Precursor Emissions Research Articles

Disentangling the mechanism of temperature and water vapor modulation on ozone under a warming climate

Temperature and water vapor have been considered as important factors affecting ozone concentrations, however, their synergistic effect on ozone, as well as its response to a warming climate remains unclear. Here, numerical experiments with different combinations of climate and emissions highlighted opposite changes of annual ozone over the contiguous U.S. under scenarios with high vs. low ozone precursor emissions in the future with warming. Comparing the occurrence of heat waves on higher vs. lower ozone days, we found significant suppression of the heat wave exacerbation of ozone by anthropogenic emissions reductions. Variations of ozone with temperature (T) and water vapor (Q) over the western U.S. and southeastern U.S. in the simulations are clearly revealed through the construction of an O3-T-Q diagram. Based on the diagram, we further identified a T-dominant regime at higher temperatures where ozone increases with temperature increase, and a Q-dominant regime at lower temperatures where ozone decreases with temperature increase due to the negative effect of water vapor. With larger anthropogenic emission reductions, ozone may increase at a lower rate with temperature in the T-dominant regime, or even decrease with temperature in the Q-dominant regime. Our results show that with large emission reductions, substantial ozone pollution due to warming and increasing heat waves may be counteracted by water vapor, partly turning the ‘climate penalty’ on ozone into ‘climate benefit’. Hence controlling anthropogenic emissions may be an increasingly important strategy in a warmer and wetter climate to improve air quality and public health.

Effects of Seasonal Variation on Spatial and Temporal Distributions of Ozone in Northeast China.

The levels of tropospheric ozone (O3) are closely related to regional meteorological conditions, precursor emissions, and geographical environments, which have a significant negative impact on human health. The concentrations of O3 were relatively low, while the spatial distribution was strongly heterogeneous in Northeast China; however, little is known about how the influencing factors affect the distribution of O3 in Northeast China. Here, the O3 concentration, meteorological observation data, precursors (NO2), and vegetation coverage data from 41 monitoring cities in Northeast China from 2017 to 2020 were collected and analyzed. The spatial-temporal distributions and evolution characteristics of O3 concentrations were investigated using statistical analysis, kriging interpolation, spatial autocorrelation analysis, cold-hot spot analysis, and geographic detectors, and the effects of meteorological factors, NO2, and green land area on O3 concentrations were evaluated seasonally and spatially. The results showed that O3 pollution in Northeast China was generally at a relatively low level and showed a decreasing trend during 2017-2020, with the highest concentrations in the spring and the lowest concentrations in the autumn and winter. May-July had relatively high O3 concentrations, and the over-standard rates were also the highest (>10%). The spatial distribution showed that the O3 concentration was relatively high in the south and low in the northeast across the study area. A globally significant positive correlation was derived from the spatial autocorrelation analysis. The cold-hot spot analysis showed that O3 concentrations exhibited spatial agglomerations of hot spots in the south and cold spots in the north. In Northeast China, the south had hot spots with high O3 pollution, the north had cold spots with excellent O3 levels, and the central region did not exhibit strong spatial agglomerations. A weak significant negative correlation between O3 and NO2 indicated that the emissions of NOx derived from human activities have weak effects on the O3 concentrations, and wind speed and sunshine duration had little effect on spatial differentiation of the O3 concentrations. Spatial variability in O3 concentrations in the spring and autumn was mainly driven by temperature, but in the summer, the influence of temperature was weakened by the relative humidity and precipitation; no factor had strong explanatory power in the winter. The temperature was the only controlling factor in hot spots with high O3 concentrations. In cold spots with low O3 concentrations, the relative humidity and green land area jointly affected the spatial distributions of O3.

First long-term surface ozone variations at an agricultural site in the North China Plain: Evolution under changing meteorology and emissions

Significant upward trends in surface ozone (O3) have been widely reported in China during recent years, especially during warm seasons in the North China Plain (NCP), exerting adverse environmental effects on human health and agriculture. Quantifying long-term O3 variations and their attributions helps to understand the causes of regional O3 pollution and to formulate according control strategy. In this study, we present long-term trends of O3 in the warm seasons (April–September) during 2006–2019 at an agricultural site in the NCP and investigate the relative contributions of meteorological and anthropogenic factors. Overall, the maximum daily 8-h average (MDA8) O3 exhibited a weak decreasing trend with large interannual variability. < 6 % of the observed trend could be explained by changes in meteorological conditions, while the remaining 94 % was attributed to anthropogenic impacts. However, the interannual variability of warm season MDA8 O3 was driven by both meteorology (36 ± 28 %) and anthropogenic factors (64 ± 27 %). Daily maximum temperature was the most essential factor affecting O3 variations, followed by ultraviolet radiation b (UVB) and boundary layer height (BLH), with rising temperature trends inducing O3 inclines throughout April to August, while UVB mainly influenced O3 during summer months. Under changes in emissions and air quality, warm season O3 production regime gradually shifted from dominantly VOCs-limited during 2006–2015 to NOx-limited afterwards. Relatively steady HCHO and remarkably rising NOx levels resulted in the fast decreasing MDA8 O3 (−2.87 ppb yr−1) during 2006–2012. Rapidly decreasing NOx, flat or slightly increasing HCHO promoted O3 increases during 2012–2015 (9.76 ppb yr−1). While afterwards, slow increases in HCHO and downwards fluctuating NOx led to decreases in MDA8 O3 (−4.97 ppb yr−1). Additionally, continuous warming trends might promote natural emissions of O3 precursors and magnify their impacts on agricultural O3 by inducing high variability, which would require even more anthropogenic reduction to compensate for.

Co-control of the haze pollution emissions in China: Insight from supply chains.

Because of rapid economic development and the increase in social demand, China has been suffering from serious air pollution, in particular, haze pollution. To mitigate haze from the source, it is essential to achieve co-control of three important haze precursors: volatile organic compounds (VOCs), sulfur dioxide (SO2 ), and nitrogen oxide (NOx ). In this study, we used the environmentally extended input-output model, structural path analysis, and structural path decomposition method to investigate changes in consumption-based emissions of three major haze precursors (i.e., NOx , SO2 , and VOCs) in China during 2007-2017. First, the results revealed that fixed capital formation was the most critical final demand to co-control the three precursors. Investment in construction was the most important behavior for co-control. Second, the most crucial common path driving the changes in emissions of the three precursors was "transportation and warehousing→household consumption" during 2007-2012, and "electricity, gas, and water supply→household consumption" during 2012-2017. Finally, direct emission intensity of transportation and warehousing, and electricity, gas, and water supplies were critical to co-control precursors. The results of this study provided a comprehensive understanding of changes in haze precursor emissions driven by demand. Therefore, China must strengthen the co-control of multiple pollutant emissions on both the production and consumption sides by adjusting supply chains. Integr Environ Assess Manag 2023;19:1048-1063. © 2022 SETAC.

Understanding recent tropospheric ozone trends in the context of large internal variability: a new perspective from chemistry-climate model ensembles

Observational records of meteorological and chemical variables are imprinted by an unknown combination of anthropogenic activity, natural forcings, and internal variability. With a 15-member initial-condition ensemble generated from the CESM2-WACCM6 chemistry-climate model for 1950–2014, we extract signals of anthropogenic (‘forced’) change from the noise of internally arising climate variability on observed tropospheric ozone trends. Positive trends in free tropospheric ozone measured at long-term surface observatories, by commercial aircraft, and retrieved from satellite instruments generally fall within the ensemble range. CESM2-WACCM6 tropospheric ozone trends are also bracketed by those in a larger ensemble constructed from five additional chemistry-climate models. Comparison of the multi-model ensemble with observed tropospheric column ozone trends in the northern tropics implies an underestimate in regional precursor emission growth over recent decades. Positive tropospheric ozone trends clearly emerge from 1950 to 2014, exceeding 0.2 DU yr−1 at 20–40 N in all CESM2-WACCM6 ensemble members. Tropospheric ozone observations are often only available for recent decades, and we show that even a two-decade record length is insufficient to eliminate the role of internal variability, which can produce regional tropospheric ozone trends oppositely signed from ensemble mean (forced) changes. By identifying regions and seasons with strong anthropogenic change signals relative to internal variability, initial-condition ensembles can guide future observing systems seeking to detect anthropogenic change. For example, analysis of the CESM2-WACCM6 ensemble reveals year-round upper tropospheric ozone increases from 1995 to 2014, largest at 30 S–40 N during boreal summer. Lower tropospheric ozone increases most strongly in the winter hemisphere, and internal variability leads to trends of opposite sign (ensemble overlaps zero) north of 40 N during boreal summer. This decoupling of ozone trends in the upper and lower troposphere suggests a growing prominence for tropospheric ozone as a greenhouse gas despite regional efforts to abate warm season ground-level ozone.

Understanding Regional Background Ozone by Multiple Methods: A Case Study in the Shandong Region, China, 2018–2020

AbstractUprising ground‐level ozone (O3) and its regional pollution in Northern China are attracting more attention. Besides local precursor emissions and photochemistry, background ozone and long‐range transport also contribute significantly to O3 concentrations. To quantify the regional background O3 concentrations and their temporal and spatial variations, multiple methods, including the principal component analysis (PCA) and the Texas Commission on Environmental Quality (TCEQ) method, were applied as a case study in Shandong province in Northern China, where serious O3 pollution occurred frequently yet the background contributions have not been well quantified. We used four methods to quantify the regional background O3: Method 1 is PCA analysis with only ambient O3 data as input; Method 2 is PCA analysis considering O3 and meteorological parameters; Method 3 combines multiple linear regression and the traditional PCA method; and Method 4 is based on TCEQ and consists of the lowest MAD8 O3 measured to represent regional background O3 concentrations. Results derived from multiple methods show an overall consistent trend with 2018–2020 averaged regional background O3 (MDA8) of 41.5 ppb, accounting for 79.4% of the total O3 in the region. From 2018 to 2020, the changes in regional MDA8 O3 estimated by Methods 1–4 are −1.8, 0.7, −2.4, and 0.4 ppb, respectively. Clear seasonal variations in the regional background O3 are observed, showing a pattern of summer &gt; spring &gt; autumn &gt; winter. In addition, the regional ozone contribution at coastal cities was larger than that for inland cities with local O3 contribution gradually increasing from coastal areas to inland areas. The 3‐year average sea‐land breeze contribution to summertime O3 in the eastern coastal cities was estimated to be around 2.1%, while the local photochemistry to O3 in the inland cities was about 29.7% during ozone pollution episodes, with maximum contribution estimated up to 55.8%. Overall, our study provides insights into the regional background ozone and local photochemistry in Northern China.

Comprehensive Assessment for the Impacts of S/IVOC Emissions from Mobile Sources on SOA Formation in China.

Semivolatile/intermediate-volatility organic compounds (S/IVOCs) from mobile sources are essential SOA contributors. However, few studies have comprehensively evaluated the SOA contributions of S/IVOCs by simultaneously comparing different parameterization schemes. This study used three SOA schemes in the CMAQ model with a measurement-based emission inventory to quantify the mobile source S/IVOC-induced SOA (MS-SI-SOA) for 2018 in China. Among different SOA schemes, SOA predicted by the 2D-VBS scheme was in the best agreement with observations, but there were still large deviations in a few regions. Three SOA schemes showed the peak value of annual average MS-SI-SOA was up to 0.6 ± 0.3 μg/m3. High concentrations of MS-SI-SOA were detected in autumn, while the notable relative contribution of MS-SI-SOA to total SOA was predicted in the coastal areas in summer, with a regional average contribution up to 20 ± 10% in Shanghai. MS-SI-SOA concentrations varied by up to 2 times among three SOA schemes, mainly due to the discrepancy in SOA precursor emissions and chemical reactions, suggesting that the differences between SOA schemes should also be considered in modeling studies. These findings identify the hotspot areas and periods for MS-SI-SOA, highlighting the importance of S/IVOC emission control in the future upgrading of emission standards.

Change in Tropospheric Ozone in the Recent Decades and Its Contribution to Global Total Ozone

AbstractTropospheric ozone is a key chemically active trace gas and radiative forcer. Understanding its long‐term changes is important to properly interpret observed changes in total column ozone and stratospheric ozone recovery. We investigate global and regional tropospheric ozone changes and their impact on total column ozone during 2005–2018 using satellite measurements and the NASA Goddard Earth Observing System Chemistry Climate Model (GEOSCCM). Global total ozone increased ∼4 DU during 2005–2018 (+0.28 ± 0.06 DU yr−1) as inferred from Ozone Monitoring Instrument (OMI). Consistent with previous studies, the OMI/MLS (Microwave Limb Sounder) derived global tropospheric ozone increased 2.2 DU during this period, 60% of the global total column ozone increase. While GEOSCCM reproduces reasonably well the total column increase, it overestimates the stratospheric ozone increase and underestimates the tropospheric ozone increase. We find that the tropospheric ozone increases are likely attributed to a growth of regional emissions of key ozone precursors, especially volatile organic compounds as reflected by the positive trends in formaldehyde (CH2O). Although carbon monoxide (CO) has been decreasing everywhere around the globe, it has relatively small impact on the tropospheric ozone trend. Trends in nitrogen dioxide (NO2) vary with regions, and these changes counteract or reinforce the positive effects of CH2O on the tropospheric ozone increases. The model underestimates the observed tropospheric ozone increase, especially over the US and Europe, because of underestimated NO2 emissions changes used in the model. The stratospheric ozone contribution increases during this period in the Northern Hemisphere and contributes to the tropospheric ozone increase.

Sequential hydrothermal carbonization and CO2 gasification of sewage sludge for improved syngas production with mitigated emissions of NOx precursors

Due to high moisture and protein contents in sewage sludge (SS), conventional thermal treatment of SS is energy-intensive and a large amount of harmful nitrogen-containing gases are emitted. In this study, a sequential hydrothermal carbonization (HTC) and CO2 gasification system has been proposed to effectively improve the gasification efficiency and considerably reduce the emissions of NOx precursors (i.e., NH3 and HCN) in SS treatment. Yield and quality of syngas were comprehensively investigated during CO2 gasification in terms of various temperatures with different dosages of CaO additive in HTC, and varied reaction temperature and CO2 percentage in gasification process. On the whole, CO2 gasification of SS derived hydrochar (HC) demonstrated obvious reduction of tar formation from 10.9 to 6.8 % and enhancement of calorific value of formed syngas from 14.1 to 15.7 MJ/Nm3. Production of NOx precursors was greatly reduced due to formed non-active quaternary-N in HC, while HTC with CaO favored the mitigated NOx precursors in syngas owing to enriched pyrrole-N and quaternary-N therein. Catalytic tar decomposition and Boudouard reaction in CO2 gasification were responsible for the distinct reduction of tar formation, and notable increase of carbon conversion ratio and syngas yield. Although facilitated catalytic gasification in CO2 atmosphere can maximize syngas yield, lower portion of CO2 (ca. 20 %) was more beneficial to drastically mitigate emissions of NOx precursors, especially the reinforced transformation of NH3 to N2. The fundamental knowledge could ultimately help to achieve significant abatement of NOx precursors during CO2 gasification for improved syngas production.

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 52.13µg/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 96.18µg/m3 (exceeding 1.60 times the WHO Guidelines 2021) and 94.86µg/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, NOx and NMVOCs, are the main contributors to O3 pollution, with the largest contribution coming from zone 1 (FAZ) with 91.5 thousand tons of NOx/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.

Assessment of summertime ozone formation in the Sichuan Basin, southwestern China

The alarming increase of ambient ozone (O3) levels across China raises an urgent need in understanding underlying mechanisms of regional O3 events for highly urbanized city clusters. Sichuan Basin (SCB) situated in southwestern China has experienced severe O3 pollution at times in summer from 2013 to 2020. Here, we use the WRF-CMAQ model with the Integrated Source Apportionment Method (ISAM) to investigate the evolution mechanism and conduct source attribution of an extreme O3 episode in the SCB from June 1 to 8, 2019. This typical summer O3 episode is associated with the synoptic-driven meteorological phenomenon and transboundary flow of O3 and precursors across the SCB. Weak ventilation in combination with stagnant conditions triggered the basin-wide high O3 concentrations and enhanced BVOC emissions substantially contribute up to 57.9 μg/m3 MDA8 O3. CMAQ-ISAM indicates that precursor emissions from industrial and transportation have the largest impacts on elevating ambient O3 concentrations, while power plant emissions exhibit insignificant contributions to basin-wide O3 episodes. These results improve the understanding of the summertime O3 episode in the SCB and contribute insights into designing O3 mitigation policy.

Disentangling Aerosol and Cloud Effects on Dimming and Brightening in Observations and CMIP6

AbstractPeriods of dimming and brightening have been recorded in observational datasets of surface solar radiation (SSR) between the mid‐20th century and present day. Atmospheric components affect SSR, including aerosols and clouds, though studies disagree somewhat about the relative effect of each component in different regions. Current Earth system models (ESMs) are unable to simulate observed trends in SSR. This study includes an investigation into observed SSR variations between 1961 and 2014 and an evaluation of the effects of cloud cover variations and impacts of aerosol extinction, using timeseries of SSR and cloud cover from in‐situ measurements. Historical simulations by 42 ESMs participating in the Coupled Model Intercomparison Project phase 6 (CMIP6) have also been studied and compared to observations. The observational study indicates that cloud cover has had a dampening effect on the variations of SSR and that emissions of aerosol and aerosol precursors are the main cause of the general trends in observed SSR in four regions—China, Japan, Europe, and the United States—during 1961–2014. The study of simulated SSR in CMIP6 yields the conclusion that current ESMs remain unable to simulate the magnitude of observed dimming and brightening in China, Japan, and the United States, but that the European SSR trends between 1961 and 2014 are fairly well reproduced in the ESMs. A rough quantification of the regional surface radiation extinction efficiency of aerosol and precursor emissions in the simulations is found to agree with observed values in Europe, but not in the other three regions.

A tool for air pollution scenarios (TAPS v1.0) to enable global, long-term, and flexible study of climate and air quality policies

Abstract. Air pollution is a major sustainability challenge – and future anthropogenic precursor and greenhouse gas (GHG) emissions will greatly affect human well-being. While mitigating climate change can reduce air pollution both directly and indirectly, distinct policy levers can affect these two interconnected sustainability issues across a wide range of scenarios. We help to assess such issues by presenting a public Tool for Air Pollution Scenarios (TAPS) that can flexibly assess pollutant emissions from a variety of climate and air quality actions, through the tool's coupling with socioeconomic modeling of climate change mitigation. In this study, we develop and implement TAPS with three components: recent global and fuel-specific anthropogenic emissions inventories, scenarios of emitting activities to 2100 from the MIT Economic Projection and Policy Analysis (EPPA) model, and emissions intensity trends based on recent scenario data from the Greenhouse Gas–Air Pollution Interactions and Synergies (GAINS) model. An initial application shows that in scenarios with less climate and pollution policy ambition, near-term air quality improvements from existing policies are eclipsed by long-term emissions increases – particularly from industrial processes that combine sharp production growth with less stringent pollution controls in developing regions. Additional climate actions would substantially reduce air pollutant emissions related to fossil fuel (such as sulfur and nitrogen oxides), while further pollution controls would lead to larger reductions for ammonia and organic carbon (OC). Future applications of TAPS could explore diverse regional and global policies that affect these emissions, using pollutant emissions results to drive global atmospheric chemical transport models to study the scenarios' health impacts.

Data for "Rapid increase in tropospheric ozone over Southeast Asia attributed to changes in precursor emission source regions and sectors"

Data for "Rapid increase in tropospheric ozone over Southeast Asia attributed to changes in precursor emission source regions and sectors"

Effects of anthropogenic precursor emissions and meteorological conditions on PM2.5 concentrations over the “2+26” cities of northern China

Elucidating the characteristics and influencing mechanisms of PM2.5 concentrations is the premise and key to the precise prevention and control of air pollution. However, the temporal and spatial heterogeneity of PM2.5 concentrations and its driving mechanism are complex and need to be further analyzed. We analyzed the temporal and spatial variations of PM2.5 concentrations in the “2 + 26” cities from 2015 to 2021, and quantified the influence of meteorological factors and anthropogenic emissions and their interactions on PM2.5 concentrations based on geographic detector model. We find the inter-annual and inter-season PM2.5 concentrations show downward trend from 2015 to 2021, and the inter-month PM2.5 concentrations present a U-shaped distribution. The PM2.5 concentrations in the “2 + 26” cities manifest a spatial distribution pattern of high in the south and low in the north, and high in the middle and low in the surroundings. Meteorological conditions have stronger effects on PM2.5 concentrations than anthropogenic emissions, and planetary boundary layer height and temperature are the two main driving factors at the annual scale. On the seasonal scale, sunshine duration is the dominant factor of PM2.5 concentrations in summer and autumn, and planetary boundary layer height is the dominant factor of PM2.5 concentrations in winter. The effect of anthropogenic emissions on PM2.5 concentration is higher in winter and spring than in summer and autumn, and ammonia and ozone have stronger effects on PM2.5 concentrations than other anthropogenic emissions. Interactions between the factors significantly enhance the PM2.5 concentrations. The interactions between planetary boundary layer height and other impacting factors play dominant roles on PM2.5 concentrations at annual scale and in winter. Our results not only provide crucial information for further developing air quality policies of the “2 + 26” cities, but also bear out several important implications for clean air policies in China and other regions of the world.

A machine learning approach to investigate the build-up of surface ozone in Mexico-City

Ground-level ozone is an important pollutant regarding air quality and climate. Mexico City frequently experiences severe ozone episodes due to a combination of strong ozone precursor emissions and its specific topographical environment which critically impacts meteorological conditions. High ozone levels during these episodes cause harmful effects to the public health and the environment. This necessitates ranking air quality and meteorological variables according to their contributions towards the build-up of ozone. In this study, three machine learning models are used to learn a prediction function with hourly data of eight predictors as input and hourly ground-level ozone mixing ratios as output. One-year hourly data of eight predictors collected in Mexico-City from March 2015 to February 2016 is employed to train and test the models. The best model, capturing ozone peak levels with 92% accuracy during 6–18 March 2016, is used to rank the predictors according to their importance in the build-up of ozone applying a shapley additive explanations approach based on the game theory shapley values. This 6–18 March 2016 period encompassed different meteorological and emission conditions and included a severe ozone smog episode from 12 to 17 March 2016. Such ranking of the air quality and meteorological variables is crucial for policy-making decisions regarding the prevention and mitigation of ozone detrimental effects during severe ozone episodes and provides insight into the functional dependency of ozone on its predictors. The proposed approach showcases Mexico City, but its principles can be applied for ozone episodes at any other location.

Changing ozone sensitivity in the South Coast Air Basin during the COVID-19 period

Abstract. The South Coast Air Basin (SoCAB), which includes the city of Los Angeles and is home to more than 15 million people, frequently experiences ozone (O3) levels that exceed ambient air quality standards. While strict regulation of O3 precursors has dramatically improved air quality over the past 50 years, the region has seen limited improvement in O3 over the past decade despite continued reductions in precursor emissions. One contributing factor to the recent lack of improvement is a gradual transition of the underlying photochemical environment from a VOC-limited regime (where VOC denotes volatile organic compound) towards an NOx-limited one. The changes in human activity prompted by COVID-19-related precautions in spring and summer of 2020 exacerbated these existing changes in the O3 precursor environment. Analyses of sector-wide changes in activity indicate that emissions of NOx decreased by 15 %–20 % during spring (April–May) and by 5 %–10 % during summer (June–July) relative to expected emissions for 2020, largely due to changes in mobile-source activity. Historical trend analysis from two indicators of O3 sensitivity (the satellite HCHO/NO2 ratio and the O3 weekend/weekday ratio) revealed that spring of 2020 was the first year on record to be on average NOx-limited, while the “transitional” character of recent summers became NOx-limited due to COVID-19-related NOx reductions in 2020. Model simulations performed with baseline and COVID-19-adjusted emissions capture this change to an NOx-limited environment and suggest that COVID-19-related emission reductions were responsible for a 0–2 ppb decrease in O3 over the study period. Reaching NOx-limited territory is an important regulatory milestone, and this study suggests that deep reductions in NOx emissions (in excess of those observed in this study) would be an effective pathway toward long-term O3 reductions.

Regional characteristics of fine aerosol mass increase elucidated from long-term observations and KORUS-AQ campaign at a Northeast Asian background site

Northeast Asia has suffered from severe PM2.5 pollution and the exact mechanisms have yet to be fully understood. Here, we investigated the transformation processes of submicron aerosols using a 4-year data set obtained at Jeju, a Northeast Asian background site. The diurnal-cycle constrained empirical orthogonal function analysis of nanoparticle size–number distribution distinguished 2 modes: burst of nucleation–Aitken particles and increase in accumulation mode particles, representing “new particle formation and growth” and “PM2.5 mass increase,” respectively. In these events, aerosol and meteorological characteristics changed progressively over several days, revealing that the PM2.5 mass increase is an episodic event occurring on a regional scale. The increase in PM2.5 mass was accompanied by an increase in aerosol liquid water content, which correlated well with SO4−2 and NO3, and a decrease in incoming solar radiation (−14.1 Wm−2 day−1) constituting a positive feedback. The “transport/haze” episode of KOREA–U.S. Air Quality campaign corresponds to “PM2.5 mass increase,” during which the vertical evolution of particles demonstrates that nanoparticles ≥3.5 nm were entrained into the shallow boundary layer upon vertical mixing and converted to accumulation-mode particles ≥0.3 μm at relative humidity (RH) exceeding the deliquescence RH of secondary inorganic aerosol (SIA). Coincidently, at ground, the coating thickness of refractory black carbon (rBC) (48 ± 39 nm) and SIA concentration increased. Furthermore, the diameter of rBC (180–220 nm)-containing particle in core–shell configuration linearly increased with PM2.5 mass, reaching 300–400 nm at PM2.5 ≥ 40 μg m−3. This observational evidence suggests that the thick coating of rBCs resulted from the active conversion of condensable gases into the particulate phase on the rBC surface, thereby increasing the mass of the accumulation-mode aerosol. Consequently, this result complies with the strategy to reduce primary emissions of gaseous precursors for SIA and particulates such as rBC as a way to effectively mitigate haze pollution as well as climate change in Northeast Asia.

Seasonal modeling analysis of nitrate formation pathways in Yangtze River Delta region, China

Abstract. Nitrate (NO3−) has been the dominant and the least reduced chemical component of fine particulate matter (PM2.5) since the stringent emission controls implemented in China in 2013. The formation pathways of NO3− vary seasonally and differ substantially in daytime vs. nighttime. They are affected by precursor emissions, atmospheric oxidation capacity, and meteorological conditions. Understanding NO3− formation pathways provides insights for the design of effective emission control strategies to mitigate NO3− pollution. In this study, the Community Multiscale Air Quality (CMAQ) model was applied to investigate the impact of regional transport, predominant physical processes, and different formation pathways to NO3− and total nitrate (TNO3, i.e., HNO3+ NO3−) production in the Yangtze River Delta (YRD) region during the four seasons of 2017. NO3-/PM2.5 and NO3-/TNO3 are the highest in the winter, reaching 21 % and 94 %, respectively. The adjusted gas ratio (adjGR = ([NH3]+ [NO3−])/([HNO3]+ [NO3−])) in the YRD is generally greater than 2 in the four seasons across most areas in the YRD, indicating that YRD is mostly in the NH3-rich regime and that NO3− is limited by HNO3 formation. Local emissions and regional transportation contribute to NO3− concentrations throughout the YRD region by 50 %–62 % and 38 %–50 %, respectively. The majority of the regional transport of NO3− concentrations is contributed by indirect transport (i.e., NO3− formed by transported precursors reacting with local precursors). Aerosol (AERO, including condensation, coagulation, new particle formation, and aerosol growth) processes are the dominant source of NO3− formation. In summer, NO3− formation is dominated by AERO and total transport (TRAN, sum of horizontal and vertical transport) processes. The OH + NO2 pathway contributes to 60 %–83 % of the TNO3 production, and the N2O5 heterogeneous (HET N2O5) pathway contributes to 10 %–36 % in the YRD region. HET N2O5 contribution becomes more important in cold seasons than warm seasons. Within the planetary boundary layer in Shanghai, the TNO3 production is dominated by the OH + NO2 pathway during the day (98 %) in the summer and spring and by the HET N2O5 pathway during the night (61 %) in the winter. Local contributions dominate the OH + NO2 pathway for TNO3 production during the day, while indirect transport dominates the HET N2O5 pathway at night.

Robust evidence for reversal of the trend in aerosol effective climate forcing

Abstract. Anthropogenic aerosols exert a cooling influence that offsets part of the greenhouse gas warming. Due to their short tropospheric lifetime of only several days, the aerosol forcing responds quickly to emissions. Here, we present and discuss the evolution of the aerosol forcing since 2000. There are multiple lines of evidence that allow us to robustly conclude that the anthropogenic aerosol effective radiative forcing (ERF) – both aerosol–radiation interactions (ERFari) and aerosol–cloud interactions (ERFaci) – has become less negative globally, i.e. the trend in aerosol effective radiative forcing changed sign from negative to positive. Bottom-up inventories show that anthropogenic primary aerosol and aerosol precursor emissions declined in most regions of the world; observations related to aerosol burden show declining trends, in particular of the fine-mode particles that make up most of the anthropogenic aerosols; satellite retrievals of cloud droplet numbers show trends in regions with aerosol declines that are consistent with these in sign, as do observations of top-of-atmosphere radiation. Climate model results, including a revised set that is constrained by observations of the ocean heat content evolution show a consistent sign and magnitude for a positive forcing relative to the year 2000 due to reduced aerosol effects. This reduction leads to an acceleration of the forcing of climate change, i.e. an increase in forcing by 0.1 to 0.3 W m−2, up to 12 % of the total climate forcing in 2019 compared to 1750 according to the Intergovernmental Panel on Climate Change (IPCC).