Comparative study on water-soluble inorganic ions in PM2.5 from two distinct climate regions and air quality

Recently, air quality has significantly improved in developed country, but that issue is of concern in emerging megacity in developing country.In this study, aerosols and their precursor gas were collected by NILU filter pack at two distinct urban sites during the winter and summer in Osaka, Japan and dry and rainy seasons in Ho Chi Minh City(HCMC),Vietnam.The aims are to investigate the contribution of water-soluble inorganic ions(WSIIs) to PM2.5, thermodynamic characterization and possible formation pathway of secondary inorganic aerosol(SIA).The PM2.5 concentration in Osaka(15.8 μg/m^3) is lower than that in HCMC(23.0 μg/m^3), but the concentration of WSIIs in Osaka(9.0 μg/m^3) is two times higher than that in HCMC(4.1 μg/m^3).Moreover, SIA including NH4^+, NO3^-and SO4^2-are major components in WSIIs accounting for 90% and 76%(in molar) in Osaka and HCMC,respectively.Thermodynamic models were used to understand the thermodynamic characterization of urban aerosols.Overall, statistical analysis results indicate that very good agreement(R2> 0.8) was found for all species, except for nitrate aerosol in HCMC.We found that when the crustal species present at high amount, those compositions should be included in model calculation(i.e.in the HCMC situation).Finally, we analyzed the characteristics of NH4^+– NO3^-– SO4^2-system.A possible pathway to produce fine nitrate aerosol in Osaka is via the homogeneous reaction between NH3 and HNO3, while nonvolatile nitrate aerosols can be formed by the heterogeneous reactions in HCMC.

Characteristics of Aerosol Pollution Under Different Visibility Conditions in Winter in a Coastal Mega-City in China

To investigate chemical profiles and formation mechanisms of aerosol particles in winter haze events,comprehensive measurements including hourly concentrations of PM2.5 and water-soluble inorganic ions and related gasphase precursors were conducted via an online monitoring system from January to March of 2016 in Shenzhen,a coastal mega-city in south China.In this study,high concentrations of PM2.5,NO2 and lower levels of O3 were observed during haze periods in comparison with clear days(Visibility>15 km).The major secondary ionic species were NH+4、NO-3 and SO2-4,which varied significantly on haze and clear days.The ratio of NO-3/SO2-4 in haze days was greater than that on clear days and tended to be larger when air pollution became more serious.At the same time,compared with previous studies,it has been found that the ratio has been increasing gradually in Shenzhen,indicating that the motor vehicle exhaust emissions have a more and more important impact on air quality in Shenzhen.Sulfur oxidation rate(SOR)and nitrogen oxidation rate(NOR)was higher during the haze period than that in clean days,indicating efficient gas to particle conversion.Further analysis shows that high concentrations of sulfate might be explained by aqueous oxidation,but gas-phase reactions might dominate nitrate formation.This study also highlights that wintertime nitrate formation can be an important contributor to aerosol particles,especially during haze periods.

Composition and size distribution of wintertime inorganic aerosols at ground and alpine regions of northwest China

Water-soluble inorganic ions(WSIIs)play a pivotal role in atmospheric chemical reactions,particularly influencing the formation of secondary particulate matter.A comprehensive grasp of the vertical distribution of atmospheric pollutants holds immense significance in understanding the diffusion and transportation of these pollutants.This study investigates the WSIIs of PM_(2.5)and size-segregated particles at the top(~2060 m a.s.l.)and foot of Mt.Hua during the winter of 2020.All the measured ions present significant higher concentrations(1.9~6.9 times)at the foot than the top.Cl^(-)and K^(+)at the foot are more than 4 times of those at the top,whereas Ca^(2+)and Mg^(2+)are only 1.3-1.9 times higher.The particle size distribution of NO_(3)^(-),SO_(4)^(2-),K^(+)and Cl^(-)demonstrate a single peak distribution(0.7-1.1μm)at the foot,but with a bimodal distribution(0.7-1.1μm and 4.7-5.8μm)at the top.These differences suggest that the aerosol at the alpine region is mainly transported via long-distance from Northwest/North China,but limited influenced by vertical transport through valley breeze.The changes of concentration and size distribution of WSIIs in dust event and non-dust period indicate that the effects of dust event on aerosols at ground surface were weaker than that of the free troposphere of Guanzhong Plain.Notably,our study underscores the dominant influence of NO_(3)^(-)in shaping the gas-particle distribution of ammonia within the winter free troposphere.Our results highlight the significant role of long-range transport on aerosols in the free troposphere in Guanzhong Plain,Northwest China.

Chemical characteristics of PM_(2.5) during a typical haze episode in Guangzhou

The chemical characteristics（water-soluble ions and carbonaceous species） of PM2.5 in Guangzhou were measured during a typical haze episode.Most of the chemical species in PM2.5 showed significant difference between normal and haze days.The highest contributors to PM2.5 were organic carbon（OC）,nitrate,and sulfate in haze days and were OC,sulfate,and elemental carbon（EC） in normal days.The concentrations of secondary species such as,NO3^-,SO4^2-,and NH4^＋ in haze days were 6.5,3.9,and 5.3 times higher than those in normal days,respectively,while primary species（EC,Ca^2＋,K^＋） show similar increase from normal to haze days by a factor about 2.2-2.4.OC/EC ratio ranged from 2.8 to 6.2 with an average of 4.7 and the estimation on a minimum OC/EC ratio showed that SOC（secondary organic carbon） accounted more than 36.6% for the total organic carbon in haze days.The significantly increase in the secondary species（SOC,NO3^-,SO4^2-,and NH4^＋）,especially in NO3^-,caused the worst air quality in this region.Simultaneously,the result illustrated that the serious air pollution in haze episodes was strongly correlated with the meteorological conditions.During the sampling periods,air pollution and visibility had a good relationship with the air mass transport distance;the shorter air masses transport distance,the worse air quality and visibility in Guangzhou,indicating the strong domination of local sources contributing to haze formation.High concentration of the secondary aerosol in haze episodes was likely due to the higher oxidation rates of sulfur and nitrogen species.

Growth of nitrate contribution to aerosol pollution during wintertime in Xi'an,northwest China:Formation mechanism and effects of NH_(3)

With the strengthened controls on SO2 emissions and extensive increases in motor vehicles'exhaust,aerosol pollution shifts from sulfate-rich to nitrate-rich in recent years in Xi'an,China.To further gain insights into the factors on nitrate formation and efficiently mitigate air pollution,highly time-resolved observations of water-soluble inorganic ions(WSIIs)in PM_(2.5) were measured in a suburban area of Xi'an,China during wintertime.Hourly concentration of total WSIIs is 39.8μg m-3 on average,accounting for 50.3%of PM_(2.5) mass.In contrast to a slight decrease in the mass fraction of SO_(4)^(2-),NO_(3)-shows a sig-nificant increase of the PM_(2.5) contribution with the aggravation of aerosol pollution.This suggests the importance of NO_(3)-formation to haze evolution.Furthermore,homogeneous reactions govern the formation of NO_(3)-,while alkali metals such as calcium and sodium play an additional role in retaining NO_(3)-in PM_(2.5) during clean periods.However,the heterogeneous hydrolysis reaction contributed more to NO_(3)-formation during the pollution periods under high relative humidity.Our investigation reveals that temperature,relative humidity,oxidant,and ammonia emissions facilitate rapid NO_(3)-formation.Using the random forest(RF)model,NO_(3)-concentrations were successfully simulated with measured variables for the training and testing datasets(R2>0.95).Among these variables,CO,NH_(3),and NO_(2) were found to be the main factors affecting the NO_(3)-concentrations.Compared with the period without vehicle re-striction,the contributions of NO_(3)-and NH4+to PM_(2.5) mass decreased by 5.3%and 3.4%in traffic re-striction periods,respectively.The vehicle restriction leads to the decreases of precursor gases of NO_(2),SO_(2),and NH_(3) by 12.8%,5.9%,and 27.6%,respectively.The results demonstrate collaborative emission reduction of NO_(x) and NH_(3) by vehicle restrictions,and using new energy vehicles(or electric vehicles)can effectively alleviate particulate matter pollution in northwest China.

Ionic composition of submicron particles(PM_(1.0)) during the long-lasting haze period in January 2013 in Wuhan, central China

In January 2013, a long-lasting severe haze episode occurred in Northern and Central China; at its maximum, it covered a land area of approximately 1.4 million km2. In Wuhan, the largest city in Central China, this event was the most severe haze episode in the 21st century. Aerosol samples of submicron particles （PMI.o） were collected during the long-lasting haze episode at an urban site and a suburban site in Wuhan to investigate the ion characteristics of PMi.0 in this area. The mass concentrations of PM1.0 and its water-soluble inorganic ions （WSIIs） were almost at the same levels at two sites, which indicates that PM1.0 pollution occurs on a regional scale in Wuhao. WSIIs （Na^＋, NH4^＋, K^＋, Mg^2＋, Ca^2＋, Cl^-, NO3 and SO2-） were the dominant chemical species and constituted up to 48.4% and 47.4% of PM1.0 at WD and TH, respectively. The concentrations of PM1.0 and WSIIs on haze days were approximately two times higher than on normal days. The ion balance calculations indicate that the particles were more acidic on haze days than on normal days. The results of the back trajectory analysis imply that the high concentrations of PM1.0 and its water-soluble inorganic ions may be caused by stagnant weather conditions in Wuhan.