Recent Progress in Atmospheric Chemistry Research in China: Establishing a Theoretical Framework for the “Air Pollution Complex”

Atmospheric chemistry research has been growing rapidly in China in the last 25 years since the concept of the“air pollution complex”was first proposed by Professor Xiaoyan TANG in 1997.For papers published in 2021 on air pollution(only papers included in the Web of Science Core Collection database were considered),more than 24000 papers were authored or co-authored by scientists working in China.In this paper,we review a limited number of representative and significant studies on atmospheric chemistry in China in the last few years,including studies on(1)sources and emission inventories,(2)atmospheric chemical processes,(3)interactions of air pollution with meteorology,weather and climate,(4)interactions between the biosphere and atmosphere,and(5)data assimilation.The intention was not to provide a complete review of all progress made in the last few years,but rather to serve as a starting point for learning more about atmospheric chemistry research in China.The advances reviewed in this paper have enabled a theoretical framework for the air pollution complex to be established,provided robust scientific support to highly successful air pollution control policies in China,and created great opportunities in education,training,and career development for many graduate students and young scientists.This paper further highlights that developing and low-income countries that are heavily affected by air pollution can benefit from these research advances,whilst at the same time acknowledging that many challenges and opportunities still remain in atmospheric chemistry research in China,to hopefully be addressed over the next few decades.

Neoarchean atmospheric chemistry and the preservation of S-MIF in sediments from the São Francisco Craton

Sulfur mass-independent fractionation(S-MIF)preserved in Archean sedimentary pyrite is interpreted to reflect atmospheric chemistry.Small ranges in Δ^(33)S that expanded into larger fractionations leading up to the Great Oxygenation Event(GOE;2.45–2.2 Ga)are disproportionately represented by sequences from the Kaapvaal and Pilbara Cratons.These patterns of S-MIF attenuation and enhancement may differ from the timing and magnitude of minor sulfur isotope fractionations reported from other cratons,thus obscuring local for global sulfur cycling dynamics.By expanding the Δ^(33)S record to include the relatively underrepresented São Francisco Craton in Brazil,we suggest that marine biogeochemistry affected S-MIF preservation prior to the GOE.In an early Neoarchean sequence(2763–2730 Ma)from the Rio das Velhas Greenstone Belt,we propose that low δ^(13)Corg(<-30‰)and dampened Δ^(33)S(-0.4‰to-0.7‰)in banded iron formation reflect the marine diagenetic process of anaerobic methane oxidation.The overlying black shale(TOC up to 7.8%)with higher δ^(13)Corg(-33.4‰to-19.2‰)and expanded Δ^(33)S(2.3‰±0.8‰),recorded oxidative sulfur cycling that resulted in enhance preservation of S-MIF input from atmospheric sources of elemental sulfur.The sequence culminates in a metasandstone,where concomitant changes to more uniform δCorg(-30‰to--25‰),potentially associated with the RuBisCO I enzyme,and near-zero Δ^(33)S(-0.04‰to 0.38‰)is mainly interpreted as evidence for local oxygen production.When placed in the context of other sequences worldwide,the Rio das Velhas helps differentiate the influences of global atmospheric chemistry and local marine diagenesis in Archean biogeochemical processes.Our data suggest that prokaryotic sulfur,iron,and methane cycles might have an underestimated role in pre-GOE sulfur minor isotope records.

A review of gas-phase chemical mechanisms commonly used in atmospheric chemistry modelling

The atmospheric chemical mechanism is an essential component of airshed models used for investigating the chemical behaviors and impacts of species.Since the first tropospheric chemical mechanism was proposed in the 1960s,various mechanisms including Master Chemical Mechanism(MCM),Carbon Bond Mechanism(CBM),Statewide Air Pollution Research Center(SAPRC)and Regional Atmospheric Chemistry Mechanism(RACM)have been developed for different research purposes.This work summarizes the development and applications of these mechanisms,introduces their compositions and lumping methods,and compares the ways the mechanisms treat radicals with box model simulations.CBM can reproduce urban pollution events with relatively low cost compared to SAPRC and RACM,whereas the chemical behaviors of radicals and the photochemical production of ozone are described in detail in RACM.The photolysis rates of some oxygenated compounds are low in SAPRC07,which may result in underestimation of radical levels.As an explicit chemical mechanism,MCM describes the chemical processes of primary pollutants and their oxidation products in detail.MCM can be used to investigate certain chemical processes;however,due to its large size,it is rarely used in regional model simulations.A box model case study showed that the chemical behavior of OH and HO_(2)radicals and the production of ozone were well described by all mechanisms.CBM and SAPRC underestimated the radical levels for different chemical treatments,leading to low ozone production values in both cases.MCM and RACM are widely used in box model studies,while CBM and SAPRC are often selected in regional simulations.

Reactions between the SO^(·-)_4 radical and some common anions in atmospheric aqueous droplets

The rate constants of reactions between the SO4^- radical and some common anions in atmospheric aqueous droplets e.g. Cl^-,NO^-, HSO3^- and HCO3^- were determined using the laser flash photolysis technique.Absorption spectra of SO4^- and the product radicals were also reported.The chloride ion was evaluated among all the anions to be the most efficient scavenger of SO4^-.The results may supply useful information for a better understanding of the vigorous radical-initiated reactions in atmospheric aqueous droplets such as clouds, rains or fogs.

A preliminary analysis of the surface chemistry of atmospheric aerosol particles in a typical urban area of Beijing

Atmospheric aerosol particle samples were collected using an Ambient Eight Stage（Non-Viable） Cascade Impactor Sampler in a typical urban area of Beijing from 27 th Sep.to 5th Oct.,2009.The surface chemistry of these aerosol particles was analyzed using Static Time of Flight-Secondary Ion Mass Spectrometry（Static TOF-SIMS）.The factors influencing surface compositions were evaluated in conjunction with the air pollution levels,meteorological factors,and air mass transport for the sampling period.The results show that a variety of organic ion groups and inorganic ions/ion groups were accumulated on the surfaces of aerosol particles in urban areas of Beijing;and hydrophobic organic compounds with short-or middle-chain alkyl as well as hydrophilic secondary inorganic compounds were observed.All these compounds have the potential to affect the atmospheric behavior of urban aerosol particles.PM1.1–2.1and PM3.3–4.7had similar elements on their surfaces,but some molecules and ionic groups demonstrated differences in Time of Flight-Secondary Ion Mass Spectrometry spectra.This suggests that the quantities of elements varied between PM1.1–2.1and PM3.3–4.7.In particular,more intense research efforts into fluoride pollution are required,because the fluorides on aerosol surfaces have the potential to harm human health.The levels of air pollution had the most significant influence on the surface compositions of aerosol particles in our study.Hence,heavier air pollution was associated with more complex surface compositions on aerosol particles.In addition,wind,rainfall,and air masses from the south also greatly influenced the surface compositions of these urban aerosol particles.

In situ monitoring of atmospheric nitrous acid based on multi-pumping flow system and liquid waveguide capillary cell

In the last four decades, various techniques including spectroscopic, wet chemical and mass spectrometric methods, have been developed and applied for the detection of ambient nitrous acid（HONO）. We developed a HONO detection system based on long path photometry which consists of three independent modules i.e., sampling module, fluid propulsion module and detection module. In the propulsion module, solenoid pumps are applied. With solenoid pumps the pulsed flow can be computer controlled both in terms of pump stroke volume and pulse frequency, which enables the attainment of a very stable flow rate. In the detection module, a customized Liquid Waveguide Capillary Cell（LWCC） is used. The customized LWCC pre-sets the optical fiber in-coupling with the liquid wave guide, providing the option of fast startup and easy maintenance of the absorption photometry. In summer 2014, our system was deployed in a comprehensive campaign at a rural site in the North China Plain. More than one month of high quality HONO data spanning from the limit of detection to 5 ppb were collected. Intercomparison of our system with another established system from Forschungszentrum Juelich is presented and discussed. In conclusion, our instrument achieved a detection limit of 10 ppt V within2 min and a measurement uncertainty of 7%, which is well suited for investigation of the HONO budget from urban to rural conditions in China.

Review of technologies and their applications for the speciated detection of RO2 radicals

Peroxy radicals(RO_(2)),which are formed during the oxidation of volatile organic compounds,play an important role in atmospheric oxidation reactions.Therefore,the measurement of RO_(2),especially distinct species of RO_(2)radicals,is important and greatly helps the exploration of atmospheric chemistry mechanisms.Although the speciated detection of RO_(2)radicals remains challenging,various methods have been developed to study them in detail.These methods can be divided into spectroscopy and mass spectrometry technologies.The spectroscopy methods contain laser-induced fluorescence(LIF),UV-absorption spectroscopy,cavity ring-down spectroscopy(CRDS)and matrix isolation and electron spin resonance(MIESR).The mass spectrometry methods contain chemical ionization atmospheric pressure interface time-of-flight mass spectrometry(CI-APi-TOF),chemical ionization mass spectrometry(CIMS),CI-Orbitrap-MS and the third-generation proton transfer reaction-time-of-flight mass spectrometer(PTR3).This article reviews technologies for the speciated detection of RO_(2)radicals and the applications of these methods.In addition,a comparison of these techniques and the reaction mechanisms of some key species are discussed.Finally,possible gaps are proposed that could be filled by future research into speciated RO_(2)radicals.

Mechanisms and Kinetics of Reaction CHCIBr＋NO2

The reaction CHClBr＋NO2 was investigated via quantum chemical methods and kinetic calculations. The reaction mechanism on the singlet potential energy surface（PES） was considered by B3LYP method, and the energies were calculated at the CCSD（T） and CASPT2 levels of theory. The rate constants and the ratios of products were obtained by utilizing VTST and RRKM methods over wide temperature and pressure ranges. Our results indicate that carbon-to-nitrogen approach via a barrierless process is preferred in the initial association of CHClBr and NO2. The dominant product is BrNO＋CHCIO（PI）, which agrees well with the experimental observation. P2（ClNO＋CHBrO） and P3（HNO＋CBrClO） may also have minor contributions to the reaction. The calculated overall rate constants are independent of pressure and consistent with the experimental data, which can be fitted with the following equation over the temperature range of 200--1500 K： k（T）=2.31 × 10^-15T^0.99exp（771/T）. Compared with reaction CH2Br＋NO2, reaction CHCIBr＋NO2 has decreased the overall rate constants.

Simulation and Validation of the Aerosol Optical Thickness over China in 2006

The Model of Atmospheric Transport and Chemistry （MATCH） developed by the US National Center for Atmospheric Research （NCAR） was used to calculate the aerosol optical thickness （AOT） over China in 2006, with emission source data of the Intercontinental Chemical Transport Experiment Phase B （INTEX-B） and NCEP/NCAR reanalysis data as inputs. The simulation results of AOT were then validated with obser- vational data from the Moderate Resolution Imaging Spectroradiometer （MODIS）, Chinese Sun Hazemeter Network （CSHNET）, Aerosol Robotics Network （AERONET）, and China Aerosol Remote Sensing Network （CARSNET） at more than 30 stations over China. The comparison results indicated that the high values of AOT in the areas such as the Sichuan basin and East and South China and the low values of AOT over the Tibetan Plateau and Northwest and Northeast China were reasonably simulated by the MATCH. This model tended to underestimate the AOT values in high-aerosol-loading areas but overestimate the AOT val- ues in less polluted areas because there are still large uncertainties in the expression of emission sources, the description of the optical properties of aerosols, the treatment of cloud and precipitation, and the selection of grid resolution. The modeling results were consistent with the CSHNET, CARSNET, AERONET, and MODIS data in most parts of China, and the correlation coefficient of the monthly mean AOT between the model and the observation was 0.79 with CSHNET data at 23 stations, 0.51 with MODIS data, and 0.88 with data at 3 CARSNET stations and 2 other stations. All of them passed the significance test with c 〈 0.0001. The results demonstrated that the MATCH has the ability to simulate the characteristics of the AOT distribution and its seasonal variation over China.

Origin of major ions in monthly rainfall events at the Bamenda Highlands, North West Cameroon

Rainwater characteristics can reveal emissions from various anthropogenic and natural sources into the atmosphere. The physico-chemical characteristics of 44 monthly rainfall events （collected between January and December 2012） from 4 weather stations （Bamenda, Ndop plain, Ndawara and Kumbo） in the Bamenda Highlands （BH） were investigated. The purpose was to determine the sources of chemical species, their seasonal inputs and suitability of the rainwater for drinking. The mean pH of 5 indicated the slightly acidic nature of the rainwater. Average total dissolved solids （TDS） were low （6.7 mg/L）, characteristic of unpolluted atmospheric moisture/air. Major ion concentrations （mg/L） were low and in the order K＋ 〉 Ca2＋ 〉 Mg2~ 〉 Na＋ for cations and NO3 〉〉 HCO3 〉 SO] 〉 CI- 〉 PO3- 〉 F- for anions. The average rainwater in the area was mixed Ca-Mg-SO4-CI water type. The CI-/Na＋ ratio （1.04） was comparable to that of seawater （1.16）, an indication that N a＋ and CI originated mainly from marine （Atlantic Ocean） aerosols. High enrichments of Ca2＋, Mg2＋ and SO2- to Na＋ ratios relative to seawater ratios （constituting 44% of the total ions） demonstrated their terrigenous origin, mainly from Saharan and Sahelian arid dusts. The K＋/Na＋ ratio （2.24）, which was similar to tropical vegetation ash （2.38）, and NO3 was essentially from biomass burning. Light （〈 100 mm） pre-monsoon and post-monsoon convective rains were enriched in major ions than the heavy （〉 100 mm） monsoon rains, indicating a high contribution of major ions during the low convective showers. Despite the acidic nature, the TDS and major ion concentrations classified the rainwater as potable based on the WHO guidelines.