Air Mass Cluster Research Articles

Seasonal trends and light extinction effects of PM2.5 chemical composition from 2021 to 2022 in a typical industrial city of central China

This study investigates the concentrations, chemical compositions, and sources of PM2.5 in Huangshi, China. Daily average PM2.5 levels ranged from 8.43 to 193.08 μg m−3, with an annual mean of 54.13 μg m−3, exceeding China's annual secondary standard of 35 μg m−3. Seasonal mean concentrations peaked in winter and were lowest in summer. Organic carbon (OC) and elemental carbon (EC) had annual means of 4.89 μg m−3 and 0.94 μg m−3, respectively. Water-soluble inorganic ions (WSIIs) accounted for 52.17% of PM2.5, with NO3−, SO42−, and NH4+ being the major components. The NO3−/SO42− ratio averaged 1.65, indicating a transition from coal combustion to vehicle emissions as the primary pollution source. Chemical mass reconstruction revealed that NH4NO3, (NH4)2SO4, and organic matter (OM) accounted for 65.3% of PM2.5 mass. Seasonal variations in light extinction (bext) highlighted the impact of secondary inorganic salts on visibility, with an annual average bext of 346.30 ± 246.98 Mm−1. Airmass clusters and potential source region analysis suggested PM2.5 and its components were primarily originated from local and nearby regions. These findings underscore the effectiveness of local pollution control measures, changing pollution sources, and the necessity for targeted emission controls to improve air quality and visibility in urban areas.

Does COVID-19 lockdown matter for air pollution in the short and long run in China? A machine learning approach to policy evaluation

This paper leverages a data-driven two-step approach to effectively evaluate the effects of COVID-19 lockdown on air pollution in both the short and long-term in China. Using air pollution, meteorological conditions, and air mass clusters from 34 air quality monitoring stations in Beijing from 2015 to 2022, this study first employs a deweathering machine learning technique to decouple the confounding effects of meteorological on the air pollution. Furthermore, a detrending percentage change indictor is applied to remove the influence of seasonal variations on air pollution. The findings reveal that: (1) Human interventions are the primary drivers of changes in air pollution concentrations, whereas meteorological factors have a relatively minor impact. (2) During the COVID-19 lockdown, significant variations in air pollution levels are observed, with the effects of city lockdown ranging from a decrease of 40.11% ± 14.81% to an increase of 20.28% ± 14.36%. Notably, there is a decline in concentrations of NO2, PM2.5, CO, and PM10, while the levels of O3 and SO2 increase even during the strictest lockdown period. (3) In the year following the COVID-19 lockdown, there is a rebound in overall air pollution levels. However, by the second year, a general decline in air pollution is observed, except for O3. Therefore, it is imperative to integrate the confounding effects of meteorological factors into air quality management policies under various future scenarios: adopt high-intensity control measures for sudden air quality deteriorations, advance green recovery initiatives for long-term emission reductions, and coordinate efforts to reduce composite atmospheric pollution.

Transport and Potential Sources Regions of Double High Pollution in Nanjing by Different Synoptic Situations

Based on the hourly concentration data of fine particulate matter （PM2.5） and ozone （O3） in Nanjing from 2015 to 2019， the synoptic situation that occurred in Nanjing， in which high PM2.5 and high O3 coexisted （hereinafter referred to as double high pollution （DHP））， was typed using T-mode principal component analysis. Additionally， the backward trajectory clustering analysis method， potential source contribution method （PSCF）， and concentration weight trajectory analysis method （CWT） were used to study the transport paths and potential source region distribution of the DHP of Nanjing by different synoptic situations. The synoptic situations favorable to the DHP in Nanjing were the control of weak low-pressure type （Type1） and high-pressure center （Type2）. Synoptic situations could have had an effect on the directional origin of the backward trajectory. In Type1， the Nanjing area was affected by two low pressures in the northeast and southwest， and the clustering trajectories of the Nanjing air mass mainly came from the eastern and western directions. The average concentrations of PM2.5 and O3 in the trajectory were 83.48 μg·m-3 and 106.85 μg·m-3， respectively. In Type 2， Nanjing and its surroundings were at the edge of the high-pressure center， and the air mass cluster trajectories mainly came from the north and east. The average concentrations of PM2.5 and O3 in the trajectory were 94.47 μg·m-3 and 92.32 μg·m-3， respectively. Most of the two types of backward trajectories belonged to short and medium-distance regional transportation， indicating that the pollution of neighboring provinces was one of the main factors affecting the DHP in Nanjing. PSCF and CWT analysis showed that the distribution of the most important potential sources of PM2.5 and O3 in Type1 and Type2 were not completely consistent， which indicates that the two pollutants did not come from the same area in the DHP.

Seasonal Characteristics of Long-Range Transport and Potential Associated Sources of Particulate Matter (Pm<sub>10</sub>) Pollution at the Station Elk, Poland, on 2021-2022 Data

The current study aimed to determine the potential sources of distant emissions of PM10 particles that significantly affect PM10 levels at a given site in southeastern Baltic. The EEA Air Quality Monitoring Station in Elk City, northeastern Poland, was selected for this study. This station is located approximately 50 km from the border of the Russian exclave (Kaliningrad Region). In this study, the NOAA HYSPLIT_4 trajectory model, potential source contribution function (PSCF), and concentration-weight trajectory (CWT) were employed to investigate the origin of the measured PM10 mass at a receptor site. PSCF and CWT utilize back-trajectory analysis and Lagrangian particle dispersion simulations to reconstruct the advection pathways of air masses arriving at the site. These reconstructed retroplumes provide detailed information regarding the geographic locations traversed by polluted air masses on their way to the receptor. By integrating trajectory information with concurrent pollutant concentration data, the PSCF and CWT enable the identification of potential source regions and quantification of their impact on the observed atmospheric levels. From January 1, 2021, to December 31, 2022, at 200 m the 72h backward trajectories of air masses entering the receptor point were calculated and categorized by clustering them into 5-4-4-5 clusters. Subsequently, the PM10 levels at the Elk site associated with each air mass cluster were examined during the observation period. The seasonal variation in PM10 was generally characterized by a peak in winter and minimum values in summer. PM10 was lower during warmer periods, particularly during summer, and significantly, higher concentrations were observed during colder periods. Cluster analyses showed that airflow followed a seasonal pattern, with different results obtained in different seasons. According to the PSCF and CWT results, in winter and spring, the receptor site was influenced more by long-range PM10 pollution, particularly from heavily industrialized areas in Central-Eastern Europe. In contrast, in summer and autumn, the receptor site was less influenced by long-range pollution. The findings demonstrate that the seasonal distributions of PM10 source areas obtained using these two methods generally share similar characteristics, suggesting the credibility and accuracy of the analytical results.

Chemical Characteristics and Source Apportionment of Organic Aerosols in Urban Shanghai During Cold Season Based on High Time-resolution Measurements of Organic Molecular Markers

Organic aerosols (OA) are closely related to the formation of both PM2.5 and O3 in the atmosphere. In this study, a thermal desorption aerosol GC/MS (TAG) online system was adopted to measure hourly concentrations of 94 total organic molecular markers in PM2.5 at an urban site in Shanghai from November 6th to December 31st, 2021. Combined with air mass cluster analysis and other online measurement data, the chemical characteristics of OA under the influence of different air masses, oxidant levels, and relative humidity (RH) levels were investigated. The results showed that OA was characterized by higher mass percentages of primary organic molecular markers (e.g., saturated fatty acids, unsaturated fatty acids, and alkanes) under the influence of local air masses. Further, high loadings of biomass burning tracers were observed in OA under the influence of long-range transported air masses. In contrast, OA impacted by marine air masses was laden with significantly higher fractions of secondary organic molecular markers, such as dicarboxylic acids and hydroxyl dicarboxylic acids, which were formed from a wide range of volatile organic precursors through photochemical and aqueous-phase processing. With the application of the positive matrix factorization (PMF) model, seven total primary source factors and five secondary source factors were resolved for PM2.5 and OA during the observation. Among them, secondary nitrate was the highest contributor to PM2.5 mass with a mass percentage of 25.2%, whereas vehicle emissions were the top contributor (24.0%) to OA mass. Primary source factors, including coal combustion, vehicle emission, and cooking emission as well as their corresponding secondary source factors (e.g., secondary nitrate, secondary organic aerosols 2, etc.) showed elevated contributions in PM2.5 and OA with the increase in PM2.5 masses, indicating that more stringent controls of local emission sources (e.g., coal combustion, vehicle emission, and cooking emission) are needed to further lower PM2.5 pollution and improve air quality in Shanghai.

A 1-year aerosol chemical speciation monitor (ACSM) source analysis of organic aerosol particle contributions from anthropogenic sources after long-range transport at the TROPOS research station Melpitz

Abstract. Atmospheric aerosol particles are a complex combination of primary emitted sources (biogenic and anthropogenic) and secondary aerosol resulting from aging processes such as condensation, coagulation, and cloud processing. To better understand their sources, investigations have been focused on urban areas in the past, whereas rural-background stations are normally less impacted by surrounding anthropogenic sources. Therefore, they are predisposed for studying the impact of long-range transport of anthropogenic aerosols. Here, the chemical composition and organic aerosol (OA) sources of submicron aerosol particles measured by an aerosol chemical speciation monitor (ACSM) and a multi-angle absorption photometer (MAAP) were investigated at Melpitz from September 2016 to August 2017. The location of the station at the frontier between western and eastern Europe makes it the ideal place to investigate the impact of long-range transport over Europe. Indeed, the station is under the influence of less polluted air masses from westerly directions and more polluted continental air masses from eastern Europe. The OA dominated the submicron particle mass concentration and showed strong seasonal variability ranging from 39 % (in winter) to 58 % (in summer). It was followed by sulfate (15 % and 20 %) and nitrate (24 % and 11 %). The OA source identification was performed using the rolling positive matrix factorization (PMF) approach to account for the potential temporal changes in the source profile. It was possible to split OA into five factors with a distinct temporal variability and mass spectral signature. Three were associated with anthropogenic primary OA (POA) sources: hydrocarbon-like OA (HOA; 5.2 % of OA mass in winter and 6.8 % in summer), biomass burning OA (BBOA; 10.6 % and 6.1 %) and coal combustion OA (CCOA; 23 % and 8.7 %). Another two are secondary and processed oxygenated OA (OOA) sources: less oxidized OOA (LO-OOA; 28.4 % and 36.7 %) and more oxidized OOA (MO-OOA; 32.8 % and 41.8 %). Since equivalent black carbon (eBC) was clearly associated with the identified POA factors (sum of HOA, BBOA, and CCOA; R2= 0. 87), eBC's contribution to each of the POA factors was achieved using a multilinear regression model. Consequently, CCOA represented the main anthropogenic sources of carbonaceous aerosol (sum of OA and eBC) not only during winter (56 % of POA in winter) but also in summer (13 % of POA in summer), followed by BBOA (29 % and 69 % of POA in winter and summer, respectively) and HOA (15 % and 18 % of POA in winter and summer, respectively). A seasonal air mass cluster analysis was used to understand the geographical origins of the different aerosol types and showed that during both winter and summer time, PM1 (PM with an aerodynamic diameter smaller than 1 µm) air masses with eastern influence were always associated with the highest mass concentration and the highest coal combustion fraction. Since during wintertime CCOA is a combination of domestic heating and power plant emissions, the summer contribution of CCOA emphasizes the critical importance of coal power plant emissions to rural-background aerosols and its impact on air quality, through long-range transportation.

Long term influence of groundwater preservation policy on stubble burning and air pollution over North-West India

Stubble burning (SB) has been a major source of seasonal aerosol loading and pollution over northern India. The aftereffects of groundwater preservation act i.e., post 2010 era (2011–2020) has seen delay in crop harvesting thereby shifting the peak SB to May (Wheat SB) and to November (Paddy SB) by 8–10 and 10–12 days compared to pre-2010. Groundwater storage depletion rate of 29.2 mm yr−1 was observed over the region. Post 2010 era shows an increase of 1.4% in wheat SB and 21% in Paddy SB fires over Punjab and Haryana with 70% of PM2.5 air mass clusters (high probability > 0.8) advecting to the downwind regions leading to 23–26% increase in PM2.5 and 4–6% in aerosol loading over National Capital Region (NCR). Although the objective of water conservation policy was supposed to preserve the groundwater by delaying the paddy transplantation and sowing, on the contrary the implementation of this policy has seen groundwater storage after 2013 depleting at a rate of 29.2 mmyr−1 over these regions. Post policy implementation has led to shift and shrinking of harvest window with increased occurrences in SB fires which also increase associated particulate matter pollution over North India.

Temporal variations, regional contribution, and cluster analyses of ozone and NOx in a middle eastern megacity during summertime over 2017-2019.

Particulate matter is usually regarded as the dominant pollutant in Tehran megacity in Iran. However, the number of ozone exceedance days significantly increased in recent years. This study analyzes simultaneous measurements of O3 and NOx (NO+NO2) concentrations to improve our understanding of ozone evolution during the summers of 2017 to 2019. The k-means clustering technique was used to select five representative air quality monitoring sites in Tehran to capture O3 and NOx concentrations' variability. The findings show that all of the investigated sites failed to meet the ozone non-attainment criterion. The ozone weekend effect is seen in the study of weekday/weekend differences in 2017 and 2018, but not in 2019, which can be due to the shift in the ozone production regime. The summer mean variation analysis can also be used to deduce this regime change. In 2017, the O3 and NO2 summer mean variations suggest a holdback in the NO2 upward trend and a reversal in the O3 downward trend that had been in place since 2012. Air mass back trajectory clustering reveals that east and north-east air mass clusters have the most significant impact on Tehran's O3 pollution and the highest regional contribution to OX. The study of OX against NOx shows that the regional contribution to OX increased from 2017 to 2018 and then decreased in 2019; however, the local contribution is the opposite. The diurnal analysis of the regional and local contributions to OX indicated that OX in Tehran might be primarily affected by pollutants from a short distance. The findings reveal critical changes in the behavior of O3 in recent years, indicating that decision-makers in Tehran should reconsider air pollution control measures.

Origin and Transport Pathway of Dust Storm and Its Contribution to Particulate Air Pollution in Northeast Edge of Taklimakan Desert, China

The Taklimakan Desert in Northwest China is the major source of dust storms in China. The northeast edge of this desert is a typical arid area which houses a fragile oasis eco-environment. Frequent dust storms cause harmful effects on the oasis ecosystem and negative impacts on agriculture, transportation, and human health. In this study, the major source region, transport pathway, and the potential contribution of dust storms to particulate air pollution were identified by using both trajectory analysis and monitoring data. To assess the source regions of dust storms, 48 h backward trajectories of air masses arriving at the Bugur (Luntai) County, which is located at the northeast edge of Taklimakan Desert, China on the dusty season (spring) and non-dusty month (August, representing non-dusty season) in the period of 1999–2013, were determined using Hybrid Single Particle Lagrangian Integrated Trajectory model version 4 (HYSPLIT 4). The trajectories were categorized by k-means clustering into 5 clusters (1a–5a) in the dusty season and 2 clusters (1b and 2b) in the non-dusty season, which show distinct features in terms of the trajectory origins and the entry direction to the site. Daily levels of three air pollutants measured at a station located in Bugur County were analyzed by using Potential Source Contribution Function (PSCF) for each air mass cluster in dusty season. The results showed that TSP is the major pollutant, with an average concentration of 612 µg/m3, as compared to SO2 (23 µg/m3) and NO2 (32 µg/m3) in the dusty season. All pollutants were increased with the dust weather intensity, i.e., from suspended dust to dust storms. High levels of SO2 and NO2 were mostly associated with cluster 1a and cluster 5a which had trajectories passing over the anthropogenic source regions, while high TSP was mainly observed in cluster 4a, which has a longer pathway over the shifting sand desert area. Thus, on strong dust storm days, not only higher TSP but also higher SO2 and NO2 levels were observed as compared to normal days. The results of this study could be useful to forecast the potential occurrence of dust storms based on meteorological data. Research focusing on this dust-storm-prone region will help to understand the possible causes for the changes in the dust storm frequency and intensity, which can provide the basis for mitigation of the negative effects on human health and the environment.

Online Measurement of PM2.5 at an Air Monitoring Supersite in Yangtze River Delta: Temporal Variation and Source Identification

To comprehensively explore the transport of air pollutants, one-year continuous online observation of PM2.5 was conducted from 1 April 2015 to 31 March 2016 at Dianshan Lake, a suburban junction at the central of Yangtze River Delta. The chemical species of PM2.5 samples mainly focused on Organic carbon (OC), Elemental carbon (EC) and Water-Soluble Inorganic Ions (WSIIs). The annual average of PM2.5 concentration was 59.8 ± 31.7 µg·m−3, 1.7 times higher than the Chinese National Ambient Air Quality Standards (CNAAQS) (35 µg·m−3). SNA (SO42−, NO3− and NH4+) was the most dominated species of PM2.5 total WSIIs, accounting for 51% of PM2.5. PM2.5 and all of its chemical species shared the same seasonal variations with higher concentration in winter and spring, lower in autumn and summer. The higher NO3−/EC and NOR occurred in winter suggested that intensive secondary formation of nitrate contributed to the higher levels of PM2.5. Cluster analysis based on 72-h backward air trajectory showed that the air mass cluster from nearby inland cities, including Zhejiang, Anhui and Jiangxi Provinces contributed mostly to the total trajectories. Furtherly, potential source contribution function (PSCF) analysis revealed that local sources, namely the emissions in the Yangtze River, were the primary sources. During haze pollution, NO3− was the most important fraction of PM2.5 and the heterogeneous formation of nitrate became conspicuous. All the results suggested that the anthropogenic emissions (such as traffic exhaust) was responsible for the relatively high level of PM2.5 at this monitoring station.

Characteristics of Surface Ozone in Five Provincial Capital Cities of China during 2014–2015

Ozone (O3) pollution has become an increasing concern in China since elevated surface O3 concentrations were observed in recent years. In this study, five provincial cities (Beijing, Shanghai, Guangzhou, Xi’an, and Hefei) located in different regions of China were selected to study the spatiotemporal variations and affecting factors of O3 concentrations during 2014–2015. Beijing, Shanghai, and Guangzhou had suffered more severe O3 pollution, yet Beijing had the highest number of days that exceeded the Chinese MDA8 (maximum daily 8 h average) standard of 160 µg m−3. MDA8 O3 exhibited different seasonal patterns among the five cities. In Beijing and Xi’an, MDA8 O3 showed the highest in summer and lowest in winter. Guangzhou also had the highest O3 concentration in summer, but had similar levels in other three seasons. The O3 levels were similarly high in Shanghai during spring, summer, and autumn, while in Hefei, O3 concentration peaked in autumn. No significant difference between weekend and weekday O3 levels was observed in all the five cities. The diurnal cycle reached a maximum in the afternoon and a minimum in the early morning, which was consistent in the five cities. Correlation analyses showed that the associations between O3 and the other five criteria air pollutants, as well as meteorological parameters, were substantially different among the five cities. Air mass cluster analyses during episodic days revealed that the short-distance transport of O3 and its precursors had a greater impact for high O3 pollution in the five cities. Overall, our results demonstrate that O3 pollution exhibited great divergence among different regions and thus region-oriented control measures are suggested to reduce O3 pollution in China.

Understanding the Chemistry and Sources of Precipitation Ions in the mid-Brahmaputra Valley of Northeastern India

ABSTRACT The chemistry of rainwater over mid-Brahmaputra Valley was studied for three consecutive years (2012–2014; n = 285). The samples were analyzed for major chemical parameters viz. pH, electrical conductivity (EC), and ions (SO42–, NO3–, Cl–, F–, Br–, Ca2+, NH4+, Mg2+, Na+, K+, and Li+), organic acids (HCOO– and CH3COO–) and dissolved organic carbon (DOC). The mean pH for the entire study period was found to be 5.66, which ranged from 4.51 to 7.68, and the volume weighted (VW) mean pH was found to be 5.16. Over 55% of the samples showed pH between 5 and 6, and a few samples had pH Ca2+ > SO42– > NO3– > Cl– > Na+ > K+ > Mg2+ > H+ > HCO3– > Br– > F– > Li+, indicating dominance of alkaline ions over acidic ions such that 94% of mineral acid was neutralized. The secondary ions, NH4+, SO42–, and NO3–, showed high wet deposition fluxes. The chemistry exhibits explicit seasonality. The airmass clusters of monsoon and non-monsoon seasons, and the associated chemistry varied, which showed influence of long-range transport. The interspecies correlations varied between the monsoon and non-monsoon time samples meaning variation in the source strengths of the contribution sources of the chemical species of the rainwater. Positive Matrix Factorization (PMF) was applied to the data which extracted six factors that explained the sources and chemistry of the rainwater constituents which are of sea, agriculture, coal burning, biomass burning, and secondary origin.

Long-term (2005–2012) measurements of near-surface air pollutants at an urban location in the Indo-Gangetic Basin

Simultaneous long-term measurements of near-surface air pollutants at an urban station, New Delhi, were studied during 2005–2012 to understand their distribution on different temporal scales. The annual mean mass concentrations of nitrogen dioxide (\(\hbox {NO}_{2})\), sulphur dioxide (\(\hbox {SO}_{2})\), particulate matter less than \(10\,\upmu \hbox {m}\) (\(\hbox {PM}_{10})\) and suspended particulate matter (SPM) were found to be \(62.0\,{\pm }\,27.6\), \(12.5\,{\pm }\,8.2\), \(253.7\,{\pm }\,134\) and \(529.2\,{\pm }\,213.1\,\upmu \hbox {g}/\hbox {m}^{3}\), respectively. The 24-hr mean mass concentrations of \(\hbox {NO}_{2}\), \(\hbox {PM}_{10}\) and SPM were exceeded on \(\sim \)27%, 87% and 99% days that of total available measurement days to their respective National Ambient Air Quality Standard (NAAQS) level. However, it never exceeded for \(\hbox {SO}_{2}\), which could be attributed to reduction of sulphur in diesel, use of cleaner fuels such as compressed natural gas, LPG, etc. The mean mass concentrations of measured air pollutants were found to be the highest during the winter/post-monsoon seasons, which are of concern for both climate and human health. The annual mean mass concentrations of \(\hbox {NO}_{2}\), \(\hbox {PM}_{10}\) and SPM showed an increasing trend while \(\hbox {SO}_{2}\) appears to be decreasing since 2008. Air mass cluster analysis showed that north–northwest trajectories accounted for the highest mass concentrations of air pollutants (more prominent in the winter/post-monsoon season); however, the lowest were associated with the southeast trajectory cluster.

Seasonal aerosol variations over a coastal city, Zhoushan, China from CALIPSO observations

This paper presents the observed seasonal aerosol variations over Zhoushan, an eastern coastal Chinese city. Data were obtained from the Cloud – Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite from the period of June 2007 to May 2017. We compared the columnar Aerosol Optical Depth (AOD) data from the CALIPSO and MODerate resolution Imaging Spectroradiometer (MODIS). Results showed good consistency, but the former was systematically lower than the latter. The temporal distribution of columnar AOD showed significant variations with the highest in spring and lowest in summer. Similarly, the seasonal scatter plots suggested that the highest correlation coefficient was 0.56 in winter and summer, followed by the autumn (0.53), and spring (0.40) seasons. In addition, the results revealed that the polluted dust and polluted continental aerosols (38.9% and 30.5%, respectively) were dominant aerosol subtypes observed in winter, whereas, the polluted dust (47.2%) aerosol subtype was found dominant in spring. The polluted continental aerosol subtype appeared dominant during the summer and autumn seasons, with the frequencies of about 56.0% and 47.4%, respectively. These findings can be reasonably explained using the air mass cluster analysis computed for the obtained backward trajectories derived from the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model. Furthermore, the aerosol vertical extinction coefficient measured at the wavelength of 532 nm was found to be highest near the surface (~0.2 km−1) in winter and autumn and decreased sharply as the altitude increased indicating that aerosols were present at an altitude <2 km. However, during spring, the values of extinction coefficient remained >0.15 km−1 at an altitude range of 0–3.5 km due to convection and strong vertical mixing lifting aerosols to slightly higher levels. Furthermore, during the spring, approximately 54% of the particulate depolarization ratio (PDR) values were ≤ 0.2, and the remaining 46% of the PDR were > 0.2, suggesting both spherical and irregular particles were present in the atmosphere.

On the Influence of Air Mass Origin on Low‐Cloud Properties in the Southeast Atlantic

AbstractThis study investigates the impact of air mass origin and dynamics on cloud property changes in the Southeast Atlantic (SEA) during the biomass burning season. The understanding of clouds and their determinants at different scales is important for constraining the Earth's radiative budget and thus prominent in climate system research. In this study, the thermodynamically stable SEA stratocumulus cover is observed not only as the result of local environmental conditions but also as connected to large‐scale meteorology by the often neglected but important role of spatial origins of air masses entering this region. In order to assess to what extent cloud properties are impacted by aerosol concentration, air mass history, and meteorology, a Hybrid Single‐Particle Lagrangian Integrated Trajectory cluster analysis is conducted linking satellite observations of cloud properties (Spinning‐Enhanced Visible and Infrared Imager), information on aerosol species (Monitoring Atmospheric Composition and Climate), and meteorological context (ERA‐Interim reanalysis) to air mass clusters. It is found that a characteristic pattern of air mass origins connected to distinct synoptical conditions leads to marked cloud property changes in the southern part of the study area. Long‐distance air masses are related to midlatitude weather disturbances that affect the cloud microphysics, especially in the southwestern subdomain of the study area. Changes in cloud effective radius are consistent with a boundary layer deepening and changes in lower tropospheric stability (LTS). In the southeastern subdomain cloud cover is controlled by a generally higher LTS, while air mass origin plays a minor role. This study leads to a better understanding of the dynamical drivers behind observed stratocumulus cloud properties in the SEA and frames potentially interesting conditions for aerosol‐cloud interactions.

High ozone episodes at a semi-urban site in India: Photochemical generation and transport

The present study focused on investigating the possible atmospheric conditions influencing the high ozone episodes over a semi-urban site (27.16° N, 78.08° E) in Agra, India during July 2014–June 2016. During the study period, average concentration of ozone was 32.3±22.7ppb, and concentrations of its precursors viz carbon monoxide (CO), nitrogen oxide (NO) and total nitrogen oxides (NOx⁎) were 527.3±482.7ppb, 6.5±8.2ppb, and 12.1±7.8ppb, respectively. The maximum monthly average ozone concentration was observed in June (48.2±31.0ppb) and it was nearly three times higher than the minimum levels observed in December (16.7±10.5ppb). Considering high ozone episodes, 78days exceed hourly ozone limit of 90 ppb specified by National Air Quality Standards (NAAQS, CPCB, 2009) for India while 75 days exceed the daily maximum 8 hourly ozone limit of 70ppb or above (NAAQS, EPA, 2015). Trajstat model was used for air-mass cluster analysis during episodic days; five clusters of air-masses were identified and among them, the cluster from northwest direction had the maximum frequency (35.4%). During the study period, four different types of high ozone episodes were identified and explained using prevailing meteorology. The episodes were attributed to local photochemistry and/or transport.

Overview of surface measurements and spatial characterization of submicrometer particulate matter during the DISCOVER-AQ 2013 campaign in Houston, TX

ABSTRACTThe sources of submicrometer particulate matter (PM1) remain poorly characterized in the industrialized city of Houston, TX. A mobile sampling approach was used to characterize PM1 composition and concentration across Houston based on high-time-resolution measurements of nonrefractory PM1 and trace gases during the DISCOVER-AQ Texas 2013 campaign. Two pollution zones with marked differences in PM1 levels, character, and dynamics were established based on cluster analysis of organic aerosol mass loadings sampled at 16 sites. The highest PM1 mass concentrations (average 11.6 ± 5.7 µg/m3) were observed to the northwest of Houston (zone 1), dominated by secondary organic aerosol (SOA) mass likely driven by nighttime biogenic organonitrate formation. Zone 2, an industrial/urban area south/east of Houston, exhibited lower concentrations of PM1 (average 4.4 ± 3.3 µg/m3), significant organic aerosol (OA) aging, and evidence of primary sulfate emissions. Diurnal patterns and backward-trajectory analyses enable the classification of airmass clusters characterized by distinct PM sources: biogenic SOA, photochemical aged SOA, and primary sulfate emissions from the Houston Ship Channel. Principal component analysis (PCA) indicates that secondary biogenic organonitrates primarily related with monoterpenes are predominant in zone 1 (accounting for 34% of the variability in the data set). The relevance of photochemical processes and industrial and traffic emission sources in zone 2 also is highlighted by PCA, which identifies three factors related with these processes/sources (~50% of the aerosol/trace gas concentration variability). PCA reveals a relatively minor contribution of isoprene to SOA formation in zone 1 and the absence of isoprene-derived aerosol in zone 2. The relevance of industrial amine emissions and the likely contribution of chloride-displaced sea salt aerosol to the observed variability in pollution levels in zone 2 also are captured by PCA.Implications: This article describes an urban-scale mobile study to characterize spatial variations in submicrometer particulate matter (PM1) in greater Houston. The data set indicates substantial spatial variations in PM1 sources/chemistry and elucidates the importance of photochemistry and nighttime oxidant chemistry in producing secondary PM1. These results emphasize the potential benefits of effective control strategies throughout the region, not only to reduce primary emissions of PM1 from automobiles and industry but also to reduce the emissions of important secondary PM1 precursors, including sulfur oxides, nitrogen oxides, ammonia, and volatile organic compounds. Such efforts also could aid in efforts to reduce mixing ratios of ozone.

Spatiotemporal distribution and source apportionment of low molecular weight organic acids in wet precipitation at a coastal city, China.

To investigate the characteristics and sources of low molecular weight (LMW) organic acids in wet precipitation at a coastal city, Xiamen, a total of 313 rainwater samples were collected at seven different functional areas from September 2012 to August 2013. Spatiotemporal characteristics of LMW organic acids as well as pH and electrical conductivity were analyzed. Meanwhile, air mass clusters in different seasons and the positive matrix factorization (PMF) source apportion model were comprehensively used to identify the sources of organic acids. In conclusion, the volume-weighted mean (VWM) concentration of formic (3.20μmol/L), acetic (1.84μmol/L), lactic (0.44μmol/L), and oxalic acid (0.53μmol/L) were obtained, which jointly contributed to 4.33% of the total free acidity (TFA). At the same time, the highest wet deposition flux of LMW organic acids and contribution of that to TFA were achieved at the forest protection area during growing season in Xiamen. In addition, biogenic emissions (77.12%), sea salts (13.77%), regional agriculture activities (3.92%), soil emissions (2.56%), biomass burning (1.47%), and secondary aerosols (1.15%) were determined as the source of LMW organic acids. Besides, the dominancy of biomass burning via long-range transport in non-growing season (NGS) and the contribution of biogenic emission in growing season (GS) were recognized. Finally, the considerable influence of sea salts on the LMW organic acids (13.77%) in Xiamen was quantified, especially for oxalic acid.

Chemical Characterization and Source Apportionment of PM2.5 during Spring and Winter in the Yangtze River Delta, China

ABSTRACTTo investigate the characteristics and sources of PM2.5 in the Yangtze River Delta (YRD) region, a total of 10 sampling sites were selected in the three major cities of Shanghai, Nanjing, and Ningbo and the regional background city of Lin’an, and 380 samples were collected in spring and winter. The spatiotemporal characteristics of the PM2.5 mass concentration and the chemical components were analyzed. Meanwhile, air mass clusters and the positive matrix factorization (PMF) source apportion model were comprehensively used to identify the sources of PM2.5. The mass concentration of PM2.5 in winter (83.51–107.64 µg m–3) was higher than that in spring (54.11–85.72 µg m–3). Sulfate, nitrate, and ammonium (SNA) dominated the PM2.5, accounting for 39.0% in spring and 46.1% in winter in the YRD region. Higher ratio of secondary organic carbon (SOC) to organic carbon (OC) generally occurred in winter due to increased emissions of organic precursors. The mean equivalent ratio of AE/CE was 0.81 ± 0.25 in winter and 0.96 ± 0.33 in spring, which indicated a slight alkalinity of the atmospheric particles in the YRD region. Secondary aerosols (29.44%) and traffic (25.66%) were identified as the main sources of PM2.5 in the YRD during spring, and they contributed 22.71% and 29.71% in winter. In addition, air mass flow from Northern China imported substantial fugitive soils to the YRD region in spring and enhanced the contribution of combustion in winter through long-range transport, while air masses originating from Southwestern China were strongly associated with biomass burning. Our results showed that the PM2.5 concentration and chemical characterization in the YRD region was significantly influenced by air mass transport.

Influence of post‐harvest crop residue fires on surface ozone mixing ratios in the N.W. IGP analyzed using 2 years of continuous in situ trace gas measurements

AbstractO3, CO, and NOx affect air quality and tropospheric chemistry but factors that control them in the densely populated N.W. Indo‐Gangetic Plain (IGP) are poorly understood. This work presents the first simultaneous 2 year long in situ data set acquired from August 2011 to September 2013 at a N.W. IGP site (30.667°N, 76.729°E; 310 m asl). We investigate the impact of emissions and meteorology on the diel and seasonal variability of O3, CO, and NOx. Regional post‐harvest crop residue fires contribute majorly to an enhancement of 19 ppb in hourly averaged ozone concentrations under similar meteorological conditions in summer and 7 ppb under conditions of lower radiation during the post monsoon. d[O3]/dt (from sunrise to daytime O3 maxima) was highest during periods influenced by post‐harvest fires in post monsoon season (9.2 ppb h−1) and lowest during monsoon season (4.1 ppb h−1). Analysis of air mass clusters revealed that enhanced chemical formation of O3 and not transport was the driver of the summertime and post monsoon ambient O3 maxima. Despite having high daytime NOx (&gt;12 ppb) and CO (&gt;440 ppb) in winter, average daytime O3 was less than 40 ppb due to reduced photochemistry and fog. Average daytime O3 during the monsoon was less than 45 ppb due to washout of precursors and suppressed photochemistry due to cloud cover. The 8 h ambient air quality O3 standard was violated on 451 days in the period August 2011–September 2013. The results show that substantial mitigation efforts are required to reduce regional O3 pollution in the N.W. IGP.