A CNN-SVR model for NO_(2)profile prediction based on MAX-DOAS observations:The influence of Chinese New Year overlapping the 2020 COVID-19 lockdown on vertical distributions of tropospheric NO_(2)in Nanjing,China

In this study,a hybrid model,the convolutional neural network-support vector regression model,was adopted to achieve prediction of the NO_(2)profile in Nanjing from January 2019to March 2021.Given the sudden decline in NO_(2)in February 2020,the contribution of the Coronavirus Disease-19(COVID-19)lockdown,Chinese New Year(CNY),and meteorologi cal conditions to the reduction of NO_(2)was evaluated.NO_(2)vertical column densities(VCDs) from January to March 2020 decreased by 59.05%and 32.81%,relative to the same period in 2019 and 2021,respectively.During the period of 2020 COVID-19,the average NO_(2)VCDs were 50.50%and 29.96%lower than those during the pre-lockdown and post-lockdown pe riods,respectively.The NO_(2)volume mixing ratios(VMRs)during the 2020 COVID-19 lock down significantly decreased below 400 m.The NO_(2)VMRs under the different wind fields were significantly lower during the lockdown period than during the pre-lockdown period This phenomenon could be attributed to the 2020 COVID-19 lockdown.The NO_(2)VMRs be fore and after the CNY were significantly lower in 2020 than in 2019 and 2021 in the same period,which further proves that the decrease in NO_(2)in February 2020 was attributed to the COVID-19 lockdown.Pollution source analysis of an NO_(2)pollution episode during the lockdown period showed that the polluted air mass in the Beijing-Tianjin-Hebei was trans ported southwards under the action of the north wind,and the subsequent unfavorable meteorological conditions(local wind speed of<2.0 m/sec)resulted in the accumulation o pollutants.

Atmospheric environment monitoring technology and equipment in China:A review and outlook

Accurate monitoring of the atmospheric environment and its evolution are important for understanding the sources,chemical mechanisms,and transport processes of air pollution and carbon emissions in China,and for regulatory and control purposes.This study gives an overview of atmospheric environment monitoring technology and equipment in China and summarizes the major achievements obtained in recent years.China has made great progress in the development of atmospheric environment monitoring technology and equipment with decades of effort.The manufacturing level of atmospheric environment monitoring equipment and the quality of products have steadily improved,and a technical&production system that can meet the requirements of routine monitoring activities has been initiated.It is expected that domestic atmospheric environment monitoring technology and equipment will be able to meet future demands for routine monitoring activities in China and provide scientific assistance for addressing air pollution problems.

Key emergency response technologies for abrupt air pollution accidents in China

Abrupt air pollution accidents can endanger people’s health and destroy the local ecological environment.The appropriate emergency response can minimize the harmful effects of accidents and protect people’s lives and property.This paper provides an overview of the key emergency response technologies for abrupt air pollution accidents around the globe with emphasis on the major achievements that China has obtained in recent years.With decades of effort,China has made significant progress in emergency monitoring technologies and equipment,source estimation technologies,pollutant dispersion simulation technologies and others.Many effective domestic emergency monitoring instruments(e.g.,portable DOAS/FT-IR systems,portable FID/PID systems,portable GC-MS systems,scanning imaging remote sensing systems,and emergency monitoring vehicles)had been developed which can meet the demands for routine emergency response activities.A monitoring layout technique combining air dispersion simulation,fuzzy comprehensive evaluation,and a post-optimality analysis was proposed to identify the optimal monitoring layout scheme under the constraints of limited monitoring resources.Multiple source estimation technologies,including the forward method and the inversion method,have been established and evaluated under various scenarios.Multi-scale dynamic pollution dispersion simulation systems with high temporal and spatial resolution were further developed.A comprehensive emergency response platform integrating database support,source estimation,monitoring schemes,fast monitoring of pollutants,pollution predictions and risk assessment was developed based on the technical idea of"source identification-model simulation-environmental monitoring"dynamic interactive feedback.It is expected that the emergency response capability for abrupt air pollution accidents will gradually improve in China.

A portable instrument for measurement of atmospheric Ox and NO2 based on cavity ring-down spectroscopy

Atmospheric O_(x)(nitrogen dioxide(NO_(2))+ozone(O_(3)))can better reflect the local and regional change character-istics of oxidants compared to O_(3)alone,so obtaining O_(x)accurately and rapidly is the basis for evaluating the O_(3)production rate.Furthermore,O_(x)has proved to be a more representative indicator and can serve as a reflection of pollution prevention efficacy.A portable instrument for measuring atmospheric O_(x)and NO_(2)based on cavity ring-down spectroscopy(O_(x)/NO_(2)-CRDS)was developed in this work.The NO_(2)concentration is accurately mea-sured according to its absorption characteristic at 407.86 nm.Ambient O_(3)is converted into NO_(2)by chemical titration of high concentrations of nitrogen oxide(NO),and the O_(3)conversion efficiencies obtained are nearly 99%.The detection limit of the O_(x)/NO_(2)-CRDS system for O_(x)is 0.024 ppbv(0.1 s),and the overall uncertainty of the instrument is±6%.Moreover,the Kalman filtering technique was applied to improve the measurement accuracy of O_(x)/NO_(2)-CRDS.The system was applied in a comprehensive field observation campaign at Hefei Sci-ence Island from 26 to 30 September 2022,and the time concentration series and change characteristics of O_(x)and NO_(2)were obtained for five days.The measured O_(x)concentrations were compared with those of two com-mercial instruments,and the consistency was good(R^(2)=0.98),indicating that this system can be deployed to accurately and rapidly obtain the concentrations of atmospheric O_(x)and NO_(2).It will be a useful tool for assessing the atmospheric oxidation capacity and controlling O_(3)pollution.

Long-term observations of tropospheric NO_2, SO_2 and HCHO by MAX-DOAS in Yangtze River Delta area, China

Yangtze River Delta （YRD） area is one of the important economic zones in China. However, this area faces increasing environmental problems. In this study, we use ground-based multi-axis differential optical absorption spectroscopy （MAX-DOAS） network in Eastern China to retrieve variations of NO2, SO2, and formaldehyde （HCHO） in the YRD area. Three cities of YRD （Hefei, Nanjing, and Shanghai） were selected for long-term observations. This paper presents technical performance and characteristics of instruments, their distribution in YRD, and results of vertical column densities （VCDs） and profiles of NO2, SO2, and HCHO. Average diurnal variations of tropospheric NO2 and SO2 in different seasons over the three stations yielded minimum values at noon or in the early afternoon, whereas tropospheric HCHO reached the maximum during midday hours. Slight reduction of the pollutants in weekends occurred in all the three sites. In general trace gas concentrations gradually reduced from Shanghai to Hefei. Tropospheric VCDs of NO2, SO2, and HCHO were compared with those from Ozone Monitoring Instrument （OMI） satellite observations, resulting in R2 of 0.606, 0.5432, and 0.5566, respectively. According to analysis of regional transports of pollutants, pollution process happened in YRO under the north wind with the pollution dissipating in the southeast wind. The feature is significant in exploring transport of tropospheric trace gas pollution in YRD, and provides basis for satellite and model validation.

Variation of temporal and spatial patterns of NO_2 in Beijing using OMI and mobile DOAS

Control policies such as "odd-and-even license plate rule" were implemented by the Chinese government to restrict traffic and suspend factory production in Beijing and neighboring cities during the Asia-Pacific Economic Cooperation summit. We use ozone monitoring instrument(OMI), mobile differential optical absorption spectroscopy(DOAS), and multi-axis differential optical absorption spectroscopy(MAX-DOAS) to measure the variation of the spatial and temporal patterns of NO2 column densities from October 24, 2014 to November 22, 2014. It is found that the NO2 column densities during the episode of control policies are significantly lower than those during other periods, and the emission flux of NO2 calculated by mobile DOAS is also lower than the results from other periods. Some daily low NO2 column densities occur with the northwest wind direction. We then compare the relationship between OMI and mobile DOAS NO2 column density observations, and the results of mobile DOAS are approximately 2.7 times larger than the OMI values. The largest discrepancy occurs in the northern part of Beijing city. In other parts, the two instruments have a better correlation coefficient(R2) of 0.61. The low NO2 column densities that occur during the episode of control policies are affected by the control policies as well as meteorological conditions.

Observations of N2O5 and NO3 at a suburban environment in Yangtze River Delta in China: Estimating heterogeneous N2O5 uptake coefficients

The nitrate radical(NO3)and dinitrogen pentoxide(N2.5)play an important role in the nocturnal atmosphere chemistry.Observations of N03 radicals and N2 O5 were performed in a semirural ground site at Tai’Zhou in polluted southern China using cavity ring down spectroscopy(CRDS)from 23 May to 15 June 2018.The observed N03 and N2 O5 concentrations were relatively low,with 1 min average value of 4.4±2.2 and 26.0±35.7 pptV,respectively.The N2 O5 uptake coefficient was determined to be from 0.027 to 0.107 based on steady state lifetime method.Fast N2 O5 hydrolysis was the largest contributor to the loss of NO3 and contributed to substantial nitrate formation,with an average value of 14.83±6.01μg/m3.Further analysis shows that the N2 O5 heterogeneous reactions dominated the nocturnal NOx loss and the nocturnal NOx loss rate is 0.14±0.02 over this region.

Nocturnal atmospheric chemistry of NO_(3) and N_(2)O_(5) over Changzhou in the Yangtze River Delta in China

Comprehensive observations of the nocturnal atmospheric oxidation of NO_(3)and N_(2)O_(5)were conducted at a suburban site in Changzhou in the YRD using cavity ring-down spectroscopy(CRDS)from 27 May to 24 June,2019.High concentrations of NO_(3)precursors were observed,and the nocturnal production rate of NO_(3)was determined to be 1.7±1.2 ppbv/hr.However,the nighttime NO_(3)and N_(2)O_(5)concentrations were relatively low,with maximum values of 17.7 and 304.7 pptv,respectively,illustrating the rapid loss ofNO_(3)andN_(2)O_(5).Itwas found that NO_(3)dominated the nighttime atmospheric oxidation,accounting for 50.7%,whileO3 andOH only contributed 34.1%and 15.2%,respectively.For the reactions of NO_(3)with volatile organic compounds(VOCs),styrenewas found to account for 60.3%,highlighting its dominant role in the NO_(3)reactivity.In general,the contributions of the reactions between NO_(3)and VOCs and the N_(2)O_(5)uptake to NO_(3)losswere found to be about 39.5%and 60.5%,respectively,indicating that N_(2)O_(5)uptake also played an important role in the loss of NO_(3)and N_(2)O_(5),especially under the high humidity conditions in China.The formation of nitrate at night mainly originated from N_(2)O_(5)uptake,and the maximum production rate of NO_(3)^(-)reached 6.5 ppbv/hr.The average NOx consumption rate via NO_(3)and N_(2)O_(5)chemistry was found to be 0.4 ppbv/h,accounting for 47.9%of the total NO_(x)removal.The predominant roles of NO_(3)and N_(2)O_(5)in nitrate formation and NO_(x)removal in the YRD region was highlighted in this study.

A hydroxyl radical detection system using gas expansion and fast gating laser-induced fluorescence techniques

An OH radical measurement instrument based on Fluorescence Assay by Gas Expansion（FAGE）has been developed in our laboratory.Ambient air is introduced into a low-pressure fluorescence cell through a pinhole aperture and irradiated by a dye laser at a high repetition rate of 8.5 k Hz.The OH radical is both excited and detected at 308 nm using A-X（0,0）band.To satisfy the high efficiency needs of fluorescence collection and detection,a 4-lens optical system and a self-designed gated photomultiplier（PMT）is used,and gating is actualized by switching the voltage applied on the PMT dynodes.A micro channel photomultiplier（MCP）is also prepared for fluorescence detection.Then the weak signal is accumulated by a photon counter in a specific timing.The OH radical excitation spectrum range in the wavelength of 307.82–308.2 nm is detected and the excited line for OH detection is determined to be Q1（2）line.The calibration of the FAGE system is researched by using simultaneous photolysis of H2O and O2.The minimum detection limit of the instrument using gated PMT is determined to be 9.4×10~5molecules/cm^3,and the sensitivity is 9.5×10^（-7）cps/（OH·cm^（-3））,with a signal-to-noise ratio of 2 and an integration time of 60 sec,while OH detection limit and the detection sensitivity using MCP is calculated to be 1.6×10~5molecules/cm^3and 2.3×10^（-6）cps/（OH·cm^（-3））.The laboratory OH radical measurement is carried out and results show that the proposed system can be used for atmospheric OH radical measurement.

Measurement of tropospheric HO_(2) radical using fluorescence assay by gas expansion with low interferences

An instrument to detect atmospheric HO_(2) radicals using fluorescence assay by gas expansion(FAGE)technique has been developed.HO_(2) is measured by reaction with NO to form OH and subsequent detection of OH by laser-induced fluorescence at low pressure.The system performance has been improved by optimizing the expansion distance and pressure,the influence factors of HO_(2) conversion efficiency are also studied.The interferences of RO_(2) radicals were investigated by determining the conversion efficiency of RO_(2) to OH during the measurement of HO_(2).The dependence of the conversion of HO_(2) on NO concentration was investigated,and low HO_(2) conversion efficiency was selected to realize the ambient HO_(2) measurement,where the conversion efficiency of RO_(2) derived by propane,ethene,isoprene and methanol to OH has been reduced to less than 6%in the atmosphere.Furthermore,no significant interferences from PM_(2.5) and NO were found in the ambient HO_(2) measurement.The detection limits for HO_(2)(S/N=2)are estimated to 4.8×10^5 cm^-3 and 1.1×10^6 cm^-3(ρHO_(2)=20%)under night and noon conditions,with 60 sec signal integration time.The instrument was successfully deployed during STORM-2018 field campaign at Shenzhen graduate school of Peking University.The concentration of atmospheric HOx radical and the good correlation of OH with j(O1D)was obtained here.The diurnal variation of HOx concentration shows that the OH maximum concentration of those days is about 5.3×10^6 cm^-3 appearing around 12:00,while the HO_(2) maximum concentration is about 4.2×10^8 cm^-3 appearing around 13:30.

Study of aerosol characteristics and sources using MAX-DOAS measurement during haze at an urban site in the Fenwei Plain

Atmospheric aerosols have effects on atmospheric radiation assessments,global climate change,local air quality and visibility.In particular,aerosols are more likely transformed and accumulated in winter.In this paper,we used the Multi-Axis Differential Optical Absorption Spectroscopy(MAX-DOAS)instrument to study the characteristics of aerosol type and contributions of PM_(2.5) chemical components to aerosol extinction(AE),vertical distribution of aerosols,and source.From December 30,2018 to January 27,2019,we conducted MAX-DOAS observations on Sanmenxia.The proportion of PM_(2.5) to PM10 was 69.48%-95.39%,indicating that the aerosol particles were mainly fine particles.By analyzing the ion data and modifying Interagency Monitoring of Protected Visual Environments(IMPROVE)method,we found that nitrate was the largest contributor to AE,accounting for 31.51%,28.98%,and 27.95%of AE on heavily polluted,polluted,and clean days,respectively.NH4+,OC,and SO42-were also major contributors to AE.The near-surface aerosol extinction retrieved from MAX-DOAS measurement the PM_(2.5) and PM10 concentrations measured by an Unmanned Aerial Vehicle(UAV)have the same trend in vertical distribution.AE increased about 3 times from surface to 500 m.With the backward trajectory of the air mass during the haze,we also found that the continuous heavy pollution was mainly caused by transport of polluted air from the northeast,then followed by local industrial emissions and other sources of emissions under continuous and steady weather conditions.

Measurement of HONO flux using the aerodynamic gradient method over an agricultural field in the Huaihe River Basin,China

To investigate nitrous acid(HONO)levels and potential HONO sources above crop rotation fields.The HONO fluxes were measured by the aerodynamic gradient(AG)method from 14 December 2019 to 2 January 2020 over an agricultural field in the Huaihe River Basin.The ambient HONO levels were measured at two different heights(0.15 and 1.5 m),showing a typical diurnal cycle with low daytime levels and high nighttime levels.The upward HONO fluxes were mostly observed during the day,whereas deposition dominated at night.The diurnal variation of HONO flux followed solar radiation,with a noontime maximum of 0.2 nmol/(m^(2)·sec).The average upward HONO flux of 0.06±0.17 nmol/(m^(2)·sec)indicated that the agricultural field was a net source for atmospheric HONO.The higher HONO/NO_(2)ratio and NO_(2)-to-HONO conversion rate close to the surface suggested that nocturnal HONO was formed and released near the ground.The unknown HONO source was derived from the daytime HONO budget analysis,with an average strength of 0.31 ppbV/hr at noontime.The surface HONO flux,which was highly correlated with the photolysis frequency J(NO_(2))(R^(2)=0.925)and the product of J(NO_(2))×NO_(2)(R^(2)=0.840),accounted for∼23%of unknown daytime HONO source.The significant correlation between HONO fluxes and J(NO_(2))suggests a light-driven HONO formation mechanism responsible for the surface HONO flux during daytime.

Direct Retrieval of NO_(2)Vertical Columns from UV-Vis(390-495nm)Spectral Radiances Using a Neural Network

Satellite retrievals of columnar nitrogen dioxide(NO_(2))are essential for the characterization of nitrogen oxides(NO_(x))processes and impacts.The requirements of modeled a priori profiles present an outstanding bottleneck in operational satellite NO_(2)retrievals.In this work,we instead use neural network(NN)models trained from over 360,000 radiative transfer(RT)simulations to translate satellite radiances across 390-495nm to total NO_(2)vertical column(NO_(2)C).Despite the wide variability of the many input parameters in the RT simulations,only a small number of key variables were found essential to the accurate prediction of NO_(2)C,including observing angles,surface reflectivity and altitude,and several key principal component scores of the radiances.In addition to the NO_(2)C,the NN training and cross-validation experiments show that the wider retrieval window allows some information about the vertical distribution to be retrieved(e.g.,extending the rightmost wavelength from 465 to 495 nm decreases the root-mean-square-error by 0.75%)under high-NO_(2)C conditions.Applying to four months of TROPOMI data,the trained NN model shows strong ability to reproduce the NO_(2)C observed by the ground-based Pandonia Global Network.The coefficient of determination(R2,0.75)and normalized mean bias(NMB,-33%)are competitive with the level 2 operational TROPOMI product(R^(2)=0:77,NMB=−29%)over clear(geometric cloud fraction<0:2)and polluted(NO_(2)C≥7:5×10^(15)molecules/cm2)regions.The NN retrieval approach is~12 times faster than predictions using high spatial resolution(~3 km)a priori profiles from chemical transport modeling,which is especially attractive to the handling of large volume satellite data.