The impacts of ship emissions on ozone in eastern China

Tropospheric ozone (O3), which is one of the main pollutants impeding air quality compliance, has received considerable attention in China. As maritime transportation continues to expand, the effect of ship emissions on air quality is becoming increasingly important. In this study, the Weather Research and Forecast model (WRF), the Community Multiscale Air Quality model (CMAQ), and the integrated process rate (IPR) module provided in the CMAQ are applied to evaluate the impacts of ship emissions on O3 concentration at a national scale in China, including the spatiotemporal characteristics and influencing pathways. Ship emissions can increase or decrease O3 concentrations, with varying effects in different seasons and regions. In the winter, spring, and fall, ship emissions were predicted to decrease O3 concentrations in most areas, whereas in the summer, they increase the O3 concentration, even in regions far away from the coastline, thus adversely affecting the Yangtze River Delta (YRD) and Pearl River Delta (YRD). Additionally, owing to differences in the emissions of volatile organic compounds and nitrogen oxides, the northern and southern regions of the YRD respond differently to ship emissions. Additionally, the influence of ship emissions on the diurnal variation of O3 in the summer was investigated, where significant differences were indicated between cities. The IPR was used to investigate the individual processes contributing to changes in the O3 concentration caused by ship emissions. The transport process appears to be the primary contributor to O3 production, whereas chemistry and dry deposition played key roles in O3 loss. This study provides an in-depth insight into the impacts of ship emissions on O3 in China, which can facilitate the formulation of corresponding environmental policies.

Chronic and acute health effects of PM2.5 exposure and the basis of pollution control targets

Ho Chi Minh City (HCMC) is changing and expanding quickly, leading to environmental consequences that seriously threaten human health. PM2.5 pollution is one of the main causes of premature death. In this context, studies have evaluated strategies to control and reduce air pollution; such pollution-control measures need to be economically justified. The objective of this study was to assess the socio-economic damage caused by exposure to the current pollution scenario, taking 2019 as the base year. A methodology for calculating and evaluating the economic and environmental benefits of air pollution reduction was implemented. This study aimed to simultaneously evaluate the impacts of both short-term (acute) and long-term (chronic) PM2.5 pollution exposure on human health, providing a comprehensive overview of economic losses attributable to such pollution. Spatial partitioning (inner-city and suburban) on health risks of PM2.5 and detailed construction of health impact maps by age group and sex on a spatial resolution grid (3.0 km × 3.0 km) was performed. The calculation results show that the economic loss from premature deaths due to short-term exposure (approximately 38.86 trillion VND) is higher than that from long-term exposure (approximately 14.89 trillion VND). As the government of HCMC has been developing control and mitigation solutions for the Air Quality Action Plan towards short- and medium-term goals in 2030, focusing mainly on PM2.5, the results of this study will help policymakers develop a roadmap to reduce the impact of PM2.5 during 2025-2030.

Assessment of rice yield and economic losses caused by ground-level O3 exposure in the Mekong delta region, Vietnam

The Lower Mekong Delta region (LMD) accounts for 90% of Vietnam's rice exports; however, the air quality in the LMD is remarkably reduced by ground-level ozone (O₃) pollution. This study aimed to quantify the relative yield and economic value losses in rice-growing crop seasons affected by ground-level O₃ concentrations across the LMD. The results of this study can serve as a basis for extensive assessments for the following years and support environmental managers to propose control measures of O₃ precursor emissions (NO_x and VOCs) from man-made sectors, as well as build protective solutions for rice farming in LMD. Two ground-level O₃ exposure metrics of M7 and AOT40 reflecting ground-level O₃ pollution impacts, combined with the model of exposure-relative yield relationship (or surface O₃-crop models), were used to assess losses of crop production (CPL) and economic cost losses (ECL) caused by rice crop yield reductions. For the M7 metric of ground-level O₃ exposure, the average value was 14.746 ppbV, with levels ranging from 13.959 ppbV to 15.502 ppbV, and the affected area was spread across 1309.39 thousand hectares. The AOT40 exposure metric reached an average value of 11.490 ppbV, with a range of 0.000-31.665 ppbV. The highest exposure level was 17.503-31.653 ppbV, covering an area of 747.01 thousand hectares. The total CPL of the three rice crops over the LMD was 9593.52 tonnes (accounting for 0.039% of the total value of rice production in the region), with a total corresponding EPL of 62.405 billion VND (equivalent to 2761.01 thousand USD). The results are considered a baseline study to serve as a basis for extensive assessments for the following years and support for the environmental managers to propose control measures of O₃ precursor emissions (NO_x and VOCs) from man-made sectors as well as build protective solutions in rice farming in LMD shortly.

Building and optimising an automatic monitoring system network for outdoor PM2.5: a case study of Ho Chi Minh City

PM2.5 exposure data are important for air quality management. Optimal planning and determination of locations where PM2.5 is continuously monitored are important for urban areas in Ho Chi Minh City (HCMC), a megacity with specific environmental problems. Objectives of the study to propose an automatic monitoring system network (AMSN) to measure outdoor PM2.5 concentrations in HCMC using low-cost sensors. Data related to the current monitoring network, population, population density, threshold reference standards set by the National Ambient Air Quality Standard (NAAQS) and the World Health Organisation (WHO), and inventory emissions from various sources, both anthropogenic and biogenic, were obtained. Coupled WRF/CMAQ models were used to simulate PM2.5 concentrations in HCMC. The simulation results were extracted from the grid cells, from which the values of points exceeding the set thresholds were determined. The population coefficient was calculated to determine the corresponding total score (TS). Optimisation of the monitoring locations was statistically performed using Student's t-test to select the official locations for the monitoring network. TS values ranged from 0.0031 to 3215.9. The TSmin value was reached in the Can Gio district and the TSmax value was reached in SG1. Based on the t-test results, 26 initial locations were proposed for a preliminary configuration, from which 10 optimal monitoring sites were selected to develop the AMSN of outdoor PM2.5 concentration measurements in HCMC towards 2025.

Ground-level ozone in the Mekong Delta region: precursors, meteorological factors, and regional transport

The Mekong Delta region (MDR), also known as Vietnam's rice bowl, produced a bountiful harvest of about 23.8 million tons in 2020, accounting for 55.7% of the country's total production, providing food security for 20% of the world population. With the rapid pace of industrialisation and urbanisation, the concentration of ozone in the lower atmosphere has risen to a level that reduces crop yields, especially rice, and is therefore the subject of research. This study aims to simulate the spatiotemporal distribution of ground-level ozone in the area and evaluate the impact of precursor emissions and meteorological factors on the spatiotemporal distributions of ozone concentrations. The study area was divided into seven zones, including six agro-ecological zones (AEZs) and one low-mountainous area, mainly to clarify the role of emissions in each AEZ. The simulation results showed that ground-level O₃ in the MDR ranged from 40.39 to 52.13 µg/m³. In six agro-ecological zones, the average annual ground-level O₃ concentration was relatively high and was the highest in zone 6 (CPZ) and zone 3 (LXZ) with values of 96.18 µg/m³ (exceeding 1.60 times the WHO Guidelines 2021) and 94.86 µg/m³ (exceeding 1.58 times the WHO Guidelines 2021), respectively. In each zone, the annual average O₃ concentration tended to gradually increase from the inner delta to coastal areas. Two types of precursors, NO_x and NMVOCs, are the main contributors to O₃ pollution, with the largest contribution coming from zone 1 (FAZ) with 91.5 thousand tons of NO_x/year and 455.2 thousand tons of NMVOCs/year. Among the meteorological factors considered, temperature (T), relative humidity (RH), and surface pressure (P) were the three main factors that contributed to the increase in ground-level ozone. The spatio-temporal distribution of ground-level O₃ in the MDR was influenced by emission precursors from different zones as well as meteorological factors. The present results can help policy-makers formulate plans for agro-industrial development in the entire region.

The impact of ship emissions on PM2.5 and the deposition of nitrogen and sulfur in Yangtze River Delta, China

Ship emissions contribute significantly to the deterioration of air quality, while their impacts on ambient PM_2.5 and depositions have not been comprehensively evaluated. This is especially true for China because it has a long coastline, busy shipping routes and many large ports. To fill this gap, this study applied the SMOKE/WRF/CMAQ modeling system to quantifying the impacts of ships on PM_2.5 compositions, annual and seasonal contribution to PM_2.5 as well as the wet and dry deposition of nitrogen and sulfur compounds over the land areas in YRD region for 2014. The results showed that 4.0% of annual PM_2.5 concentrations over the land areas could be explained by ship emissions and the largest contribution could reach up to 35.0% in port areas. Temporally, the contribution to PM_2.5 exhibited an obviously seasonal variation. The highest contribution was predicted in autumn (6.2%), followed by summer (5.4%), spring (3.6%) and winter (1.2%) for the land areas. Spatially, the contribution reached up to 13.6% along the coastline and dropped to 2.1% 300 km inland. As for the impacts on PM_2.5 components, the primary components were relatively small and increased mainly along the shipping routes and the Yangtze River, whereas the secondary components played a more important role in both water and land areas. The sulfur deposition due to ship emissions was occurred generally along the shipping routes and was dominated by the dry SO₂ deposition. The nitrogen depositions, on the contrary, was observed not only along the shipping routes but also extend to wide land areas. Further investigation revealed that ship emissions have caused an evident increase of dry nitrogen deposition in NO₂ and HNO₃, while a slight decrease in NH₃ over YRD region. These results indicated that comprehensive regulations of ship emissions are required considering their adverse effects on the ambient concentration of PM_2.5 and the deposition of sulfur and nitrogen.

Numerical simulations for the sources apportionment and control strategies of PM2.5 over Pearl River Delta, China, part I: Inventory and PM2.5 sources apportionment

This article uses the WRF-CMAQ model to systematically study the source apportionment of PM_2.5 under typical meteorological conditions in the dry season (November 2010) in the Pearl River Delta (PRD). According to the geographical location and the relative magnitude of pollutant emission, Guangdong Province is divided into eight subdomains for source apportionment study. The Brute-Force Method (BFM) method was implemented to simulate the contribution from different regions to the PM_2.5 pollution in the PRD. Results show that the industrial sources accounted for the largest proportion. For emission species, the total amount of NO_x and VOC in Guangdong Province, and NH₃ and VOC in Hunan Province are relatively larger. In Guangdong Province, the emission of SO₂, NO_x and VOC in the PRD are relatively larger, and the NH₃ emissions are higher outside the PRD. In northerly-controlled episodes, model simulations demonstrate that local emissions are important for PM_2.5 pollution in Guangzhou and Foshan. Meanwhile, emissions from Dongguan and Huizhou (DH), and out of Guangdong Province (SW) are important contributors for PM_2.5 pollution in Guangzhou. For PM_2.5 pollution in Foshan, emissions in Guangzhou and DH are the major contributors. In addition, high contribution ratio from DH only occurs in severe pollution periods. In southerly-controlled episode, contribution from the southern PRD increases. Local emissions and emissions from Shenzhen, DH, Zhuhai-Jiangmen-Zhongshan (ZJZ) are the major contributors. Regional contribution to the chemical compositions of PM_2.5 indicates that the sources of chemical components are similar to those of PM_2.5. In particular, SO₄^2- is mainly sourced from emissions out of Guangdong Province, while the NO^3- and NH⁴⁺ are more linked to agricultural emissions.

Numerical simulations for the sources apportionment and control strategies of PM2.5 over Pearl River Delta, China, part II: Vertical distribution and emission reduction strategies

The contribution of various emission sources to the vertical structure of the PM_2.5 concentration and the modeling of emission reduction strategies are emphasized in this study. Analysis of vertical distribution of PM_2.5 concentration in the planetary boundary layer (PBL) reveals that strong diurnal cycle exists during the pollution episodes, with heavier surface pollution in nocturnal periods. Contributions from transportation and agriculture are mainly restricted to the surface, while contributions from industry and power are distributed in a relatively higher layer. In the northerly-controlled episodes, the contribution of local emissions mainly accumulates below 300 m and the impact of the emissions from surrounding cities can reach 500-600 m during nocturnal periods. The contributions outside of Guangdong are uniformly distributed within 1000 m altitude. In the daytime, the contribution of emissions is basically uniform throughout the PBL. In the southerly-controlled episodes, the contribution of local emission mainly concentrates below 400 m during the nocturnal periods. Emissions from surrounding cities can exert the influence below 1000 m height, and the contribution outside of Guangdong reaches even 1500 m. In the daytime, the contribution of emissions in the PBL is distributed evenly. The highest altitude of the contribution from different subdomains that can reach is closely related to the physical property of the PBL. The industrial and agricultural emissions are the most important contributors for the surface PM_2.5 concentration. Results from emission reduction experiments show that PM_2.5 reduces significantly near the pollution center. Although control efficiency decreases with the increasing reduction ratio, the efficiency differences between 30% and 50% reduction is limited. In particular, 10% reduction in industrial emission causes PM_2.5 concentration to be slightly higher in the afternoon. Furthermore, below 200-m height, emission reduction experiments perform the effective reduction in PM_2.5 concentration, and higher reduction ratio results in larger reduced PM_2.5 concentration on almost all layers in the PBL.

Impacts of transportation sector emissions on future U.S. air quality in a changing climate. Part I: Projected emissions, simulation design, and model evaluation

Emissions from the transportation sector are rapidly changing worldwide; however, the interplay of such emission changes in the face of climate change are not as well understood. This two-part study examines the impact of projected emissions from the U.S. transportation sector (Part I) on ambient air quality in the face of climate change (Part II). In Part I of this study, we describe the methodology and results of a novel Technology Driver Model (see graphical abstract) that includes 1) transportation emission projections (including on-road vehicles, non-road engines, aircraft, rail, and ship) derived from a dynamic technology model that accounts for various technology and policy options under an IPCC emission scenario, and 2) the configuration/evaluation of a dynamically downscaled Weather Research and Forecasting/Community Multiscale Air Quality modeling system. By 2046-2050, the annual domain-average transportation emissions of carbon monoxide (CO), nitrogen oxides (NO_x), volatile organic compounds (VOCs), ammonia (NH₃), and sulfur dioxide (SO₂) are projected to decrease over the continental U.S. The decreases in gaseous emissions are mainly due to reduced emissions from on-road vehicles and non-road engines, which exhibit spatial and seasonal variations across the U.S. Although particulate matter (PM) emissions widely decrease, some areas in the U.S. experience relatively large increases due to increases in ship emissions. The on-road vehicle emissions dominate the emission changes for CO, NO_x, VOC, and NH₃, while emissions from both the on-road and non-road modes have strong contributions to PM and SO₂ emission changes. The evaluation of the baseline 2005 WRF simulation indicates that annual biases are close to or within the acceptable criteria for meteorological performance in the literature, and there is an overall good agreement in the 2005 CMAQ simulations of chemical variables against both surface and satellite observations.

Impacts of transportation sector emissions on future U.S. air quality in a changing climate. Part II: Air quality projections and the interplay between emissions and climate change

In Part II of this work we present the results of the downscaled offline Weather Research and Forecasting/Community Multiscale Air Quality (WRF/CMAQ) model, included in the "Technology Driver Model" (TDM) approach to future U.S. air quality projections (2046-2050) compared to a current-year period (2001-2005), and the interplay between future emission and climate changes. By 2046-2050, there are widespread decreases in future concentrations of carbon monoxide (CO), nitrogen oxides (NO_x = NO + NO₂), volatile organic compounds (VOCs), ammonia (NH₃), sulfur dioxide (SO₂), and particulate matter with an aerodynamic diameter ≤ 2.5 μm (PM_2.5) due mainly to decreasing on-road vehicle (ORV) emissions near urban centers as well as decreases in other transportation modes that include non-road engines (NRE). However, there are widespread increases in daily maximum 8-hr ozone (O₃) across the U.S., which are due to enhanced greenhouse gases (GHG) including methane (CH₄) and carbon dioxide (CO₂) under the Intergovernmental Panel on Climate Change (IPCC) A1B scenario, and isolated areas of larger reduction in transportation emissions of NO_x compared to that of VOCs over regions with VOC-limited O₃ chemistry. Other notable future changes are reduced haze and improved visibility, increased primary organic to elemental carbon ratio, decreases in PM_2.5 and its species, decreases and increases in dry deposition of SO₂ and O₃, respectively, and decreases in total nitrogen (TN) deposition. There is a tendency for transportation emission and CH₄ changes to dominate the increases in O₃, while climate change may either enhance or mitigate these increases in the west or east U.S., respectively. Climate change also decreases PM_2.5 in the future. Other variable changes exhibit stronger susceptibility to either emission (e.g., CO, NO_x, and TN deposition) or climate changes (e.g., VOC, NH₃, SO₂, and total sulfate deposition), which also have a strong dependence on season and specific U.S. regions.

Factors dominating 3-dimensional ozone distribution during high tropospheric ozone period

Data from an in situ monitoring network and five ozone sondes are analysed during August of 2012, and a high tropospheric ozone episode is observed around the 8th of AUG. The Community Multi-scale Air Quality (CMAQ) model and its process analysis tool were used to study factors and mechanisms for high ozone mixing ratio at different levels of ozone vertical profiles. A sensitive scenario without chemical initial and boundary conditions (ICBCs) from MOZART4-GEOS5 was applied to study the impact of stratosphere-troposphere exchange (STE) on vertical ozone. The simulation results indicated that the first high ozone peak near the tropopause was dominated by STE. Results from process analysis showed that: in the urban area, the second peak at approximately 2 km above ground height was mainly caused by local photochemical production. The third peak (near surface) was mainly caused by the upwind transportation from the suburban/rural areas; in the suburban/rural areas, local photochemical production of ozone dominated the high ozone mixing ratio from the surface to approximately 3 km height. Furthermore, the capability of indicators to distinguish O₃-precursor sensitivity along the vertical O₃ profiles was investigated. Two sensitive scenarios, which had cut 30% anthropogenic NO_X or VOC emissions, showed that O₃-precursor indicators, specifically the ratios of O₃/NOy, H₂O₂/HNO₃ or H₂O₂/NO_Z, could partly distinguish the O₃-precursor sensitivity between VOCs-sensitive and NOx-sensitive along the vertical profiles. In urban area, the O₃-precursor relationship transferred from VOCs-sensitive within the boundary layer to NOx-sensitive at approximately 1-3 km above ground height, further confirming the dominant roles of transportation and photochemical production in high O₃ peaks at the near-ground layer and 2 km above ground height, respectively.

Effectiveness of SO2 emission control policy on power plants in the Yangtze River Delta, China-post-assessment of the 11th Five-Year Plan

Facing the air pollution problems in China, emission control strategies have been implemented within the framework of national Five-Year Plan (FYP). According to the lack of post-assessment studies in the literature, this study assessed the effectiveness of the SO₂ emission control policies on power plants after the 11th FYP (2006-2010) by modeling emission control scenarios. The idealized emission control policy (the PS90 scenario with assumption of 90% SO₂ emission reduction from power plants) could reduce the SO₂ and SO₄^2- concentrations by about 51 and 14%, respectively, over the Yangtze River Delta region. While the actual emission control condition (the P2010 scenario based on the actual emissions from power plants in 2010) demonstrated that the actual reduction benefits were 30% of SO₂ and 9% of SO₄^2-. On the city scale, the P2010 scenario imposed positive benefits on Shanghai, Nanjing, Nantong, and Hangzhou with SO₂ reductions of about 55, 12, 30, and 21%, respectively, while an 11% increase of SO₂ concentration was found in Ningbo. The number of days exceeding China's National Ambient Air Quality Standard of Class I daily SO₂ concentration was estimated to be 75, 52, 7, 77, and 40 days for Shanghai, Nanjing, Nantong, Ningbo, and Hangzhou under the real SO₂ control condition (P2010). The numbers could be decreased by 16, 11, 2, 21, and 11% if the control effect reaches the level of the PS90 scenario. This study serves as a scientific basis to design capable enforcement of emission control strategies in China in the future national plans.

Assessment of regional air quality resulting from emission control in the Pearl River Delta region, southern China

To evaluate the impact of emission control measures on the air quality in the Pearl River Delta (PRD) region of South China, statistic data including atmospheric observations, emissions and energy consumptions during 2006-2014 were analyzed, and a Weather Research and Forecasting - Community Multi-scale Air Quality (WRF-CMAQ) model was used for various scenario simulations. Although energy consumption doubled from 2004 to 2014 and vehicle number significantly increased from 2006 to 2014, ambient SO₂, NO₂ and PM₁₀ were reduced by 66%, 20% and 24%, respectively, mainly due to emissions control efforts. In contrast, O₃ increased by 19%. Model simulations of three emission control scenarios, including a baseline (a case in 2010), a CAP (a case in 2020 assuming control strength followed past control tendency) and a REF (a case in 2020 referring to the strict control measures based on recent policy/plans) were conducted to investigate the variations of air pollutants to the changes in NO_x, VOCs and NH₃ emissions. Although the area mean concentrations of NO_x, nitrate and PM_2.5 decreased under both NO_x CAP (reduced by 1.8%, 0.7% and 0.2%, respectively) and NO_x REF (reduced by 7.2%, 1.8% and 0.3%, respectively), a rising of PM_2.5 was found in certain areas as reducing NO_x emissions elevated the atmospheric oxidizability. Furthermore, scenarios with NH₃ emission reductions showed that nitrate was sensitive to NH₃ emissions, with decreasing percentages of 0-10.6% and 0-48% under CAP and REF, respectively. Controlling emissions of VOCs reduced PM_2.5 in the southwestern PRD where severe photochemical pollution frequently occurred. It was also found that O₃ formation in PRD was generally VOCs-limited while turned to be NO_x-limited in the afternoon (13:00-17:00), suggesting that cutting VOCs emissions would reduce the overall O₃ concentrations while mitigating NO_x emissions in the afternoon could reduce the peak O₃ levels.

Impact of the Loess Plateau on the atmospheric boundary layer structure and air quality in the North China Plain: a case study

The North China Plain (NCP), to the east of the Loess Plateau, experiences severe regional air pollution. During the daytime in the summer, the Loess Plateau acts as an elevated heat source. The impacts of such a thermal effect on meteorological phenomena (e.g., waves, precipitation) in this region have been discussed. However, its impacts on the atmospheric boundary layer structure and air quality have not been reported. It is hypothesized that the thermal effect of the Plateau likely modulates the boundary layer structure and ambient concentrations of pollutants over the NCP under certain meteorological conditions. Thus, this study investigates such effect and its impacts using measurements and three-dimensional model simulations. It is found that in the presence of daytime westerly wind in the lower troposphere (~1 km above the NCP), warmer air above the Loess Plateau was transported over the NCP and imposed a thermal inversion above the mixed boundary layer, which acted as a lid and suppressed the mixed layer growth. As a result, pollutants accumulated in the shallow mixed layer and ozone was efficiently produced. The downward branch of the thermally-induced Mountain-Plains Solenoid circulation over the NCP contributed to enhancing the capping inversion and exacerbating air pollution. Previous studies have reported that low mixed layer, a factor for elevated pollution in the NCP, may be caused by aerosol scattering and absorption of solar radiation, frontal inversion, and large scale subsidence. The present study revealed a different mechanism (i.e., westerly warm advection) for the suppression of the mixed layer in summer NCP, which caused severe O3 pollution. This study has important implications for understanding the essential meteorological factors for pollution episodes in this region and forecasting these severe events.