Study On The Relationship Between Atmospheric Aerosol PH And Secondary Nitrate Based On Online Observed Data

With the rapid development of China's economy and civilization,air pollution has increased more and more attention because of its'adverse effects on environment and human health.PM_2.5 is the primary pollutant in many cities,and secondary inorganic ions like NO₃^-increase largely during haze periods.The formation pathways of secondary species are closely related with aerosol acidity?pH?.Particle pH is difficult to directly measured,indirect methods such as thermodynamic model ISORROPIA is widely used in the world.Acidity play a critical role on human health,the secondary formation pathways and the solubility of trace nutrients,so it's important to accurately measure particle pH.As the most abundant component in PM_2.5,NO₃^-concentration and particle-gas partitioning???NO₃^-?,NO₃^-/?NO₃^-+HNO₃?g???are influenced by pollutant sources'emissions,temperature,relative humidity,aerosol pH,liquid water content,and other species'concentration.Meanwhile,NO₃^-concentration,pollutant sources'emissions,temperature and relative humidity can also effect aerosol pH.There are four modes in ISORROPIA,including reverse,forward,stable and metastable.In order to choose the most appropriate mode in pH calculating,online datasets with 1-h time resolution collected in Tianjin,China,2015,were applied in this study.For metastable and forward modes,the input and output data were correlated well,indicating that the results were reasonable.While for stable-forward and reverse-metastable modes,the pH changed largely.For stable-forward mode,73%data were concentrated on 7.7,may be because that O1 O2 subcases don't consider the ammonia partitioning when calculating liquid H⁺concentration.For revised stable-forward mode?the coding error of O1 O2 were corrected?,the aerosol pH were correlated well with metastable-forward result,but the results didn't meet the ion balance acquirement well.In order to explore the influence of T,RH,chemical species'concentration and pollutant sources'contributions on pH,a number of model tests were carried out in this study.Compared with NO₃^-,SO₄^2-had a larger influence on pH;larger ammonia concentration could lead to higher pH,but it didn't elevate pH to near neutral acidity because higher ammonia would be partitioned to phase state.RH can influence particle pH by influencing the liquid water content and semi-volatile species'concentration.Higher RH could increase water content and semi-volatile species'concentration while higher T could decrease water content and semi-volatile species'concentration.Secondary sulfate contribution had the most influence on particle pH and diurnal variation of pH;dust can increase pH by emitting crustal species.NO₃^-was the most important species of PM_2.5,its'fast formation pathway was not clear,and its'concentration and gas-particle partitioning???NO₃^-??were influenced by meteorological conditions,source contributions and particle pH.In order to investigate the complicated relationship between these influencing factors,this study collected online datasets with 1-h time resolution in summer of Tianjin,2015.The results showed that secondary nitrate?SN?and particle pH presented triangle relationship,probably closely related with nitrate precursors'emissions and its'formation pathways.Under different meteorological conditions and particle pH,the NO₃^-was formed by different formation mechanism.Besides,when SS contributed high,the nitrate gas-particle partitioning exhibited a characteristic“S-curve”relationship with pH.An index I_TL?T:temperature;L:liquid water content?was defined,and it was helpful for exploring the major influencing factors of??NO₃^-?and identifying the sensitive region of the“S-curve”relationship.