Formation Of Secondary Pollutants Research Articles

Reactive Chlorine Species Advancing the Atmospheric Oxidation Capacities of Inland Urban Environments.

Chlorine (Cl) radicals from photolabile chlorine species are highly reactive and can affect the fate of air pollutants in the atmosphere. Although several campaigns have been conducted, typically in coastal environments, long-term observations of reactive chlorine species and their impacts on atmospheric oxidation capacities (AOCs) are lacking. Here, we report nearly full-year observations of Cl2 and ClNO2 levels in Beijing and evaluate their impacts on the AOC with a box model coupled with Cl chemistry. Cl radicals promote the circulation of OH-HO2-RO2 by accelerating the OH chain lengths by up to 12.6% on average, hence boosting the AOC, especially in the winter or spring. This promotion effect is nonlinearly dependent on the VOC and NOx concentrations, thus leading to a slight shift in ozone formation from a VOC-sensitive regime to a transition regime with seasonal differences. Given the ubiquitous reactive chlorines in polluted inland urban regions, the AOCs and the formation of secondary pollutants will be underestimated if the reactive chlorine species are neglected.

Evolution Characteristics of Atmospheric Formaldehyde Emissions in Guangdong Province from 2006 to 2020

Atmospheric formaldehyde, a key precursor for ozone (O3) and secondary PM2.5, is carcinogenic and plays an important role in atmospheric photochemistry and the formation of secondary pollution. However, the lack of understanding of the emission sources of atmospheric formaldehyde limits the study on the formation mechanism of secondary pollution and the formulation of pollution control strategies. This study used the emission factor and source profile methods to establish the emission inventories of formaldehyde in Guangdong Province from 2006 to 2020 and identified the main emission sources of formaldehyde and spatial and temporal evolution characteristics. The results showed that the formaldehyde emissions in Guangdong Province fluctuated in the range of 39000-56000 tons during 2006 to 2020, exhibiting a very weak downward trend. Biomass combustion is an important source of formaldehyde emission in Guangdong Province, of which the contribution decreased from 58% in 2006 to 27% in 2020 owing to effective control measures implemented in Guangdong Province. The solvent use source became the predominant emission source of formaldehyde in 2020 by contributing up to 28%, primarily through plastic products and asphalt paving sources. The construction machinery and trucks fueled by diesel were important contributors of formaldehyde emissions from mobile sources. Although the formaldehyde emissions in the Pearl River Delta and the non-Pearl River Delta were equivalent, the spatial distributions showed that formaldehyde emission hotspots were concentrated in the center of the Pearl River Delta and the eastern and western areas of the non-Pearl River Delta. This was primarily because the solvent use and mobile sources were the main sources of formaldehyde emissions in the Pearl River Delta, whereas the biomass combustion source was the dominant source in the non-Pearl River Delta. Therefore, the formaldehyde emission mitigations of the industrial and mobile sources in the central region of the Pearl River Delta and the biomass combustion source in the western area of Guangdong should be further strengthened in the future.

Multi-objective optimization study of hydrothermal aging on NOx-assisted soot catalytic combustion and secondary pollution emission in regeneration of CeO2 catalytic diesel particulate filter

The present study aimed to explore the effect of hydrothermal aging on the heterogeneous catalytic combustion process of soot in CeO2 catalytic diesel particular filter and the formation of secondary pollutants (N2O and CO).The present study was based on a fixed-bed experimental bench for NOx-assisted soot catalytic oxidation in CeO2-CDPF with different soot loading and catalyst coating amounts. A non-linear regression surrogate model and a multi-objective optimization decision approach were used to analyze the trade-off relationship between the design variables and the optimization objectives. The results showed that lower levels of soot loading (2.5 g/L), CeO2 catalyst coating amount (2.5 g/L) and higher NOx concentration (1000 ppm) in fresh catalysts were favorable for improving the performance of soot-assisted catalytic combustion and minimizing the formation of N2O and CO. However, it was observed that higher levels of catalyst coating amount (10 g/L) resulted in better soot catalytic combustion performance with lower N2O and CO emissions. Global sensitivity analysis showed a positive correlation between N2O and CO peak concentrations and peak temperatures with soot loading, but a negative correlation with catalyst loading. Over time, hydrothermal aging of CDPF caused the inhibitory effect of CeO2 catalyst on N2O to shift from negative to positive correlation, while also decreasing the synergistic performance of NOx in soot catalytic combustion.

Atmospheric oxidation capacity and secondary pollutant formation potentials based on photochemical loss of VOCs in a megacity of the Sichuan Basin, China

Volatile organic compounds (VOCs) are significant precursors to photochemical pollution. However, reactive VOC species are easily oxidized during transportation, resulting in a systematic underestimate of the measured concentrations. To address this, we applied an improved calculation method to correct the measured VOC concentrations into photochemical initial concentrations (PICs) in Chengdu, a megacity in the Sichuan Basin, China, which is highly vulnerable to complex pollution. In this study, 56 VOC species on the Photochemical Assessment Monitor Station (PAMS) target list were quantitatively monitored throughout all four seasons. Comparing to directly measured values, photochemically initialized total mixing ratios of VOCs increased by 18.6 % in general. The photochemical loss percentages of alkenes and aromatics were prominent in summer (68.6 %, 28.7 %) and spring (65.9 %, 24.7 %), respectively. Furthermore, we examined contributions of VOCs to atmospheric oxidation capacity (AOC) depending on PICs and found that maximum daily total AOC showed a surge in spring and summer. Besides hydroxyl radicals, daytime O3 in spring and late-afternoon nitrate radicals in summer were essential for AOC with PICs. As expected, alkenes and aromatics dominated PIC-based ozone formation potentials (OFPs). Furthermore, contribution of alkenes to secondary organic aerosol formation potentials reached 15.5 % and 7.6 % in spring and summer, respectively. Using positive matrix factorization model, we identified five VOC sources including vehicular exhaust, industrial emissions, solvent usage, biogenic sources, and liquefied petroleum gas/natural gas use. Based on PICs, biogenic sources were significantly underestimated in spring and summer. Meanwhile, m,p-xylene from solvent usage and isoprene from biogenic sources were the primary contributors to OFPs. Consequently, these results emphasize the significance of photochemically oxidized VOC concentrations, especially for reactive species in typical seasons.

Recent advances in heavy metal removal by thin film nanocomposite membrane

Abstract Rapid industrialization has become one of the root causes of environmental problems, particularly heavy metal pollution. Numerous methods, including chemical precipitation and ion exchange, have been used to remove heavy metal ions from aqueous solutions, but all of them have drawbacks, such as low metal removal effectiveness, significant reagent loss, excessive energy use, and the need for further development of the current techniques. Membrane-based technologies such as reverse osmosis (RO) and nanofiltration (NF) are gaining popularity in the removal of heavy metals due to their high rejection and low formation of secondary pollutants. Among the methods under consideration, the membrane-based approach holds the most promise for wastewater treatment with a focus on heavy metal ion removal due to its ease of manufacture, superior qualities, and increased separation effectiveness. Since membrane performance is typically hampered by fouling, low permeability, and significant contaminant permeation when compared to strict selectivity criteria, the development of novel membrane materials has emerged as an important area of research for academia, industry, and national laboratories. We have therefore reviewed previous initiatives and technological developments in order to achieve more effective heavy metal removal and recovery from industrial wastewater.

OH measurements in the coastal atmosphere of South China: possible missing OH sinks in aged air masses

Abstract. The hydroxyl radical (OH) is the main atmospheric oxidant responsible for the removal of many reduced trace gases and the formation of secondary air pollutants. However, due to technical difficulties in measuring OH, the existing measurements of atmospheric OH concentrations are limited, and its sources and sinks are not well understood under low-nitrogen-oxide (NOx) conditions. In this study, we observed the OH concentrations using chemical ionization mass spectrometry at a coastal site in Hong Kong from October to November 2020. The average noontime OH concentration over the study period was measured at 4.9±2.1×106 cm−3. We found that a box model with comprehensive observational constraints reproduced the observed daytime OH concentrations when air parcels originated from the continental regions. However, this model overpredicted the observed daytime OH concentrations for coastal air parcels by 142 % on average. Unaccounted-for OH sinks in the model are proposed to be the cause of this overprediction. A missing OH reactivity, which is defined as the pseudo-first-order rate coefficient for OH loss by unmeasured trace gases, was estimated as 5.0±2.6 s−1 (lower limit) in the coastal air, and the missing reactivity increased with decreasing concentrations of NOx and volatile organic compounds (VOCs). Further studies are needed to find out the exact cause of the model overestimation and to identify the suspected unmeasured chemical species that contribute to the OH budget, in order to better quantify the formation of secondary air pollutants.

Secondary aerosol formation in incense burning particles by O3 and OH oxidation via single particle mixing state analysis

Incense burning is a common religious activity that emits abundant gaseous and particulate pollutants into the atmosphere. During their atmospheric lifetime, these gases and particles are subjected to oxidation, leading to the formation of secondary pollutants. We examined the oxidation of incense burning plumes under O3 exposure and dark condition using an oxidation flow reactor connected to a single particle aerosol mass spectrometer (SPAMS). Nitrate formation was observed in incense burning particles, mainly attributable to the ozonolysis of nitrogen-containing organic compounds. With UV on, nitrate formation was significantly enhanced, likely due to HNO3/HNO2/NOx uptake triggered by OH chemistry, which is more effective than ozone oxidation. The extent of nitrate formation is insensitive to O3 and OH exposure, possibly due to the diffusion limitation on interfacial uptake. The O3-UV-aged particles are more oxygenated and functionalized than O3-Dark-aged particles. Oxalate and malonate, two typical secondary organic aerosol (SOA) components, were found in O3-UV-aged particles. Our work reveals that nitrate, accompanied by SOA, can rapidly form in incense-burning particles upon photochemical oxidation in the atmosphere, which could deepen our understanding of air pollution caused by religious activities.

Unequal Health Risks of Integrated Volatile Organic Compounds Emitted From Various Anthropogenic Sources

AbstractVolatile organic compounds (VOCs) emitted from anthropogenic sources are critical precursors to ozone and secondary organic aerosol (SOA), and potential contributors to health risks. However, quantitatively estimating the impact of these source‐derived VOCs on air quality and health is limited by their diverse chemical compositions, resulting in nonselective mass‐based VOC control strategies. To address this issue, we conducted a comprehensive study of typical VOC samples freshly emitted from various flue gases via field‐based measurements and chemical analysis under the same quality control. Our results showed that different source‐specific VOC profiles have unequal sensitivities to chemical reactivity on ozone and SOA formation potential, as well as potential health risks. Mobile sources and residential solid fuel burning were identified as the most sensitive sources of potential secondary pollutants and potential inhalation health risks, respectively. Despite accounting for only 22.7% of the total mass, residential VOCs contributed significantly to both inhalation non‐cancer risk (NCR) (57.4%) and cancer risk (CR) (47.3%). The large inequality between residential VOC emissions and their health risks highlights the necessity of continuously promoting the transition from mass‐based VOC control to effect‐oriented VOC control. This suggests that mobile sources is worth more attention for secondary pollutant formation, while residential solid fuel burning sources should receive more policy priority to reduce health risks.

Observation of nitrogen oxide-influenced chlorine chemistry and source analysis of Cl2 in the Yangtze River Delta, China

Atmospheric chlorine (Cl)- and nitrogen (N)-containing species such as Cl2, ClNO2, and N2O5 participate actively in atmospheric chemistry by triggering radical reactions that lead to the formation of secondary pollutants. Despite of their importance in atmospheric chemistry, the measurements of these species are scarce, due to analytical challenges. Herein, we report observation results of Cl2, ClNO2, and N2O5 from measurements using high-resolution time-of-flight chemical ionization mass spectrometry (HR-ToF-CIMS) in Shanghai and Changzhou in the highly industrialized and populated Yangtze River Delta (YRD) of East China. In Changzhou, the maximum concentrations of N2O5, ClNO2, and Cl2, were 0.4, 1.3, and 0.5 ppbv (1-min averages), respectively, while they were 0.5, 5.7, and 1.1 ppbv, respectively, in Shanghai. Much higher ClNO2 levels were found in Shanghai compared to Changzhou, although the concentrations of the precursor (N2O5) are similar, indicating different heterogenous N2O5 uptake and Cl activation efficiency at these two sites. The diurnal variation of Cl2 in Shanghai shows a strong daytime peak, which is contradictory to its fast lost by photolysis during daytime; the diurnal pattern of Cl2 in Changzhou shows a prominent night peak and a much smaller daytime peak than that in Shanghai. The heterogeneous uptake of ClNO2 and the photolysis of O3 on chloride-containing aerosols are believed to contribute to efficient Cl2 formation at night and day, respectively. The estimated heterogeneous uptake coefficient of O3 is 0.004 ± 0.007, with a maximum of 0.034. Further analysis on these heterogeneous reactions with the help of aerosol composition data (chloride, liquid water content, pH etc.) was also performed to ascertain the role of the secondary formation of Cl2 in daytime Shanghai. Through simple estimation methods, the maximum daytime production rate of Cl2 was estimated to be 0.95 ppbv h−1, while daytime photolysis of Cl2 contributed to 98% of Cl formation. Our results indicate that there is a large secondary source of Cl2 in the YRD region during the day, which can contribute substantially to Cl formation and requires more investigations.

AEROSPACE TECHNOLOGIES FOR ASSESSING SOIL CONTAMINATION

Actuality. Studies of soil pollution assessment are based on scientific principles that define a complex system of environmental safety management in the context of increased exposure to sources of secondary dust pollution of the atmosphere. To ensure environmental safety under conditions of high levels of dust pollution, it is necessary to apply and improve the relevant models. Among the many types of environmental pollution, dust pollution of the atmospheric air and the deposition of harmful substances on the soil are particularly dangerous. This pollution can take two forms: direct emissions from industrial enterprises (primary) or the formation of secondary pollution through physical and chemical processes in places where dust-like waste is stored. Fine waste after air purification with dimensions of less than 100 microns is particularly hazardous. In modern environmental monitoring and assessment of soil pollution, special attention is paid to remote methods that allow for more effective monitoring of the impact of human activity and solving environmental problems. The use of unmanned aerial vehicles is one such method that has positive results. The purpose of the article is to solve the scientific problem of improving aerospace methods based on unmanned aerial vehicles (UAVs) for monitoring and assessing the quality of soil pollution. The object of the study is the use of aerospace tools for monitoring and assessing the condition of soil cover. To achieve this goal, the following tasks have been defined: to study the current state and ways to improve the efficiency of UAVs in the system of environmental monitoring of soils; to develop models of environmental assessment; to analyze existing approaches to the use of aerospace assets for monitoring and assessing the state of soil cover. Conclusions: a methodological approach based on a modified method of the comprehensive assessment of the level of technogenic hazard of industrial facilities is proposed to assess the state of environmental safety in conditions of intense dust pollution of the atmospheric air.

Aging of pollution air parcels acts as the dominant source for nocturnal HONO

Atmospheric HONO acts as a major source for OH radicals in polluted areas, playing an important role in formation of secondary pollutants. However, the atmospheric HONO sources remain unclear. Here we propose that the heterogeneous reaction of NO2 on aerosols during aging processes acts as the dominant source for nocturnal HONO. Based on the nocturnal variations of HONO and related species in Tai'an city of China, we firstly developed new method to estimate the localized HONO dry deposition velocity (v(HONO)). The estimated v(HONO) of 0–0.077 m/s was in a good agreement with the reported ranges. Additionally, we set up a parametrization to reflect the HONO formation from the aged air parcels based on the variation of HONO/NO2 ratio. The detailed variation of nocturnal HONO could be well reproduced by a complete budget calculation coupled with above parameterizations, with the difference between the observed and calculated HONO levels being <5 %. The results also revealed the average contribution of HONO formation from aged air parcels to atmospheric HONO could achieve to be ~63 % in average.

Atmospheric NOx oxidation as major sources for nitrous acid (HONO)

Nitrous acid (HONO) is the major precursor of hydroxyl (OH) radicals to initiate tropospheric chemistry leading to formation of secondary pollutants. The sources of atmospheric HONO, however, are not fully understood. Here we show two additional HONO sources that stem from atmospheric oxidation of nitrogen oxide (NOx = NO + NO2). Nitric acid (HNO3) formed from photooxidation of NO2 can be converted into HONO with a yield of ~53%, and dark NO oxidation by NO3 radicals in the presence of H2O produces HONO with a yield of 2%. The diurnal variations of HONO levels from field observations in the urban (Beijing) and rural (Wangdu) areas of the North China Plain can be well reproduced by the WRF-Chem model when the two new HONO sources are taken into account. The findings imply that atmospheric NOx oxidation pathways are the major sources for HONO, which can significantly accelerate ozone formation in polluted regions as well.

Characterization of Volatile Organic Compound Emissions and CO2 Uptake from Eco-roof Plants.

Vegetation plays an important role in biosphere-atmosphere exchange, including emission of biogenic volatile organic compounds (BVOCs) that influence the formation of secondary pollutants. Gaps exist in our knowledge of BVOC emissions from succulent plants, which are often selected for urban greening on building roofs and walls. In this study, we characterize the CO2 uptake and BVOC emission of eight succulents and one moss using proton transfer reaction - time of flight - mass spectrometry in controlled laboratory experiments. CO2 uptake ranged 0 to 0.16 μmol [g DW (leaf dry weight)]-1 s-1 and net BVOC emission ranges -0.10 to 3.11 μg [g DW]-1 h-1. Specific BVOCs emitted or removed varied across plants studied; methanol was the dominant BVOC emitted, and acetaldehyde had the largest removal. Isoprene and monoterpene emissions of studied plants were generally low compared to other urban trees and shrubs, ranging 0 to 0.092 μg [g DW]-1 h-1 and 0 to 0.44 μg [g DW]-1 h-1, respectively. Calculated ozone formation potentials (OFP) of the succulents and moss range 4×10-7 - 4×10-4 g O3 [g DW]-1 d-1. Results of this study can inform selection of plants used in urban greening. For example, on a per leaf mass basis, Phedimus takesimensis and Crassula ovata have OFP lower than many plants presently classified as low OFP and may be promising candidates for greening in urban areas with ozone exceedances.

Sources Causing Long-Term and Seasonal Changes in Combustion-Derived Particulate Matter in the Urban Air of Sapporo, Japan, from 1990 to 2002

Fifty-one samples were collected seasonally to estimate the amounts of total suspended particulate (TSP) in Sapporo, Japan, from 1990 to 2002. The atmospheric concentration of combustion-derived particulate (Pc) was calculated based on the NP method using 1-nitropyrene and pyrene. The atmospheric TSP and Pc concentration ranges were between 31–121 µg m−3 of air (Mean ± standard deviation (SD) = 58.2 ± 20.2 µg m−3) and 31–121 µg m−3 (Mean ± SD = 8.2 ± 6.0 µg m−3), respectively. First-order linear regression equations suggested that the Pc fraction decreased faster than TSP. The highest and lowest Pc concentrations were observed in winter and summer, respectively, whereas the highest and lowest TSP concentrations were observed in spring and winter, respectively. The largest and smallest Pc/TSP concentration ratios were observed in winter (0.324) and summer (0.075), respectively. The seasonal fractions of high-temperature combustion-derived particulate (Ph) in Pc ranged from 0.56 (winter) to 0.75 (summer), suggesting that the contribution of vehicle emissions to Pc was always larger than those of coal and biomass combustion. The sources of long-term and seasonal change in Pc were elucidated by analyzing organic source markers. Atmospheric concentrations of polycyclic aromatic hydrocarbons (PAHs), nitropolycyclic aromatic hydrocarbons (NPAHs) and hopanes showed long-term and seasonal changes similar to those of Pc, although biomarkers of biomass and coal combustion, such as levoglucosan, mannosan, and galactosan were not as strongly correlated. These results suggest that the change in the Pc concentration was mainly affected by vehicle emissions rather than by coal and biomass combustion or secondary pollutant formation. The decrease in the Pc over the study period was mainly a result of the Japanese particulate matter/NOx regulations on vehicle exhaust.

Real-time measurements of non-methane volatile organic compounds in the central Indo-Gangetic basin, Lucknow, India: source characterisation and their role in O3 and secondary organic aerosol formation

Abstract. Lucknow is the capital of India's largest state, Uttar Pradesh, one of South Asia's most polluted urban cities. Tropospheric photochemistry relies on non-methane volatile organic compounds (NMVOCs), which are ozone and secondary organic aerosol precursors. Using the proton-transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS) at an urban background site in Lucknow, the chemical characterisation of NMVOCs was performed in real time from December 2020 to May 2021. About ∼ 173 NMVOCs from m/z 31.018 to 197.216 were measured during the study period, including aromatics, non-aromatics, oxygenates, and nitrogen-containing compounds. The campaign daily mean concentrations of the NMVOCs were 125.5 ± 37.5 ppbv. The NMVOC daily average concentrations were about ∼ 30 % higher during the winter months (December–February) than in summer (March–May). The oxygenated volatile organic compounds and aromatics were the dominant VOC families, accounting for ∼ 57 %–80 % of the total NMVOC concentrations. Acetaldehyde, acetone, and acetic acid were the major NMVOC species, 5–15 times higher than the other species. An advanced multi-linear engine (ME-2) model was used to perform the NMVOC source apportionment using positive matrix factorisation (PMF). It resolves the five main sources contributing to these organic compounds in the atmosphere. They include traffic (23.5 %), two solid fuel combustion factors, SFC 1 (28.1 %) and SFC 2 (13.2 %), secondary volatile organic compounds (SVOCs) (18.6 %), and volatile chemical products (VCPs) (16.6 %). Aged and fresh emissions from solid fuel combustion (SFCs 1 and 2) were the dominant contributors to the total NMVOCs, and compounds related to these factors had a high secondary organic aerosol (SOA) formation potential. Interestingly, the traffic factor was the second-highest contributor to the total NMVOCs, and compounds related to this factor had a high ozone formation potential. Significant differences in the composition of the two solid fuel combustions indicate the influence of local emissions and transport of regional pollution to the city. The high temperature during summer leads to more volatilisation of oxygenated VOCs, related to the VCP factor. The study is the first attempt to highlight the sources of NMVOCs and their contribution to secondary pollutant (SOA and O3) formation in the city of Lucknow during winter and summer. The insights from the study would help various stakeholders to manage primary and secondary pollutants within the city.

Compositional Characteristics of Volatile Organic Compounds in Typical Industrial Areas of the Pearl River Delta: Importance of Oxygenated Volatile Organic Compounds

Volatile organic compounds (VOCs) are key components of tropospheric chemistry, of which industrial emissions are an important source of atmospheric VOCs. In this study, online measurements of 74 VOCs were made in a typical industrial area of the Pearl River Delta in southern China during the early summer of 2021. The mean volume mixing ratio of total volatile organic compounds (TVOC) was (81.9±45.4)×10-9 during the campaign. Among them, oxygenated volatile organic compounds (OVOCs) accounted for the largest fraction of TVOC, with an average of 51.5%, followed by aromatics, accounting for 19.4% of TVOC. The proportion of OVOCs in TVOC gradually increased with the increase in TVOC concentration. Industry-related emissions were the main contributors to aromatics and OVOCs in this region. Aromatics and OVOCs were the two major contributors to the ozone formation potential (OFP), accounting for 56.4% and 26.7%, respectively. Furthermore, OVOCs also contributed 40.0% of the total ·OH reactivity from VOCs. Xylenes, toluene, acrolein, and ethyl acetate had a greater contribution to the formation of secondary pollution; thus, these species should be given priority for controlling secondary pollution. Our results underscore the severity of OVOCs pollution in industrial areas and the important roles of OVOCs in secondary pollution.

Investigation of HO2 uptake mechanisms onto multiple-component ambient aerosols collected in summer and winter time in Yokohama, Japan

The heterogeneous loss of HO2 radicals onto ambient aerosols plays an important role in tropospheric chemistry. However, sparse investigation of the dominating parameters controlling the HO2 uptake coefficients onto ambient aerosols (γHO2) has largely hindered the application of the measured γHO2 to the global spatial prediction. Here we induced an offline method using LFP-LIF technique to measure the kinetics of HO2 uptake onto ambient aerosols collected in summertime and wintertime in Yokohama city, a regional urban site near Tokyo, Japan. By controlling the dominating parameters which influence γHO2, we were able to investigate the detailed HO2 uptake mechanism. We characterized the chemical composition of the collected ambient aerosols, including organics, inorganics, transition metals ions, etc. and modeled γHO2 using different mechanisms. Results show that γHO2 increased with the increase in RH, and the aerosol states (“dry” or wet/aqueous) have large effects on γHO2. With fixed RH and aerosol chemical composition, γHO2was highly dependent on pH and inversely correlated with [HO2]0. By combing the measured γHO2 values with the modeled ones, we found that both the HO2 self-reaction and transition metal-catalyzed reactions should be accounted for to yield a single parameterization to predict γHO2, and different chemical compositions may have collective effects on γHO2. Results may serve for extending the γHO2 values measured at one observation site to different environmental conditions, which will help us to achieve more accurate modeling results concerning secondary pollutant formation (i.e., ozone).

Development of an integrated machine-learning and data assimilation framework for NOx emission inversion

As major air pollutants, nitrogen oxides (NOx, mainly comprising NO and NO2) not only have adverse effects on human health but also contribute to the formation of secondary pollutants, such as ozone and particulate nitrate. To acquire reasonable NOx simulation results for further analysis, a reasonable emission inventory is needed for three-dimensional chemical transport models (3D-CTMs). In this study, a comprehensive emission adjustment framework for NOx emission, which integrates the simulation results of the 3D-CTM, surface NO2 measurements, the three-dimensional variational data assimilation method, and an ensemble back propagation neural network, was proposed and applied to correct NOx emissions over China for the summers of 2015 and 2020. Compared with the simulation using prior NOx emissions, the root-mean-square error, normalized mean error, and normalized mean bias decreased by approximately 40 %, 40 %, and 60 % in NO2 simulation using posterior NOx emissions corrected by the framework proposed in this work. Compared with the emissions for 2015, the NOx emission generally decreased by an average of 5 % in the simulation domain for 2020, especially in Henan and Anhui provinces, where the percentage reductions reached 24 % and 19 %, respectively. The proposed framework is sufficiently flexible to correct emissions in other periods and regions. The framework can provide reliable and up-to-date emission information and can thus contribute to both scientific research and policy development relating to NOx pollution.

Photodegradation of xylene isomers: Kinetics, mechanism, secondary pollutant formation potential and health risk evaluation

Photodegradation technology has been widely applied in the purification of industrial aromatic hydrocarbons. However, whether this technology efficiently removes the pollutants to prevent secondary pollution and health risk is still unclear. Here, the photodegradation processes of three xylenes were compared under designed reaction atmospheres and light sources. Xe lamp showed poor photodegradation ability toward xylenes, no matter in N2 or N2+O2 system, while much higher photodegradation performance of xylenes were obtained under ultraviolet (UV) and vacuum ultraviolet (VUV) irradiation, especially in N2+O2+VUV system, where 97.9% of m-xylene, 99.0% of o-xylene or 87.5% of p-xylene with the initial concentration of 860 mg/m3 was removed within 240 min. The xylenes underwent three processes of photo-isomerization, photodecomposition and photo-oxidation to produce intermediates of aromatics, alkanes and carbonyls. Among them, the photo-isomerization products showed the highest concentration percentage (e.g., ≥50% in o-xylene system), confirming that photo-isomerization reaction was the dominated photodegradation process of xylenes. Moreover, these isomerized products not only contributed about 97% and 91% to the formation potential of O3 (OFP) and secondary organic aerosols (SOAFP), but also displayed obvious non-carcinogenic risk, although one of photodecomposition product—benzene showed the highest occupational exposure risk. Therefore, the secondary pollution and health risks of photodegradation products of xylenes were non-ignorable, although the OFP, SOAFP and health risks of the generated products reduced at least 4.5 times in comparison with that of the degraded xylenes. The findings are helpful for the appropriate application of this technology in the purification of industrial organic waste gas.

State of gaseous air pollutants and resulting health effects in Karachi, Pakistan.

Karachi, Pakistan, is a priority site for air pollution research due to high emissions of air pollutants from vehicular traffic, industrial activities, and biomass burning, as well as rapid growth in population. The objectives of this study were to investigate the levels of gaseous pollutants (NO, NO2, O3, HNO3, and SO2) in Karachi, to determine temporal and seasonal variations, to compare Karachi's air quality with other urban centers, to identify relationships with meteorological conditions, to identify source characterization, and to perform a backward-in-time trajectory analysis and a health impact assessment. Daily samples of gaseous pollutants were collected for six consecutive weeks in each of the four seasons for a year. Daily maximum concentrations of NO (90 parts per billion by volume (ppbv)), NO2 (28.1 ppbv), O3 (57.8 ppbv), and SO2 (331 ppbv) were recorded in fall, while HNO3 (9129 parts per trillion by volume (pptv)) was recorded in spring. Seasonal average concentrations were high in winter for NO (9.47 ± 7.82 ppbv), NO2 (4.84 ± 3.35 ppbv), and O3 (8.92 ± 7.65 ppbv), while HNO3 (629 ± 1316 pptv) and SO2 (20.2 ± 39.4 ppbv) were high in spring and fall, respectively. The observed SO2 seasonal average concentration in fall (20.2 ± 39.4) was 5 times higher than that in summer (3.97 ± 2.77) with the fall 24-h average (120 ppbv) exceeding the WHO daily guideline (7.64 ppbv) by a factor of about 15.7. A health impact assessment estimated an increase of 1200 and 569 deaths due to short-term exposure to SO2 in fall and spring, respectively. Chronic daily intake estimated risk per 1000 was 0.99, 0.47, 0.45, and 0.26 for SO2 in fall, NO in winter, O3 in winter, and NO2 in spring, respectively. This study confirms the effect of poor urban air quality on public health and demonstrated the influence of photochemical reactions as well as unfavorable meteorological conditions on the formation of secondary pollutants.