Effects Of Soil Drought And Nitrogen Deposition On VOC Emissions And Their O₃ And SOA Formation Potential For Pinus Thunbergii

Biogenic volatile organic compounds（BVOCs）are important precursors for ozone（O₃）and secondary organic aerosol（SOA）generation,which can react photochemically with nitrogen oxides（NO_x）and hydroxide radicals（-OH）under the action of ultraviolet light to generate secondary pollutants,causing serious harm to the atmospheric environment.In recent years,due to global warming and human activities,severity and occurrence frequency of drought events increased particularly in China,atmospheric nitrogen（N）deposition has increased.Soil drought and N deposition can affect the synthesis and release of BVOCs by affecting the metabolism and growth of plants.To investigate the effects of soil drought and N deposition on BVOC emissions and their contribution to O₃ and SOA production,this study set up a laboratory simulation experiment with Pinus thunbergii,a dominant tree species in China,by using a dynamic closed sampling system with thermal desorption-gas chromatography and mass spectrometry（TD-GC/MS）technique to investigate the response of single and combined treatments with different levels of soil drought（60%,50%,40%,30%,20%,10%and re-water treatment）and N deposition（0,2,5 and 10 g N/（m²·yr））on BVOC emission rates and composition.We also calculated the contribution of each BVOC species to the O₃formation potential（OFP）and SOA formation potential（SOAP）of Pinus thunbergii under different treatments.The study will provide theoretical basis and basic data for assessing the effects of climate change and atmospheric pollution on BVOC emissions and their atmospheric secondary pollutants formation.The main findings are as follows:（1）Soil drought and nitrogen deposition single treatment had a significant effect on VOC emissions from Pinus thunberg.Isoprene,monoterpene,sesquiterpene and total VOC emissions increased and then decreased with increasing drought stress（except for sesquiterpene in 20%VWC）.The re-water treatment only restored the isoprene emission to pre-drought treatment levels.N deposition promoted isoprene and sesquiterpene emission and suppressed other VOCs emission.The contribution of isoprene and sesquiterpene emissions increased under soil drought and nitrogen deposition,and monoterpenes consistently accounted for the highest percentage of total VOC emissions.（2）N deposition increased isoprene and sesquiterpene emissions and decreased other VOC emissions.Total VOCs and monoterpene emissions were stimulated in LN（nitrogen addition of 2 g N/（m²·yr））and inhibited in MN and HN（nitrogen addition of 5 and 10 g N/（m²·yr））.The contribution of isoprene and sesquiterpenes increased,the contribution of monoterpenes decreased,and the contribution of other VOCs decreased in the LN（17.69%of the total）and MN（13.99%of the total）treatments and increased in the HN（28.21%of the total）treatment.（2）The overall treatment of soil drought and N deposition had a significant effect on VOC emissions from Pinus thunbergii.The overall treatment reduced the total VOC,monoterpene and other VOC emissions,and had no significant effect on sesquiterpene emissions.The overall treatment reduced the total VOC,monoterpene and other VOC emissions,and had no significant of soil drought and N deposition inhibited total VOC and monoterpene emissions more than the single effect of soil drought.The contribution of isoprene and sesquiterpene in total VOC emissions increased under the overall treatment（except for moderate drought combined with moderate N deposition）,while the contribution of monoterpene and other VOCs decreased under all overall treatments.（3）Under soil drought stress,the OFP and SOAP of total VOCs,sesquiterpene（except for mild drought and when VWC was 20%）and other VOCs decreased;OFP and SOAP of isoprene and sesquiterpene increased and OFP and SOAP of other VOCs decreased under the N deposition treatment;OFP and SOAP of total VOCs,monoterpene and other VOCs decreased under the combined treatment of soil drought and N deposition.In the combination of soil drought and N deposition,the OFP and SOAP of total VOCs,monoterpene and other VOCs decreased.Soil drought stress and N deposition also had significant effects on the compound composition of OFP and SOAP of Pinus thunbergii,and the compound composition of OFP and SOAP of Pinus thunbergii differed from that of VOCs due to the different chemical activities.