Characteristics And Source Of Carbonyls And Effects On Photochemistry Pollution In North China Plain

Carbonyls are an important class of volatile organic compounds(VOCs)that play critical roles in tropospheric chemistry,and have detrimental effects to human health and ecological environment.Hence,investing the characteristics,sources and their effects on ozone formation as well as their impact on atmospheric oxidation chemistry is very significant.In the present study,several typical field campaigns were conducted at urban site,mountain site and oil extraction region.with the aid of multiple data analysis methods,as well as modeling tools such as a photochemical box model,Weather Research and Forecasting model coupled with Chemistry(WRF-Chem),Meteorological and Lagrangian particle dispersion model(LPDM),Positive matrix factorization(PMF)anda multiple linear regression mode to better understand the characteristics and sources of carbonyl compounds and the formation mechanisms of carbonyl compounds as well as their effects on O3 formation and ROx radicals.Based on the field experiment data obtained at an urban site of Beijing,we identified of key precursor species of secondary carbonyls formation and evaluated their impact on atmospheric oxidation chemistry.Time series accompanied by high concentrations of carbonyls and O3 in Beijing over the period clearly revealed severe photochemical air pollution in Beijing.HCHO,acetone and CH3CHO were the principal carbonyl compounds,accounting for approximately 82%of the total top ten carbonyls.In addition to these three abundant carbonyls discussed above,two dicarbonyls,CHOCHO and MGLY,also presented abundant levels(0.68 ± 0.26 and 1.10 ± 0.44 ppbv,respectively)despite their high reactivity.The clear day-to-day variation patterns of O3 and carbonyl concentrations with nearly simultaneous peaks at noon were seen,which demonstrates the contribution of photo-oxidation reactions to O3 and carbonyls in the daytime.To dissect the photochemical formation mechanisms of the major carbonyls in Beijing,detailed modeling analyses were conducted to identify the major formation pathways and principal precursor species of HCHO,CH3CHO,CHOCHO and MGLY.The reactions of RO+O2 and OH+OVOCs dominated the photochemical formation of carbonyls.Specifically,the oxidation of alkenes(mainly C5Hs and C2H4)is the major formation route of HCHO.The degradation of alkenes,such as C3H6,i-butene and cis-2-pentene,and that of alkanes,predominantly i-pentane,played an important role in the CH3CHO formation.The oxidation of isoprene and aromatics dominated the secondary formation of CHOCHO and MGLY.Therefore,reducing the emissions of alkenes and aromatics should be an efficient way to mitigate carbonyl formation.Meanwhile,the strong atmospheric oxidizing capacity was predicted(7.36 to 7.72×107 molecules cm-3 s-1)during the smog episodes using the MCM model,with OH contributing most to the daytime oxidation.The photolysis of OVOCs was the dominant primary ROx source.Sensitivity analyses showed that O3 production was under a mixed-control regime with OVOCs(mainly carbonyls)being the key species.This study also found that the carbonyls pollution is not only serious at urban region,at Mount.Tai,the carbonyls pollution was also serious and had an important influence to O3 formation.C1-C8 carbonyls were measured at Mount Tai.Acetone(3.57±0.55 ppbv),formaldehyde(3.48±0.98 ppbv)and acetaldehyde(1.27±0.78 ppbv)are the three most abundant species.Isovaleraldehyde(0.37±0.17 ppbv)presents another important carbonyl compound despite its high reactivity.The sources of carbonyls are dominated by photo-oxidation of VOCs during the daytime and regional transport at night.Secondary formation contributed to the majority of the ambient carbonyls,especially for the higher carbon compounds,which was consistent with the conclusions obtained in our Beijing experiment.Compared with Beijing,the carbonyls concentrations maintained high levels at Mount.Tai.By using the LPDM model to calculate the history of air masses arriving at the measurement site we found that the observed high levels of carbonyls at night was ascribed to the air masses transport form the southeast sector industrial zones.Formaldehyde,acetaldehyde and isovaleraldehyde played important roles in OH removal as well as ozone formation,and hence should be paid much attention for the future control of O3 pollution.We also found that the extraction of oil had important impacts on carbonyl pollution as well as regional air quality.Based on the field experiments performed at the downwind of oil extraction source region,we found that although there are limited pollution source around the study site,the carbonyls concentration were comparable to those at urban region.Carbonyls showed obvious seasonal variations,and the concentration of carbonyls in summer was significantly greater than that in winter,with carbonyls concentration being twice times higher in summer as compared to winter.In addition to these three major carbonyl compounds,formaldehyde,acetaldehyde and acetone,in winter the butenal also has a high level,indicating the potential source of 1-pentene,while in summer n-butyl aldehyde and valeraldehyde have high concentrations,which indicated potential 1-hexene source.The carbonyls compositions in the oil exploration area are similar to those at regional site,with concentration slightly higher than that of the regional stations.Formaldehyde,acetaldehyde and acetone are the most abundant carbonyls,accounting for more than 74%of the total carbonyls.The daily variation patterns of major carbonyls showed that the concentration of carbonyls increased and peaked in the noon under daily photo-oxidation reactions and decreased in the afternoon,but increased again in the evening,which should be the result of transport of regional photochemical aged oil extraction plumes.Then we dissect the photochemical formation mechanisms and principal precursor species of HCHO and CH3CHO using MCM model.The results showed that the reactions of RO(from Alkenes)+O2 and CH3O+O2 dominated the photochemical formation of HCHO.As to CH3CHO,three types reactions,namely OH + OVOCs、C2H5O+O2 and RO2(from Alkenes)+O2 dominated the photochemical CH3CHO formation.Sensitivity analyses revealed that the in situ HCHO production was limited by Alkenes in winter but by BHC in summer,and that the in situ CH3CHO production was limited by Alkenes and Alkanes in winter but mainly alkanes in summer.This conclusion is generally consistent with the result obtained at Beijing which indicated that the HCHO production was limited by Alkenes and by Alkenes and Alkanes for CH3CHO.The chemical production and loss of ozone was also estimated using the MCM model by constraining the measured trace gases and meteorological variables.The O3 production is dominated by the oxidation of NO by HO2 and the NO oxidation by RO2 reaction.Sensitivity analyses showed that O3 production was VOCs limited in winter and VOCs and NOx common limited in summer with OVOCs(mainly carbonyls)being the key species in both winter and summer.Overall,this study obtained the characteristics and source of carbonyls,dissected the photochemical formation mechanisms and principal precursor species of major carbonyls and underlined their effects on atmospheric oxidation photochemistry pollution in typical urban,mountain and oil extraction regions in North China Plain.The findings of this study will be helpful in the development of effective control strategies to mitigate the severe photochemical air pollution in urban,mountain and oil extraction regions in China.