[Study of Peak Carbon Emission of a City in Yangtze River Delta Based on LEAP Model].

Based on the LEAP model framework, a LEAP-X sub-sector calculation model suitable for X City was constructed in this study. Four scenarios including a baseline scenario, low-carbon scenario, enhanced low-carbon scenario, and peak in 2023 scenario were set up to predict and analyze the carbon emission situation. The calculation and analysis results showed that it could achieve the carbon peak before 2030 only under the enhanced low-carbon scenario and peak in 2023 scenario. The peak year of the enhanced low-carbon scenario was around 2025 with a peak carbon emission of approximately 170 million tons, but the peak time may actually be delayed. Industry was the largest sector of carbon emissions, and the petrochemical industry was the largest portion of industry, the proportion of which was always maintained at approximately 30% under different scenarios. However, the proportion of power generation and steel industry decreased annually, whereas the proportion of the net imported power gradually increased. Industrial structure optimization and energy structure adjustment were the main driving factors of carbon peak in X City. Carbon emissions per unit of GDP will fall by approximately 41% in 2030 compared with that in 2020 under the enhanced low-carbon scenario.

[Elucidating the Impacts of Meteorology and Emission Changes on Concentrations of Major Air Pollutants in Major Cities in the Yangtze River Delta Region Using a Machine Learning De-weather Method].

This study applied a de-weather method based on a machine learning technique to quantify the contribution of meteorology and emission changes to air quality from 2015 to 2021 in four cities in the Yangtze River Delta Region. The results showed that the significant reductions in PM2.5, NO2, and SO2 emissions(57.2%-68.2%, 80.7%-94.6%, and 81.6%-96.1%, respectively) offset the adverse effects of meteorological conditions, resulting in lower pollutant concentrations. The meteorological contribution of maximum daily 8-h average O3(MDA8_O3) showed a stronger effect than that of others(23.5%-42.1%), and meteorological factors promoted the increase in MDA8_O3 concentrations(4.7%); however, emission changes overall resulted in a decrease in MDA8_O3 concentrations(-3.2%). NO2 and MDA8_O3 decreased more rapidly from 2019 to 2021, mainly because the emissions played a stronger role in reducing pollutant concentrations than from 2015 to 2018. However, emissions changes had weaker reduction effects on PM2.5 and SO2 from 2019 to 2021 than from 2015 to 2018. De-weather methods could effectively seperate the effects of meteorology and emission changes on pollutant trends, which helps to evaluate the real effects of emission control policies on pollutant concentrations.

[Analysis of O3 Pollution Characteristics, Formation Sensitivity, and Transport Impact in Southern Nanjing].

As one of the most important city clusters in China, surface ozone (O3) pollution in the Yangtze River Delta (YRD) Region has become a prominent air quality problem in recent years. Online observations of ambient O3, nitrogen oxides (NOx), and volatile organic compounds (VOCs) were conducted in southern Nanjing from July-September 2020 and April-May 2021. On this basis, the pollution characteristics of O3 were analyzed. The O3-VOCs-NOx sensitivity and the transport influence of its precursors were further discussed using models. The frequency statistics of the daily maximum 8-hour moving average (DMA-8h O3) and hourly concentration (O3-1h) showed that O3 pollution in southern Nanjing was more serious than that in urban areas. Three typical O3 pollution episodes were selected during the whole observation period, which included August 16th-27th, 2020; September 3rd-11th, 2020; and May 17th-25th, 2021. The O3-VOCs-NOx sensitivities in these three pollution episodes were analyzed using the observation-based model (OBM). The results of the relative incremental reactivity (RIR) and empirical kinetics modeling approach (EKMA) showed that the sensitivities of O3 formation in the three pollution episodes were in the NOx-limited regime, the transition regime, and the VOCs-limited regime, respectively. This difference in O3-VOCs-NOx sensitivity reflects that the site may have been influenced by transport. Therefore, the potential source contribution function (PSCF) and the concentration weighted trajectory (CWT) method were further used to analyze the potential source areas of NOx, VOCs, and O3 in these three pollution episodes. The results showed that there were obvious regional transport effects of NOx, VOCs, and O3 in these three pollution episodes. The location of potential sources differed in these three pollution episodes, which were affected by the eastern cities of the Lishui site; the urban area of Nanjing and eastern area of Anhui Province; and the urban area of Nanjing and the middle of the YRD Region, respectively. The impact of transport on O3 and its precursors also indicated the necessity of regional joint prevention and control of O3 pollution in the YRD Region.

[Transport Influence and Potential Sources of PM2.5 Pollution for Nanjing].

In this study, 24-hour backward trajectories of the air mass in Nanjing were calculated by using the HYSPLIT model with the NCEP global reanalysis data from March 2019 to February 2020. The backward trajectories combined with the hourly concentration data of PM2.5 were then utilized in the trajectory clustering analysis and potential pollution source analysis. The results showed that the average concentration of PM2.5 in Nanjing was(36±20) µg·m-3 during the study period, with 17 days exceeding the grade Ⅱ national ambient air quality standards (75 µg·m-3). PM2.5 concentration exhibited clear seasonal variation, with winter (49 µg·m-3)>spring (42 µg·m-3)>autumn (31 µg·m-3)>summer (24 µg·m-3). PM2.5 concentration was significantly positively correlated with surface air pressure but significantly negatively correlated with air temperature, relative humidity, precipitation, and wind speed. Based on the trajectories, seven transport routes were identified in spring, and six routes for the other seasons. The northwest and south-southeast routes in spring, southeast route in autumn, and southwest route in winter were the main pollution transport routes in each season, with the characteristics of short transport distance and slow air mass movement, indicating that local accumulation was one of the main reasons for the high value of PM2.5 in quiet and stable weather. The distance of the northwest route in winter was large, and the PM2.5 concentration was 58 µg·m-3, which was the 2nd highest concentration in all routes, indicating that the cities in the northeast of Anhui had a great transport influence on Nanjing PM2.5. The distribution of PSCF and CWT was relatively consistent, and the main potential source areas were mainly local and adjacent areas of Nanjing, indicating that PM2.5 control is needed to strengthen local control and carry out joint prevention and control with adjacent areas. Winter was most affected by transport, its main potential source area was located at the junction of northwest Nanjing and Chuzhou, and the main source origin was in Chuzhou; therefore joint prevention and control should be expanded to Anhui.

Understanding the nocturnal ozone increase in Nanjing, China: Insights from observations and numerical simulations.

Surface ozone (O3) is mainly photochemically formed by nitrogen oxides (NOX) and volatile organic compounds (VOCs), and therefore O3 usually has a distinct diurnal variation with high concentrations in the afternoon and low values at night. However, eight nocturnal O3 increase (NOI) events were identified in Nanjing in June 2021. To understand the mechanism of NOI events, we selected two events (June 6-7, and 24-25) for observational data analysis. The Community Multiscale Air Quality (CMAQ) model was employed for the process analysis (PA) and regional transport of O3. By analyzing the O3 observation data and meteorological factors, we found that there were clear southeastward O3 transport paths. The O3 peak clearly moved from the upwind to the downwind cities in both events. Model simulations showed that when nocturnal O3 enhancement occurred, horizontal transport resulted in a negative to positive net O3 production rate. O3 continued to get accumulated in Nanjing. Nocturnal O3 in the first event was dominated by long-range transport, with the top two contributing cities being Huzhou (5.6 %) and Jiaxing (4.7 %). NOI during the second event was from the nearby upwind cities. The top three contributing cities were Shanghai (18.3 %), Wuxi (9.1 %), and Suzhou (8.8 %). We conclude that the June NOI events in Nanjing were mainly driven by the horizontal transport of southeasterly winds. This study provides scientific support for O3 prevention and control in Nanjing in the summer.

Diagnostic analysis of regional ozone pollution in Yangtze River Delta, China: A case study in summer 2020.

A regional ozone (O3) pollution event occurred in the Yangtze River Delta region during August 17-23, 2020 (except on August 21). This study aims to understand the causes of O3 pollution during the event using an emission-based model (i.e., the Community Multiscale Air Quality (CMAQ) model) and an observation-based model (OBM). The OBM was used to investigate O3 sensitivity to its precursors during the O3 pollution, concluding that O3 formation was limited by volatile organic compounds (VOCs) on August 19, but was co-limited by VOCs and nitrogen oxides (NOx) on other polluted days. Aromatics and alkenes were the two main VOC groups contributing to the O3 formation, with trans-2-butene and m/p-xylene as the key species among the VOCs measured at the Nanjing urban site. The source apportionment results estimated using the source-oriented CMAQ model suggest that the transportation and industry sources dominated the non-background O3 production in Nanjing, which were responsible for 52% and 24.7%, respectively. The O3 concentration attributed to NOx (~70%) was significantly higher than that attributed to VOCs (approximately 30%). The process analysis revealed that vertical mixing increased the O3 concentrations in the early morning, and photochemical reactions promoted O3 formation and accumulation during the daytime within the planetary boundary layer. At night, outflow from horizontal transport and nocturnal chemistry jointly resulted the O3 depletion. The contributions of inter-city transport during the O3 pollution period in Nanjing were also estimated. The predicted O3 concentration was largely recorded from long-distance regions, reaching 46%, followed by local sources (38%) and surrounding cities (16%). The results indicate that both NOx and VOCs contributed significantly to O3 pollution during this event, and the emissions controls of NOx and the key VOC species of aromatics and alkenes from a cooperative regional perspective should be considered to mitigate O3 pollution.

Reconciling the bottom-up methodology and ground measurement constraints to improve the city-scale NMVOCs emission inventory: A case study of Nanjing, China.

Reliable emission estimate of non-methane volatile organic compounds (NMVOCs) is important for understanding the atmospheric chemistry and formulating control policy of ozone (O3). In this study, a speciated emission inventory of anthropogenic NMVOCs was developed with the refined "bottom-up" methodology and best available information of individual sources for Nanjing in 2017. The total NMVOCs emissions were calculated at 163.2 Gg. It was broken down into the emissions of over 500 individual species and aromatics took the largest fraction (33.3% of the total emissions). Meanwhile, 105 compounds were measured at 5 sites representing different functional zones of Nanjing for one year. The annual mean concentration of totally 105 species varied from 48.5 ppbv to 63.7 ppbv, and alkanes was the most abundant group with its mass fractions ranging 37.2-40.1% at different sites. Constrained by the emission ratios of individual species versus carbon monoxide (CO) based on ground measurement, the total emissions of 105 species (NMVOCs-105) was estimated at 195.6 Gg, 81.1% larger than the bottom-up estimate of NMVOCs-105 (108.0 Gg). The constrained emissions indicated an overestimation of aromatics and underestimation of OVOCs and halocarbons in the bottom-up emission inventory because of the uncertainties in source profiles. O3 simulation with Community Multi-scale Air Quality (CMAQ) model was conducted for January, April, July and October in 2017 to evaluate the bottom-up and constrained emission estimates. The mean normal bias (MNB) and mean normal error (MNE) values were generally within the criteria (MNB ≤ ±15% and MNE ≤ 30%) for both inventories. The model performance was improved when the constrained estimates were applied, indicating the benefit of ground observation constraints on NMVOCs emission estimation and O3 simulation. Based on the O3 formation potential (OFP), 12 key NMVOCs species mainly from surface coating, on-road vehicles and oil exploitation and refinery were identified as the priority compounds for O3 reduction.

Real-world emissions of construction mobile machines and comparison to a non-road emission model.

This study implemented real-world tests in Nanjing, China for measuring emission factors (EFs) of air pollutants, including Carbon Monoxide (CO), Hydrocarbon (HC), Nitrogen Oxides (NOx), and Particulate Matter (PM) from ten construction machines in three operational modes (idling, moving, and working) with a Portable Emission Measurement System. The idling mode shows the least variation of EFs, and its average CO EFs can be higher than the moving and working modes by 43% and 34%, respectively. The working mode generates the highest emission for all other pollutants with the highest variation. The EFs suggested by the Guide (an official guidebook for developing emission inventory in China) are in general lower than the measured EFs, and the gap becomes larger for older machines. The EFs of CO, NOx, and PM of China Stage II machines are 24%, 120%, and 66% higher than those of the Guide, respectively. The differences go up as high as 126%, 1066%, and 559% for China Stage I machines, indicating the upgrade of engine technology from Stage I to Stage II, as well as the effect of machine deterioration. The result of this study reveals the effectiveness of stringent emission standards in controlling emissions from construction machines. High emissions from older machines emphasize the importance of a more rigorous machine replacement policy and a regulated maintenance strategy. The result also stresses the need to update the Guide with differentiated activity modes, region variations, and machine deterioration effects.

[Transport Influence and Potential Sources of Ozone Pollution for Nanjing During Spring and Summer in 2017].

In this study, the 24-hour backward trajectories of air mass at ground level(10 m)in Nanjing were calculated by using the HYSPLIT model with the NCEP global reanalysis data from April 1st to October 31st, 2017. The backward trajectories were then combined with the hourly concentration data of O3 in Nanjing for trajectories clustering analysis and potential pollution sources analysis. The results show that in 2017, the maximum daily 8 h running average O3 level in Nanjing was around 12-261 µg·m-3 with 58 days of O3 pollution in Nanjing, mainly in the spring and summer. The monthly variation of O3 showed a single peak, with the highest O3 concentration, as well as the most days exceeding the standard, occurring in June; the diurnal variation of O3 was unimodal and reached its peak around 14:00. A total number of 5136 trajectories were obtained by simulation, among which the exceeded trajectories accounted for approximately 10%. The exceedance trajectories in May and June were significantly higher, accounting for 60% of the total exceedance trajectories. Six ground-level air mass transporting pathways were identified through clustering analysis, from the NNE, NW, SW, SSE, SE, and NE directions. The SE and SSE directions with higher O3 levels were the dominant transport routes of O3 pollution, contributing to 23.33% and 20.76% of backward trajectories, respectively. As for the potential pollution source analysis, the area with high WCWT value distribution matched the WPSCF result, indicating that the potential sources of O3 pollution were mainly distributed in Changzhou, Wuxi, Suzhou, Huzhou, and other cities around Taihu Lake. Additionally, cities located around Nanjing, such as Taizhou, Ma'anshan, Wuhu, Chuzhou, Nantong, and Lianyungang, were considered the secondary potential sources. The results indicate that O3 pollution in Nanjing is a regional issue and its control requires joint prevention and control strategies in the Yangtze River Delta.

Impact of COVID-19 lockdown on ambient levels and sources of volatile organic compounds (VOCs) in Nanjing, China.

A lot of restrictive measures were implemented in China during January-February 2020 to control rapid spread of COVID-19. Many studies reported impact of COVID-19 lockdown on air quality, but little research focused on ambient volatile organic compounds (VOCs) till now, which play important roles in production of ozone and secondary organic aerosol. In this study, impact of COVID-19 lockdown on VOCs mixing ratios and sources were assessed based on online measurements of VOCs in Nanjing during December 20, 2019-Feburary 15, 2020 (P1-P2) and April 15-May 13, 2020 (P3). Average VOCs levels during COVID-19 lockdown period (P2) was 26.9 ppb, about half of value for pre-lockdown period (P1). Chemical composition of VOCs also showed significant changes. Aromatics contribution during decreased from 13% during P1 to 9% during P2, whereas alkanes contribution increased from 64% to 68%. Positive matrix factorization (PMF) was then applied for non-methane hydrocarbons (NMHCs) sources apportionment. Five sources were identified, including a source related to transport and background air masses, three sources related to petrochemical industry or chemical industry (petrochemical industry#1-propene/ethene, petrochemical industry#2-C7-C9 aromatics, and chemical industry-benzene), and a source attributed to gasoline evaporation and vehicular emission. During P2, NMHCs levels from petrochemical industry#2-C7-C9 aromatics showed the largest relative decline of 94%, followed by petrochemical industry#1-propene/ethene (67%), and gasoline evaporation and vehicular emission (67%). Furthermore, ratios of OH reactivity of NMHCs versus NO2 level (ROH,NMHCs/NO2) and total oxidant production rate (P (OX)) were calculated to assess potential influences of COVID-19 lockdown on O3 formation.

A new approach to evaluate regional inequity determined by PM2.5 emissions and concentrations.

PM2.5 is one of the most severe types of air pollution that threatens human health. Its emissions have a notable spillover effect once released into the atmosphere and transported. In domestic trade, PM2.5 emissions can be indirectly imported from external regions. Thus, regional inequity caused by PM2.5 needs to be integrated and comprehensively estimated. Based on PM2.5 emissions/concentrations grid maps and an input-output model, this study first examined the temporal-spatial changes in PM2.5 emissions/concentrations across China. Additionally, a detailed relationship between PM2.5 emissions and concentrations was examined at multiple scales, both temporal and spatial. Finally, this study developed a new approach with which to evaluate regional inequity. The results show that PM2.5 emissions and concentrations increased between 1990 and 2012 and 1998 and 2016, respectively; the increase was more obvious for PM2.5 emissions. Spatially, a rapid increase in PM2.5 emissions was observed in the North China Plain and the Sichuan Basin. Between 1998 and 2012, the distribution of PM2.5 concentrations was similar to that of emissions; however, between 2013 and 2016, 46.6% of the total area showed a decrease, mainly in the central and southern parts of China. Relationship analysis revealed that PM2.5 emissions and concentrations are closely correlated in both time and space. There was obvious regional inequity among provinces; developed regions always imported considerably more PM2.5 emissions from undeveloped regions than they exported. Overall, the regional inequity estimation framework shows that provinces along the coastline, especially developed provinces, have advantages under the regional inequity estimation framework, while most of the inland regions have disadvantages, especially in the west and north.

Impacts of model resolution on predictions of air quality and associated health exposure in Nanjing, China.

Air quality models have been used in health studies to provide spatial and temporal information of various air pollutants. Model resolution is an important factor affecting the accuracy of exposure assessment using model predictions. In this study, the WRF/CMAQ model system was applied to quantitatively estimate the impacts of the model resolution on the predictions of air quality and associated health exposure in Nanjing, China in 2016. Air quality was simulated with a grid resolution of 1, 4, 12, and 36 km respectively. Predictions with 1 or 4 km resolution are slightly better for particulate matter with an aerodynamic diameter ≤2.5 µm (PM2.5) and its compositions and predictions with 12 km are slightly better for daily 8-h maximum ozone (O3-8 h). Model resolution does not significantly improve predictions for PM2.5 and O3-8 h in Nanjing, however, the spatial distributions of PM2.5 and O3-8 h are better captured with finer resolutions. Population weighted concentrations (PWCs) of PM2.5 with different model resolutions are similar to the average of observations, but PWCs of O3-8 h with all resolutions are obviously larger than the observations, indicating that the current sites may well represent the population exposure to PM2.5, but under-estimate the exposure to O3. Model resolution results in about 6% in the estimated premature mortality due to exposure to PM2.5 but more than 20% difference in premature mortality due to exposure to O3. Future studies are needed to evaluate the impacts of the resolution on the exposure of PM2.5 compositions in the city scale when PM2.5 composition measurements available at multiple sites.

[Scenario Simulation Study Constrained by the Ambient Air Quality Standards in Nanjing].

With the constraint that all six major pollutants in Nanjing must meet the air quality standards by 2030, on the basis of the 2015 emission inventory, the CMAQ air quality model was used to conduct PM2.5 sensitivity tests, and scenario analysis was used to predict the emission inventory and the air quality of four emission reduction scenarios were simulated. Finally, the total control index under the constraint of meeting the standards was obtained. The results show that primary particulate matter (PPM) reduction is the most effective at reducing the concentration of PM2.5 in the atmosphere, on the basis of emission reduction in surrounding areas, PPM emission reduction accounts for 88% of the total reduction of the annual average concentration of PM2.5, followed by NH3, NOx, SO2, and VOCs, which contribute to 10.3%, 5.5%, 3.2%, and 0.5%, respectively. Compared to 2015, the reduction ratios of the major pollutants are between 22% and 53%. Controlling the activity level is more effective for SO2, NH3 and CO emissions reduction, while there is still more opportunity for NOx and VOCs end treatment. When the emissions of SO2, NOx, PM10, PM2.5, BC, OC, CO, VOCs, and NH3 are controlled to 2.43×104, 8.47×104, 9.42×104, 3.74×104, 0.19×104, 0.30×104, 26.56×104, 13.08×104, and 1.50×104 t, respectively, it is expected that the levels of the six pollutants in Nanjing can meet the national ambient air quality level 2 standards.