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In order to reduce the uncertainty of aerosol radiative forcing in global climate models, we need to better understand natural aerosol sources which are important to constrain the current and pre-industrial climate. Here, we analyze Particle Number Size Distributions (PNSD) collected during a year (2015) across four coastal and inland Antarctic research bases (Halley, Marambio, Concordia/Dome C and King Sejong). We find that the four Antarctic locations have striking differences in PNSD, stressing multiple aerosol sources and processes likely exist. We utilise k-means cluster analysis to separate the PNSD data into six main categories. Nucleation and Bursting PNSDs occur 28-48% of the time between sites, most commonly at coastal sites Marambio and King Sejong where air masses mostly come from the west and travel over extensive regions of sea ice, marginal ice, and open ocean, and likely arise from new particle formation. Aitken high, Aitken low, and bimodal PNSDs occur 37-68% of the time, most commonly at Concordia/Dome C on the Antarctic Plateau, and likely arise from atmospheric transport and aging from aerosol originating likely in both coastal boundary layer and free troposphere. Pristine PNSDs with low aerosol concentrations occur 12-45% of the time, most common at Halley located at low altitudes and far from the coastal melting ice, and influenced by air masses from the west. The Antarctic Atmospheric circulation has a strong control on the air mass origin type. Most of the time Marambio and King Sejong stations are affected by Easterly air masses, whereas Halley gets air masses mainly from the Weddell sea marginal and consolidated pack ice. Not only the sea spray primary aerosols and gas to particle secondary aerosols sources, but also the different air masses impacting the research stations should be kept in mind when deliberating upon different aerosol precursors sources across research stations. We provide evidence that both primary and secondary components from pelagic and sympagic regions strongly contribute to the annual seasonal cycle of Antarctic aerosols which add insight on the possible sources of aerosol production/activity in the whole Antarctic region. Our simultaneous aerosol measurements stress the importance of the variation in atmospheric biogeochemistry across the Antarctic region. [ABSTRACT FROM AUTHOR]

Lockdown restrictions in response to the COVID--19 pandemic led to the curtailment of many activities and reduced emissions of primary air pollutants. Here, we applied Positive Matrix Factorization to particle size distribution (PSD) data from six monitoring sites (three urban background and three roadside) between four European cities (Helsinki, Leipzig, Budapest, and London) to evaluate how particle number concentrations (PNCs) and their sources changed during the respective 2020 lockdown periods compared to the reference years 2014--2019. A number of common factors were resolved between sites, including nucleation, road traffic semi--volatile fraction (road trafficsvf), road traffic solid fraction (road trafficsf), diffuse urban (woodsmoke + aged traffic), ozone--associated secondary aerosol (O3--associated SA), and secondary inorganic aerosol (SIA). Nucleation, road traffic, and diffuse urban factors were the largest contributors to mean PNCs during the reference years and respective lockdown periods. However, SIA factors were the largest contributors to particle mass concentrations, irrespective of environment type. Total mean PNCs were lower at two of the urban background and all roadside sites during lockdown. Nucleation and road trafficsvf factors response to lockdown restrictions were highly variable, although road trafficsf factors were consistently lower at roadside sites. The responses of diffuse urban factors were largely consistent and were mostly lower at urban background sites. Secondary aerosols (O3-associated SA and SIA) exhibited extensive reductions to their mean PNCs at all sites. These variegated responses to lockdowns across Europe point to a complex network of sources and aerosol sinks contributing to PSDs. [ABSTRACT FROM AUTHOR]

The Arctic environment is rapidly changing due to accelerated warming in the region. The warming trend is driving a decline in sea ice extent, which thereby enhances feedback loops in the surface energy budget in the Arctic. Arctic aerosols play an important role in the radiative balance, and hence the climate response, in the region; yet direct observations of aerosols over the Arctic Ocean are limited. In this study, we investigate the annual cycle in the aerosol particle number size distribution (PNSD), particle number concentration (PNC), and black carbon (BC) mass concentration in the central Arctic during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. This is the first continuous, year-long dataset of aerosol PNSD ever collected over the sea ice in the central Arctic Ocean. We use a k-means cluster analysis, FLEXPART simulations, and inverse modeling to evaluate seasonal patterns and the influence of different source regions on the Arctic aerosol population. Furthermore, we compare the aerosol observations to land-based sites across the Arctic, using both long-term measurements and observations during the year of the MOSAiC expedition (2019 - 2020), to investigate interannual variability and to give context to the aerosol characteristics from within the central Arctic. Our analysis identifies that, overall, the central Arctic exhibits typical seasonal patterns of aerosols, including anthropogenic influence from Arctic Haze in winter and secondary aerosol processes in summer. The seasonal pattern corresponds with the global radiation, surface air temperature, and the timing of sea ice melting/freezing, which drives changes in transport patterns and secondary aerosol processes. In winter, the Norilsk region in Russia/Siberia was the dominant source of Arctic Haze signal in the PNSD and BC observations, which contributed to higher accumulation mode PNC and BC mass concentration in the central Arctic than at land-based observatories. We also show that the wintertime Arctic Oscillation (AO) phenomenon, which was reported to achieve a record-breaking positive phase during January - March 2020, explains the unusual timing and magnitude of Arctic Haze across the Arctic region compared to longer-term observations. In summer, the PNC of nucleation and Aitken mode aerosol is enhanced, but concentrations were notably lower in the central Arctic over the ice pack than at land-based sites further south. The analysis presented herein provides a current snapshot of Arctic aerosol processes in an environment that is characterized by rapid changes, which will be crucial for improving climate model predictions, understanding linkages between different environmental processes, and investigating the impacts of climate change in future Arctic aerosol studies. [ABSTRACT FROM AUTHOR]

The Arctic is a rapidly changing ecosystem, with complex ice-ocean-atmosphere feedbacks. An important process new particle formation (NPF) from gas phase precursors, which provide a climate forcing effect. NPF has been studied comprehensively at different sites in the Arctic ranging from those in the high Arctic, those at Svalbard, and those in the continental Arctic, but no harmonized analysis has been performed on all sites simultaneously, with no calculations of key NPF parameters available for some sites. Here, we analyse the formation and growth of new particles from six long-term ground-based stations in the Arctic (Alert, Villum, Tiksi, Mt. Zeppelin, Gruvebadet. & Utqiagvik). Our analysis of particle formation and growth rates, as well as back trajectory analysis shows summertime maxima in frequency of NPF and particle formation rate at all sites, although the mean frequency and particle formation rates themselves vary greatly between sites, highest at Svalbard, and lowest in the high Arctic. Growth rate, condensational sinks and vapour source rates show a slight bias towards the southernmost sites, with vapour source rates varying by around an order of magnitude between the northernmost and southernmost sites. Air masse back trajectories during NPF at these northernmost sites are associated with large areas of sea ice and snow, whereas events at Svalbard are associated with more sea ice and ocean regions. Events at the southernmost sites are associated with large areas of land, and sea ice. These results emphasize how understanding the geographical variation in surface type across the Arctic is key to understanding secondary aerosol sources, and provide harmonised analysis of NPF across the Arctic. [ABSTRACT FROM AUTHOR]

Delhi is one of the world's most polluted cities, with very high concentrations of airborne particulate matter. However, little is known on the factors controlling the characteristics of particle number size distributions. Here, new measurements are reported from three field campaigns conducted in winter, pre-monsoon and post-monsoon seasons on the Indian Insitute of Technology campus in the south of the city. Particle number size distributions were measured simultaneously using a Scanning Mobility Particle Sizer and a Grimm optical particle monitor, covering 15 nm to >10 µm diameter. The merged, wide-range size distributions were categorised into five size ranges: nucleation (15-20 nm), Aitken (20-100 nm), accumulation (100 nm-1 µm), large fine (1-2.5 µm) and coarse (2.5-10 µm) particles. The ultrafine fraction (15-100 nm) accounts for about 52 % of all particles by number (PN10), but just 1 % by PM10 volume (PV10). The measured size distributions are markedly coarser than most from other parts of the world, but are consistent with earlier cascade impactor data from Delhi. Our results suggest substantial aerosol processing by coagulation, condensation and water uptake in the heavily polluted atmosphere, which takes place mostly at nighttime and in the morning hours. Total number concentrations are highest in winter, but the mode of the distribution is largest in the post-monsoon (autumn) season. The accumulation mode particles dominate the particle volume in autumn and winter, while the coarse mode dominates in summer. Polar plots show a huge variation between both size fractions in the same season and between seasons for the same size fraction. The diurnal pattern of particle numbers is strongly reflective of a road traffic influence upon concentrations, especially in autumn and winter. There is a clear influence of diesel traffic at nighttime when it is permitted to enter the city, and also indications in the size distribution data of a mode <15 nm, probably attributable to CNG/LPG vehicles. New particle formation appears to be infrequent, and in this dataset is limited to one day in the summer campaign. Our results reveal that the very high emissions of airborne particles in Delhi, particularly from traffic, determine the variation of particle number size distributions. [ABSTRACT FROM AUTHOR]

The Indian megacity of Delhi suffers from some of the poorest air quality in the world. While ambient NO2 and particulate matter (PM) concentrations have received considerable attention in the city, high ground level ozone (O3) concentrations are an often overlooked component of pollution. O3 can lead to significant ecosystem damage, agricultural crop losses, and adversely affect human health. During October 2018, concentrations of speciated non-methane hydrocarbons volatile organic compounds (C2 -- C13), oxygenated volatile organic compounds (o-VOCs), NO, NO2, HONO, CO, SO2, O3, and photolysis rates, were continuously measured at an urban site in Old Delhi. These observations were used to constrain a detailed chemical box model utilising the Master Chemical Mechanism v3.3.1. VOCs and NOx (NO + NO2) were varied in the model to test their impact on local O3 production rates, P(O3), which revealed a VOC-limited chemical regime. When only NOx concentrations were reduced, a significant increase in P(O3) was observed, thus VOC co-reduction approaches must also be considered in pollution abatement strategies. Of the VOCs examined in this work, mean morning P(O3) rates were most sensitive to monoaromatic compounds, followed by monoterpenes and alkenes, where halving their concentrations in the model led to a 15.6 %, 13.1 % and 12.9 % reduction in P(O3), respectively. P(O3) was not sensitive to direct changes in aerosol surface area but was very sensitive to changes in photolysis rates, which may be influenced by future changes in PM concentrations. VOC and NOx concentrations were divided into emission source sectors, as described by the EDGAR v5.0 Global Air Pollutant Emissions and EDGAR v4.3.2_VOC_spec inventories, allowing for the impact of individual emission sources on P(O3) to be investigated. Reducing road transport emissions only, a common strategy in air pollution abatement strategies worldwide, was found to increase P(O3), even when the source was removed in its entirety. Effective reduction in P(O3) was achieved by reducing road transport along with emissions from combustion for manufacturing and process emissions. Modelled P(O3) reduced by ~ 20 ppb h-1 when these combined sources were halved. This study highlights the importance of reducing VOCs in parallel with NOx and PM in future pollution abatement strategies in Delhi. [ABSTRACT FROM AUTHOR]

Understanding aerosol-cloud-climate interactions in the Arctic is key to predict the climate in this rapidly changing region. Whilst many studies have focused on submicron aerosol (diameter less than 1 µm), relatively little is known about the climate relevance of supermicron aerosol (diameter above 1 µm). Here, we present a cluster analysis of multiyear (2015-2019) aerodynamic volume size distributions with diameter ranging from 0.5 to 20 µm measured continuously at the Gruvebadet Observatory in the Svalbard archipelago. Together with aerosol chemical composition data from several online and offline measurements, we apportioned the occurrence of the coarse-mode aerosols to anthropogenic (two sources, 27%) and natural (three sources, 73%) origins. Specifically, two clusters are related to Arctic haze with high levels of black carbon, sulfate and accumulation mode (0.1-1 µm) aerosol. The first cluster (9%) is attributed to ammonium sulfate-rich Arctic haze particles, whereas the second one (18%) to larger-mode aerosol mixed with sea salt. The three natural aerosol clusters were: open ocean sea spray aerosol (34%), mineral dust (7%), and an unidentified source of sea spray-related aerosol (32%). The results suggest that sea spray-related aerosol in polar regions may be more complex than previously thought due to short/long-distance origins and mixtures with Arctic haze, biogenic and likely snow-blowing aerosols. Studying supermicron natural aerosol in the Arctic is imperative for understanding the impacts of changing natural processes on Arctic aerosol. [ABSTRACT FROM AUTHOR]

New particle formation (NPF) events occur almost everywhere in the world and can play an important role as a particle source. The frequency and characteristics of NPF events vary spatially and this variability is yet to be fully understood. In the present study, long term particle size distribution datasets (minimum of three years) from thirteen sites of various land uses and climates from across Europe were studied and NPF events, deriving from secondary formation and not traffic related nucleation, were extracted and analysed. The frequency of NPF events was consistently found to be higher at rural background sites, while the growth and formation rates of newly formed particles were higher at roadsides, underlining the importance of the abundance of condensable compounds of anthropogenic origin found there. The growth rate was higher in summer at all rural background sites studied. The urban background sites presented the highest uncertainty due to greater variability compared to the other two types of site. The origin of incoming air masses and the specific conditions associated with them greatly affect the characteristics of NPF events. In general, cleaner air masses present higher probability for NPF events, while the more polluted ones show higher growth rates. However, different patterns of NPF events were found even at sites in close proximity (< 200 km) due to the different local conditions at each site. Region-wide events were also studied and were found to be associated with the same conditions as local events, although some variability was found which was associated with the different seasonality of the events at two neighbouring sites. NPF events were responsible for an increase in the number concentration of ultrafine particles of more than 400% at rural background sites on the day of their occurrence. The degree of enhancement was less at urban sites due to the increased contribution of other sources within the urban environment. It is evident that, while some variables (such as solar radiation intensity, relative humidity or the concentrations of specific pollutants) appear to have a similar influence on NPF events across all sites, it is impossible to predict the characteristics of NPF events at a site using just these variables, due to the crucial role of local conditions. [ABSTRACT FROM AUTHOR]

Atmospheric aerosols contribute some of the greatest uncertainties to estimates of global radiative forcing, and have significant effects on human health. New particle formation (NPF) is the process by which new aerosols of sub-2 nm diameter form from gas-phase precursors and contributes significantly to particle numbers in the atmosphere, accounting for approximately 50 % of cloud condensation nuclei globally. Here, we study summertime NPF in urban Barcelona in NE Spain. The rate of formation of new particles is seen to increase linearly with sulphuric acid concentration in a manner similar to systems studied in chamber studies involving sulphuric acid, water and dimethylamine (DMA), as well as sulphuric acid, water and the oxidation products of pinanediol. The sulphuric acid dimer : monomer ratio is significantly lower than that seen in experiments involving sulphuric acid and DMA in chambers, indicating that stabilization of sulphuric acid clusters by bases is weaker in this dataset than in chambers, and thus another mechanism, likely involving the plentiful highly oxygenated organic molecules (HOMs) is plausible. The high concentrations of HOMs arise largely from both alkylbenzene and monoterpene oxidation, with the former providing greater concentrations of HOMs due to significant local sources. The concentration of these HOMs shows a dependence on both temperature and precursor VOC concentration. New particle formation without growth past 10 nm is also observed, and on these days the highly oxygenated organic compound concentration is significantly lower than on days with growth, and thus high concentrations of low volatility oxygenated organics appear to be a necessary condition for the growth of newly formed particles in Barcelona. These results are consistent with prior observations of new particle formation in both chambers and the real atmosphere, and these results are likely representative of the urban background of many European Mediterranean cities. [ABSTRACT FROM AUTHOR]

Mercury is ubiquitous in the atmosphere and atmospheric transport is an important source for this element in the Arctic. Measurements of gaseous elemental mercury (GEM) have been carried out at the Villum Research Station (Villum) at Station Nord, situated in north Greenland. The measurements cover the period 1999–2017 with a gap in the data for the period 2003–2008 (for a total of 11 years). The measurements were compared with model results from the Danish Eulerian Hemispheric Model (DEHM) model that describes the contribution from direct anthropogenic transport, marine emission and general background concentration. The percentage of time spent over different surfaces was calculated by back-trajectory analysis and the reaction kinetics was determined by comparison with ozone. The GEM measurements were analysed for trends, both seasonal and annually. The only significant trend found was a negative one for the winter months. Comparison of the measurements to simulations using the Danish Eulerian Hemispheric Model (DEHM) indicated that direct transport of anthropogenic emissions of mercury accounts for between 14 and 17 % of the measured mercury. Analysis of the kinetics of the observed Atmospheric Mercury Depletion Events (AMDEs) confirms the results of a previous study at Villum of the competing reactions of GEM and ozone with Br, which suggests a lifetime of GEM on the order of a month. However, a GEM lifetime of 12 months gave the best agreement between model and measurements. The chemical lifetime is shorter and thus the apparent lifetime appears to be the result of deposition followed by reduction and reemission; for this reason the term "relaxation time" is preferred to "lifetime" for GEM. The relaxation time for GEM causes a delay between emission reductions and the effect on actual concentrations. No annual trend was found for the measured concentrations of GEM over the measurement period despite emission reductions. This is interesting and together with low direct transport of GEM to Villum, as found by the DEHM model, it shows that the dynamics of GEM is very complex. Therefore in the coming years, intensive measurement networks is highly needed to describe the global distribution of mercury in the environment as the use of models to predict future levels will still be highly uncertain. The situation is increasingly complex due to global change that most likely will change the transport patterns of mercury not only in the atmosphere but also between matrixes. [ABSTRACT FROM AUTHOR]

The Southern Ocean and Antarctic region currently best represent one of the few places left on our planet with conditions similar to the preindustrial age. Currently, climate models have low ability to simulate conditions forming the aerosol baseline; a major uncertainty comes from the lack of understanding of aerosol size distributions and their dynamics. Contrasting studies stress that primary sea-salt aerosol can contribute significantly to the aerosol population, challenging the concept of climate biogenic regulation by new particle formation (NPF) from dimethyl sulphide marine emissions. We present a statistical cluster analysis of the physical characteristics of particle size distributions (PSD) collected at Halley (Antarctica) for the year 2015 (89% data coverage). By applying the Hartigan-Wong k-Means method we find 8 clusters describing the entire aerosol population. Three clusters show pristine average low particle number concentrations (< 121-179 cm-3) with three main modes (30 nm, 75-95 nm, 135-160 nm) and represent 57% of the annual PSD (up to 89-100% during winter, 34-65% during summer based upon monthly averages). Nucleation and Aitken mode PSD clusters dominate summer months (Sep-Jan, 59-90%), whereas a clear bimodal distribution (43 and 134 nm, respectively, min Hoppel mode 75 nm) is seen only during the Dec-Apr period (6-21%). Major findings of the current work include: (1) NPF and growth events originate from both the sea ice marginal zone and the Antarctic plateau, strongly suggesting multiple vertical origins, including marine boundary layer and free troposphere; (2) very low particle number concentrations are detected for a substantial part of the year (57%), including summer (34-65%), suggesting that the strong annual aerosol concentration cycle is driven by a short temporal interval of strong NPF events; (3) a unique pristine aerosol cluster is seen with a bimodal size distribution (75 nm and 160 nm, respectively), strongly correlating with wind speed and possibly associated with blowing snow and sea spray sea salt, dominating the winter aerosol population (34-54%). A brief comparison with two other stations (Dome C Concordia and King Sejong Station) during the year 2015 (240 days overlap) shows that the dynamics of aerosol number concentrations and distributions are more complex than the simple sulphate-sea spray binary combination, and it is likely that an array of additional chemical components and processes drive the aerosol population. A conceptual illustration is proposed indicating the various atmospheric processes related to the Antarctic aerosols, with particular emphasis on the origin of new particle formation and growth. [ABSTRACT FROM AUTHOR]

Particle nucleation is one of the main sources of atmospheric particulate matter by number, with new particles having great relevance for human health and climate. Highly oxidised multifunctional organic molecules (HOMs) have been recently identified as key constituents in the growth, and, sometimes, in initial formation of new particles. While there have been many studies of HOMs in atmospheric chambers, flow tubes and clean environments, analyses of data from polluted environments are scarce. Here, measurements of HOMs and particle size distributions down to small molecular clusters are presented alongside VOC and trace gas data from a campaign in Beijing. Many gas phase HOMs have been characterised and their temporal trends and behaviours analysed in the context of new particle formation. The HOMs identified have a comparable degree of oxidation to those seen in other, cleaner, environments, likely due to an interplay between the higher temperatures facilitating rapid hydrogen abstractions and the higher concentrations of NOx and other RO2. terminators ending the autoxidation sequence more rapidly. Our data indicate that alkylbenzenes, monoterpenes, and isoprene are important precursor VOCs for HOMs in Beijing. Many of the C5 and C10 compounds derived from isoprene and monoterpenes have a slightly greater degree of average oxidation state of carbon compared to those from other precursors. Most HOMs except for large dimers have daytime peak concentrations, indicating the importance of OH. chemistry in the formation of HOMs, as O3 is lower on the days with higher HOM concentrations; similarly, VOC concentrations are lower on the days with higher HOM concentrations. The daytime peaks of HOMs coincide with the growth of freshly formed new particles, and their initial formation coincides with the peak in sulphuric acid vapours, suggesting that the nucleation process is sulphuric acid-dependent, with HOMs contributing to subsequent particle growth. [ABSTRACT FROM AUTHOR]

NPF events have different patterns of development depending on the conditions of the area in which they occur. In this study, NPF events occurring at three sites of differing characteristics (rural Harwell (HAR), urban background North Kensington (NK), urban roadside Marylebone Road (MR), London, UK) were studied (seven years of data). The different atmospheric conditions in each study area not only have an effect on the frequency of the events, but also affect their development. The frequency of NPF events is similar at the rural and urban background locations (about 7% of days), with a high proportion of events occurring at both sites on the same day (45%). The frequency of NPF events at the urban roadside site is slightly less (6% of days), and higher particle growth rates (average 5.5nmh−1 at MR compared to 3.4nmh−1 and 4.2nmh−1 at HAR and NK respectively) must result from rapid gas to particle conversion of traffic-generated pollutants. A general pattern is found in which the condensation sink increases with the degree of pollution of the site, but this is counteracted by increased particle growth rates at the more polluted location. A key finding of this study is that the role of the urban environment leads to an increment of 20% in N16–20nm in the urban background compared to that of the rural area in NPF events occurring at both sites. The relationship of the origin of incoming air masses is also considered and an association of regional events with cleaner air masses is found. Due to lower availability of condensable species, NPF events that are associated with cleaner atmospheric conditions have lower growth rates of the newly formed particles. The decisive effect of the condensation sink in the development of NPF events and the survivability of the newly formed particles is underlined, and influences the overall contribution of NPF events to the number of ultrafine particles in an area. The other key factor identified by this study is the important role that urban pollution plays in new particle formation events. [ABSTRACT FROM AUTHOR]

APHH-Beijing (Atmospheric Pollution and Human Health in a Chinese Megacity) is an international collaborative project to examine the emissions, processes and health effects of air pollution in Beijing. The four research themes of APHH-China are: (1) sources and emissions of urban atmospheric pollution; (2) processes affecting urban atmospheric pollution; (3) exposure science and impacts on health; and (4) interventions and solutions to reduce health impacts. Themes 1 and 2 are closely integrated and support Theme 3, while Themes 1-3 provide scientific data for Theme 4 on the development of cost-effective solutions. A key activity within APHH-Beijing was the two month-long intensive field campaigns at two sites: (i) central Beijing, and (ii) rural Pinggu. The coordinated campaigns provided observations of the atmospheric chemistry and physics in and around Beijing during November-December 2016 and May-June 2017. The campaigns were complemented by numerical air quality modelling and air quality and meteorology data at the 12 national monitoring stations in Beijing. This introduction paper provides an overview of (i) APHH-Beijing programme, (ii) the measurement and modelling activities performed as part of it in Beijing, and (iii) the air quality and meteorological conditions during the two field campaigns. The winter campaign was characterized by high PM2.5 pollution events whereas the summer experienced high ozone pollution events. Air quality was poor during the winter campaign, but less severe than in the same period in 2015 when there were a number of major pollution episodes. PM2.5 levels were relatively low during the summer period, matching the cleanest periods over the previous five years. Synoptic scale meteorological analysis suggests that the greater stagnation and weak southerly circulation in November/December 2016 may have contributed to the poor air quality. [ABSTRACT FROM AUTHOR]

Particle number size distributions have been measured simultaneously by Scanning Mobility Particle Sizers (SMPS) at five sites in Central London for a one month campaign in January-February 2017. These measurements were accompanied by condensation particle counters (CPC) to measure total particle number count at four of the sites and aethalometers measuring Black Carbon (BC) at five sites. The spatial distribution and inter-relationships of the particle size distribution and SMPS total number counts with CPC total number counts and Black Carbon measurements have been analysed in detail as well as variations in the size distributions. One site (Marylebone Road) was in a heavily-trafficked street canyon, one site (Westminster University) was on a rooftop adjacent to the Marylebone Road sampler, a further sampler was located at Regent's University within a major park to the north of Marylebone Road. A fourth sampler was located nearby at 160 m above ground level on the BT tower and a fifth sampler was located 4 km to the west of the main sampling region at North Kensington. Consistent with earlier studies it was found that the mode in the size distribution had shifted to smaller sizes at the Regent's University (park) site, the mean particle shrinkage rate being 0.04 nm s-1 with slightly lower values at low wind speeds and some larger values at higher wind speeds. There was evidence of complete evaporation of the semi-volatile nucleation mode under certain conditions at the elevated BT Tower site. Whereas SMPS total count and Black Carbon showed typical traffic-dominated diurnal profiles, the CPC count data typically peaked during nighttime as did CPC/SMPS and CPC/BC ratios. This is thought to be due to the presence of high concentrations of small particles (2.5-15 nm diameter) probably arising from condensational growth from traffic emissions during the cooler nighttime conditions. Such behaviour was most marked at the Regent's University and Westminster University sites and less so at Marylebone Road, while at the elevated BT Tower site the ratio of particle number (CPC) to Black Carbon peaked during the morning rush hour and not at nighttime, unlike the other sites. An elevation in nucleation mode particles associated with winds from the West and WSW sector was concluded to result from emissions from London Heathrow Airport, despite a distance of 22 km from the Central London sites. [ABSTRACT FROM AUTHOR]

Some air pollution datasets contain multiple variables with a range of measurement units, and combined analysis by Positive Matrix Factorization (PMF) is problematic, but can offer benefits from the greater information content. In this work, a novel method is devised and the source apportionment of a mixed unit data set (PM10 mass and Number Size Distribution NSD) is achieved using a novel two-step approach to PMF. In the first step the PM10 data is PMF analysed using a source apportionment approach in order to provide a solution which best describes the environment and conditions considered. The time series G values (and errors) of the PM10 solution are then taken forward into the second step where they are combined with the NSD data and analysed in a second PMF analysis. This results in apportioned NSD data associated with the PM10 factors. We exemplify this approach using data reported in the study of Beddows et al. (2015), producing one solution which unifies the two separate solutions for PM10 and NSD data datasets together. We also show how regression of the NSD size bins and the G time series can be used to elaborate the solution by identifying NSD factors (such as nucleation) not influencing the PM10 mass. [ABSTRACT FROM AUTHOR]

Understanding aerosol size distributions is crucial to our ability to predict aerosol number concentrations. When of favourable size and composition, both long range transported particles as well as locally formed ones may serve as Cloud Condensation Nuclei (CCN); small changes may have a large impact on the low CCN concentrations currently characteristic of the Arctic environment. Here, we present a cluster analysis of particle size distributions (PSD, size range 8-500 nm) simultaneously collected from three high Arctic sites across Europe during a three year period (2013-2015). Two sites are located in the Svalbard archipelago: Zeppelin research station (474 m above ground), and the nearby Gruvebadet Observatory (about 2 km distance from Zepplelin, 67 m above ground). The third site (Villum Research Station - Station Nord, 30 m above ground) is 600 km to the west-northwest of Zeppelin, at the tip of north-eastern Greenland. An inter-site comparison exercise is carried out for the first time including the Gruvebadet site. K-means analysis provided eight specific aerosol categories, further combined into broad PSD with similar characteristics, namely: pristine low concentrations (12-14 %), new particle formation (16-32 %), Aitken (21-35 %) and accumulation (20-50 %). Confined for longer time periods by consolidated pack sea ice regions, the Greenland site shows PSD with lower ultrafine mode aerosol concentrations during summer, but higher accumulation mode aerosol concentrations during winter relative to the Svalbard sites. By association with chemical composition and Cloud Condensation Nuclei properties, further conclusions can be derived. Three distinct types of accumulation mode aerosol are observed during winter months, associated with sea spray (largest detectable sizes), Arctic haze (main mode at 150 nm) and aged accumulation mode (main mode at 220 nm) aerosols. In contrast, locally produced and most likely of marine biogenic origin particles exhibit size distributions dominated by the nucleation and Atiken mode aerosol during summer months. The obtained data and analysis set now the stage for future studies; including apportioning the relative contribution of primary and secondary aerosol formation processes to the aerosol size distribution in high Arctic, and elucidating anthropogenic aerosol dynamics, transport and removal processes across the Greenland sea. In a region of enormous importance for future climate on Earth, it is imperative to continue strengthening international scientific cooperation, in order to address important research questions on scales beyond singular station or measurement events. [ABSTRACT FROM AUTHOR]

Atmospheric new particle formation (NPF) is an important phenomenon in terms of the global particle number concentrations. Here we investigated the frequency of NPF, formation rates of 10 nm particles and growth rates in the size range of 10–25 nm using at least one year of aerosol number size-distribution observations at 36 different locations around the world. The majority of these measurement sites are in the Northern Hemisphere. We found that the NPF frequency has a strong seasonal variability, taking place on about 30 % of the days in March–May and on about 10 % of the days in December–February. The median formation rate of 10 nm particles varies by about three orders of magnitude (0.01–10 cm−3 s−1) and the growth rate by about an order of magnitude (1–10 nm h−1). The smallest values of both formation and growth rates were observed at polar sites and the largest ones in urban environments or anthropogenically influenced rural sites. The correlation between the NPF event frequency and the particle formation and growth rate was at best moderate between the different measurement sites, as well as between the sites belonging to a certain environmental regime. For a better understanding of atmospheric NPF and its regional importance, we would need more observational data from different urban areas in practically all parts of the world, from additional remote and rural locations in Northern America, Asia and most of the Southern Hemisphere (especially Australia), from polar areas, and from at least a few locations over the oceans. [ABSTRACT FROM AUTHOR]

The vertical profile of new particle formation (NPF) events was studied by comparing the aerosol size number distributions measured aloft and at surface level in a suburban environment in Madrid, Spain using airborne instruments. The horizontal distribution and regional impact of the NPF events was investigated with data from three urban and suburban stations in the Madrid metropolitan area. Intensive regional NPF episodes followed by particle growth were simultaneously recorded at three stations in and around Madrid, in a field campaign in July 2016. On some days a marked decline in particle size (shrinkage) was observed in the afternoon, associated with a change in air masses. Additionally, a few nocturnal nucleation mode bursts were observed in the urban stations, which could be related to aircraft emissions transported from the airport. Considering all simultaneous diurnal NPF events registered, growth rates were significantly lower at the urban stations, ranging 2.0–3.9 nm h−1, compared to the suburban station (2.9–10.0 nm h−1). Total concentration of 9.1–25 nm particles reached 2.8 x 104 cm−3 at the urban station and 1.7 x 104 cm−3 at the suburban station, the mean daily values being 3.7 x 104 cm−3 (2.2 x 104 cm−3 at the suburban station) during event days. The formation rates of 9–25 nm particles peaked around noon and recorded a median value of 2.0 cm−3 s−1 and 1.1 cm−3 s−1 at the urban and suburban stations, respectively. The condensation and coagulation sinks presented minimum values shortly before sunrise, increasing after dawn reaching the maximum value at 14:00 UTC, with average daily mean values of 3.4 x 10−3 s−1 (2.5 x 10−3 s−1 at the suburban station) and 2.4 x 10−5 s−1, respectively, during event days. The vertical soundings demonstrated that ultrafine particles (UFP) are transported from surface levels to higher levels, thus newly-formed particles ascend from surface to the top of the mixing layer. The morning soundings revealed the presence of a residual layer in the upper levels in which aged particles (nucleated and grown on previous days) prevail. The particles in this layer also grow in size, with growth rates significantly smaller than those inside the mixed layer. Under conditions with strong enough convection, the soundings revealed homogeneous number size distributions and growth rates at all altitudes, which follow the same evolution in the other stations considered in this study. This indicates that NPF occurs quasi-homogenously in an area spanning at least 17 km horizontally. The NPF events extend over the full vertical extension of the mixed layer reaching as high as 3000 m. This can have consequences in the radiative balance of the atmosphere and affect the climate. Results also evidenced that total particle concentration in and around Madrid in summer is dominated by NPF during summer, thus it may obscure the impact of vehicle exhaust emissions on levels of UFP. [ABSTRACT FROM AUTHOR]

Major airports are often located within or close to large cities; their impacts on the deterioration of air quality at ground level are amply recognised. The international airport of Heathrow is a major source of nitrogen oxides in the Greater London area, but its contribution to the levels of submicrometre particles is unknown, and is the objective of this study. Two sampling campaigns were carried out during warm and cold seasons at a site close to the airfield (1.2 km). Size spectra were largely dominated by ultrafine particles: nucleation particles (< 30 nm) were found to be ~ 10 times higher than those commonly measured in urban background environments of London. A set of chemometric tools was used to discern the pollution arising from aircraft operations and those from other sources within the city or from the traffic generated by the airport. Five clusters and 6 factors were identified by applying k-means cluster analysis and positive matrix factorization (PMF) respectively to particle number size distributions; their interpretation was based on their modal structures, wind directionality, diurnal patterns, road and airport traffic volumes and on the relationship with weather and other air pollutants. Airport emissions, fresh and aged road traffic, urban accumulation mode and two secondary sources were then identified and apportioned. The comparison of cluster and PMF analyses allowed extraction of further information. The analysis of a strong regional nucleation event was also performed to detect its effect upon concentrations. The fingerprint of Heathrow has a characteristic modal structure peaking at < 20 nm and accounts for 30-35 % of total particles in both the seasons. Other main contributors are fresh (24-36 %) and aged (16-21 %) road traffic emissions and urban accumulation from London (around 10 %). Secondary sources accounted for less than 6 % in number concentrations but for more than 50 % in volume concentration. In 2016, the UK government provisionally approved the construction of a third runway; therefore the direct and indirect impact of Heathrow on local air quality is expected to increase unless mitigation strategies are applied successfully. [ABSTRACT FROM AUTHOR]

A chemical reaction chamber system has been developed for the processing of oleic acid aerosol particles with ozone under two relative humidity conditions: dry and humidified to 65% R.H.. The apparatus consists of an aerosol flow tube, in which the ozonolysis occurs, coupled to a scanning mobility particle sizer (SMPS) and an aerosol time-of-flight mass spectrometer (ATOFMS) which measure the evolving particle size and composition. Under both relative humidity conditions, ozonolysis results in a significant decrease in particle size and mass which is consistent with the formation of volatile products that partition from the particle to the gas phase. Mass spectra derived from the ATOFMS reveal the presence of the typically observed reaction products: azaleic acid, nonanal, oxononanoic acid and nonanoic acid, as well as a range of higher molecular weight products deriving from the reactions of reaction intermediates with oleic acid and its oxidation products. These include octanoic acid, and 9- and 10-oxooctadecanoic acid, as well as products of considerably higher molecular weight. Quantitative evaluation of product yields with the ATOFMS shows a marked dependence upon both particle size association (from 0.3 to 2.1 μm diameter) and relative humidity. Under dry conditions, the percentage residual oleic acid increases with increasing particle size, as does the percentage of higher molecular weight products, due to the poorer internal mixing of the larger particles. The main lower molecular weight products are nonanal and oxonononic acid. Under humidified conditions, the percentage unreacted oleic acid is greater, except in the smallest particle fraction, and oxononanoic acid dominates the product distribution, with little formation of high molecular weight products relative to the dry particles. It is postulated that water reacts with reactive intermediates, competing with the processes which produce high molecular weight products. Whilst the oleic acid model aerosol system is of limited relevance to complex internally mixed atmospheric aerosol, the generic findings presented in this paper give useful insights into the nature of heterogeneous chemical processes. [ABSTRACT FROM AUTHOR]

A chemical reaction chamber system has been developed for the processing of oleic acid aerosol particles with ozone under two relative humidity conditions: dry and humidified to 65% R.H. The apparatus consists of an aerosol flow tube, in which the ozonolysis occurs, coupled to a scanning mobility particle sizer (SMPS) and an aerosol time-of-flight mass spectrometer (ATOFMS) which measure the evolving particle size and composition. Under both relative humidity conditions, ozonolysis results in a significant decrease in particle size and mass which is consistent with the formation of volatile products that partition from the particle to the gas phase. Mass spectra derived from the ATOFMS reveal the presence of the typically observed reaction products: azaleic acid, nonanal, oxononanoic acid and nonanoic acid, as well as a range of higher molecular weight products deriving from the reactions of reaction intermediates with oleic acid and its oxidation products. These include octanoic acid, and 9- and 10-oxooctadecanoic acid, as well as products of considerably higher molecular weight. Quantitative evaluation of product yields with the ATOFMS shows a marked dependence upon both particle size association (from 0.3 to 2.1 µm diameter) and relative humidity. Under dry conditions, the percentage residual oleic acid increases with increasing particle size, as does the percentage of higher molecular weight products, due to the poorer internal mixing of the larger particles. The main lower molecular weight products are nonanal and oxonononic acid. Under humidified conditions, the percentage unreacted oleic acid is greater, except in the smallest particle fraction, and oxononanoic acid dominates the product distribution, with little formation of high molecular weight products relative to the dry particles. It is postulated that water reacts with reactive intermediates, competing with the processes which produce high molecular weight products. Whilst the oleic acid model aerosol system is of limited relevance to complex internally mixed atmospheric aerosol, the generic findings presented in this paper give useful insights into the nature of heterogeneous chemical processes. [ABSTRACT FROM AUTHOR]

The Aerosol Time-of-Flight Mass Spectrometer (ATOFMS) provides size resolved information on the chemical composition of single particles with high time resolution. Within SAPUSS (Solving Aerosol Problems by Using Synergistic Strategies), continuous ATOFMS measurements of ambient particles were made simultaneously at two urban locations: urban background (UB) site and road side (RS) site in the city of Barcelona (Spain) from 17th September to 18th October 2010. Two different instrumental configurations were used: ATOFMS (TSI 3800) with a converging nozzle inlet (high efficiency at about 800-2000nm) at the UB site and ATOFMS (TSI 3800-100) with an aerodynamic lens inlet (high efficiency at about 300-700nm) at the RS site. This is the first time, to our knowledge, that two ATOFMS instruments have been deployed in the same field study. The different instrument configurations had an impact on the observed particle types at the two sites. Nevertheless, ten particle types were detected at both locations, including local and regional elemental carbon (22.7-58.9% of total particles), fresh and aged sea salt (1.0-14.6%), local and regional nitrate -containing aerosols (3-11.6%), local lead-containing metallic particles (0.1-0.2%) and transported Fe-nitrate particles (0.8-2.5%). The ATOFMS at the UB also characterised four particle types: calcium-containing dust (0.9%), Saharan dust (1.3%), vanadium-containing particles (0.9%) and vegetative debris (1.7%). By contrast, the high statistical counts of fine particles detected at the RS allowed identification of eight particle types. Four of these contained amines of primary and secondary origin. Aminium salts were found related to coarse sulphate rich particle types, suggesting heterogeneous reaction mechanisms for their formation. The other four particle types mainly containing organic carbon were found spiking at different types of the day, showing a complex single particle mixing state relationship between organic carbon and nitrate. This ATOFMS study clearly shows that the composition of atmospheric fine particles in Barcelona, and likely other Mediterranean urban areas, is complex, with a wide range of local and regional sources combining with chemical processing to produce at least twenty-two different particle types exhibiting different temporal behaviour. The advantage of using two ATOFMS instruments is also demonstrated, with the nozzle-skimmer configuration enabling detection of coarse dust particles and the aerodynamic lens configuration allowing better identification of particles rich in organic carbon and amines. [ABSTRACT FROM AUTHOR]