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Cities face twin challenges of changing climate and urban heat island (UHI) effect, with the environmental implications of urbanization still unclear. This study employs the Weather Research and Forecasting (WRF) model to examine the impacts of urbanization in the Greater Bay Area (GBA) during warm and cold seasons in past (2000) and near-future (2030) scenarios [i.e., under shared socioeconomic pathways (SSP): SSP1-26, SSP2-45, and SSP5-85]. Datasets from the World Urban Database and Access Portal Tools (WUDAPT) were incorporated into the WRF model to improve the representation of urbanization effects on weather patterns. Our findings indicate that urban expansion significantly increases urban temperatures and decreases wind speed across selected climate change scenarios. The spatially averaged urban temperatures in the fine resolution domain for 2030 could rise by 0.9 °C (warm season) and 2.4 °C (cold season), respectively, and urban wind speed decreases by ∼ 3 m/s under the SSP5-85 scenario. In cold season, the UHI effect could last over 20 h, while urban area may experience cooler nighttime temperatures than rural areas in warm season. Moreover, the future scenario predicts higher daytime O 3 levels due to the warming effects of climate change and urbanization, but lower nighttime levels in warm season, attributed to intensified south-easterly sea breeze and background winds, when compared to the past scenario. This study highlights the importance of incorporating urbanization and climate change in future urban atmospheric environment studies, and underscores that urban climate change adaptation and mitigation should consider extra impacts on built-environment caused by urbanization. • Climate change and urbanization impacts on meteorological and O 3 fields were explored in GBA. • Urbanization and climate change are expected to raised temperatures by 0.9 °C in warm season over the study area. • Urbanization and climate change are expected to raised temperatures by 2.4 °C in cold season over the study area. • Urbanization is expected to reduce wind speed by 3 m/s and intensify sea breeze penetration over the study area. [ABSTRACT FROM AUTHOR]

Efforts to enhance greenhouse gas (GHG) emission reduction in East Asia play a pivotal role on both global and regional scales in advancing climate mitigation strategies. This study aimed to better constrain anthropogenic CO 2 emission estimates by expanding the network of near-surface in-situ stations for CO 2 observations across South Korea. To achieve an optimal CO 2 network design, we conducted an Observing System Simulation Experiment (OSSE) coupled with the Stochastic Lagrangian Transport model (STILT), utilizing meteorological data from the Korean Integrated Model (KIM). Our inversion setup incorporated two CO 2 emission datasets with a 0.1o resolution: EDGAR v6 for prior emissions and GRACED for truth emissions. A uniform model-mismatch error of 3 ppm was introduced across sites. The effectiveness of the existing five in-situ stations, termed the base network, in South Korea was evaluated to gauge their ability to constrain CO 2 surface flux estimates. However, the findings revealed a reduction in flux uncertainty of only 29.2%, which fell short of the desired uncertainty reduction goal. In this base network, the Lotte World Tower (LWT: 37.5126°E, 127.1025°E) in Seoul and the Anmyeondo (AMY: 36.538576° N, 126.330071° E) site in Taean county stood as major contributors, with estimated reductions of 17.48% and 6.35%, respectively. Consequently, we proposed and developed an extended network, identifying seven candidate sites based on consideration of logistical factors, existing infrastructures, and proximity to the emission source regions. An incremental optimization scheme ranked their contributions, resulting in an additional 25% reduction, bringing the total to 54.13%. However, it is noteworthy that diminishing returns (ranging from 13% to less than 0.1%) were observed with an increase in station count mainly due to the possibility that adding a station earlier in the sequence might render subsequent stations redundant. Despite this, the proposed CO 2 network successfully reduced uncertainty in emissions, narrowing the gap with the objectives of the Global Greenhouse Gas Watch (G3W). • Designing CO 2 near-surface observation network over South Korea. • New stations selected considering logistic factors, existing infrastructures, and proximity to emission sources. • The base network was able to reduce 29.18% of the uncertainty in CO 2 emissions. • Extending network, by adding new seven candidate sites, led to a total uncertainty reduction of 54.13%. [ABSTRACT FROM AUTHOR]

Hong Kong has used a risk-based Air Quality Health Index (AQHI) system for nearly a decade, which aggregates hospitalization risks associated with short-term exposure to pollutants. Serving as an air pollution risk communication tool, it keeps the public informed about the current air quality, anchoring around the World Health Organization's (WHO) 2005 Air Quality Guidelines (AQGs). Given the WHO's recent update to the AQGs in 2021, it is pertinent to examine how Hong Kong's air quality risk communication might be enhanced using these updated guidelines. The AQHI currently focuses only on short-term health risks, potentially leading to an underestimation of cumulative impacts. To address this, we propose enhancing the AQHI to include both long-term and short-term health risks. Short-term risks are estimated based on excess hospitalization risks from various pollutants using local data, while long-term risks are estimated using beta coefficients from the WHO due to limited local data. We further suggest using the WHO's Interim Targets (ITs) as thresholds for health risk categories, eliminating arbitrary cut-points. Promoting both public health protection and climate mitigation is important but challenging, particularly with the surge in ozone levels due to enhanced NOx controls in VOC-limited regions like Hong Kong and Guangdong. Nevertheless, despite the rise in ozone, combined health risks in these areas have decreased due to reductions in NO 2 and PM concentrations. Shifting to an AQHI-like total health risk assessment emphasizes the harmonization of health protection and climate mitigation goals. This health-focused framework posits that rigorous NOx controls can effectively serve both policy objectives in tandem. • After a decade's review, HK's AQHI advised to report on long-term health risks. • Original short-term AQHI helps daily plans but may overlook crucial long-term risks. • Long-term health risk-based AQHI clarifies air quality trends for easier public grasp. • Aligning AQHI bands with WHO AQGs & ITs ensures clear, transparent risk communication. • Drastic fossil fuel cuts boost health, aid climate, despite temporary ozone rise. [ABSTRACT FROM AUTHOR]

Tropospheric Ozone (O 3) is both an air pollutant and a greenhouse gas. Predicting changes to O 3 is therefore important for both air quality and near-term climate forcing. It is computationally expensive to predict changes in tropospheric O 3 from every possible future scenario in composition climate models like those used in the 6th Coupled Model Intercomparison Project (CMIP6). Here we apply the different emission pathways used in CMIP6 with a model based on source-receptor relationships for tropospheric O 3 to predict historical and future changes in O 3 and its radiative forcing over a 300 year period (1750–2050). Changes in regional precursor emissions (nitrogen oxides, carbon monoxide and volatile organic compounds) and global methane abundance are used to quantify the impact on tropospheric O 3 globally and across 16 regions, neglecting any impact from changes in climate. We predict large increases in global surface O 3 (+8 ppbv) and O 3 radiative forcing (+0.3 W m−2) over the industrial period. Nine different Shared Socio-economic Pathways are used to assess future changes in O 3. Scenarios involving weak air pollutant controls and climate mitigation are inadequate in limiting the future degradation of surface O 3 air quality and enhancement of near-term climate warming over all regions. Middle-of-the-road and strong mitigation scenarios reduce both surface O 3 concentrations and O 3 radiative forcing by up to 5 ppbv and 0.17 W m−2 globally, providing benefits to future air quality and near-term climate forcing. Sensitivity experiments show that targeting mitigation measures towards reducing global methane abundances could yield additional benefits for both surface O 3 air quality and near-term climate forcing. The parameterisation provides a valuable tool for rapidly assessing a large range of future emission pathways that involve differing degrees of air pollutant and climate mitigation. The calculated range of possible responses in tropospheric O 3 from these scenarios can be used to inform other modelling studies in CMIP6. • Large regional response in future tropospheric ozone to CMIP6 scenarios. • Weak mitigation scenarios degrade air quality and enhance climate warming. • Strong mitigation scenarios reduce surface ozone and ozone radiative forcing. • Largest contributions to ozone are from the energy, industry and transport sectors. • Targeting mitigation measures at methane yield additional benefits to ozone. [ABSTRACT FROM AUTHOR]

Abstract A comprehensive overview is provided evaluating direct real-world CO 2 emissions of both diesel and petrol cars newly registered in Europe between 1995 and 2015. Before 2011, European diesel cars emitted less CO 2 per kilometre than petrol cars, but since then there is no appreciable difference in per-km CO 2 emissions between diesel and petrol cars. Real-world CO 2 emissions of diesel cars have not declined appreciably since 2001, while the CO 2 emissions of petrol cars have been stagnant since 2012. When adding black carbon related CO 2 -equivalents, such as from diesel cars without particulate filters, diesel cars were discovered to have had much higher climate relevant emissions until the year 2001 when compared to petrol cars. From 2001 to 2015 CO 2 -equivalent emissions from new diesel cars and petrol cars were hardly distinguishable. Lifetime use phase CO 2 -equivalent emissions of all European passenger vehicles were modelled for 1995–2015 based on three scenarios: the historic case, another scenario freezing percentages of diesel cars at the low levels from the early 1990s (thus avoiding the observed "boom" in new diesel registrations), and an advanced mitigation scenario based on high proportions of petrol hybrid cars and cars burning gaseous fuels. The difference in CO 2 -equivalent emissions between the historical case and the scenario avoiding the diesel car boom is only 0.4%. The advanced mitigation scenario would have been able to achieve a 3.4% reduction in total CO 2 -equivalent emissions over the same time frame. The European diesel car boom appears to have been ineffective at reducing climate-warming emissions from the European transport sector. Graphical abstract Image 1 Highlights • Diesel cars had much higher CO 2 -equivalent emissions than petrol cars until the year 2001. • From 2001 to 2015 CO 2 -equivalent emissions from new diesel cars and petrol cars became hardly distinguishable. • Difference in modelled CO 2 -equivalent emissions between 1995 and 2015 and a scenario for the same period avoiding the diesel car boom is negligible. • An advanced mitigation scenario would have been able to achieve a 3.4% reduction in total CO 2 -equivalent emissions. • The European diesel car boom was ineffective at reducing climate-warming emissions from the European transport sector. [ABSTRACT FROM AUTHOR]

Black carbon mitigation has received increasing attention for its potential contribution to both climate change mitigation and air pollution control. Although different bottom-up models concerned with unit mitigation costs of various technologies allow the assessment of alternative policies for optimized cost-effectiveness, the lack of adequate data often forced many reluctant explicit and implicit assumptions that deviate away from actual situations of rural residential energy consumption in developing countries, where most black carbon emissions occur. To gauge the theory-practice gap in black carbon mitigation – the unit cost differences that lie between what is estimated in the theory and what is practically achieved on the ground – this study conducted an extensive field survey and analysis of nine mitigation technologies in rural China, covering both northern and southern regions with different residential energy consumption patterns. With a special focus on two temporal characteristics of those technologies – lifetimes and annual utilization rates, this study quantitatively measured the unit cost gaps and explain the technical as well as sociopolitical mechanisms behind. Structural and behavioral barriers, which have affected the technologies’ performance, are discussed together with policy implications to narrow those gaps. [ABSTRACT FROM AUTHOR]

Haze pollution in China, dominated by the aerosol particulate matters smaller than 2.5 μm (PM2.5) has raised much concern over the past decade. PM2.5 is principally derived from secondary new particulate formation (NPF) with main precursors SO 2 , NO 2 , NH 3 and organic compounds (OC), where NH 3 is the only alkaline inorganic gas. However, due to the lack understanding of NH 3 's role in NPF, few attention has been paid to NH 3 , which hinders the haze mitigation in China. In this work, the role of NH 3 in NPF is investigated theoretically with a new kinetic model. Combining the oxidation of SO 2 /NO 2 in SO 2 /NO 2 /NH 3 /H 2 O/air system and the aggregation of clusters in H 2 SO 4 /HNO 3 /NH 3 /OC system, this model has been established based on gas-kinetic theory, and tested upon uncertainty analysis with Monte Carlo method. The results of NPF calculations at the atmospheric concentration of each precursor show that NH 3 is able to enhance NPF indirectly by facilitating conversions of SO 2 and NO 2 , and directly by promoting aggregations of H 2 SO 4 , HNO 3 , NH 3 and OC. The major effect of NH 3 on NPF is found to be the enhancement of conversion fractions for SO 2 and NO 2 during oxidation processes. In addition, the promotion in contribution of HNO 3 to NPF due to NH 3 has been particularly observed. Therefore, controlling the emission of NH 3 strictly as current restrictions on SO 2 and NO x is reasonable to mitigating the haze pollution in China which has much higher atmospheric NH 3 concentration over the global average level. [ABSTRACT FROM AUTHOR]

In this paper, we present ten scenarios developed using the IMAGE2.4 framework (Integrated Model to Assess the Global Environment) to explore how different assumptions on future climate and air pollution policies influence emissions of greenhouse gases and air pollutants. These scenarios describe emission developments in 26 world regions for the 21st century, using a matrix of climate and air pollution policies. For climate policy, the study uses a baseline resulting in forcing levels slightly above RCP6.0 and an ambitious climate policy scenario similar to RCP2.6. For air pollution, the study explores increasingly tight emission standards, ranging from no improvement, current legislation and three variants assuming further improvements. For all pollutants, the results show that more stringent control policies are needed after 2030 to prevent a rise in emissions due to increased activities and further reduce emissions. The results also show that climate mitigation policies have the highest impact on SO 2 and NO X emissions, while their impact on BC and OC emissions is relatively low, determined by the overlap between greenhouse gas and air pollutant emission sources. Climate policy can have important co-benefits; a 10% decrease in global CO 2 emissions by 2100 leads to a decrease of SO 2 and NO X emissions by about 10% and 5%, respectively compared to 2005 levels. In most regions, low levels of air pollutant emissions can also be achieved by solely implementing stringent air pollution policies. The largest differences across the scenarios are found in Asia and other developing regions, where a combination of climate and air pollution policy is needed to bring air pollution levels below those of today. [ABSTRACT FROM AUTHOR]

Black carbon (BC) aerosol has relatively short atmospheric lifetimes yet plays a unique and important role in the Earth's climate system, making it an important short-term climate mitigation target. Globally, biomass burning is the largest source of BC emissions into the atmosphere. This study investigated the mass absorption efficiency (MAE) of biomass burning BC generated by controlled combustion of various wildland fuels during the Fire Laboratory at Missoula Experiments (FLAME). MAE values derived from a photoacoustic spectrometer (∼7.8 m 2 /g at a wavelength of 532 nm) were in good agreement with those suggested for uncoated BC when the emission ratios of organic carbon (OC) to elemental carbon (EC) were extremely low (i.e., below 0.3). With the increase of OC/EC, two distinct types of biomass smoke were identified. For the first type, MAE exhibited a positive dependence on OC/EC, while the overestimation of the light absorption coefficient ( b abs ) by a filter-based method was less significant and could be estimated by a nearly constant correction factor. For the second type, MAE was biased low and correlated negatively with OC/EC, while the overestimation of b abs by the filter-based method was much more significant and showed an apparent OC/EC dependence. This study suggests that BC emission factors determined by the commonly used thermal-optical methods might be sustantially overestimated for some types of biomass burning emissions. Our results also indicate that biomass burning emissions may include some liquid-like organics that can significantly bias filter-based b abs measurements. [ABSTRACT FROM AUTHOR]

Exposure to surface ozone has detrimental impacts on vegetation and crop yields. In this study, we estimate ozone impacts on crop production and subsequent impacts on land use in the 2005–2050 period using results of the TM5 atmospheric chemistry and IMAGE integrated assessment model. For the crops represented in IMAGE, we compute relative yield losses based on published exposure-response functions. We examine scenarios with either constant or declining emission factors in a weak climate policy future (radiative forcing target of 6.0 W/m 2 at the end of the century), as well as co-benefits of stringent climate policy (targeted at 2.6 W/m 2 ). Without a large decrease in air pollutant emissions, higher ozone concentrations could lead to an increase in crop damage of up to 20% locally in 2050 compared to the situation in which the changes in ozone are not accounted for. This may lead to a 2.5% global increase in crop area, and a regional increase of 8.9% in Asia. Implementation of air pollution policies could limit crop yield losses due to ozone to maximally 10% in 2050 in the most affected regions. Similar effects can be obtained as a result of co-benefits from climate policy (reducing ozone precursor emissions). We also evaluated the impact of the corresponding land-use changes on the carbon cycle. Under the worst-case scenario analysed in this study, future ozone increases are estimated to increase the cumulative net CO 2 emissions between 2005 and 2050 by about 3.7 Pg C, which corresponds to about 10% of baseline land use emissions over the same period. [ABSTRACT FROM AUTHOR]

Diesel vehicles are one of many sources of black carbon (BC) emissions in Europe; nevertheless, due to recent changes in fuel composition towards renewable fuel such as biodiesel, advances in engine design and pollution control technology, BC and organic carbon (OC) emissions may worsen air quality and encourage global warming. As there are no regulatory terms on BC mass concentration levels, assessment of pollution sources is crucial for air quality improvement policies and mitigation of climate change. BC mass concentration and aerosol optical absorption were derived using 7-wavelenght Aethalometer during 3 measurement campaigns in 2014, 2017, and 2020. BC source apportionment analysis was performed using 18 combinations of absorption Ångström exponent (AAE) values for assessing BC to fossil fuel combustion and biomass burning. The choice of the most suitable AAE combination (0.9 and 2.2, respectively) was supported by organic aerosol (OA) source apportionment results. AAE value for transport exhaust emission (0.95) from locally available diesel fuel was established using (CI) 1.9 TDI engine with different engine torque regimes and 7-wavelenght Aethalometer. Statistically and experimentally obtained values were compared and showed great agreement. Subsequently, a statistically obtained AAE tr value was used for BC source apportionment and long-term analysis. A significant increase in the contribution of transport-related BC contribution was observed between years 2014 and 2020 (from 51% to 65%) which was associated with a boost in the use of diesel vehicles (by 83.9% of diesel passenger cars comparing 2014 and 2020) and growing diesel fuel consumption. These findings underline the diesel transport-induced problem for local air quality together with its impact on atmospheric radiative balance and raise concerns for public health. • The choice of AAE combination was supported by OA source apportionment results. • BC source apportionment revealed increasing trend of transport-related pollution. • A significant increase in BC tr light absorption was observed during 2014–2020. [ABSTRACT FROM AUTHOR]

Nitrogen oxides (NO x = NO + NO 2) are key precursors of tropospheric ozone (O 3) together with volatile organic compounds (VOC) and carbon monoxide (CO). Since O 3 has positive radiative forcing and is harmful to human health, the reduction of anthropogenic emissions of NO x is thought to be beneficial from the perspectives of climate change and air pollution in principle. However, there have been discussions contending that the reduction of NO x emissions is not necessarily beneficial for the mitigation of climate change and improvement of air quality, since 1) it decreases the atmospheric mixing ratio of hydroxyl radicals (OH), which increases the atmospheric lifetime of methane (CH 4), and 2) O 3 formation is VOC-limited in urban areas and the decrease of NO x emission would increases urban O 3 by facilitating the NO titration effect. In order to scrutinize such discussion, literature review have been made on the temporal variations of the increasing rate of tropospheric CH 4 in the last 30 years, and on urban/rural O 3 issues related to the NO x -limited/VOC-limited regime. Based on the review, it may be concluded that the variation of emissions of CH 4 itself paly a dominant role, and the variation of consumption rate by OH play a minor role for the recent variation of CH 4. It has been suggested that NO x and NMVOC should be reduced simultaneously in order to avoid the adverse effect on climate change mitigation. From the review on policy-related discussion of NO x -limited and VOC-limited O 3 formation, the increase of O 3 by the decrease in NO x emissions has generally been seen in winter and nighttime when photochemical production is minimal, and the higher percentile or diurnal maximum mixing ratios of O 3 in summer tends to decrease with the decrease in NO x emissions. We suggested that the NO x -limited/VOC-limited approach is not appropriate as a long-term policy guideline for ozone control, since it is unreasonable that NO x reduction is not recommended when ambient NO x levels are high, while further NO x reduction is recommended only when the VOC/NO x ratio gets high after NO x control has been achieved based on other policy principle. Simultaneous reduction of NO x and NMVOC would be beneficial for reducing global, regional, and urban O 3 to alleviate climate change and human health impacts. The ultimate reduction of anthropogenic emissions of NO x can be envisioned toward a denitrified (de-NO x) society along with a decarbonized (de-CO 2) society. [ABSTRACT FROM AUTHOR]

The mass absorption efficiency (MAE) and mass scattering efficiency (MSE) of aerosols are critical to their light extinction capacity. In winter in northern China, episodes of clear air and heavy haze alternately occur due to the high primary emission and secondary formation of aerosols, coupled with changing weather conditions. These occurrences facilitate detailed investigation of how significant changes in the pollution level impact the MAE and MSE, they also enable the determination of whether current air quality improvement measures are also beneficial to the mitigation of climate change, to avoid climatic side effects of attempts to clean the air. In the present study, a one-month observation campaign was conducted in Beijing, China, during the 2016–2017 winter season. A photoacoustic extinctiometer, which is an in situ measurement instrument, was used to investigate the aerosol absorption coefficient (σ a) and scattering coefficient (σ s) at an infra-red wavelength of 870 nm. The MAE and MSE were found to vary differently with increasing pollution level. A positive correlation between the particulate matter concentration (PM 2.5) and the MAE was observed for PM 2.5 < 100 μg m−3, with the increase in the MAE declining for PM 2.5 = 100–200 μg m−3, and the MAE subsequently stabilizing. In contrast, the MSE continuously increased with increasing PM 2.5. The single scattering albedo (SSA), which governs the climatic effect of aerosols, was observed to be a function of both σ a and σ s , increasing from 0.827 ± 0.018 (average ± standard deviation) for a bottom particulate matter concentration group of (0, 25] μg m−3 to 0.924 ± 0.007 for a top particulate matter concentration group of (300, 400] μg m−3, indicating a positive correlation between the SSA and PM 2.5. These observations suggest a risk of current clean air measures weakening or eliminating the cooling effect of ambient aerosols in northern China. It seems that no previous study has involved the simultaneous acquisition of σ a and σ s at the same wavelength with the purpose of determining how increasing air pollution differently impacts the MAE and MSE. Practical measures that can be used to decrease the black carbon-to-PM 2.5 ratio include phasing out or reducing black carbon-rich sources and promoting technologies that decrease the black carbon-to-organic carbon ratio. Image 1 • MAE and MSE were found to show different responses to escalating pollution levels. • A risk of weakened cooling effect of ambient aerosol with the clean air process. • Practical solution was proposed to against the risks in China's clean air process. [ABSTRACT FROM AUTHOR]