Long-range Transport Of Air Masses Research Articles

An unexpected increase in PM2.5 levels in Xi'an during the COVID-19 pandemic restrictions: The interplay of anthropogenic and natural factors

This study investigated the variations in summer and winter PM2.5 concentrations and chemical composition in urban Xi'an before and during the COVID-19 pandemic restrictions. During the pandemic restrictions, summer daytime PM2.5 concentrations remained comparable to pre-pandemic levels, while a reduction was noted at nighttime. Conversely, winter experienced a significant increase in both daytime and nighttime PM2.5 concentrations. Chemical composition analysis revealed reductions in secondary inorganic ion concentrations but notable increases in crustal matter concentrations during the pandemic restrictions, particularly evident in winter. The reductions in secondary inorganic ion concentrations were likely due to decreased emissions of corresponding anthropogenic precursors in summer, while linked to reductions in transformation efficiencies in winter. The heightened crustal matter concentrations were likely attributed to increased contributions of long-range air mass transport from dusty regions, especially prevalent in winter. Source apportionment using positive matrix factorization analysis provided quantitative insights into the distinct source profiles contributing to PM2.5 before and during the pandemic restrictions, with secondary inorganic-rich sources decreasing and dust-related sources increasing during the pandemic restrictions. Additionally, combustion sources, primarily from coal and biomass burning, showed higher contributions during winter. In conclusion, this study underscores the complex interplay between anthropogenic and natural factors influencing PM2.5 levels in Xi'an. Efforts to mitigate PM2.5 pollution should prioritize reducing anthropogenic emissions and implementing measures to control dust emissions, particularly when dust-related sources significantly contribute to elevated PM2.5 concentrations. These findings provide valuable insights into developing effective strategies for addressing the PM2.5 pollution problem in Xi'an.

Urban air PCDD/Fs: Atmospheric concentrations, temporal changes, gas/particle partitioning, possible sources and cancer risks

Polychlorinated dibenzo-p-dioxins/furans (PCDD/Fs) are pollutants of concern due to their toxic effects. No active sampling study on PCDD/Fs has been conducted in Bursa. This study aimed to fill this gap by measuring PCDD/F levels in the region. Accordingly, the samples were collected from an urban area in Bursa, covering four seasons between June 2022 and April 2023. The total (gas+particulate) ambient air concentrations were between 312.23 and 829.80 fg/m3 (mean: 555.05 ± 173.62 fg/m3). In terms of toxic equivalents (TEQ), the average concentration was 43.29 ± 9.18 fg WHOTEQ/m3. Based on the concentration values obtained, cancer and non-carcinogenic risk values of PCDD/Fs were calculated for three different age groups. The results indicated negligible health risks for all age groups. In addition, a seasonal assessment was also made and it was observed that PCDD/F concentration values varied with the ambient air temperatures. In general, higher values were measured in colder months compared to warmer months. This was probably due to the additional sources and adverse meteorological conditions. Moreover, the gas/particle partitioning of PCDD/Fs was investigated in detail. The average gas and particulate phase concentrations for PCDD/Fs were 101.81 ± 20.77 and 453.24 ± 172.50, respectively. It was found that an equilibrium state was not reached in the gas/particle partitioning. Two different gas/particle partition models based on adsorption and absorption mechanisms were compared, and the absorption model gave more consistent predictions. The Principal Component Analysis (PCA) was employed to identify the possible PCDD/F sources. The results indicated that the region was influenced by vehicle emissions, residential heating, organized industrial zones and metal recycling facilities. In addition, 72-hour backward air mass trajectory analyses were performed to understand the long-range transported air masses. However, it was found that the transported air masses did not significantly affect the concentration values measured in the sampling site.

Seasonal variations of metals and metalloids in atmospheric particulate matter (PM2.5) in the urban megacity Hanoi

Fine particulate matter (PM2.5) in the atmosphere is of particular concern due to its adverse effects on human health and its impact on global warming. Southeast Asia is a hot spot for fossil fuel combustion with recurrent release of large plumes spreading over the ocean and neighbouring countries. Due to the complex mixture of PM2.5, the atmospheric sources contribution related to local and regional emissions in Hanoi (northern Vietnam) is still ill-constrained. Here, we present a year-round study (November 2019 to December 2020) with measurements of 18 metals and metalloids (MM) and lead isotopes in the PM2.5 fraction to quantify weather-related atmospheric inputs and to assess risk to human health. Anthropogenic inputs from fossil fuel combustion accounted for about 80% in PM2.5. We found high PM2.5-bound MM concentrations often exceeding national and global standards with a low risk of chronic inhalation and carcinogenicity, mainly attributable to Cr. During winter monsoon (northeastern winds), stable weather conditions led to the enrichment of long-range air mass transport of local particulate emissions. During the summer monsoon (southeastern winds), warm and moist winds reduced coal contribution in PM2.5. Our study highlights the need for a strict implementation of policies to control hazardous MM emissions by reducing fossil fuel combustion. On the one hand, reducing coal-related activities could reduce Cr emissions and therefore improve the risks to human health. On the other hand, public policies should encourage conversion to green transport in order to reduce petrol combustion and thus limit global warming.

Exploring the size-dependent dynamics of photosynthetic cells in rainwater: The influence of atmospheric variables and rain characteristics

The presence of microalgae in the atmosphere raises health and environmental concerns. Despite recent scientific advances, our knowledge of the origins and dynamics of photosynthetic cells in relation to atmospheric processes is limited due to a lack of empirical data. To address this gap, we conducted a one-year survey, collecting and analyzing rainwater samples. This study proposes to investigate the temporal dynamics of photosynthetic cells based on their size in combination with a unique dataset of variables of interest: type of rain and its characteristics, local meteorology, concentrations of inorganic chemical species, and long-range air mass transport. The analysis of the biochemical composition of rainwater, along with its correlation with the origin of air masses using ions as tracers, provides evidence of the long-range transport of photosynthetic cells. Additionally, our study reveals distinct removal mechanisms from the atmosphere for photosynthetic cells depending on their size. Our results suggest that convective events with high-intensity rainfall led to the efficient removal of medium-sized photosynthetic cells (4–15 μm) from the atmosphere. However, removal mechanisms for small (<4 μm) and large-sized cells (>15 μm) are not influenced by microphysical rainfall characteristics and seem to be governed by different atmospheric processes: dry deposition is proposed to be a significant mechanism for the removal of large-sized photosynthetic cells, while small-sized cells detected in rain are correlated with the horizontal wind speed and duration of rainfall, particularly during stratiform events. This implies that the removal of photosynthetic cells from the atmosphere is strongly influenced by environmental variables, which are expected to vary in response to global change. Therefore, it is crucial to enhance the monitoring of photosynthetic cells in relation to atmospheric processes and investigate the potential impact of the dissemination of genetic material from distant sources on recipient ecosystems.

Variability of airborne microbiome at different urban sites across seasons: a case study in Rome

Introduction: Biogenic fraction of airborne PM10 dominated by bacteria and fungi, has been recognized as serious environmental and human health issues in cities.Methods: In the present study, we combined a high-throughput amplicon sequencing of the bacterial 16S rRNA gene and the fungal internal transcribed spacer (ITS) region, with elemental analysis of airborne particulate matter (PM10) to investigate the community compositions and structures of PM10-associated bacteria and fungi across four different seasons in three urban sites of Rome with differential pollution rate.Results: In this study, а clear seasonal shift of bacterial and fungal community structure driven by PM10 mass concentrations and environmental factors, such as temperature and precipitations, has been identified. In addition, the seasonal impact of local sources and long-range transported air masses on the community structures of the microbes has been also postulated. Our data revealed that the lack of precipitation and the subsequent resuspension of dust produced by vehicular traffic might contribute to the maximum abundance of soil-associated microbes in winter and summer. However, the increase of PM10 concentrations favoured also by climatic conditions, domestic heating and dust advection event from African desert further shaped the community structure of winter. Across three seasons, the pollutant removal-hydrogen oxidation bacteria and the opportunist-human pathogenic fungi progressively increased with pollution levels, in the sequence from green to residential and/or polluted area close to the traffic roads, with highest fraction during winter.Discussion: Hence, our results highlight a close interrelationship between pollution, climatic factors and abundance of certain bacterial and fungal predicted functional groups also with potential implications for human health.

Seasonal variations in the production of singlet oxygen and organic triplet excited states in aqueous PM2.5 in Hong Kong SAR, South China

Abstract. Photooxidants drive many atmospheric chemical processes. The photoexcitation of light-absorbing organic compounds (i.e., brown carbon, BrC) in atmospheric waters can lead to the generation of reactive organic triplet excited states (3C∗), which can undergo further reactions to produce other photooxidants such as singlet oxygen (1O2∗). To determine the importance of these aqueous photooxidants in secondary organic aerosol (SOA) formation and transformation, we must know their steady-state concentrations and quantum yields. However, there have been limited measurements of aqueous 3C∗ and 1O2∗ in atmospheric samples outside of North America and Europe. In this work, we report the first measurements of the steady-state concentrations and quantum yields of 3C∗ and 1O2∗ produced in aerosols in South China. We quantified the production of 3C∗ and 1O2∗ in illuminated aqueous extracts of PM2.5 collected in different seasons at two urban sites and one coastal semi-rural site during a year-round study conducted in Hong Kong SAR, South China. The mass absorption coefficients at 300 nm for BrC in the aqueous PM2.5 extracts ranged from 0.49 to 2.01 m2 g-C−1 for the three sites. Both 1O2∗ and 3C∗ were produced year-round. The steady-state concentrations of 1O2∗ ([1O2∗]ss) in the illuminated aqueous extracts ranged from 1.56×10-14 to 1.35×10-12 M, with a study average of (4.02±3.52)×10-13 M. At nearly 2 orders of magnitude lower than [1O2∗]ss, the steady-state concentrations of 3C∗ ([3C∗]ss) ranged from 2.93×10-16 to 8.08×10-14 M, with a study average of (1.09±1.39)×10-14 M. The quantum yields of 1O2∗ and 3C∗ also spanned wide ranges across samples, with a range of 1.19 % to 13.74 % and an average of (5.19±2.63) % for 1O2∗ and a range of 0.05 % to 3.24 % and an average of (0.56±0.66) % for 3C∗. The [1O2∗]ss and [3C∗]ss correlated with the concentration and absorbance of BrC, thus implying that the amount of BrC drives the steady-state concentrations of these photooxidants. The locations (urban vs. semi-rural) did not have a significant effect on [3C∗]ss and [1O2∗]ss, which indicated that BrC from local sources did not have a significant influence on the year-round 3C∗ and 1O2∗ production. 3C∗ and 1O2∗ production were found to be the highest in winter and the lowest in summer for all three sites. The observed seasonal trends of 1O2∗ and 3C∗ production could be attributed to the seasonal variations in the long-range air mass transport. Our analysis highlighted the key role that regional sources play in influencing the composition and concentrations of water-soluble BrC in winter PM2.5 in Hong Kong SAR, which contributed to their highest 3C∗ and 1O2∗ production. The current results will be useful for modeling seasonal aqueous organic aerosol photochemistry in the South China region.

Water-soluble organic aerosols over South Asia – Seasonal changes and source characteristics

Water-soluble organic carbon (WSOC) has been identified as a key component in atmospheric aerosols due to its ability to act as cloud condensation nuclei (CCN) owing to their highly hygroscopic nature. This paper discusses about the spatio-temporal variability in WSOC mass concentration, sources (primary and secondary contributions), the role of long-range air-mass transport in modulating their abundance, at distinct sectors over South Asia. We found from our observations that, photochemical ageing of primary organic aerosols that are derived from biomass emissions, significantly contribute to the total WSOC budget over South Asia. The wide range of water-soluble compounds released by biomass burning can contribute directly to the WSOC fraction or undergo further atmospheric processing, such as oxidation or ageing, leading to the formation of additional WSOC. WSOC/OC (organic carbon) ratio and the correlation between the WSOC and secondary organic carbon (SOC) are used for assessing the contribution from secondary sources. The three different ratios are used to delineate different source processes; OC/EC (elemental carbon) for source identification, WSOC/OC for long-range atmospheric transport (ageing) and WSOC/SOC to understand the primary and secondary contribution of WSOC. The present investigation revealed that, the primary OC that have undergone significant chemical processing as a result of long-range transport have a substantial influence on WSOC formation over South Asia, especially in Indo Gangetic Plain outflow regions such as southern peninsular and adjacent marine regions. Overall, oxidation and ageing of primary organic aerosols emitted from biomass burning was found to serve as an important source of WSOC over South Asia.

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.

Investigation of sources and atmospheric transformation of carbonaceous aerosols from Shyamnagar, eastern Indo-Gangetic Plains: Insights from δ13C and carbon fractions

This study reports the chemical characterization of the carbonaceous component of PM2.5 (particulate matter with aerodynamic diameter ≤2.5 μm) collected over a year-long campaign from a regional site in Shyamnagar, West Bengal, in the Indo-Gangetic Plains (IGP), India. The carbonaceous fractions (elemental and organic carbon), mass concentrations, and stable carbon isotopic composition (δ13C value) of aerosols were measured and utilized to characterize the sources and understand the atmospheric processing of aerosols. Cluster analysis, Potential Source Contribution Function (PSCF) modeling, and fire count data were analyzed to decipher the pattern of air masses, source contributions, and extent of burning activities. The PM2.5 mass concentrations were significantly higher during winter (168.3 ± 56.3 μg m−3) and post-monsoon (109.8 ± 59.1 μg m−3) compared to the monsoon (29.8 ± 10.7 μg m−3) and pre-monsoon (55.1 ± 23.0 μg m−3). Organic carbon (OC), elemental carbon (EC), and total carbon (TC) concentrations were also several factors higher during winter and post-monsoon compared to monsoon and pre-monsoon. The winter and post-monsoon experienced the impact of air masses from upwind IGP. On the other hand, long-range transported air masses from the South-West direction dominated during monsoon and pre-monsoon, which are also relatively cleaner periods. The average δ13C during post-monsoon and winter was ∼1‰ higher compared to monsoon and pre-monsoon. The vehicular exhaust and biomass/biofuel burning contributed dominantly in winter and post-monsoon. In comparison, lower δ13C in pre-monsoon and monsoon might be attributed to the dominance of biomass/biofuel combustion. Photochemical-induced aging of the anthropogenic aerosols resulted in a higher δ13C of TC in winter and post-monsoon, whereas the mixing of different local sources in pre-monsoon and monsoon resulted in lower δ13C values. These findings benefit policymakers in strategizing proper and effective management of biomass/biofuel burning in the IGP to minimize air pollution.

Measurement report: Abundance and fractional solubilities of aerosol metals in urban Hong Kong – insights into factors that control aerosol metal dissolution in an urban site in South China

Abstract. Water-soluble metals are known to produce greater adverse human health outcomes than their water-insoluble forms. Although the concentrations of water-soluble aerosol metals are usually limited by atmospheric processes that convert water-insoluble metals to water-soluble forms, factors that control the solubilities of aerosol metals in different environments remain poorly understood. In this study, we investigated the abundance and fractional solubilities of different metals in size-fractionated aerosols collected at an urban site in Hong Kong and identified the factors that modulated metal solubilities in fine aerosols. The concentrations of total and water-soluble metals in fine and coarse aerosols were the highest during the winter and spring seasons due to the long-range transport of air masses by northerly prevailing winds from emission sources located in continental areas north of Hong Kong. The study-averaged metal fractional solubilities spanned a wide range for both fine (7.8 % to 71.2 %) and coarse (0.4 % to 47.9 %) aerosols, but higher fractional solubilities were typically observed for fine aerosols. Sulfate was found to be strongly associated with both the concentrations of water-soluble Cr, Fe, Co, Cu, Pb, and Mn and their fractional solubilities in fine aerosols, which implied that sulfate-driven acid processing likely played an important role in the dissolution of the water-insoluble forms for these six metals. Further analyses revealed that these strong associations were due to sulfate providing both the acidic environment and liquid water reaction medium needed for the acid dissolution process. Thus, the variability in the concentrations of water-soluble Cr, Fe, Co, Cu, Pb, and Mn and their fractional solubilities were driven by both the aerosol acidity levels and liquid water concentrations, which in turn were controlled by sulfate. These results highlight the roles that sulfate plays in the acid dissolution of metals in fine aerosols in Hong Kong. Our findings will likely also apply to other urban areas in South China, where sulfate is the dominant acidic and hygroscopic component in fine aerosols.

Effect of Meteorological Conditions and Long-Range Air Mass Transport on Surface Aerosol Composition in Winter Moscow

Effect of Meteorological Conditions and Long-Range Air Mass Transport on Surface Aerosol Composition in Winter Moscow

Variabilities of δ13C and carbonaceous components in ambient PM2.5 in Northeast India: Insights into sources and atmospheric processes

A year-long sampling campaign of ambient PM2.5 (particulate matter with aerodynamic diameter ≤2.5 mm) at a regional station in the North-Eastern Region (NER) of India was performed to understand the sources and formation of carbonaceous aerosols. Mass concentration, carbon fractions (organic and elemental carbon), and stable carbon isotope ratio (δ13C) of PM2.5 were measured and studied along with cluster analysis and Potential Source Contribution Function (PSCF) modelling. PM2.5 mass concentration was observed to be highest during winter and post-monsoon seasons when the meteorological conditions were relatively stable compared to other seasons. Organic carbon (OC) concentration was more than two times higher in the post-monsoon and winter seasons than in the pre-monsoon and monsoon seasons. Air mass back trajectory cluster analysis showed the dominance of local and regional air masses during winter and post-monsoon periods. In contrast, long-range transported air masses influenced the background site in pre-monsoon and monsoon. Air mass data and PSCF analysis indicated that aerosols during winter and post-monsoon are dominated by freshly generated emissions from local sources along with the influence from regional transport of polluted aerosols. On the contrary, the long-range transported air masses containing aged aerosols were dominant during pre-monsoon. No significant variability was observed in the range of δ13C values (−28.2‰ to −26.4‰) during the sampled seasons. The δ13C of aerosols indicates major sources to be combustion of biomass/biofuels (C3 plant origin), biogenic aerosols, and secondary aerosols. The δ13C variability and cluster/PSCF modelling suggest that aged aerosols (along with enhanced photo-oxidation derived secondary aerosols) influenced the final δ13C during the pre-monsoon. On the other hand, lower δ13C in winter and post-monsoon is attributed to the freshly emitted aerosols from biomass/biofuels.

Temporal variation of gaseous elemental mercury in a northern coastal city in China: Monsoon and COVID-19 lockdown effects

Continuous measurements of gaseous elemental mercury (GEM) were conducted in Qingdao from March 2020 to March 2021. The average concentration of GEM was (2.39 ± 1.07 ng/m3) with a variation range of 0.27–10.78 ng/m3. GEM exhibited a clear pattern of daily variation, with daily peaks occurring between 11:00–13:00. GEM concentrations were higher in winter (2.80 ± 1.28 ng/m3) than that in summer (2.18 ± 1.05 ng/m3). The high winter concentrations were related to coal-fired heating and the increased frequency of polluted weather in northern China. Principal component analysis showed that the main factors affecting GEM concentration were fossil fuel combustion, natural source release and atmospheric diffusion conditions. The anthropogenic emission sources were the main source of GEM in spring and winter, and natural sources of GEM was large in summer. The potential source contribution function suggested that North and Central China were the main potential sources of GEM, and there were large differences in the potential sources of GEM in different seasons. Comparing the GEM in the same time periods in 2018, 2020, and 2021, government policies, temporary lockdown measures for the COVID-19 epidemic, and urban village renovation led to a decreasing trend of GEM concentrations. This study contributes to a better understanding of the effects of long-range transport of air masses and anthropogenic emissions on atmospheric mercury in eastern coastal cities and offshore areas.

Low-molecular-weight carbonyl volatile organic compounds on the North China Plain

To understand the photochemical pollution characteristics of low-molecular-weight carbonyl compounds on the North China Plain (NCP), 2,4-dinitrophenylhydrazine (DNPH) was used to sample low-molecular-weight carbonyl compounds in the atmosphere in Beijing, Tianjin, Shijiazhuang, Xianghe and Xinglong from June 2019 to January 2020, and the samples were then analyzed by a high-performance liquid chromatography (HPLC) to obtain the concentration, daytime variations, seasonal variations, OH radical loss rate, ozone formation potential and sources of five low-molecular-weight carbonyl compounds in the NCP region. The total concentrations at the five sites were 7.70 ± 1.94 ppbv (Beijing), 10.20 ± 1.83 ppbv (Tianjin), 15.31 ± 2.35 ppbv (Shijiazhuang), 10.53 ± 1.82 ppbv (Xianghe) and 5.42 ± 1.15 ppbv (Xinglong), respectively. The contribution of primary source from anthropogenic emission was higher in Tianjin, Shijiazhuang and Xianghe than that in Beijing. The primary and secondary generation contributions were similar in Xinglong. The concentration in Beijing was highest in autumn and lowest in winter. For Tianjin, Shijiazhuang, Xianghe and Xinglong, the highest concentrations occurred in summer. At the five sites, formaldehyde and acetaldehyde contributed the most to L·OH and OFP, while acetone contributed the least. Both L·OH and OFP in Shijiazhuang were the highest, and those in Xinglong were the lowest. The North China Plain was mainly influenced by short-range airflow from the sea and coastal areas during the summer, with the airflow mainly coming from the south in autumn, while the long-range air mass transport was more influential during the winter. In addition, most pollutants were found to be generated locally. By utilizing a source-tracer-ratio method in the source analysis, with the change of season, the anthropogenic source contribution of each site was found to gradually increase while that of other sources gradually decreased.

An aerosol classification scheme for global simulations using the K-means machine learning method

Abstract. The K-means machine learning algorithm is applied to climatological data of seven aerosol properties from a global aerosol simulation using EMAC-MADE3. The aim is to partition the aerosol properties across the global atmosphere in specific aerosol regimes; this is done mainly for evaluation purposes. K-means is an unsupervised machine learning method with the advantage that an a priori definition of the aerosol classes is not required. Using K-means, we are able to quantitatively define global aerosol regimes, so-called aerosol clusters, and explain their internal properties and their location and extension. This analysis shows that aerosol regimes in the lower troposphere are strongly influenced by emissions. Key drivers of the clusters' internal properties and spatial distribution are, for instance, pollutants from biomass burning and biogenic sources, mineral dust, anthropogenic pollution, and corresponding mixtures. Several continental clusters propagate into oceanic regions as a result of long-range transport of air masses. The identified oceanic regimes show a higher degree of pollution in the Northern Hemisphere than over the southern oceans. With increasing altitude, the aerosol regimes propagate from emission-induced clusters in the lower troposphere to roughly zonally distributed regimes in the middle troposphere and in the tropopause region. Notably, three polluted clusters identified over Africa, India, and eastern China cover the whole atmospheric column from the lower troposphere to the tropopause region. The results of this analysis need to be interpreted taking the limitations and strengths of global aerosol models into consideration. On the one hand, global aerosol simulations cannot estimate small-scale and localized processes due to the coarse resolution. On the other hand, they capture the spatial pattern of aerosol properties on the global scale, implying that the clustering results could provide useful insights for aerosol research. To estimate the uncertainties inherent in the applied clustering method, two sensitivity tests have been conducted (i) to investigate how various data scaling procedures could affect the K-means classification and (ii) to compare K-means with another unsupervised classification algorithm (HAC, i.e. hierarchical agglomerative clustering). The results show that the standardization based on sample mean and standard deviation is the most appropriate standardization method for this study, as it keeps the underlying distribution of the raw data set and retains the information of outliers. The two clustering algorithms provide similar classification results, supporting the robustness of our conclusions. The classification procedures presented in this study have a markedly wide application potential for future model-based aerosol studies.

Chemical characterisation of natural and anthropogenic inputs of dust in the seasonal snowpack (2017–2020) at Calderone Glacier (Gran Sasso d’Italia)

Environmental context We present a chemical characterisation of the seasonal snowpack sampled for four consecutive years at the Calderone, the southernmost glacier still surviving in peninsular Italy. This debris covered glacier recently split into two little ice bodies, whose evolution could be influenced by the snowpack properties. In particular the impact of long-range aerosol advections on concentrations of impurities in the snowpack over the local background is discussed. Rationale The Calderone Glacier (Central Apennine, Gran Sasso d’Italia mountain group) is the southernmost glacial apparatus in Europe, split into two glacierets (Upper and Lower Calderone) since the end of the last millennium. Because of its location and altitude, this site is mainly characterised by the long-range transport of air masses which arise from different Mediterranean source regions. Therefore, the seasonal snowpack’s chemistry is strongly affected by the dry and wet deposition of contaminants associated with anthropogenic and natural sources. Methodology In the present study, the seasonal snowpack stratified on the Calderone glacier has been characterised for four consecutive years (2017–2020) in the same monitoring site (2700 m asl), where a snow pit has been dug yearly, to observe the modification of chemical and physical properties depending on local and long-range atmospheric contributions. We determined the concentrations and fluxes of major inorganic ions (MIs) by ion chromatography and of 31 trace elements (TEs) by triple quadrupole ICP-MS. Results Major and trace element concentration profiles along the snowpack allowed to discriminate the snow layers contaminated by long range advections from the uncontaminated ones. The uncontaminated snow layers’ concentrations were used to calculate regional background values. The results have been compared to other remote sites to assess their robustness. Discussion Different source contributions have been recognised using enrichment factors for the trace elements, particularly crustal, marine and anthropogenic. Deposited atmospheric aerosols, found in the snowpack as distinct layers generated during intense air mass advections, have been correlated to these contributions.

The influence of meteorological conditions and long-range air mass transport on the winter near-surface aerosol composition in Moscow

The influence of meteorological conditions and long-range air mass transport on the winter near-surface aerosol composition in Moscow

Characterization of atmospheric total gaseous mercury at a remote high-elevation site (Col Margherita Observatory, 2543 m a.s.l.) in the Italian Alps

The Col Margherita (MRG) Observatory is a high-altitude background station located in the Eastern Italian Alps. Its elevation and distance from major anthropogenic and natural sources make it ideal for monitoring baseline mercury levels and investigating its geochemical cycles. In this work, total gaseous mercury (TGM), ozone (O3) and meteorological variables were studied to investigate seasonal and diurnal variability of TGM measurements from March 2018 to May 2019. We found that the year-round mean TGM concentration was 1.8 times higher than the annual atmospheric mercury measurements previously reported during the GMOS project at the same measurement site. The seasonal variation of TGM was characterized by high values in spring and summer and lower values in winter. A systematic diel pattern of TGM was obtained, with low concentrations during the daytime and higher concentrations in the late evening. Spatial patterns and temporal changes in TGM, back-trajectories (BTs) and planetary boundary layer (PBL) analysis showed that total gaseous mercury levels were influenced by local meteorology, as well as regional and long-range transport of air masses. The lowest TGM concentrations in winter are associated with high wind speeds, low boundary layer height and cleaner air masses originating from the Western sector. Conversely, the highest TGM concentrations in spring and summer were attributed to polluted air masses passing over North-eastern and Continental Europe and were probably favoured by the higher PBL height. During the snow cover season, investigation of TGM peak events also demonstrated the potential influence of re-emission processes of previously reactive mercury deposition over snow surfaces.

Isotopic composition (δ15N, δ18O) of nitrate in high-frequency precipitation events differentiate atmospheric processes and anthropogenic NOx emissions

Reactive nitrogen (Nr) species from the atmosphere contribute to adverse impacts like acid deposition and eutrophication; thus, information about local or distal emission N sources towards wet deposition of nitrogen oxides (NOx) is important information. Given N emissions source regions and atmospheric chemistry processes can change on an hourly basis, a thorough assessment of Nr removal from the atmosphere requires high resolution precipitation sampling. We measured nitrate isotopes (δ15Ν, δ18O of NO3−) and δ18Ο-H2O of all precipitation events at high-frequency (5–30 min) for Vienna, Austria, in 2019. Average monthly δ15N of NO3− ranged from −7.0‰ (AIR) to −0.1‰ with lowest and higher values in summer and winter, respectively. Similarly, the average δ18O of NO3− was highest in winter (+75.0‰, VSMOW) and lowest in summer (+61.2‰). Nitrate in nighttime precipitation and intense rainouts had higher δ15N-NO3− values (−4.4‰ and − 4.6‰, respectively) compared to daytime and washout samples (−5.7‰ and − 5.5‰). Atmospheric oxidation and cloud scavenging on a diel basis were correlated to air temperature, with an opposite non-linear effect between the δ18O of NO3− and H2O. The δ15N of NO3− showed a negative correlation with rain intensity, but a positive relationship to the number of days without rain prior to a precipitation event. Overall, nitrate source partitioning with and without photochemical equilibrium isotope fractionation suggested strong year-round influences from vehicle emissions and biomass burning by day and night. Biogenic, fertilized soil, and coal combustion N sources (power plants) showed a lower contribution to precipitation NO3− and was linked to both harvest periods and long-range transport of air masses from North-Eastern and southern Europe. N emissions from local municipal waste incineration were significant in the autumn. Our study reveals that nitrate isotopes in wet deposition combined with δ18O-H2O contribute new information to inform NOx source management strategies aimed at improving environmental air and water quality.

A multi-year source apportionment of PM2.5 at multiple sites in the southern Po Valley (Italy)

A source apportionment study was carried out at four sites in Emilia-Romagna region, southern Po Valley, one of the most critical regions in Europe in terms of atmospheric pollution. PM2.5 daily samples were collected during 4 years from April 2013 to October 2017 at one rural site (San Pietro Capofiume) and three urban background locations in the cities of Bologna, Rimini, Parma which show different features and are located in the central, coastal and inner part of the investigated region. Samples were analyzed to achieve a complete chemical characterization (carbon fractions, ions, and elements). A source apportionment analysis by Positive Matrix Factorization (PMF) was performed and 6 PM2.5 factors were identified at all sites but the rural one (where 5 out of 6 of them were detected); the factors were associated to traffic with dust resuspension, biomass burning, oil combustion/ship emission, mix anthropogenic (not found at the rural site), ammonium nitrate and ammonium sulfate with organics. Chemical profiles of factors were very similar among all the 4 sites, indicating that main pollution sources are basically the same at the 4 sites, while some differences emerged with regard to source contributions. Factors related to secondary components seem to explain almost 50% or even more of PM2.5 mass concentration in all seasons. Traffic and biomass burning are the most relevant contributors to PM2.5 in terms of primary components. A not negligible contribution of biomass burning results in Rimini during the summer, suggesting other possible sources of wood combustion, such as cooking or open burning of agricultural pruning bonfires. Agriculture is not singled out as a PMF factor, but a rough estimate based on ammonium concentrations and ammonia data from emission inventory indicates a contribution from this source of about 10% of PM2.5 mass, thus resulting the single productive activity with the highest impact on PM2.5 at the investigated sites. Back trajectory analysis points out the relevant extra-regional contributions of two factors; indeed, oil combustion/ship emission is related to long-range transport of air masses overpassing the Mediterranean sea and secondary sulfate from Eastern Europe countries occasionally impacts on the Po Valley.