)on SOA formation with different[H2O2]o/[cyclohexene]o were investigated in a smog chamber.It was found that the cyclohexene SOA yield increases with increasing initial OH· concentration at both high and low RH conditions.The increased rate of SOA formation was lower at wet conditions than that at dry conditions.For[H2O2]o/[cyclohexene]0= 0.4,the SOA yield increased from 1.5%at dry condition to 7%at wet condition.For[H2O2]o/[cyclohexene]o = 0.8,the SOA yield increased from 8%at dry condition to 12%at wet condition.The oligomers were detected in the SOA formed at high RH conditions,which was responsible for the increase of the SOA yield.In contrast,at high RH the SOA yield fell from 54%to 52%when[H2O2]o/[cyclohexene]0=1.3.The SOA mass loss was higher at high RH at the same OH· exposure.For[H2O2]o/[cyclohexene]o =1.3,the relative intensity proportion of both low and high molecular weight substances reduced more at wet conditions,which indicates that the inhibition of oligomers formation and the further reaction of SOA with OH· result in a slightly lower SOA yield at wet condition when OH· was sufficient.