Secondary Inorganic Aerosol Concentrations Research Articles

Causes of the unremitting high ambient levels of PM10 in a suburban background site in NE Spain

Atmospheric PM10 and benzo(a)pyrene (BaP) concentrations in Manlleu (NE Spain) have remained high from 2008 to 2023, frequently exceeding EU limit/target values, and reaching BaP levels up to six times higher than urban averages in Spain. Furthermore, PM speciation campaigns were carried out in 2013, 2014–2015, 2016–2017 and 2021–2022. Chemical mass closure for autumn-winter showed a consistent PM10 composition for the different PM speciation campaigns, comprising 46–53% organic matter (OM), 18–26% secondary inorganic aerosol (SIA), 13–23% mineral matter (MM), and 5–9% elemental carbon (EC). Trend analysis revealed very light decrease and constant concentrations for PM10 and BaP, respectively over the study period, emphasizing the need for compliance with current and forthcoming EU air quality directives, the last aiming to halve PM10 limit values. Source apportionment of samples of the sporadic campaigns identified biomass burning (BB, 17.5 μg m−3, 48%) and MM and industry (16.3 μg m−3, 44%) as the main autumn-winter PM10 contributors, with high SIA concentrations attributed to several factors, including high ammonia (NH3) emissions. Local topography and meteorological conditions contribute to aggravate PM10 pollution. While metal concentrations have decreased since 2013, suggesting reduced industrial emissions, persistently high OM and EC concentrations indicate ongoing issues with BB emissions from domestic, commercial, and agricultural sources. Online analysis of black carbon (BC) and non-refractory PM1 components during winter 2016–2017 confirmed domestic and commercial BB as the primary sources of the BB contributions. These findings highlight the need of the implementation of more effective measures in reducing BB and agricultural/farming NH3 emissions. This study highlights the relevance of these issues for similar towns, the probable unremitting problem over the last decade, and the necessity of enhanced monitoring in small cities and policy actions to meet air quality standards under the new EU directive.

Atmospheric NH3 in urban Beijing: long-term variations and implications for secondary inorganic aerosol control

Abstract. Ammonia (NH3) has major effects on the environment and climate. In situ measurements of NH3 concentrations taken between June 2009 and July 2020 at an urban site in Beijing were analyzed to study its long-term behavior, responses to meteorological conditions, and influences on the formation of secondary inorganic aerosols (SIAs). The 11-year average NH3 mixing ratio was 26.9±19.3 ppb (median 23.5 ppb). The annual average NH3 mixing ratio increased from 2009 to 2017 by 50 % and then decreased by 49 % from 2017 to 2020. Notably, the long-term trend for NH3 at the ground level did not align with the trends derived from satellite observations and emission estimates. The NH3 concentration exhibited a stronger correlation with the daily variation in water vapor (H2O) concentration than with air temperature. Thermodynamic modeling revealed the nonlinear response of SIAs to NH3, with increased sensitivity when its concentration was reduced to 40 % of the initial level. Although reducing NH3 concentrations can improve air quality during winter, controlling acid gas concentrations has a greater effect than controlling NH3 concentrations on reducing SIA concentrations, until NH3 and acidic gas concentrations are reduced below 80 % of their current levels. Nevertheless, the increased mass proportion of ammonium salts in SIAs during the observation period indicates that future control measures for NH3 concentrations may need to be prioritized in Beijing.

Characteristics and Cause of PM2.5 During Haze Pollution in Winter 2022 in Zhoukou, China

The characteristics and main factors of causes of haze in Zhoukou in January 2022 were analyzed. Six air pollutants, water-soluble ions, elements, OC, EC, and other parameters in fine particulate matter were monitored and analyzed using a set of online high-time-resolution instruments in an urban area. The results showed that the secondary inorganic aerosols(SNA), carbonaceous aerosols(CA, including organic carbon OC and inorganic carbon EC), and reconstructed crustal materials(CM, such as Al2O3, SiO2, CaO, and Fe2O3, etc.) were the three main components, accounting for 61.3%, 24.3%, and 9.72% in PM2.5, respectively. The concentrations of SNA, CA, CM, and SOA were increased, accompanied with higher AQI. The sulfur oxidation rate(SOR) and nitrogen oxidation rate(NOR) in January were 0.53 and 0.46, respectively. The growth rates[μg·(m3·h)] of sulfate and nitrate were 0.027(-5.89-9.47, range) and 0.051(-23.1-12.4), respectively. During the haze period, the growth rates of sulfate and nitrate were 0.13 μg·(m3·h)-1and 0.24 μg·(m3·h)-1, which were 4.8 and 4.7 times higher than the average value of January, respectively. Although the sulfur oxidation rate was greater than the nitrogen oxidation rate, the growth rate of nitrate was approximately 1.8 times that of sulfate owing to the difference in the concentration of gaseous precursors and the influence of relative humidity. The growth rates of nitrate in SNA were significantly higher than those of sulfate on heavily polluted days. The values of SOR, NOR, and concentrations of SNA and SOA during higher AQI and humidity periods were higher than those in lower AQI and humidity periods. The Ox(NO2+O3) decreased with the increase in relative humidity. The SOA was higher at nighttime, increasing faster with the humidity than that in daytime. Under the situation of lower temperature, higher humidity, and lower wind speed, the emission of gaseous precursors of SNA requires further attention in Zhoukou in winter. Advanced control strategies of emissions of SO2 and NO2, such as mobile sources and coal-burning sources, could reduce the peak of haze in winter efficiently.

Modeling PM2.5 During Severe Atmospheric Pollution Episode in Lagos, Nigeria: Spatiotemporal Variations, Source Apportionment, and Meteorological Influences

AbstractIn 2021, the World Health Organization ranked Nigeria among the most polluted nations in the world, an indication of a deteriorating air quality, especially in the major urban areas of the country, which might pose adverse human health impacts. In this study, the Integrated Source Apportionment Method (ISAM) tool in the Community Multiscale Air Quality (CMAQ) model (CMAQ‐ISAM) was employed to quantify the contributions of eight emissions sectors to fine particulate matter (PM2.5) and its major components in Lagos during a prolonged severe atmospheric pollution episode (APE) in January 2021. The influence of meteorological conditions on the formation and dispersion of PM2.5 during the APE was also elucidated. Spatially, elevated PM2.5 concentrations were found in the northwestern region of Lagos, an urban area with larger anthropogenic emissions. Residential and industry were the two major sources of PM2.5. Residential contributed the most to total PM2.5 (∼40 μg/m3), followed by industry (∼20 μg/m3). High concentrations of secondary inorganic aerosols (SIA) at the northwest and upper northern areas of Lagos were majorly attributed to residential and industry sectors. In addition, sulfate accounted for the largest fraction of PM2.5, with residential, industry, and energy being its major sources. Residential, industry, and on‐road sectors dominated the contributions to nitrate, while residential and industry were the major contributors to ammonium. Furthermore, the elevated PM2.5 concentrations during the APE were greatly enhanced by unfavorable meteorological conditions. This study provides insights for designing effective emissions control strategies to mitigate future severe PM2.5 pollution episode in Lagos.

Separating Daily 1 km PM2.5 Inorganic Chemical Composition in China since 2000 via Deep Learning Integrating Ground, Satellite, and Model Data.

Fine particulate matter (PM2.5) chemical composition has strong and diverse impacts on the planetary environment, climate, and health. These effects are still not well understood due to limited surface observations and uncertainties in chemical model simulations. We developed a four-dimensional spatiotemporal deep forest (4D-STDF) model to estimate daily PM2.5 chemical composition at a spatial resolution of 1 km in China since 2000 by integrating measurements of PM2.5 species from a high-density observation network, satellite PM2.5 retrievals, atmospheric reanalyses, and model simulations. Cross-validation results illustrate the reliability of sulfate (SO42-), nitrate (NO3-), ammonium (NH4+), and chloride (Cl-) estimates, with high coefficients of determination (CV-R2) with ground-based observations of 0.74, 0.75, 0.71, and 0.66, and average root-mean-square errors (RMSE) of 6.0, 6.6, 4.3, and 2.3 μg/m3, respectively. The three components of secondary inorganic aerosols (SIAs) account for 21% (SO42-), 20% (NO3-), and 14% (NH4+) of the total PM2.5 mass in eastern China; we observed significant reductions in the mass of inorganic components by 40-43% between 2013 and 2020, slowing down since 2018. Comparatively, the ratio of SIA to PM2.5 increased by 7% across eastern China except in Beijing and nearby areas, accelerating in recent years. SO42- has been the dominant SIA component in eastern China, although it was surpassed by NO3- in some areas, e.g., Beijing-Tianjin-Hebei region since 2016. SIA, accounting for nearly half (∼46%) of the PM2.5 mass, drove the explosive formation of winter haze episodes in the North China Plain. A sharp decline in SIA concentrations and an increase in SIA-to-PM2.5 ratios during the COVID-19 lockdown were also revealed, reflecting the enhanced atmospheric oxidation capacity and formation of secondary particles.

Investigating Drivers of Particulate Matter Pollution Over India and the Implications for Radiative Forcing With GEOS‐Chem‐TOMAS15

AbstractAmbient fine particulate matter (PM2.5) concentrations in India frequently exceed 100 μg/m3 during fall and winter pollution episodes. We use the GEOS‐Chem chemical transport model with the TwO‐Moment Aerosol Sectional microphysics scheme with 15 size bins (TOMAS15) to assess PM2.5 composition and impacts on radiation and cloud condensation nuclei (CCN) during pollution episodes as compared to the seasonal (October‐December) average. We conduct high resolution (0.25° × 0.3125°) nested‐domain simulations over India for short‐duration, high‐PM2.5 episodes in the fall of 2015 and 2017. The simulations capture the magnitude and spatial patterns of pollution episodes measured by surface monitors (r2PM2.5 = 0.69) although aerosol optical depth is underestimated. During the episodes, near‐surface organic matter (OM), black carbon (BC), and secondary inorganic aerosol concentrations increase from seasonal averages by up to 36, 7, and 7 μg/m3, respectively. Episodic aerosol increases enhance cooling by lowering the top‐of‐atmosphere clear‐sky direct radiative effect (DRETOA) during the 2015 episode (−6 W/m2), with a smaller impact during the 2017 episode (−1 W/m2). Differences in DRETOA reflect larger increases in scattering aerosols in the column during the 2015 episode (+17 mg/m2) than in 2017 (+13 mg/m2), while absorbing aerosol column enhancements are smaller (+3 mg/m2) in both years. Changes in shortwave radiation at the surface (SWsfc) are spatially similar to DRETOA and mostly negative during both episodes. CCN enhancements (0.2% supersaturation) during these episodes occur across the western Indo‐Gangetic Plain, coincident with higher PM2.5 concentrations. Changes in DRETOA, SWsfc, and CCN during high‐PM2.5 episodes may have implications for crops, the hydrologic cycle, and surface temperature.

Reduced-form and complex ACTM modelling for air quality policy development: A model inter-comparison

Simulation models can be valuable tools in supporting development of air pollution policy. However, exploration of future scenarios depends on reliable and robust modelling to provide confidence in outcomes which cannot be tested against measurements. Here we focus on the UK Integrated Assessment Model, a fast reduced-form model with a purpose to support policy development with modelling of multiple alternative future scenarios, and the EMEP4UK model which is a complex Eulerian Atmospheric Chemistry Transport Model requiring significant computing resources. The EMEP4UK model has been used to model selected core scenarios to compare with UKIAM, and to investigate sensitivity studies such as the interannual variability in response to meteorological differences between years. This model intercomparison addresses total PM2.5, primary PM2.5 and Secondary Inorganic Aerosol concentrations for a baseline of 2018 and selected scenarios for projections to 2040. This work has confirmed the robustness of the UK Integrated Assessment Model for assessing alternative futures through a direct comparison with EMEP4UK. Both models have shown good agreement with measurements, and EMEP4UK shows an ability to replicate past trends. These comparisons highlight how a combination of reduced-form modelling (UKIAM) and complex chemical transport modelling (EMEP4UK) can be effectively used in support of air pollution policy development, informing understanding of projected futures in the context of emerging evidence and uncertainties.

Factors influencing aerosol and precipitation ion chemistry in urban background of Moscow megacity

In-depth study of aerosol and precipitation ion chemistry was carried out in order to cover an existing data gap in the seasonal-dependent ionic data in Moscow urban background atmosphere. Literature about atmospheric pollution in this megacity is, indeed, still poor, despite its peculiar climatological characteristics that make it a very interesting site for atmospheric research. Particulate matter (PM) and precipitation were collected at Moscow urban background during spring of 2018, summer, and autumn of 2019. Aerosol inorganic ions are characterized for the whole period, carboxylic acids and sugars for summer. Inorganic ions constitute a major PM10 fraction of 27% in autumn, it drops down to 16.5% in summer. Dominance of secondary inorganic aerosol (SIA) over all the other inorganic ion species is determined. Na+ and Ca2+ shows the largest variability between seasons, with maximum in summer and spring, respectively. Excess of anions in autumn is indicated by ion balance while adding carbonates, spring and summer aerosol is found electroneutral. Chloride depletion phenomenon follows the snow melt and remobilization of salt applied to roads for deicing, demonstrating the biggest impact in summer. Degree of neutralization of ammonia as sulfate and nitrate is found by the correlation between the concentration of NH4+ and calculated from SO42− and NO3−. Inorganic fraction in PM10 is higher (16.5%) than total carboxylic acids (3.5%), and total sugars and anhydrosugars (0.8%). Concentration polar plots show ion variation jointly with wind speed and wind direction and the source origin. Highest SIA concentrations relate a cluster of the prevalent air mass transportation and indicate the source in the northwestern direction in spring and summer. K+ acts as a marker of fires in spring and summer as well as of the domestic biomass burning in the region around a megacity in autumn. Source apportionment of inorganic ion and BC by Varimax analysis reveals major sources namely SIA, soil resuspension, biomass burning, deicing salt, and fossil fuel combustion. In summer, organic ions, anhydrosugars, and sugars shows additional impact of photochemistry and biological activities. Rainfall is heavier in summer than in autumn, with pH of acid rain during spring and summer. The highest concentrations for almost all ions were observed in spring, except HCO3−. Precipitation composition is described with aerosol wet removal by seasonal variability of SIA scavenging factors, decreasing in summer and increasing in autumn and spring.

Observed Evidence That Subsidence Process Stabilizes the Boundary Layer and Increases the Ground Concentration of Secondary Pollutants

AbstractSubsidence in atmospheric boundary layer (ABL) appears to be rare and short‐lived, so it is very difficult to observe. In this paper, field experimental data from a large tethered balloon (1,900 m3) synchronously measuring meteorological parameters and air pollutants, a ground‐based aerosol lidar, a Doppler wind lidar, and some other ground observations were used to analyze the formation mechanism of subsidence and its influences on the variation of meteorological parameters and pollutant concentrations of the ABL. Results show that the occurrence of subsidence was closely correlated with the cold advection accompanied by cold front. Pollutants were horizontally transported to the observation site driven by southwest low‐level jet. Under the influence of subsidence behind the cold front and turbulent kinetic energy transported toward the surface, the vertical downward transport of pollutants aloft lasted for nearly 5 hr. The observation data of the tethered balloon suspending at 500 m show that the concentrations of PM2.5, NO3−, SO42−, and NH4+ increased simultaneously. O3 concentration in the lower atmosphere also increased. Subsidence had significant effects on the secondary inorganic aerosol concentrations but the concentration of organic aerosols was not affected. Subsidence led to a stronger inversion layer, further suppressing the vertical dispersion of pollutants. Both the stronger subsidence‐induced inversion layer and the downward transport of pollutants have led to an increase in the ground PM2.5 concentration. Our findings demonstrate the close connection between subsidence and air pollution process and may provide the scientific reference for air quality potential forecast.

Trends in secondary inorganic aerosol pollution in China and its responses to emission controls of precursors in wintertime

Abstract. The Chinese government recently proposed ammonia (NH3) emission reductions (but without a specific national target) as a strategic option to mitigate fine particulate matter (PM2.5) pollution. We combined a meta-analysis of nationwide measurements and air quality modeling to identify efficiency gains by striking a balance between controlling NH3 and acid gas (SO2 and NOx) emissions. We found that PM2.5 concentrations decreased from 2000 to 2019, but annual mean PM2.5 concentrations still exceeded 35 µg m−3 at 74 % of 1498 monitoring sites during 2015–2019. The concentration of PM2.5 and its components were significantly higher (16 %–195 %) on hazy days than on non-hazy days. Compared with mean values of other components, this difference was more significant for the secondary inorganic ions SO42-, NO3-, and NH4+ (average increase 98 %). While sulfate concentrations significantly decreased over this period, no significant change was observed for nitrate and ammonium concentrations. Model simulations indicate that the effectiveness of a 50 % NH3 emission reduction for controlling secondary inorganic aerosol (SIA) concentrations decreased from 2010 to 2017 in four megacity clusters of eastern China, simulated for the month of January under fixed meteorological conditions (2010). Although the effectiveness further declined in 2020 for simulations including the natural experiment of substantial reductions in acid gas emissions during the COVID-19 pandemic, the resulting reductions in SIA concentrations were on average 20.8 % lower than those in 2017. In addition, the reduction in SIA concentrations in 2017 was greater for 50 % acid gas reductions than for the 50 % NH3 emission reductions. Our findings indicate that persistent secondary inorganic aerosol pollution in China is limited by emissions of acid gases, while an additional control of NH3 emissions would become more important as reductions of SO2 and NOx emissions progress.

PM2.5 and water-soluble inorganic ion concentrations decreased faster in urban than rural areas in China

We investigated variations of PM2.5 and water-soluble inorganic ions chemical characteristics at nine urban and rural sites in China using ground-based observations. From 2015 to 2019, mean PM2.5 concentration across all sites decreased by 41.9 µg/m3 with a decline of 46% at urban sites and 28% at rural sites, where secondary inorganic aerosol (SIAs) contributed to 21% (urban sites) and 17% (rural sites) of the decreased PM2.5. SIAs concentrations underwent a decline at urban locations, while sulfate (SO42–), nitrate (NO3–), and ammonium (NH4+) decreased by 49.5%, 31.3% and 31.6%, respectively. However, only SO42– decreased at rural sites, NO3– increased by 21% and NH4+ decreased slightly. Those changes contributed to an overall SIAs increase in 2019. Higher molar ratios of NO3– to SO42– and NH4+ to SO42– were observed at urban sites than rural sites, being highest in the heavily polluted days. Mean molar ratios of NH3/NHx were higher in 2019 than 2015 at both urban and rural sites, implying increasing NHx remained as free NH3. Our observations indicated a slower transition from sulfate-driven to nitrate-driven aerosol pollution and less efficient control of NOx than SO2 related aerosol formation in rural regions than urban regions. Moreover, the common factor at urban and rural sites appears to be a combination of lower SO42– levels and an increasing fraction of NO3– to PM2.5 under NH4+-rich conditions. Our findings imply that synchronous reduction in NOx and NH3 emissions especially rural areas would be effective to mitigate NO3–-driven aerosol pollution.

Synergistic effect of reductions in multiple gaseous precursors on secondary inorganic aerosols in winter under a meteorology-based redistributed daily NH3 emission inventory within the Beijing-Tianjin-Hebei region, China

Secondary inorganic aerosols (SIA) account for 20–60% of the total fine particulates in the Beijing-Tianjin-Hebei (BTH) region of China, indicating an urgent need to clarify the relationship among such compounds. The purpose of this study was to quantify the relationship between emissions of NH3, NOx, SO2, VOCs and SIA concentrations during a severe winter haze episode using an air quality model and a meteorology-based redistributed NH3 emission inventory within the BTH region. The results showed that the model performance regarding the NH3 simulations in January by the four emission inventories improved after the redistribution of daily NH3 emissions, with an increase of 0.02–0.13 in R, a 9–56% decrease in NMB, and a 7–51% decrease in NME. The updated simulations reproduced the daily observations of SIA, SO2, and NO2 well. A total of 125 sets of sensitivity simulations showed that a synergistic reduction in NH3 and VOCs was more efficient in terms of SIA control than simply reducing SO2 or NOx in the BTH region. If only NOx emissions were reduced, the SIA concentration would first increase and then decrease, and it could decline by another 0.86–8.03% in parallel with an equal NH3 emission cut. SIA could be reduced by approximately 22.68% with the most stringent inorganic precursors' control. Moreover, VOCs emission reductions could lead to a decrease in SIA, and the impact of VOCs on SIA was similar to that of NH3. The collaborative control of both inorganic precursors and VOCs was more effective than single-factor control measures for decreasing SIA, and the decline rate was approximately 29.26% under minimum emission conditions. This improved effectiveness was obtained because VOCs mitigation effectively decreases the ozone concentration, which in turn influences SIA formation. Finally, on the premise of a 60% SO2 cut, the reduction scheme NH3:VOCs:NOx = 4:4:1 was suggested for SIA control.

Estimation and variation analysis of secondary inorganic aerosols across the Greater Bay Area in 2005 and 2015

As the concentrations of primary components of fine particulate matter (PM2.5) have substantially decreased, the contribution of secondary inorganic aerosols to PM2.5 pollution has become more prominent. Therefore, understanding the variations in and characteristics of secondary inorganic aerosols is vital to further reducing PM2.5 concentrations in the future. In this study, an ensemble back-propagation neural network model was built by combining 3D numerical models, observation data, and machine learning methods, to estimate the concentrations of secondary inorganic aerosols (SO2-4, NO-3, and NH+4) across the Greater Bay Area (GBA) in 2005 and 2015. The ensemble model provided a better estimation than the 3D numerical air quality model, with higher correlation coefficients (approximately 0.85) and lower root mean square errors. The model revealed that the concentrations of the SO2-4, NO-3, and NH+4 decreased by 1.91, 0.20, and 0.49 μg/m3, respectively, from 2005 to 2015. To investigate the oxidation and acidy of sulfate, the sulfur oxidation ratio (SOR), degree of sulfate neutralization (DSN), and particle neutralization ratio (PNR) were calculated and analyzed for 2005 and 2015 across the GBA region. The SOR slightly increased in summer, but decreased in other seasons in 2015, indicating the overall weaker sulfate chemical formation due to sulfur emission control measures. The increasing DSN and PNR indicated that more sulfate was neutralized due to reduced sulfur emission and increased ammonia availability. Our study suggests that more effort is needed to control ammonia emission to further reduce the concentrations of SO2-4, NO-3, and NH+4 across the GBA region in the future.

Evaluation of the ammonia emission sensitivity of secondary inorganic aerosol concentrations measured by the national reference method

A national reference method for fine particulate matter mass measurement usually specifies Teflon-membrane filter for the collection of ambient particles. Ammonium nitrate, one of the major components of fine particulate matter, is semi-volatile and susceptible to evaporation loss during and after field sampling by Teflon-membrane filter. The goal of this study is to evaluate the ammonia emission sensitivity of secondary inorganic aerosol (SIA) concentrations measured by the national reference method. The evaporation loss of particulate ammonium nitrate during and after field sampling was included in the analysis to cover a wide range of emission scenarios and meteorological conditions. The global to mesoscale air quality forecast and analysis system was used to predict the PM2.5 concentrations and chemical compositions. The predicted relative humidity and temperature were used to calculate the amount of ammonium nitrate evaporation loss from the thermodynamic equilibrium model. Except for the cool months, the particulate ammonium nitrate evaporation loss significantly decreases the ammonium nitrate contents in the SIA and subsequently degraded the effectiveness of ammonia emission control over the SIA. The SIA concentration decrease achieved by a 30% reduction of ammonia emission is lessened by 27–33% in the spring, 49–79% in the summer, 21–41% in the fall when the ammonium nitrate evaporation loss is included. We also proposed the normalized ammonia emission sensitivity of ammonium nitrate concentrations (γ(NH4NO3)) as a measure of the effectiveness of ammonia control and derived a mathematical relationship between γ(NH4NO3) and ammonia reduction rates in ammonia-rich conditions.

Variations in characteristics and transport pathways of PM2.5 during heavy pollution episodes in 2013–2019 in Jinan, a central city in the north China Plain

The characteristics and transport pathways of air masses vary during heavy pollution episodes (HPEs). Three categories of HPEs have been defined: HPE Ι, II, and III, corresponding to HPE durations of 1, 2, and at least 3 days, respectively. Sixty HPEs were investigated in this study. The number of HPEs decreased from 2013 to 2017 and then increased from 2017 to 2019, dominated by emission reductions and meteorological conditions. The average and maximum PM2.5 (i.e., aerodynamic diameter of <2.5 μm) concentrations during those HPEs in 2019 decreased by 5.6%–11.8% and 11.9%–38.5%, respectively, compared with those in 2013. The longer the duration of an HPE, the higher the PM2.5 concentration. Secondary inorganic aerosol concentrations and their contents in PM2.5 during HPE Ⅲ were found to be higher than those during HPEs Ι and Ⅱ, as secondary transformations of precursor gases are more intense during long-term HPEs. The dominant trajectories of airflow arriving in Jinan originated from the southern and southeastern regions during HPEs, realized using the Hybrid Single Particle Lagrangian Integrated Trajectory. The trajectories from the north and west of Jinan contained the highest PM2.5 concentrations of 323.3–432.1 μg/m3 during HPE Ⅲ, although these trajectories only contributed 5.6%–11.1% of the total dominant transport pathways, while those in trajectories from the northwest were highest during HPEs Ι and Ⅱ. The highest contributions of air masses from short distances were found during HPE Ⅲ, of 77.8%, while they were only 65.6% and 47.8% during HPEs Ι and II, respectively. More attention should be given to transport pathways within the short distance from Jinan. Therefore, enhancing regional cooperation in Jinan and surrounding regions (particularly in the south, southeast, northwest, west, and north) is critical for improving air quality in the North China Plain.

Nonlinear response of SIA to emission changes and chemical processes over eastern and central China during a heavy haze month

This study used a chemical transport model to investigate the response of secondary inorganic aerosols (SIA) to chemical processes and its precursor emissions over northern and southern city-clusters of China in January 2014. Unexpectedly, SIA concentrations with low levels of precursor emissions were much higher over the southern regions than those over the northern region with high levels of precursor emissions, based on ground observations and high-precision simulations. The sensitivity analysis of chemical processes suggests that the gas-phase chemistry was a critical factor determining the SIA pattern, especially the higher efficiency of nitrogen conversion to nitrate in southern cities controlled by favorable meteorological elements than that in northern city. However, the heterogeneous process led to the decrease of SIA in southern regions by 3% to 36% and the increasing of SIA in NCP by 26.9%, mainly attributing to the impact on nitrate. The reason was that sulfate enhancement by the heterogeneous reactions can compete ammonia (NH3) and the excessive nitric acid converted into nitrogen oxide (NOx), leading to nitrate decrease in southern regions under NH3-deficient regimes. Moreover, through sensitivity experiments of precursor emission reduction by 20%, NH3 control was found to be the most effective for reducing SIA concentrations comparing to sulfur dioxide (SO2) and NOx reduction and a more remarkable decrease of SIA was in southern regions by 10% to 15% than that in northern region by 6.7%. The effect of the synergy control of precursors emission varied in different city-clusters, inferring that the control strategy aimed at improving air quality should be implemented based on specific characteristics of precursors emission in different regions of China.

Characteristics of water-soluble inorganic aerosol pollution and its meteorological response in Wuhan, Central China

Due to the rapid economic development in Central China, PM2.5 (particles not larger than 2.5 μm) pollution is serious in recent years. To fully understand the causes of PM2.5 in this region, we conducted a long-term tracking (2015–2018) on water-soluble inorganic ions (WSIIs) in PM2.5, including sulfate (SO42−), nitrate (NO3−), ammonium (NH4+), chlorine (Cl−), potassium (K+), calcium (Ca2+), sodium (Na+), and magnesium (Mg2+). Results indicate that each ion has significant seasonal variations, with the maximum concentration in winter and minimum in summer. The secondary inorganic aerosols (SIAs) have remarkably higher concentrations in haze periods than non-haze periods, which indicates that the outbreak of haze is closely related to the massive generation of SIAs. Moreover, each ion shows a different response to the variation of meteorology, implying the different formation mechanisms. Generally, high pollution levels of Ca2+, Mg2+, K+, Cl−, and Na+ are correlated to prevailing winds, suggesting the significant contribution of regional transport. High concentrations of SIAs are usually accompanied by the low temperature (T < 15 °C), high relative humidity (RH > 60%), and calm wind, which means they are mainly locally generated. Under conditions with low T and high RH, more gaseous sulfur and nitrogen tend to be converted into solid inorganic salts, proved by the higher oxidation efficiency. Different to NO3−, SO42− is also easily generated at high T through photochemical oxidization. This study interprets fine particle pollution from the perspective of particle composition, and the results provide an reference for regional air pollution prevention.

Role of ammonia in European air quality with changing land and ship emissions between 1990 and 2030

Abstract. The focus of this modeling study is on the role of ammonia in European air quality in the past as well as in the future. Ammonia emissions have not decreased as much as the other secondary inorganic aerosol (SIA) precursors – nitrogen oxides (NOx) and sulfur dioxide (SO2) – since the 1990s and are still posing problems for air quality and the environment. In this study, air quality simulations were performed with a regional chemical transport model at decadal intervals between 1990 and 2030 to understand the changes in the chemical species associated with SIA under varying land and ship emissions. We analyzed the changes in air concentrations of ammonia, nitric acid, ammonium, particulate nitrate and sulfate as well as changes in the dry and wet deposition of ammonia and ammonium. The results show that the approximately 40 % decrease in SIA concentrations between 1990 and 2010 was mainly due to reductions in NOx and SO2 emissions. The ammonia concentrations on the other hand decreased only near the high-emission areas such as the Netherlands and northern Italy by about 30 %, while there was a slight increase in other parts of Europe. Larger changes in concentrations occurred mostly during the first period (1990–2000). The model results indicate a transition period after 2000 for the composition of secondary inorganic aerosols due to a larger decrease in sulfate concentrations than nitrate. Changes between 2010 and 2030 – assuming the current legislation (CLE) scenario – are predicted to be smaller than those achieved earlier for all species analyzed in this study. The scenario simulations suggest that if ship emissions will be regulated more strictly in the future, SIA formation will decrease especially around the Benelux area, North Sea, Baltic Sea, English Channel and the Mediterranean region, leaving more ammonia in the gas phase, which would lead to an increase in dry deposition. In the north of the domain, the decrease in SIA would be mainly due to reduced formation of particulate nitrate, while the change around the Mediterranean would be caused mainly by decreased sulfate aerosol concentrations. One should also keep in mind that potentially higher temperatures in the future might increase the evaporation of ammonium nitrate to form its gaseous components NH3 and HNO3. Sensitivity tests with reduced NOx and NH3 emissions indicate a shift in the sensitivity of aerosol formation from NH3 towards NOx emissions between 1990 and 2030 in most of Europe except the eastern part of the model domain.

Roles of RH, aerosol pH and sources in concentrations of secondary inorganic aerosols, during different pollution periods

Abstract Secondary inorganic aerosols (SIA, including sulfate, nitrate, and ammonium) were found to be the most dominant aerosol components during haze period. Elucidating the driving factors during liquid-phase reaction formation process of SIA is a key step for understanding the origin of airborne particulate pollution. In this work, we used online instruments and collected hourly concentrations of ions, gases, meteorological parameters in a northern city of China. Source emissions were estimated by receptor model PMF/ME2 pulling run solution (Partial Target Transformation-PMF: PTT-PMF), and six sources were identified (dust, coal combustion, vehicle exhaust, secondary sulfate, secondary nitrate and biomass burning & SOC). pH values were calculated by ISORROPIA-II, with an average value of 4.68. The relationships between RH, pH, source behaviors and concentrations of SIA during different pollution episodes were discussed. Source emissions were found to influence pH and subsequently the SIA; high RH could result in high SIA because aerosol water content in atmosphere increases with RH, which is favorable for liquid-phase reactions. Finally, we employed statistical and machine learning (Random forest classification: RFC) methods to analysis importance of driving factors on SIA formation during different pollution episodes. Results showed precursors, wet conditions, atmospheric oxidation capacity and acidity favor formation of sulfate and nitrate at different pollution levels; changes of gaseous precursors, RH and temperature are more important to determine PM2.5 pollution classification (clean, slightly polluted, moderately polluted, heavily polluted episodes). The findings of this work can provide useful information for better understanding formation mechanism of SIA, mitigating air pollution and improving human health.

The impact of secondary inorganic aerosol emissions change on surface air temperature in the Northern Hemisphere

Using the Community Earth System Model (CESM) version 1.2, this study investigates the changes in secondary inorganic aerosols (SIOAs) over the Northern Hemisphere from 1850 to 2007, regional contributions, and consequences on surface air temperature. Results show that SIOAs changes can be divided into two stages. At the first stage (1850–1980), European and North American SIOAs concentrations increase, with a cooling effect especially over Europe and Eastern Siberia. At the second stage (1980–2007), SIOAs concentrations over Europe and North America are reduced with a warming effect in the mid-high latitudes, whereas SIOAs increase over East Asia and South Asia leading to a cooling effect there. The temperature changes over the emission source regions are mainly driven by radiative forcing. Horizontal transfer of heat leads to a temperature response in the Siberian region.