Formation Of Secondary Pollutants Research Articles

Investigating the sources of atmospheric nitrous acid (HONO) in the megacity of Beijing, China

Nitrous acid (HONO) can powerfully influence atmospheric photochemistry by producing hydroxyl radical (OH), which is a crucial oxidant that controls the fate of atmospheric trace species. To deduce HONO formation mechanisms in polluted regions, two field observations were conducted in urban Beijing during the early summer of 2017 and the winter of 2018. These two seasons bore distinguishing pollution characteristics with a higher degree of ageing and heavier aerosol loading in the early summer and more abundant NOx (NOx = NO + NO2) in the winter. Elevated concentrations of HONO were observed during these two seasons, with the mean ± standard deviation (maximum) concentrations of 1.25 ± 0.94 (6.69) ppbv and 1.04 ± 1.27 (9.55) ppbv in early summer and winter, respectively. The observed daytime (08:00–17:00 h, local time) HONO production rate was several times higher in early summer than in winter (4.44 ± 1.93 ppbv h−1 vs. 0.88 ± 0.49 ppbv h−1). Budget analysis revealed distinct daytime HONO formation mechanisms during these two seasons. Photo-induced heterogeneous conversion of NO2 on the ground surface dominated in early summer, and homogeneous reaction of NO + OH was dominant in winter. Photolysis of HONO was the major source of primary OH in both seasons, and thus, played a key role in the regulation of atmospheric oxidising capacity. This study demonstrates the significant seasonal variations in HONO budget and underlines the predominant role of HONO in primary OH production in Beijing. Our findings will be helpful to gain an understanding of the chemical mechanisms underlying the formation of secondary pollution in metropolitan areas.

Recent Breakthroughs and Advancements in NOx and SOx Reduction Using Nanomaterials-Based Technologies: A State-of-the-Art Review.

Nitrogen and sulpher oxides (NOx, SOx) have become a global issue in recent years due to the fastest industrialization and urbanization. Numerous techniques are used to treat the harmful exhaust emissions, including dry, traditional wet and hybrid wet-scrubbing techniques. However, several difficulties, including high-energy requirement, limited scrubbing-liquid regeneration, formation of secondary pollutants and low efficiency, limit their industrial utilization. Regardless, the hybrid wet-scrubbing technology is gaining popularity due to low-costs, less-energy consumption and high-efficiency removal of air pollutants. The removal/reduction of NOx and SOx from the atmosphere has been the subject of several reviews in recent years. The goal of this review article is to help scientists grasp the fundamental ideas and requirements before using it commercially. This review paper emphasizes the use of green and electron-rich donors, new breakthroughs, reducing GHG emissions, and improved NOx and SOx removal catalytic systems, including selective/non-catalytic reduction (SCR/SNCR) and other techniques (functionalization by magnetic nanoparticles; NP, etc.,). It also explains that various wet-scrubbing techniques, synthesis of solid iron-oxide such as magnetic (Fe3O4) NP are receiving more interest from researchers due to the wide range of its application in numerous fields. In addition, EDTA coating on Fe3O4 NP is widely used due to its high stability over a wide pH range and solid catalytic systems. As a result, the Fe3O4@EDTA-Fe catalyst is projected to be an optimal catalyst in terms of stability, synergistic efficiency, and reusability. Finally, this review paper discusses the current of a heterogeneous catalytic system for environmental remedies and sustainable approaches.

Tracer-based characterization of source variations of ambient isoprene mixing ratios in a hillocky megacity, India, influenced by the local meteorology

The ambient biogenic volatile organic compounds (BVOCs), mainly isoprene, are potentially involved in the formation of secondary pollutants, hence, they are significant in terms of air quality and climate. Although the largest sources of BVOCs are tropical regions, the measurements of isoprene in the Indian subcontinent are limited. We conducted the measurements of isoprene, benzene, and toluene at an urban site in a hillocky megacity of India using a high-sensitivity proton transfer reaction quadrupole mass spectrometer (PTR-QMS). The mixing ratios of isoprene were compared with those of aromatic compounds like benzene and toluene, which represent typical anthropogenic VOCs. Isoprene and isoprene/benzene (>5 ppbv ppbv−1) showed higher levels in the pre-monsoon months, most likely due to large emissions by urban vegetation during physiological activities in plants which was enhanced by the high ambient temperatures and solar radiation. While Benzene and toluene showed higher mixing ratios during winter, which were due to shallower boundary layer depths and transport of air masses from polluted Indo-Gangetic Plain during this season. The mixing ratios of VOCs show significant diurnal variation as a result of their different origins and the role of different meteorological parameters. The robust emission ratios of isoprene/benzene obtained from nighttime data were used to separate the non-anthropogenic and anthropogenic isoprene emissions. ∼30% enhancement observed in non-anthropogenic emissions to isoprene from winter to pre-monsoon season when temperatures and solar radiation were stronger, although traffic in the city. Isoprene/benzene ratio at lower temperatures (<25 °C) and solar radiation (<100 W m−2) was predominantly controlled by anthropogenic sources. Overall, toluene and isoprene are the most frequent species in terms of having the highest ozone-forming potential (OFP) values but biogenic isoprene became more important to ozone formation during the afternoon hours in the pre-monsoon months with high air temperatures (>25 °C).

Nighttime and daytime dark oxidation chemistry in wildfire plumes: an observation and model analysis of FIREX-AQ aircraft data

Abstract. Wildfires are increasing in size across the western US, leading to increases in human smoke exposure and associated negative health impacts. The impact of biomass burning (BB) smoke, including wildfires, on regional air quality depends on emissions, transport, and chemistry, including oxidation of emitted BB volatile organic compounds (BBVOCs) by the hydroxyl radical (OH), nitrate radical (NO3), and ozone (O3). During the daytime, when light penetrates the plumes, BBVOCs are oxidized mainly by O3 and OH. In contrast, at night or in optically dense plumes, BBVOCs are oxidized mainly by O3 and NO3. This work focuses on the transition between daytime and nighttime oxidation, which has significant implications for the formation of secondary pollutants and loss of nitrogen oxides (NOx=NO+NO2) and has been understudied. We present wildfire plume observations made during FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality), a field campaign involving multiple aircraft, ground, satellite, and mobile platforms that took place in the United States in the summer of 2019 to study both wildfire and agricultural burning emissions and atmospheric chemistry. We use observations from two research aircraft, the NASA DC-8 and the NOAA Twin Otter, with a detailed chemical box model, including updated phenolic mechanisms, to analyze smoke sampled during midday, sunset, and nighttime. Aircraft observations suggest a range of NO3 production rates (0.1–1.5 ppbv h−1) in plumes transported during both midday and after dark. Modeled initial instantaneous reactivity toward BBVOCs for NO3, OH, and O3 is 80.1 %, 87.7 %, and 99.6 %, respectively. Initial NO3 reactivity is 10–104 times greater than typical values in forested or urban environments, and reactions with BBVOCs account for &gt;97 % of NO3 loss in sunlit plumes (jNO2 up to 4×10-3s-1), while conventional photochemical NO3 loss through reaction with NO and photolysis are minor pathways. Alkenes and furans are mostly oxidized by OH and O3 (11 %–43 %, 54 %–88 % for alkenes; 18 %–55 %, 39 %–76 %, for furans, respectively), but phenolic oxidation is split between NO3, O3, and OH (26 %–52 %, 22 %–43 %, 16 %–33 %, respectively). Nitrate radical oxidation accounts for 26 %–52 % of phenolic chemical loss in sunset plumes and in an optically thick plume. Nitrocatechol yields varied between 33 % and 45 %, and NO3 chemistry in BB plumes emitted late in the day is responsible for 72 %–92 % (84 % in an optically thick midday plume) of nitrocatechol formation and controls nitrophenolic formation overall. As a result, overnight nitrophenolic formation pathways account for 56 %±2 % of NOx loss by sunrise the following day. In all but one overnight plume we modeled, there was remaining NOx (13 %–57 %) and BBVOCs (8 %–72 %) at sunrise.

Analysis of regional heavy PM2.5 pollution events in Beijing-Tianjin-Hebei and the surrounding area in China during 2015–2018

&lt;p&gt;Beijing-Tianjin-Hebei and the surrounding area (BTHS) experiences the most serious PM2.5 pollution in China. To investigate PM2.5 pollution characteristics in regional heavy air pollution, forty-three events in BTHS during 2015–2018 were selected in this study. These regional events generally lasted five days and covered nineteen cities, with an average PM2.5 concentration of 204 μg·m-3 and a maximum daily average concentration of 358 μg·m-3. The pollution events occurred mainly during October to March of the following year, which is the heating season in this region. The occurrence of pollution events and the PM2.5 concentration displayed a decreasing trend during 2015–2018. Of the forty-three events, thirteen started in Baoding city, and the peak PM2.5 concentration was recorded in Baoding city eleven times, indicating that it was the most polluted city in BTHS. A comprehensive classification method determined that southwest channel pollution was the main type of pollution event in BTHS. It occurred nine, seven, four, and eight times in 2015–2018, accounting for 56.3%, 53.8% 80.0%, and 88.9% of all pollution events, respectively. A typical regional heavy PM2.5 pollution event occurred in March 2016. The analysis revealed that it was initiated by pollutant transport from regions south of BTHS. Local pollutant emissions and regional pollutants accumulated, and secondary pollutant formation occurred in association with weak winds and a high relative humidity, which maintained the heavy pollution levels. Relative humidity had the most significant influence on the two most important stages of a heavy PM2.5 pollution event, which were the initial and high concentration stages, respectively.&lt;/p&gt;&#x0D;

Atmospheric oxidation capacity and its contribution tosecondary pollutants formation

<p indent="0mm">Atmospheric oxidation capacity (AOC) is defined as the oxidation reaction rates of primary pollutants in the atmospheric chemical process, which is characterized by the oxidants concentration or the reaction rates. Atmospheric oxidation plays an important role in the formation of secondary pollutants, and works as an important indicator for the study of secondary pollution. Since 2013, the Chinese Government implemented unprecedented strident regulations to improve air quality. Although the overall levels of fine particulate matter (PM_2.5) have dropped considerably throughout China, it remains unclear how secondary pollutants have responded to the emission reductions in view of complex gas-precursor relationship and gas-particle interactions. Previous studies also reported the positive correlation between the AOC level and secondary pollutant concentrations. The updated Community Multi-scale Air Quality (CMAQ) model with the SAPRC 11 photo-chemical mechanism as well as the observation data was applied in this study to investigate the relationship of the secondary pollutants (ozone and secondary aerosols) and their precursors with the major oxidants (HO₂, OH and NO₃ radicals) in years 2013 and 2020 in China. And the process analysis tool was implemented in the CMAQ model to determine the key oxidants budget, which could better understand the primary sources and sinks of these oxidants. This analysis was able to calculate the chemical reaction rates at each integration time step. Radical source reactions were generally photolysis reactions, and the sink reactions removed radicals through the formation of stable products. The anthropogenic emissions were from Multiresolution Emission Inventory for China (MEIC) database, and the biogenic emissions were calculated from the Model of Emissions of Gas and Aerosols from Nature (MEGAN). The meteorology inputs were generated from the Weather Research and Forecasting (WRF) model. The CMAQ model predictions agree well with the observations, with most of the model performance within the criteria. The results show that the AOC level keeps identical from 2013 to 2020 throughout China. In the North China Plain (NCP) and the Pearl River Delta (PRD), the slightly increasing trends of AOC are even reported, which may aggravate the secondary pollution. The enhanced AOC level in the NCP is mainly from the reduced PM_2.5 concentration, which is regarded as the important sink of AOC. In addition, the concentration of major oxidants also represents different regional characteristics. The highest concentrations of HO_{<italic>x</italic>} and NO₃ radical are reported in the Sichuan Basin (SCB) and NCP, respectively. The ozone photolysis and the formation of HNO₃ (OH+NO₂) are reported as the most important AOC primary sources and sinks, respectively. In the NCP, the HONO photolysis is found as an important AOC source, which is consistent with previous studies. The AOC level increases significantly during the high ozone episode (increasing rate up to 475%). Also, the high level of AOC is corresponding to the significant increase of the secondary organic aerosols, indicating its important role in the formation of secondary pollutants. In summer, the reduction of NO_{<italic>x</italic>} emissions can reduce the AOC level. Meanwhile, a more complex response between the NO_{<italic>x</italic>} emissions and AOC level is found in winter. More rational emission reduction ratio of NO_{<italic>x</italic>} and volatile organic compounds (VOCs) should be applied to better control the AOC in China. In the future, more effective AOC control strategies should be established to reduce the PM_2.5 and O₃ coordinately in the 14th Five-Year Plan of China.

Emissions and Secondary Formation of Air Pollutants from Modern Heavy-Duty Trucks in Real-World Traffic-Chemical Characteristics Using On-Line Mass Spectrometry.

Complying with stricter emissions standards, a new generation of heavy-duty trucks (HDTs) has gradually increased its market share and now accounts for a large percentage of on-road mileage. The potential to improve air quality depends on an actual reduction in both emissions and subsequent formation of secondary pollutants. In this study, the emissions in real-world traffic from Euro VI-compliant HDTs were compared to those from older classes, represented by Euro V, using high-resolution time-of-flight chemical ionization mass spectrometry. Gas-phase primary emissions of several hundred species were observed for 70 HDTs. Furthermore, the particle phase and secondary pollutant formation (gas and particle phase) were evaluated for a number of HDTs. The reduction in primary emission factors (EFs) was evident (∼90%) and in line with a reduction of 28–97% for the typical regulated pollutants. Secondary production of most gas- and particle-phase compounds, for example, nitric acid, organic acids, and carbonyls, after photochemical aging in an oxidation flow reactor exceeded the primary emissions (EFAged/EFFresh ratio ≥2). Byproducts from urea-selective catalytic reduction systems had both primary and secondary sources. A non-negative matrix factorization analysis highlighted the issue of vehicle maintenance as a remaining concern. However, the adoption of Euro VI has a significant positive effect on emissions in real-world traffic and should be considered in, for example, urban air quality assessments.

Modeling sensitivities of BVOCs to different versions of MEGAN emission schemes in WRF-Chem (v3.6) and its impacts over eastern China

Abstract. Biogenic volatile organic compounds (BVOCs) simulated by current air quality and climate models still have large uncertainties, which can influence atmospheric chemistry and secondary pollutant formation. These modeling sensitivities are primarily due to two sources. One originates from different treatments in the physical and chemical processes associated with the emission rates of BVOCs. The other is errors in the specification of vegetation types and their distribution over a specific region. In this study, the version of the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) updated by the University of Science and Technology of China (USTC version of WRF-Chem) from the public WRF-Chem(v3.6) is used. The modeling results over eastern China with different versions (v1.0, v2.0, v3.0) of the Model of Emissions of Gases and Aerosols from Nature (MEGAN) in WRF-Chem are examined or documented. Sensitivity experiments with these three versions of MEGAN and two vegetation datasets are conducted to investigate the difference of three MEGAN versions in modeling BVOCs and its dependence on the vegetation distributions. The experiments are also conducted for spring (April) and summer (July) to examine the seasonality of the modeling results. The results indicate that MEGAN v3.0 simulates the largest amount of biogenic isoprene emissions over eastern China. The different performance among MEGAN versions is primarily due to their different treatments of applying emission factors and vegetation types. In particular, the results highlight the importance of considering the sub-grid vegetation fraction in estimating BVOC emissions over eastern China, which has a large area of urbanization. Among all activity factors, the temperature-dependent factor dominates the seasonal change of activity factor in all three versions of MEGAN, while the different response to the leaf area index (LAI) change determines the difference among the three versions in seasonal variation of BVOC emissions. The simulated surface ozone concentration due to BVOCs can be significantly different (ranging from 1 to more than 10 ppbv in some regions) among the experiments with three versions of MEGAN, which is mainly due to their impacts on surface VOCs and NOx concentrations. Theoretically MEGAN v3.0 that is coupled with the land surface scheme and considers the sub-grid vegetation effect should overcome previous versions of MEGAN in WRF-Chem. However, considering uncertainties of retrievals and anthropogenic emissions over eastern China, it is still difficult to apply satellite retrievals of formaldehyde and/or limited sparse in situ observations to constrain the uncertain parameters or functions in BVOC emission schemes and their impacts on photochemistry and ozone production. More accurate vegetation distribution and measurements of biogenic emission fluxes and species concentrations are still needed to better evaluate and optimize models.

RTEII: A new high-resolution (0.1° × 0.1°) road transport emission inventory for India of 74 speciated NMVOCs, CO, NOx, NH3, CH4, CO2, PM2.5 reveals massive overestimation of NOx and CO and missing nitromethane emissions by existing inventories

21 of 30 most polluted cities for particulate matter (PM 2.5 ) are in India, yet the distribution, identity and emissions of volatile organic compounds (VOCs) from traffic, which are PM 2.5 and ozone precursors, remain unknown. Here, we measured emission factors (EFs) of 74 VOCs from a range of Indian vehicle-technology and fuel types. When combined with 0.1 ° × 0.1 ° spatially resolved activity data for the year 2015, toluene (137 ± 39 Gg yr 1 ), isopentane (111 ± 38 Ggyr −1 ), and acetaldehyde (41 ± 6 Ggyr −1 ) were top 3-VOC emissions. Petrol-2-wheelers and LPG-3-wheelers emitted the highest VOCs (EFs> 50 gVOC/L) and had highest secondary pollutant formation potential, so their replacement with electric vehicles would improve air quality. EDGARv4.3.2 and REASv.2.1 emission inventories overestimated total road sector emitted VOCs due to obsolete EFs and activity data, in particular over-estimating ethene, propene, ethyl benzene, 2,2- dimethyl butane, CO, NO x while significantly under-estimating acetaldehyde. Nitromethane emissions were missing from previous inventories and with isocyanic acid and benzene contributed significantly to toxic emissions (summed total ~41 ± 4 Ggyr −1 ). Knowledge of key VOCs emitted from the world's third largest road-network provides critical new data for mitigating secondary pollutant formation over India and will enable more accurate modelling of atmospheric composition over South Asia. • Emission factors of 74 VOCs measured for Indian vehicular categories including LPG and CNG vehicles reveal high acetaldehyde. • 0.1 × 0.1° speciated VOC traffic emission inventory over India with updated activity data shows overestimate by inventories. • Petrol 2-wheelers and LPG 3-wheelers had highest pollutant emissions.

Probing wintertime air pollution sources in the Indo-Gangetic Plain through 52 hydrocarbons measured rarely at Delhi & Mohali

During wintertime, the Indo-Gangetic Plain suffers from severe air pollution affecting several hundred million people. Here we present unprecedented measurements and source analyses of 52 NMHCs (25 alkanes, 16 aromatics, 10 alkenes and one alkyne) in the cities of Delhi and Mohali (300 km north of Delhi) during wintertime (Dec 2016–Jan 2017). NMHCs were measured using a thermal desorption gas chromatograph equipped with flame ionisation detectors with data traceable to WMO standards. The ten most abundant NMHCs that were measured were the same at both Delhi and Mohali: propane, n-butane, acetylene, ethane, toluene, i-butane, ethene, i-pentane, benzene and propene and accounted for >50% of total measured NMHC mass concentration (137 ± 5.8 μg m−3 in Mohali and 239 ± 7.7 μg m−3 in Delhi). Ambient NMHCs and calculated hydroxyl radical reactivity were approximately twice as high in Delhi relative to Mohali, and 2–12 times higher than most other mega-cities, except Lahore and Karachi. Using chemical source signatures, traffic and LPG usage emissions were identified as the major contributor of these reactive NMHCs at both sites during nighttime, with additional minor contributions of garbage burning in Mohali, and evaporative fuel and biomass burning emissions in Delhi. Comparison of NMHC/CO and NMHC/C2H2 ratios over Mohali and Delhi, to other cities, suggested gasoline/petrol-fuelled vehicles were major NMHC emitters within the traffic source. The data from both Mohali and Delhi suggest that a large fraction of the fleet comprised vehicles with older emission control in both Mohali and Delhi. Analyses revealed poor representation of propene, ethene and trimethylbenzenes in the emission inventory (EDGARv4.3.2) over Mohali and Delhi. This study provides key data and new insights into the sources of reactive NMHCs (lifetime < few days) that drive regional wintertime pollution through direct effects and the formation of secondary pollutants.

Local emissions and secondary pollutants cause severe PM2.5 elevation in urban air at the south edge of the North China Plain: Results from winter haze of 2017–2018 at a mega city

Severe haze occurrence in the north of the North China Plain (NCP) is recognized as a consequence of the regional transport of pollutants initially from the south and then the rapid formation of secondary pollutants in the local air. However, the origin of pollutants causing haze in the southern NCP has not yet been elucidated even through careful data observation. Based on the contents of water-soluble inorganic ions in PM2.5 samples collected during two severe haze episodes in Zhengzhou, a mega city located on the southern edge of the NCP, we estimated the contributions of local primary emissions and secondary pollutants to haze occurrence. On average, Na+, K+, and Ca2+ mainly originated from anthropogenic sources, and their anthropogenic fractions had proportions of 97.5%, 93.9%, and 76.5% in their respective total mass. Anions Cl− and SO42− substantially originated from not only produced substantially via secondary formation but also from primary emissions, and their primary proportions in their respective total mass were 51.1% and 30.8%. In contrast, NH4+ and NO3− were dominated by secondary formation. The increase in PM2.5 was mainly caused by the formation of secondary inorganic (29.1%) and organic species (57.2%) and the primary anthropogenic emissions (12.5%). These results indicated that the haze at the southern edge of the NCP was mainly caused by pollutants in the local areas. Compared to the haze in the northern NCP, the haze in the southern NCP edge had a higher PM2.5 mass concentration and a higher proportion of secondary species, but a lower proportion of primary species, indicating the high heterogeneity of winter haze over the NCP.

Characterizing nitrate radical budget trends in Beijing during 2013–2019

Nitrate (NO3) radical is an important oxidant in the atmosphere as it regulates the NOx budget and impacts secondary pollutant formation. Here, a long-term observational dataset of NO3-related species at an urban site in Beijing was used to investigate changes in the NO3 budget and their atmospheric impacts during 2013–2019, in this period the Clean Air Actions Plan was carried out in China. We found that (1) changes in NO3 precursors (NO2 and O3) led to a significant increase in NO3 formation in the surface layer in winter but a decrease in summer; (2) a reduction in NOx promoted thermal equilibrium, favoring the formation of NO3 rather than dinitrogen pentoxide (N2O5). The simultaneous decrease in PM2.5, during these years, further weakened the N2O5 heterogeneous uptake; (3) a box model simulation revealed that both the reactions of NO3 with volatile organic compounds (VOC) and N2O5 uptake were weakened in summer, implying that the policy actions implemented help to moderate secondary aerosol formation caused by NO3 and N2O5 chemistry in summer; and (4) during winter, both NO3 + VOC and N2O5 uptake were enhanced. Specifically, for the N2O5 uptake, the rapid increase in NO3 production, or to some extent, NO3 oxidation capacity, far outweighed the negative shift effect, leading to a net enhancement of N2O5 uptake in winter, which indicates that the action policy implemented led to an adverse effect on particulate nitrate formation via N2O5 uptake in winter. This may explain the persistent winter particulate nitrate pollution in recent years. Our results highlight the systematic changes in the NO3 budget between 2013 and 2019 in Beijing, which subsequently affect secondary aerosol formation in different seasons.

Progress on the vertical observation methods of volatile organic compounds and their applications within the atmospheric boundary layer

<p indent="0mm">Volatile organic compounds (VOCs) are important trace gases in the troposphere that play vital roles in the formation of secondary air pollutants such as ozone and organic aerosol components. An in-depth characterization of the spatiotemporal variability, particularly in three-dimensions, of VOCs in the atmospheric boundary layer is an essential prerequisite for understanding the formation mechanisms of regional pollution episodes. However, vertically resolved VOCs measurements are seldom made during field campaigns due to the limitation of available observation techniques. This review summarizes the progress in vertical observation of VOCs and their applications to the atmospheric boundary layer, focusing on the introduction of platforms for vertical measurement techniques for VOCs, including high-rise buildings, tethered balloons, aircraft, unmanned aerial vehicles (UAVs), and remote sensing. All of these platforms have been used in previous studies and have their respective advantages and disadvantages. High-rise buildings are generally characterized by low altitudes and fixed locations but could serve as long-term platforms for both online and offline VOCs measurements. Tethered balloons, generally used for offline VOCs measurements, have flexible launch locations and altitude ranges. Aircraft can provide platforms for both offline and online VOCs measurements over large spatial scales, but aircrafts are not suitable for long-term observations because of their high operating cost. Similar to tethered balloons, UAVs are used for offline VOCs measurements, while they are much more flexible in utilization and can be launched in complex environments. However, small payload, short flight duration, and availability of airspace are key factors that limit the wide application of UAVs in the field of environmental monitoring. Remote sensing platforms (including ground-based and space-based techniques) could provide vertical measurements of several VOCs species over large temporal and spatial scales. However, the vertical VOCs profiles provided by remote sensing techniques are frequently characterized by large uncertainty. With the rapid development of VOCs measurement techniques (such as UAVs and proton-transfer-reaction time-of-flight mass spectrometry), future studies are expected to obtain vertical VOCs observations with higher spatiotemporal resolutions as well as lower operating costs. Vertical observations of VOCs concentrations could provide valuable opportunities for a thorough analysis of VOCs emissions and variation of their related chemical processes within the atmospheric boundary layer. Therefore, this review also summarizes the progress in applications of vertical VOCs observations in the atmospheric boundary layer, focusing on introducing the key factors influencing VOCs vertical distribution patterns, determination of VOCs emission fluxes, evaluation and calibration of VOCs emission inventories, investigation of the vertical distribution of photochemical ozone production sensitivity, and the vertical distribution of VOCs degradation mechanisms at night. At present, vertical observations are predominantly used to identify impacts of transport and boundary-layer dynamics on the vertical distributions of different VOCs species and to estimate speciated VOCs emissions. However, the vertical variability of the oxidation and degradation mechanisms for different VOCs species and the environmental effects of this variability have been reported in only a few studies. More effort should be made to elucidate how the interactions between VOCs chemistry and vertical transport affect the formation and concentration of secondary air pollutants at ground level. In summary, there is an urgent need to obtain reliable vertical observations of key VOCs species in the atmospheric boundary layer for better investigating the formation mechanisms of regional atmospheric pollution.

Improvement in Modeling of OH and HO2 Radical Concentrations during Toluene and Xylene Oxidation with RACM2 Using MCM/GECKO-A

Due to their major role in atmospheric chemistry and secondary pollutant formation such as ozone or secondary organic aerosols, an accurate representation of OH and HO2 (HOX) radicals in air quality models is essential. Air quality models use simplified mechanisms to represent atmospheric chemistry and interactions between HOX and organic compounds. In this work, HOX concentrations during the oxidation of toluene and xylene within the Regional Atmospheric Chemistry Mechanism (RACM2) are improved using a deterministic–near-explicit mechanism based on the Master Chemical Mechanism (MCM) and the generator of explicit chemistry and kinetics of organics in the atmosphere (GECKO-A). Flow tube toluene oxidation experiments are first simulated with RACM2 and MCM/GECKO-A. RACM2, which is a simplified mechanism, is then modified to better reproduce the HOX concentration evolution simulated by MCM/GECKO-A. In total, 12 reactions of the oxidation mechanism of toluene and xylene are updated, making OH simulated by RACM2 up to 70% more comparable to the comprehensive MCM/GECKO-A model for chamber oxidation simulations.

Explicit modeling of isoprene chemical processing in polluted air masses in suburban areas of the Yangtze River Delta region: radical cycling and formation of ozone and formaldehyde

Abstract. In recent years, ozone pollution has become one of the most severe environmental problems in China. Evidence from observations have showed increased frequency of high O3 levels in suburban areas of the Yangtze River Delta (YRD) region. To better understand the formation mechanism of local O3 pollution and investigate the potential role of isoprene chemistry in the budgets of ROx (OH+HO2+RO2) radicals, synchronous observations of volatile organic compounds (VOCs), formaldehyde (HCHO), and meteorological parameters were conducted at a suburban site of the YRD region in 2018. Five episodes with elevated O3 concentrations under stagnant meteorological conditions were identified; an observation-based model (OBM) with the Master Chemical Mechanism was applied to analyze the photochemical processes during these high O3 episodes. The high levels of O3, nitrogen oxides (NOx), and VOCs facilitated strong production and recycling of ROx radicals with the photolysis of oxygenated VOCs (OVOCs) being the primary source. Our results suggest that local biogenic isoprene is important in suburban photochemical processes. Removing isoprene could drastically slow down the efficiency of ROx recycling and reduce the concentrations of ROx. In addition, the absence of isoprene chemistry could further lead to a decrease in the daily average concentrations of O3 and HCHO by 34 % and 36 %, respectively. Therefore, this study emphasizes the importance of isoprene chemistry in the suburban atmosphere, particularly with the participation of anthropogenic NOx. Moreover, our results provide insights into the radical chemistry that essentially drives the formation of secondary pollutants (e.g., O3 and HCHO) in suburban areas of the YRD region.

Characteristics and Cause Analysis of Heavy Air Pollution in a Mountainous City During Winter

Taking the typical heavy air pollution process in Yangquan from December 26, 2018 to January 20, 2019 as an example, the characteristics and cause analysis of heavy air pollution in a mountainous city in winter were analyzed in this study. The results showed that fine particle mass (PM2.5) was the primary pollutant during the heavy pollution period. The water-soluble ions and carbonaceous components were the main components of PM2.5. The secondary ions of SO42-, NO3-, and NH4+ had the lager contribution to water-soluble ions (87.7%), and the secondary organic carbon (SOC) was the main component of the carbonaceous components (71.6%). The concentration of the secondary ions during the heavy pollution period increased by 5.3 times compared to levels before the heavy pollution period, and was an important component resulting in the fast increase of PM2.5. An analysis of meteorological conditions showed that PM2.5 and its main components had a significantly positive relationship with humidity and a significantly negative relationship with wind speed. And that pollution became stronger with an increase in humidity and a decrease in wind speed. The typical meteorological characteristics of mountainous cities are high relative humidity and large temperature variations, which can accelerate the formation of secondary pollutants and are the main reasons for the rapid aggravation of PM2.5. In addition, the lower average wind speed caused by the relatively closed terrain in mountainous cities makes the diffusion conditions of air pollutants relatively poor, which is one of the reasons for the accumulation of pollutants. The source apportionment results showed that the secondary sources (46.0%) were the most important source of PM2.5, followed by coal combustion (32.6%), vehicle exhaust (19.8%), and fugitive dust (1.6%). Therefore, mountainous cities should pay more attention to controlling secondary components, especially secondary ions.

Characterization of a chemical modulation reactor (CMR) for the measurement of atmospheric concentrations of hydroxyl radicals with a laser-induced fluorescence instrument

Abstract. Precise and accurate hydroxyl radical (OH) measurements are essential to investigate mechanisms for oxidation and transformation of trace gases and processes leading to the formation of secondary pollutants like ozone (O3) in the troposphere. Laser-induced fluorescence (LIF) is a widely used technique for the measurement of ambient OH radicals and was used for the majority of field campaigns and chamber experiments. Recently, most LIF instruments in use for atmospheric measurements of OH radicals introduced chemical modulation to separate the ambient OH radical concentration from possible interferences by chemically removing ambient OH radicals before they enter the detection cell (Mao et al., 2012; Novelli et al., 2014a). In this study, we describe the application and characterization of a chemical modulation reactor (CMR) applied to the Forschungszentrum Jülich LIF (FZJ-LIF) instrument in use at the atmospheric simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber). Besides dedicated experiments in synthetic air, the new technique was extensively tested during the year-round Jülich Atmospheric Chemistry Project (JULIAC) campaign, in which ambient air was continuously flowed into the SAPHIR chamber. It allowed for performing OH measurement comparisons with differential optical absorption spectroscopy (DOAS) and investigation of interferences in a large variety of chemical and meteorological conditions. Good agreement was obtained in the LIF–DOAS intercomparison within instrumental accuracies (18 % for LIF and 6.5 % for DOAS) which confirms that the new chemical modulation system of the FZJ-LIF instrument is suitable for measurement of interference-free OH concentrations under the conditions of the JULIAC campaign (rural environment). Known interferences from O3+H2O and the nitrate radical (NO3) were quantified with the CMR in synthetic air in the chamber and found to be 3.0×105 and 0.6×105 cm−3, respectively, for typical ambient-air conditions (O3=50 ppbv, H2O = 1 % and NO3=10 pptv). The interferences measured in ambient air during the JULIAC campaign in the summer season showed a median diurnal variation with a median maximum value of 0.9×106 cm−3 during daytime and a median minimum value of 0.4×106 cm−3 at night. The highest interference of 2×106 cm−3 occurred in a heat wave from 22 to 29 August, when the air temperature and ozone increased to 40 ∘C and 100 ppbv, respectively. All observed interferences could be fully explained by the known O3+H2O interference, which is routinely corrected in FZJ-LIF measurements when no chemical modulation is applied. No evidence for an unexplained interference was found during the JULIAC campaign. A chemical model of the CMR was developed and applied to estimate the possible perturbation of the OH transmission and scavenging efficiency by reactive atmospheric trace gases. These can remove OH by gas phase reactions in the CMR or produce OH by non-photolytic reactions, most importantly by the reaction of ambient HO2 with NO. The interfering processes become relevant at high atmospheric OH reactivities. For the conditions of the JULIAC campaign with OH reactivities below 20 s−1, the influence on the determination of ambient OH concentrations was small (on average: 2 %). However, in environments with high OH reactivities, such as in a rain forest or megacity, the expected perturbation in the currently used chemical modulation reactor could be large (more than a factor of 2). Such perturbations need to be carefully investigated and corrected for the proper evaluation of OH concentrations when applying chemical scavenging. This implies that chemical modulation, which was developed to eliminate interferences in ambient OH measurements, itself can be subject to interferences that depend on ambient atmospheric conditions.

Non-methane hydrocarbons in the vicinity of a petrochemical complex in the Metropolitan Area of São Paulo, Brazil

Volatile organic compounds (VOCs) are fundamental in the atmospheric reactions, producing tropospheric ozone (O3) and secondary organic aerosols (SOA). We evaluated the hydrocarbons (HCs) near a petrochemical and industrial complex (PIC) in the metropolitan area of Sao Paulo (MASP). Air samples were hourly collected in two different sites during different days, 2016–2017. The most abundant compounds were toluene (1.5 ± 1.1 ppbv), cis-2-hexene (1.4 ± 1.9 ppbv), benzene (0.55 ± 0.66 ppbv), and m+p-xylene (0.58 ± 0.3 ppbv). HC concentrations observed at the BTP site (industrial) were approximately two times higher than those at the UFABC site (traffic). The aromatics presented the highest contribution at BTP (58%) and UFABC (56%). In addition, our comparison with other industrial areas worldwide showed a good agreement in the HC profile with Japan and the USA, suggesting the presence of similar emission sources. BTEX (benzene, toluene, ethylbenzene, m+p- and o-xylenes) correlations and ratios showed that despite vehicular emissions were the main sources of these compounds around PIC, there is the influence from industrial sources among others. Finally, to evaluate the potential impacts associated to PIC emissions, the formation of secondary pollutants was analyzed. Aromatic compounds represented ~98% and ~68% of the total SOA and O3 formation estimation, respectively. The lifetime cancer risk (LCR) for benzene was 5.75 × 10−6 and 4.25 × 10−6 for BTP and UFABC, respectively, which exceeds the US EPA recommendation (<1 × 10−6) and could negatively affect the populations health in this region

Microbial Enzymes Used in Bioremediation

Emerging pollutants in nature are linked to various acute and chronic detriments in biotic components and subsequently deteriorate the ecosystem with serious hazards. Conventional methods for removing pollutants are not efficient; instead, they end up with the formation of secondary pollutants. Significant destructive impacts of pollutants are perinatal disorders, mortality, respiratory disorders, allergy, cancer, cardiovascular and mental disorders, and other harmful effects. The pollutant substrate can recognize different microbial enzymes at optimum conditions (temperature/pH/contact time/concentration) to efficiently transform them into other rather unharmful products. The most representative enzymes involved in bioremediation include cytochrome P450s, laccases, hydrolases, dehalogenases, dehydrogenases, proteases, and lipases, which have shown promising potential degradation of polymers, aromatic hydrocarbons, halogenated compounds, dyes, detergents, agrochemical compounds, etc. Such bioremediation is favored by various mechanisms such as oxidation, reduction, elimination, and ring-opening. The significant degradation of pollutants can be upgraded utilizing genetically engineered microorganisms that produce many recombinant enzymes through eco-friendly new technology. So far, few microbial enzymes have been exploited, and vast microbial diversity is still unexplored. This review would also be useful for further research to enhance the efficiency of degradation of xenobiotic pollutants, including agrochemical, microplastic, polyhalogenated compounds, and other hydrocarbons.

Impact of the COVID-19 pandemic on air pollution in Chinese megacities from the perspective of traffic volume and meteorological factors

During 2020, the COVID-19 pandemic resulted in a widespread lockdown in many cities in China. In this study, we assessed the impact of changes in human activities on air quality during the COVID-19 pandemic by determining the relationships between air quality, traffic volume, and meteorological conditions. The megacities of Wuhan, Beijing, Shanghai, and Guangzhou were selected as the study area, and the variation trends of air pollutants for the period January–May between 2016 and 2020 were analyzed. The passenger volume of public transportation (PVPT) and the passenger volume of taxis (PVT) along with data on precipitation, temperature, relative humidity, wind speed, and boundary layer height were used to identify and quantify the driving force of the air pollution variation. The results showed that the change rates of fine particulate matter (PM2.5), NO2, and SO2 before and during the lockdown in the four megacities ranged from −49.9% to 78.2% (average: −9.4% ± 59.3%), −55.4% to −32.3% (average: −43.0% ± 9.7%), and − 21.1% to 11.9% (average: −10.9% ± 15.4%), respectively. The response of NO2 to the lockdown was the most sensitive, while the response of PM2.5 was smaller and more delayed. During the lockdown period, haze from uninterrupted industrial emissions and fireworks under the effect of air mass transport from surrounding areas and adverse climate conditions was probably the cause of abnormally high PM2.5 concentrations in Beijing. In addition, the PVT was the most significant factor for NO2, and meteorology had a greater impact on PM2.5 than NO2 and SO2. There is a need for more national-level policies for limiting firework displays and traffic emissions, as well as further studies on the formation and transmission of secondary air pollutants.