Characterization Of Organosulfates And Organonitrates In The Pearl River Delta(PRD) Region, South China

Secondary organic aerosol(SOA) is an important fraction of fine particles(with aerodynamic diameter within 2.5 μm, PM2.5), it has adverse effect on human health, affects regional air quality and influences global climate change. SOA mainly formed from the atmospheric oxidation of biogenic volatile organic compounds(BVOCs, e.g. isoprene, monoterpenes and sesquiterpenes) and anthropogenic precursors(e.g. aromatic hydrocarbons). Globally, biogenic SOA(BSOA) contributed much higher SOA burden than the anthropogenic SOA(ASOA). However, lacking of knowledge about the formation mechanism and gas to particle transformation, modelled SOA was about two orders of magnitudes lower than the observed result. Recently, both filed observations and modelling works have shown that the conversion of BVOCs to SOA can be significantly enhanced in the presence of high anthropogenic emissions. Isoprene and monoterpenes, the most prevalent and abundant species of BVOCs, could react with anthropogenic pollutants(e.g. NOx, SO2) to form ubiquitous organosulfates(OSs) and organosulfates(ONs). Especially in heavily polluted region with adequately anthropogenic pollutants, this atmospheric process would lead to significant BSOA production. Studying on OS and ON provides us a new sight on understanding the influence of anthropogenic emissions on BSOA formation.The Pearl River Delta region, one of the most urbanized and industrialized megacities in South China, has experienced remarkable economic development and rapid urbanization accompanied by large increases in emissions of anthropogenic pollutants. Located in the subtropics, PRD has an annual temperature of 25 °C and high biogenic emissions throughout the year. Thus PRD serves as an ideal place to study the influence of anthropogenic emissions on BSOA formation. In this study, fine particles were collected during summer and fall-winter in 2008 and 2010 at an regional background site in PRD. OS and ON were analyzed together with SOA tracers from BVOCs and water soluble ions. On this basis, to investigate the influence of anthropogenic emissions on BSOA formation. Moreover, we expanded our analysis to nitrated-aromatics from biomass burning(BB), to fully characterize the contribution of BB to organic aerosol from primary and secondary process. The main results were listed below:1.We established the pretreatment method using polar solvent to extract target compounds from filter-based aerosol samples under sonification, and optimized the instrument parameters of liquid chromatography coupled with a triple quadrupole tandem mass spectrometry operated in negative electrospray ionization mode(LC-ESI-MS/MS) for OS and ON quantification. The most abundant OS and ON were measured, including pinene-derived nitrooxy-organosulfates(p NOS) with MW 295, isoprene derived 2-methyltetrol sulfate ester(2-MTOOS) with MW 216 from isoprene epoxydiols(IEPOX), 2-methylglyceric acid sulfate ester(2-MGAOS) with MW 200 from methacrylic acid epoxide(MAE), and nitrooxy 2-methyltetrol sulfate ester(i NOS) with MW 261.2. In 2008, among all the detected organosulfates in the aerosol samples collected at the regional background site(Wanqingsha), p NOS was the dominant, with high level of 85.9 ng m-3, 2-MTOOS and 2-MGAOS were in the comparable levels of 1.13 and 1.31 ng m-3 while i NOS were only detected in the range of nd-1.45 ng m-3. In 2010, p NOS was still the most abundant specie, with atmospheric concentration of 117 ng m-3, 2-MTOOS was only detected in 48 samples of the total 64 samples(nd-2.09 ng m-3), 2-MGAOS could be quantified in 8 samples collected during fall-winter with levels of 0.54-1.23 ng m-3. The i NOS could not be detected. As can be seen from the analyzing results, OS in PRD were mainly from monoterpenes, isoprene derived OS were with much lower abundance compared to OS from monoterpenes. Moreover, OSs with nitrate ester group were the dominant. The OS composition in PRD was quite different from that in American or Europe where OS from isoprene and OS without nitrate ester group was dominant.3. Chamber simulation study results have proved that all the p NOS isomers could be produced through daytime OH radical initialed photo-oxidation pathway(OH-pathway), and one of them could also form via NO3 radical initialed night chemical evolution pathway(NO3-pathway). Studies in US and Europe found that levels of p NOS in nighttime were much higher than that in daytime and suggested NO3-pathway played as the dominate role in p NOS formation. On the contrast, observation in PRD showed that p NOS were with much higher levels in daytime than in nighttime and the isomer compositions exhibited no diurnal variations that suggested OH-pathway should be more important in p NOS formation in PRD. Moreover, the production of the hydroxynitrate that is the key precursor for this specific p NOS was found to be significant through photochemistry but negligible through NO3 chemistry based on the mechanisms in the Master Chemical Mechanism(MCM). These new findings provide new insights on our understanding of p NOS formation in atmospheric aerosol especially in heavily polluted regions.4. Theoretical calculation base on the kinetic parameters showed that under the extremely acidified aerosols in PRD, alcohol could be removed by esterification reaction with a life time of 0.23 hr. The calculation suggested that alcohol sulfate esterification reactions would be effective in OS production in highly polluted regions. This study illustrated that esterification is still need to be further investigated in various types of environments and conditions.5. For i OSs, 2-methyltetrol sulfate ester which could be formed from IEPOX generated by isoprene under low-NOx conditions showed low concentrations, largely due to the depression of IEPOX formation by the high NOx levels over the PRD. The high temperature throughout the year did not favor gas phase MAE production, thus low levels of 2-MGAOS would be found in the aerosol samples. Both 2-MTOOS and 2-MGAOS correlated well with their branching products formed by the nucleophilic addition of water to IEPOX and MAE. And the two OSs correlated with aerosol sulfate loading as well, that suggested isoprene OSs mainly formed from the acid catalyzed ring-opening reaction of isoprene epoxides.6. Using the kinetics model(Kintecus) we estimated the contributions of aerosol phase ring-opening reactions, gas phase OH radical oxidation and day deposition in isoprene epoxides’ exclusion. It was found that IEPOX was mainly terminated through aerosol phase ring-opening reaction(accounted for 84%) that quite high levels of IEPOX-derived SOA tracers were observed though gas phase IEPOX formation was depressed under high NOx condition in PRD. Differently, MAE was mainly exclude by dry deposition that only 3% would be uptake by the aerosols to go ring-opening reaction. Together with the depression effect of high ambient temperature on MAE production, even in highly polluted PRD with high NOx emissions, isoprene SOA products formed from high NOx pathway were still in trace amount. Thus, even in highly polluted PRD, isoprene SOA was still formed mainly through low NOx pathway. This study renewed our understanding on isoprene SOA formation in highly polluted regions.7. We expanded our analyzing method in organic tracer characterization. Using the LC-ESI-MS/MS, we recognized and quantified the nitrated phenols oxidized from precursors emitted during biomass burning(BB) events. Applying the SOA tracer method, SOA from BB emitted phenols was estimated 93.1 ng m-3 during fall-winter. This fraction was much smaller than the primary organic aerosol emitted during BB. However, the estimated SOA significantly correlated with the gasps between SOC estimated by EC-tracer method and SOA tracers method(△SOC), suggesting SOA from BB maybe partially responsible for the △SOC especially when BB events frequently occurred.