Atmospheric pollutants transport tracks revealed from ^(131)I, ^(137)Cs, and ^(134)Cs leaked from Fukushima accident and ~7Be and ^(210)Pb observed at Guiyang of China

A massive earthquake measuring 9.0 on the Richter scale that occurred on March 11, 2011, on Honshu Island, Japan, caused radioactivity leakage from the Fukushima Nuclear Power Plant, which led to the leakage of artificial nuclides(131I, 137 Cs, and 134Cs) and their global transportation by atmospheric circulation. This paper reports a systematic comparative observation on radioactive concentrations of natural nuclides(7Be and 210Pb) and artificial nuclides(131I, 137 Cs, and 134Cs) at the surface level, measured in weekly continuous aerosol sampling at Mount Guanfeng, Guiyang, China, from March 17, 2011 to April 28, 2011. During this period, the variations in the nuclide concentrations associated with their transport paths were analyzed with 315 hour back-trajectories of air mass initialized 500 m above the surface level at Guiyang. The results show that the pollutants of nuclear leakage from the Fukushima accident were transported to the Guiyang region of China via two significant pathways. In the first pathway the first wave of nuclear pollutants were transported from west to east in air masses at higher altitudes via global atmospheric circulation. The nuclear pollutants encircled the Earth almost once and after about 10 days to two weeks, between March 24 and March 31, 2011, intruded Guiyang from the northwestern region of China. In the second pathway, the nuclear pollutants from the Fukushima region arrived at Guiyang between April 7 and April 14, 2011, via air masses at lower altitudes that moved southwards because of the squeezing of the northeast Asian weather system and then by the influence, in succession, of the northeastern and southeastern air currents in the low-latitude region. The first transport pathway for atmospheric pollutants is on a global scale and based on air masses at higher altitudes, and the second transport pathway is on an eastern Asia regional scale and based on the air masses at lower altitude.

Application of a Neural Network to Store and Compute the Optical Properties of Non-Spherical Particles

Radiative transfer simulations and remote sensing studies fundamentally require accurate and efficient computation of the optical properties of non-spherical particles.This paper proposes a deep learning(DL)scheme in conjunction with an optical property database to achieve this goal.Deep neural network(DNN)architectures were obtained from a dataset of the optical properties of super-spheroids with extensive shape parameters,size parameters,and refractive indices.The dataset was computed through the invariant imbedding T-matrix method.Four separate DNN architectures were created to compute the extinction efficiency factor,single-scattering albedo,asymmetry factor,and phase matrix.The criterion for designing these neural networks was the achievement of the highest prediction accuracy with minimal DNN parameters.The numerical results demonstrate that the determination coefficients are greater than 0.999 between the prediction values from the neural networks and the truth values from the database,which indicates that the DNN can reproduce the optical properties in the dataset with high accuracy.In addition,the DNN model can robustly predict the optical properties of particles with high accuracy for shape parameters or refractive indices that are unavailable in the database.Importantly,the ratio of the database size(~127 GB)to that of the DNN parameters(~20 MB)is approximately 6810,implying that the DNN model can be treated as a highly compressed database that can be used as an alternative to the original database for real-time computing of the optical properties of non-spherical particles in radiative transfer and atmospheric models.

Sorely reducing emissions of non-methane short-lived climate forcers will worsen compound flood-heatwave extremes in the Northern Hemisphere

Non-methane short-lived climate forcer(SLCF)or near-term climate forcer(NTCF)emissions,as a significant driver of climate change,can be reduced to improve air quality.These reductions may contribute to additional warming of the climate system in the short term,thereby strongly affecting the likelihood of climate extremes.However,there has been no quantitative assessment of the impact of non-methane SLCF mitigation on compound flood-heatwave extremes(CFHEs).This study quantitatively investigates the changes in future(2031-2050 versus 1995-2014)CFHEs and the resulting population exposure in the Northern Hemisphere(NH)due to non-methane SLCF reductions.We used multi-model ensemble simulations under two future scenarios from the Aerosol and Chemistry Model Intercomparison Project(AerChemMIP)in the Coupled Model Intercomparison Project Phase 6(CMIP6).The two future scenarios share the same greenhouse gas(GHG)emissions but have weak(Shared Socioeconomic Pathway(SSP)3-7.0)versus strong(SSP3-7.0-lowNTCF)levels of air quality control measures.The results show that future non-methane SLCF reductions during 2031-2050 results in about a 7.3%±2.3%increase in grid exposure to CFHEs in the NH relative to the period 1995-2014.The frequency,intensity,and duration of CFHEs increase by varying degrees.During the period 2031-2050,the frequency of CFHEs across the NH increases by 2.9±0.9 events per decade due to non-methane SLCF reductions.The increases in CFHE frequency are more pronounced in East Asia,South Asia,Siberia,and northern and eastern North America.In East and South Asia,the in-tensities of both heatwaves and floods corresponding to CFHEs increase markedly,where heatwave magnitude(HWM)increases by 0.3±0.2 K in East Asia and weighted average precipitation(WAP)increases by 18.3%±15.3%and 12.0%±4.5%in East Asia and South Asia,respectively.In other regions,rising temperatures dominate the increase in CFHEs.With regard to the duration of CFHEs,future reductions in non-methane SLCFs increases the duration of CFHEs in the NH by O.3±0.1 d.Regionally,the sensitivity of CFHE frequency to global warming caused by non-methane SLCF mitigation is 1.2-1.9 times higher than that caused by GHG forcing.Non-methane SLCFs results in NH-averaged increases in population exposure to CFHEs of(5.0±2.0)×10^(5)person·event in the period 2031-2050.This study emphasizes the importance of considering the impacts of cleaner air in future responses to compound extremes and corresponding societal planning.

Relative Contributions of Boundary-Layer Meteorological Factors to the Explosive Growth of PM2.5 during the Red-Alert Heavy Pollution Episodes in Beijing in December 2016

Based on observations of urban mass concentration of fine particulate matter smaller than 2.5 μm in diameter （PM2.5）, ground meteorological data, vertical measurements of winds, temperature, and relative humidity （RH）, and ECMWF reanalysis data, the major changes in the vertical structures of meteorological factors in the boundary layer （BL） during the heavy aerosol pollution episodes （HPEs） that occurred in winter 2016 in the urban Beijing area were analyzed. The HPEs are divided into two stages： the transport of pollutants under prevailing southerly winds, known as the transport stage （TS）, and the PM2.5 explosive growth and pollution accumulation period characterized by a temperature inversion with low winds and high RH in the lower BL, known as the cumulative stage （CS）. During the TS, a surface high lies south of Beijing, and pollutants are transported northwards. During the CS, a stable BL forms and is characterized by weak winds, temperature inversion, and moisture accumulation. Stable atmospheric stratifica- tion featured with light/calm winds and accumulated moisture （RH 〉 80%） below 250 m at the beginning of the CS is closely associated with the inversion, which is strengthened by the considerable decrease in near-surface air temperat- ure due to the interaction between aerosols and radiation after the aerosol pollution occurs. A significant increase in the PLAM （Parameter Linking Aerosol Pollution and Meteorological Elements） index is found, which is linearly re- lated to PM mass change. During the first 10 h of the CS, the more stable BL contributes approximately 84% of the explosive growth of PM2.5 mass. Additional accumulated near-surface moisture caused by the ground temperature de- crease, weak turbulent diffusion, low BL height, and inhibited vertical mixing of water vapor is conducive to the sec- ondary aerosol formation through chemical reactions, including liquid phase and heterogeneous reactions, which fur- ther increases the PM2.5 concentration levels. The contribution of these reaction mechanisms to the explosive growth of PM2,5 mass during the early CS and subsequent pollution accumulation requires further investigation.

Aerosol Hygroscopicity during the Haze Red-Alert Period in December 2016 at a Rural Site of the North China Plain

A humidification system was deployed to measure aerosol hygroscopicity at a rural site of the North China Plain during the haze red-alert period 17–22 December 2016. The aerosol scattering coefficients under dry [relative humidity（RH） 〈 30%] and wet（RH in the range of 40%–85%） conditions were simultaneously measured at wavelengths of450, 550, and 700 nm. It is found that the aerosol scattering coefficient and backscattering coefficient increased by only 29% and 10%, respectively when RH went up from 40% to 80%, while the hemispheric backscatter fraction went down by 14%, implying that the aerosol hygroscopicity represented by the aerosol scattering enhancement factor f（RH） is relatively low and RH exerted little effects on the aerosol light scattering in this case. The scattering enhancement factors do not show significant differences at the three wavelengths, only with an approximate 2% variation, suggesting that the aerosol hygroscopicity is independent of the wavelength. Aerosol hygroscopicity is highly dependent on the aerosol chemical composition. When there is a large mass fraction of inorganics and a small mass fraction of organic matter, f（RH） reaches a high value. The fraction of NO3^- was strongly correlated with the aerosol scattering coefficient at RH = 80%, which suggests that NO3^- played an important role in aerosol hygroscopic growth during the heavy pollution period.

Temporal Variation and Source Identification of Black Carbon at Lin'an and Longfengshan Regional Background Stations in China

Black carbon （BC） is a component of fine particulate matter （PM2.5）, associated with climate, weather, air quality, and people＇s health. However, studies on temporal variation of atmospheric BC concentration at background stations in China and its source area identification are lacking. In this paper, we use 2-yr BC observations from two background stations, Lin＇an （LAN） and Longfengshan （LFS）, to perform the investigation. The results show that the mean diurnal variation of BC has two significant peaks at LAN while different characteristics are found in the BC vari- ation at LFS, which are probably caused by the difference in emission source contributions. Seasonal variation of monthly BC shows double peaks at LAN but a single peak at LFS. The annual mean concentrations of BC at LAN and LFS decrease by 1.63 and 0.26 μg m 3 from 2009 to 2010, respectively. The annual background concentration of BC at LAN is twice higher than that at LFS. The major source of the LAN BC is industrial emission while the source of the LFS BC is residential emission. Based on transport climatology on a 7-day timescale, LAN and LFS stations are sensitive to surface emissions respectively in belt or approximately circular area, which are dominated by summer monsoon or colder land air flows in Northwest China. In addition, we statistically analyze the BC source regions by using BC observation and FLEXible PARTicle dispersion model （FLEXPART） simulation. In summer, the source regions of BC are distributed in the northwest and south of LAN and the southwest of LFS. Low BC concentration is closely related to air mass from the sea. In winter, the source regions of BC are concentrated in the west and south of LAN and the northeast of the threshold area of stot at LFS. The cold air mass in the northwest plays an important role in the purification of atmospheric BC. On a yearly scale, sources of BC are approximately from five provinces in the northwest/southeast of LAN and the west of LFS. These findings are helpful in reducing BC emission and con- trolling air pollution.

Comparison of Submicron Particles at a Rural and an Urban Site in the North China Plain during the December 2016 Heavy Pollution Episodes

An extensive field experiment for measurement of physical and chemical properties of aerosols was conducted at an urban site in the Chinese Academy of Meteorological Sciences（CAMS） in Beijing and at a rural site in Gucheng（GC）, Hebei Province in December 2016. This paper compares the number size distribution of submicron particle matter（PM1, diameter 〈 1 μm） between the two sites. The results show that the mean PM1 number concentration at GC was twice that at CAMS, and the mass concentration was three times the amount at CAMS. It is found that the accumulation mode（100–850 nm） particles constituted the largest fraction of PM1 at GC, which was significantly correlated with the local coal combustion, as confirmed by a significant relationship between the accumulation mode and the absorption coefficient of soot particles. The high PM1 concentration at GC prevented the occurrence of new particle formation（NPF） events, while eight such events were observed at CAMS. During the NPF events, the mass fraction of sulfate increased significantly, indicating that sulfate played an important role in NPF. The contribution of regional transport to PM1 mass concentration was approximately 50% at both sites, same as that of the local emission. However, during the red-alert period when emission control took place, the contribution of regional transport was notably higher.

Influences of Meteorological Conditions on Interannual Variations of Particulate Matter Pollution during Winter in the Beijing–Tianjin–Hebei Area

To investigate the interannual variations of particulate matter （PM） pollution in winter, this paper examines the pollution characteristics of PM with aerodynamic diameters of less than 2.5 and 10 μm （i.e., PM2.5 and PM10）, and their relationship to meteorological conditions over the Beijing municipality, Tianjin municipality, and Hebei Province--an area called Jing-Jin-Ji （JJJ, hereinafter）-in December 2013-16. The meteorological conditions during this period are also analyzed. The regional average concentrations of PM2.5 （PM10） over the JJJ area during this period were 148.6 （236.4）, 100.1 （166.4）, 140.5 （204.5）, and 141.7 （203.1） μg m^-3, respectively. The high occurrence frequencies of cold air outbreaks, a strong Siberian high, high wind speeds and boundary layer height, and low temperature and relative humidity, were direct meteorological causes of the low PM concentration in December 2014. A combined analysis of PM pollution and meteorological conditions implied that control measures have resulted in an effective improvement in air quality. Using the same emissions inventory in December 2013-16, a modeling analysis showed emissions of PM2.5 to decrease by 12.7%, 8.6%, and 8.3% in December 2014, 2015, and 2016, respectively, each compared with the previous year, over the JJJ area.

Source characteristics of O_3 and CO_2 at Mt. Waliguan Observatory,Tibetan Plateau implied by using ~7Be and ^(210)Pb

The weekly averages of near-surface ^7Be, ^210pb, 03, and CO2 concentrations at the Global Atmospheric Watch Observatory, Mt. Waliguan （101.98°E, 36.287°N, 3810 m a.s.l.）, from October 2002 to January 2004 are presented. With the establishment of the new datasets of DCCW （Differential Concentrations in Contiguous Weeks） of ^7Be,^210pb, and O3, CO2 （△^7Be, △^210pb, △O3, △CO2, respectively, the impacts of upper-level downward transports and land-surface emissions on O3 and CO2 concentrations are implied by ^7Be and ^210pb being as independent tracers. The relations among △^7Be, △^210pb, and △O3, △CO2 are examined statistically and compared. The results indicate that with the DCCWs, the interferences with the tracing significance of ^7Be and ^210Pb from the seasonal wet scavenging of atmospheric aerosol are greatly reduced, and the weighting sources of O3 or CO2 variations are more pronounced. Basically, the variability of surface O3 is controlled predominately by air mass transported from the upper atmosphere levels while the emission from the Continent Boundary Layer （CBL） has an obvious input for CO2. The relation between △^210pb and △O3 reflects that influences of CBL emission are generally positive/negative for surface O3 budget in summer/winter, and the relation of △^7Be and △CO2 also reveals that upper level downward transport has positive/negative inputs for CO2 in summer/winter. With the highly correlated relations between ^7Be and O3, a quantitative estimation is made of the stratospheric contributions to the budget of surface O3 at WLG： the monthly averages of stratospheric O3 range from 6 ×10^-9 to 8 ×10^-9 （volume mixing ratio） in April and from June to August, and 2 ×10^-9 to 4 ×10^-9 in the remaining months. For the ultimate sources of the baseline concentration of surface 03, which consist of only stratospheric transport and tropospheric photochemistry production, the contribution from stratospheric transport is estimated to be about 20 ×10^-9 from May to July, and （12-15） ×10^-9 in the remaining months, and the total relative contribution rate is about 35% to 40%.

Chemical Components, Variation, and Source Identification of PM1 during the Heavy Air Pollution Episodes in Beijing in December 2016

Air pollution is a current global concern. The heavy air pollution episodes（HPEs） in Beijing in December 2016 severely influenced visibility and public health. This study aims to survey the chemical compositions, sources, and formation processes of the HPEs. An aerodyne quadruple aerosol mass spectrometer（Q-AMS） was utilized to measure the non-refractory PM1（NR-PM1） mass concentration and size distributions of the main chemical components including organics, sulfate, nitrate, ammonium, and chloride in situ during 15–23 December 2016. The NR-PM1 mass concentration was found to increase from 6 to 188 μg m–3 within 5 days. During the most serious polluted episode, the PM1 mass concentration was about 2.6 times that during the first pollution stage and even 40 times that of the clean days. The formation rates of PM2.5 in the five pollution stages were 26, 22, 22, 32, and 67 μg m^（–3） h–1, respectively. Organics and nitrate occupied the largest proportion in the polluted episodes, whereas organics and sulfate dominated the submicron aerosol during the clean days. The size distribution of organics is always broader than those of other species, especially in the clean episodes. The peak sizes of the interested species grew gradually during different HPEs. Aqueous reaction might be important in forming sulfate and chloride, and nitrate was formed via oxidization and condensation processes. PMF（positive matrix factorization） analysis on AMS mass spectra was employed to separate the organics into different subtypes. Two types of secondary organic aerosol with different degrees of oxidation consisted of 43% of total organics. By contrast, primary organics from cooking, coal combustion, and traffic emissions comprised 57% of the organic aerosols during the HPEs.

Equilibrium Climate Response of the East Asian Summer Monsoon to Forcing of Anthropogenic Aerosol Species

We used an online aerosol-climate model to study the equilibrium climate response of the East Asian summer monsoon （EASM） to increases in anthropogenic emissions of sulfate, organic carbon, and black carbon aerosols from 1850 to 2000. Our results show that each of these aerosol species has a different effect on the EASM as a result of changes in the local sea-land thermal contrast and atmospheric circulation. The increased emission of sulfate aerosol leads to a decrease in the thermal contrast between the land and ocean, a southward shift of the East Asian subtropical jet, and significant northerly wind anomalies at 850 hPa over eastern China and the ambient oceans, markedly dampening the EASM. An increase in organic carbon aerosol results in pronounced surface cooling and the forma- tion of an anomalous anticyclone over the oceans north of 30°N. These effects cause a slight increase in the sea-land thermal contrast and southerly flow anomalies to the west of the anticyclonic center, strengthening the northern EASM. An increase in organic carbon emission decreases the sea-land thermal contrast over southern China, which weakens the southern EASM. The response of the summer 850-hPa winds and rainfall over the East Asian monsoon region to an increase in black carbon emission is generally consistent with the response to an increase in organic carbon. The increase in black carbon emission leads to a strengthening of the northern EASM north of 35°N and a slight weakening of the southern EASM south of 35°N. The simulated response of the EASM to the increase in black carbon emission is unchanged when the emission of black carbon is scaled up by five times its year 2000 levels, although the intensities of the response is enhanced. The increase in sulfate emission primarily weakens the EASM, whereas the increases in black carbon and organic carbon emissions mitigate weakening of the northern EASM.

The macroscopic mechanisms and associated atmospheric precursor environmental capacities that lead to secondary fine particle pollution

This paper establishes the kinetic equations in atmospheric chemistry that describe the macroscopic mechanisms of secondary fine particle pollution generated by precursors during atmospheric self-purification.The dynamic and static solutions of these equations can be applied to calculate quantitative relationships between the concentration ratio of precursors and secondary fine particles as well as the physical clearance power of the atmosphere,chemical reaction rate,and the scale of a contaminated area.The dynamic solution presented here therefore corresponds with a theoretical formula for calculating the overall rate constant for the oxidation reaction of reducing pollutants in the actual atmosphere based on their local concentrations and meteorological monitoring data.In addition,the static solution presented in this paper reveals the functional relationship between the concentration of secondary fine particles and precursor emission rate as well as atmospheric self-purification capacity.This result can be applied to determine the atmospheric environmental capacity of a precursor.Hourly records collected over the last 40 years from 378 weather stations in China's Mainland as well as the spatiotemporal distribution sequence of overall oxidation reaction rates from precursors show that when the reference concentration limit of secondary fine particles is100μmol m-3,the atmospheric environmental capacity of total precursors canbe calculated as 24890×1010 mol yr-1.Thus,when the annual average concentration limit of given fine particles is 35μg m-3 and the ratio of sulfate and nitrate to 30%and 20%of the total amount of fine particles,the capacities of SO2,NOx and NH3 are 1255,1344,and 832(1010g yr-1),respectively.The clearance density of precursors for different return periods across China's Mainland under above conditions are also provided in this study.

大气CO_(2)观测支持核校我国2018~2021年人为碳排放与陆地碳交换

准确估算人为碳排放和自然碳交换对我国实现碳中和目标至关重要.当前自下而上的排放清单在区域尺度上存在不确定性,且缺乏大气观测的独立验证.本文利用我国39个高精度CO_(2)地基观测站,自上而下反演获得45 km×45 km分辨率的人为碳排放与自然碳交换,实现了对排放清单的核校支持.结果表明,2018~2021年反演获得的我国年均人为碳排放总量约为125亿吨CO_(2),高于自下而上的五套排放清单约15%.扣除人和动物呼吸排放后,2018~2021年我国年均人为碳排放约为115亿吨CO_(2).从逐年变化看,2021年我国人为二氧化碳排放总量较前一年增加4%,高于2020年同比增速(1.1%),略高于2019年同比年增速(3.7%),与我国观测到的大气中的CO_(2)年增量趋势一致.扣除农田碳汇、非CO_(2)碳排放、水蚀及火点排放后,2018-2021年我国年均陆地生态系统碳汇约为21亿吨CO_(2),高于大多数全球反演模式,占同期人为碳排放约17%.陆地生态系统自然碳交换受气候变化年际波动影响较大,其中2019年受弱厄尔尼诺事件影响,碳汇同比降幅最大,2020年碳汇有所增加,2021年同比2020年增加13.7%.自上而下反演给出了大气观测视角下CO_(2)源汇时空变化,并重点突出了不同省份在实现碳中和目标时面临的挑战.

Simulating Aerosol Size Distribution and Mass Concentration with Simultaneous Nucleation,Condensation/Coagulation, and Deposition with the GRAPES–CUACE

A coupled aerosol–cloud model is essential for investigating the formation of haze and fog and the interaction of aerosols with clouds and precipitation. One of the key tasks of such a model is to produce correct mass and number size distributions of aerosols. In this paper, a parameterization scheme for aerosol size distribution in initial emission,which took into account the measured mass and number size distributions of aerosols, was developed in the GRAPES–CUACE [Global/Regional Assimilation and Pr Ediction System–China Meteorological Administration（CMA） Unified Atmospheric Chemistry Environment model]—an online chemical weather forecast system that contains microphysical processes and emission, transport, and chemical conversion of sectional multi-component aerosols. In addition, the competitive mechanism between nucleation and condensation for secondary aerosol formation was improved, and the dry deposition was also modified to be in consistent with the real depositing length. Based on the above improvements, the GRAPES–CUACE simulations were verified against observational data during 1–31 January 2013, when a series of heavy regional haze–fog events occurred in eastern China. The results show that the aerosol number size distribution from the improved experiment was much closer to the observation, whereas in the old experiment the number concentration was higher in the nucleation mode and lower in the accumulation mode. Meanwhile, the errors in aerosol number size distribution as diagnosed by its sectional mass size distribution were also reduced. Moreover, simulations of organic carbon, sulfate, and other aerosol components were improved and the overestimation as well as underestimation of PM2.5 concentration in eastern China was significantly reduced,leading to increased correlation coefficient between simulated and observed PM2.5 by more than 70%. In the remote areas where bad simulation results were produced previously, the correlation coefficient grew from 0.35 to 0.61, and the mean mass concentration went up from 43% to 87.5% of the observed value. Thus, the simulation of particulate matters in these areas has been improved considerably.

Tracking a Severe Pollution Event in Beijing in December 2016 with the GRAPES–CUACE Adjoint Model

We traced the adjoint sensitivity of a severe pollution event in December 2016 in Beijing using the adjoint model of the GRAPES–CUACE（Global/Regional Assimilation and Prediction System coupled with the China Meteorological Administration Unified Atmospheric Chemistry Environmental Forecasting System）. The key emission sources and periods affecting this severe pollution event are analyzed. For comaprison, we define 2000 Beijing Time 3 December 2016 as the objective time when PM2.5 reached the maximum concentration in Beijing. It is found that the local hourly sensitivity coefficient amounts to a peak of 9.31 μg m^–3 just 1 h before the objective time, suggesting that PM2.5 concentration responds rapidly to local emissions. The accumulated sensitivity coefficient in Beijing is large during the 20-h period prior to the objective time, showing that local emissions are the most important in this period.The accumulated contribution rates of emissions from Beijing, Tianjin, Hebei, and Shanxi are 34.2%, 3.0%, 49.4%,and 13.4%, respectively, in the 72-h period before the objective time. The evolution of hourly sensitivity coefficient shows that the main contribution from the Tianjin source occurs 1–26 h before the objective time and its peak hourly contribution is 0.59 μg m^-3 at 4 h before the objective time. The main contributions of the Hebei and Shanxi emission sources occur 1–54 and 14–53 h, respectively, before the objective time and their hourly sensitivity coefficients both show periodic fluctuations. The Hebei source shows three sensitivity coefficient peaks of 3.45, 4.27, and 0.71 μg m^–3 at 4, 16, and 38 h before the objective time, respectively. The sensitivity coefficient of the Shanxi source peaks twice, with values of 1.41 and 0.64 μg m^–3 at 24 and 45 h before the objective time, respectively. Overall, the adjoint model is effective in tracking the crucial sources and key periods of emissions for the severe pollution event.