Vertical Evolution Mechanism And Environmental Effects Of Dicarboxylic Acid Organics In Mount Hua

Water-soluble organic aerosol（WSOA）is an important component of organic aerosols（OA）and plays an important role in the formation of secondary organic aerosols（SOA）,and the content of dicarboxylic acids acid compounds is the highest in WSOAs.Dicarboxylic acids containing two carboxylic functional groups are characterized by high water solubility and strong hygroscopicity.Therefore,it is easy to form cloud condensation nuclei（CCN）or ice nuclei（IN）,which can significantly change the cloud formation process and indirectly affect radiative forcing in the region.At present,some studies have reported the distribution of dicarboxylic acid in the alpine region,but the influence of the surface on the vertical transport of dicarboxylic acid in the troposphere is still unclear.In addition,dicarboxylic acids are a group of compounds that are significantly affected by atmospheric photochemical processes,the stable carbon isotopic(δ¹³C)value can well trace the secondary reaction and aging process in the atmosphere.In this study,the molecular distribution andδ¹³C compositions of dicarboxylic acids and related organic compounds in PM_2.5 were measured simultaneously at the top（c.2060 m a.s.l.）and foot（c.400 m a.s.l.）of Mount（Mt.）Hua during the summer and winter of 2020.Due to the strong anthropogenic emissions at ground level,the concentrations of dicarboxylic acids and related compounds at foot of Mt.Hua were generally higher than those at the top in the summer.Oxalic acid（C₂）was the predominant diacid in both sites,whose concentrations at foot and top of Mt.Hua were 87～852 and 40～398 ng m^-3,respectively.Ratios of adipic acid to azelaic acid（C₆/C₉）,phthalic aid to azelaic acid（p H/C₉）,glyoxal to methylglyoxal（Gly/m Gly）,and lowerδ¹³C values（-21.0±2.3‰and-21.9±2.7‰）of C₂ indicated that the contributions of anthropogenic sources to dicarboxylic acids in PM_2.5 in the mountain region are more significant than biogenic sources.Aerosols from the foot of Mt.Hua could affect the atmosphere on the top of the mountain via vertical transport under the influence of daytime valley wind,even though the altitude of Mt.Hua is beyond the boundary layer most of time.The valueδ¹³C of C₂ is linearly correlated with C₂/m Gly,C₂/pyruvic acid（Pyr）,C₂/glyoxylic acid（ωC₂）at the top of the mountain,and C₂/Gly,C₂/ωC₂ at the foot of the mountain,indicating that the formation pathway of C₂ is m Gly-Pyr-ωC₂-C₂ at the top of Mt.Hua and Gly-ωC₂-C₂ at the foot of Mt.Hua.Consistent with summer,the concentrations of dicarboxylic acid and related compounds at the foot of Mt.Hua was generally higher than that at the top of the Mt.Hua in winter due to intense anthropogenic emissions.Oxalic acid was still the main monomer molecule of dicarboxylic acid at the top and foot of Mt.Hua.Different from summer,C₉ at the foot of Mt.Hua in winter became the second highest concentration of dicarboxylic acid,which was mainly caused by burning biomass straw for heating in the countryside at the foot of the mountain in winter.In addition,during the whole observation period in winter,Ph concentrations at the top and foot of the Mt.Hua increased significantly,and Ph was mainly the secondary product formed by atmospheric photochemical oxidation of naphthalene,polycyclic aromatic hydrocarbons and other aromatic hydrocarbons produced by coal burning.The characteristic ratio of dicarboxylic acid indicated that the top and foot of Mt.Hua were mainly caused by anthropogenic contribution in winter.The difference was that the top of Mt.Hua was mainly from long-distance transport,while the foot of Mt.Hua was mainly from local emission.The formation path of C₂in winter was still mainly liquid phase oxidation.The difference was that the correlation between C₂and NO₃^-and NH₄⁺is better than that between C₂ and SO₄^2-at the foot of mountain in winter,while the correlation between C₂ and SO₄^2-at the top of mountain is better than that between C₂and NO₃^-,which may indicate that the liquid water content（LWC）at the foot of mountain in winter was mainly determined by NO₃^-.In addition,the vertical transmission from the ground to the top of Mt.Hua is weakened by the meteorological conditions in winter.Theδ¹³C value of oxalic acid decreased with the increase of WSOC and C₂concentrations,which further indicated that the SOA of Mt.Hua in winter was mainly generated by secondary reaction.Field observations show that biomass burning（BB）is an important contributor to dicarboxylic acids and related compounds in winter.To investigate the effect of atmospheric aging process on the formation of dicarboxylic acid and the change ofδ¹³C values of emitted dicarboxylic acid monomers,we used a combined combustion chamber and Potential Aerosol Mass（PAM）Oxidation Flow Reactor to simulate the combustion of rice,maize and wheat straw to produce fresh and aged aerosols.Succinic acid（C₄）was the most abundant species in fresh samples;while,C₂ became dominant after atmospheric aging.Of all diacids,C₂ had the highest aged to fresh emission ratios（A/F）,suggesting that C₂ is largely produced through secondary photochemical processes.Compared with fresh samples,the emission factors of ketocarboxylic acids andα-dicarbonyls increased after 2-day but decreased after 7-day aging,indicating short residence time and further atmospheric degradation from 2-to 7-days.Theδ¹³C values of C₂ for 7-day aged biomass samples were higher than those of urban aerosols but lower than marine or mountain aerosols,and theδ¹³C values of C₂ became isotopically heavier during aging.Relationships between the reduction in volatile organic compounds（VOCs）,such as toluene,benzene,and isoprene,and increase in diacids after 2-day aging indicate that these VOCs led to the formation of diacids.However,no significant correlation was found between decreases in VOCs and increases in 7-day aged diacids.In addition,the A/F of C₂ was 50.8 at 2 days and 64.5at 7 days,indicating that the conversion of VOCs to C₂ was almost completed within 2days.For the longer aging times,the particulate phase compounds may undergo further degradation in the oxidation processes.Liquid phase oxidation and in-cloud processes are important ways of WSOA formation.In order to investigate the processes of intra-cloud,the water-soluble ions and the stable carbon isotopes of dicarboxylic acids and their monomers in Mt.Hua cloud water samples during cloud events were determined.The results showed that cloud water was slightly alkaline（p H=7.6）and particulate matter was acidic（p H=3.2）,which was mainly related to their chemical compositions.Sulfate was dominant in particulate matter（63.4%）,but decreased significantly in cloud water（only 30.1%）,while nitrate and ammonium increased（from 28.5%and 8.2%to 39.8%and 30.2%）.The concentration of organic acids in cloud water was 1237.4μg L^-1,even higher than the concentration of Cl^-,K⁺and Mg²⁺(0.11～0.69 mg L^-1),which easily formed salt complexes with NH₄⁺and affected the acidity of cloud water.Our results also highlighted the important role of organic acids in the acidity of cloud water.