Coarse Mode Research Articles

On the concentration of SARS-CoV-2 in outdoor air and the interaction with pre-existing atmospheric particles

The spread of SARS-CoV-2 by contact (direct or indirect) is widely accepted, but the relative importance of airborne transmission is still controversial. Probability of outdoor airborne transmission depends on several parameters, still rather uncertain: virus-laden aerosol concentrations, viability and lifetime, minimum dose necessary to transmit the disease. In this work, an estimate of outdoor concentrations in northern Italy (region Lombardia) was performed using a simple box model approach, based on an estimate of respiratory emissions, with a specific focus for the cities of Milan and Bergamo (Italy). In addition, the probability of interaction of virus-laden aerosol with pre-existing particles of different sizes was investigated. Results indicate very low (<1 RNA copy/m3) average outdoor concentrations in public area, excluding crowded zones, even in the worst case scenario and assuming a number of infects up to 25% of population. On average, assuming a number of infects equal to 10% of the population, the time necessary to inspire a quantum (i.e. the dose of airborne droplet nuclei required to cause infection in 63% of susceptible persons) would be 31.5 days in Milan (range 2.7–91 days) and 51.2 days in Bergamo (range 4.4–149 days). Therefore, the probability of airborne transmission due to respiratory aerosol is very low in outdoor conditions, even if it could be more relevant for community indoor environments, in which further studies are necessary to investigate the potential risks. We theoretically examined if atmospheric particles can scavenge virus aerosol, through inertial impact, interception, and Brownian diffusion. The probability was very low. In addition, the probability of coagulation of virus-laden aerosol with pre-existing atmospheric particles resulted negligible for accumulation and coarse mode particles, but virus-laden aerosol could act as sink of ultrafine particles (around 0.01 μm in diameter). However, this will not change significantly the dynamics behaviour of the virus particle or its permanence time in atmosphere.

Factors Influencing New Atmospheric Particle Formation in Ordos During Summer and Autumn 2019

In this study, the aerosol number size distribution in the range of 10 nm-10 μm was collected from August 16 to October 04, 2019 at Ordos using a wide-range particle spectrometer (WPS). Combined with PM (PM2.5 and PM10), pollution gases, meteorological data, and the HYSPLIT model, the characteristics and impact factors of new particle formation (NPF) were discussed. The results indicated that there were 19 NPF events during the observation period, which have different effects on diurnal variation in aerosol number concentration in different modes. The NPF events caused a sharp increase in the number concentration of nucleation and Aitken mode aerosols, but had little effect on the number concentration of accumulation and coarse mode aerosols. The temperature, wind speed, and total solar radiation during NPF days were usually higher than those in non-NPF days, and the RH during NPF days was lower. On NPF days, the mass concentrations of PM2.5, PM10, CO, and NO2 were lower than those on non-NPF days, while the mass concentrations of O3 and SO2 were higher. NPF events were observed in 40.0% of northern air masses and 29.6% of southern air masses. There were significant differences in meteorological elements in different NPF event air mass types. The southern NPF event air mass type had the lowest wind speed and the highest RH, with averages of (2.4±1.5) m·s-1 and (48.8±10.8)%, respectively. The northern NPF event air mass type had the highest wind speed and total solar radiation, with averages of (4.2±1.9) m·s-1 and (664.5±255.6) W·m-2, respectively. The western air mass type of NPF event had the lowest RH, with an average of (29.8±12.7)%. The formation rates of new particles in the different air mass types of NPF events were similar, ranging from 1.5 to 1.8 cm-3·s-1. The largest growth rate was (12.7±13.6) nm·h-1 in the southern NPF event air mass type, which was 1.2 times and 1.4 times higher than the NPF events of northern air masses and western air masses.

Sensitivity of aerosol optical properties to the aerosol size distribution over central Europe and the Mediterranean Basin using the WRF-Chem v.3.9.1.1 coupled model

Abstract. The size distribution of atmospheric aerosols plays a key role for understanding and quantifying the uncertainties related to aerosol–radiation and aerosol–cloud interactions. These interactions ultimately depend on the size distribution through optical properties (such as aerosol optical depth, AOD) or cloud microphysical properties. Hence, the main objective of this contribution is to disentangle the impact of the representation of aerosol size distribution on aerosol optical properties over central Europe, particularly over the Mediterranean Basin, during a summertime aerosol episode. To fulfill this objective, a sensitivity test has been conducted using the coupled chemistry–meteorology model WRF-Chem (Weather Research Forecast model coupled with Chemistry). The test modified the parameters defining a lognormal size distribution (geometric diameter and standard deviation) by 10 %, 20 %, and 50 %. Results reveal that the reduction in the standard deviation of the accumulation mode leads to the largest impacts on AOD due to a transfer of particles from the accumulation mode to the coarse mode. A reduction in the geometric diameter of the accumulation mode also has an influence on AOD representation since particles in this mode are assumed to be smaller. In addition, an increase in the geometric diameter of the coarse mode produces a redistribution through the total size distribution by relocating particles from the finer modes to the coarse.

Dust-Dominated Coarse Particles as a Medium for Rapid Secondary Organic and Inorganic Aerosol Formation in Highly Polluted Air.

Secondary aerosol (SA) frequently drives severe haze formation on the North China Plain. However, previous studies mostly focused on submicron SA formation, thus our understanding of SA formation on supermicron particles remains poor. In this study, PM2.5 chemical composition and PM10 number size distribution measurements revealed that the SA formation occurred in very distinct size ranges. In particular, SA formation on dust-dominated supermicron particles was surprisingly high and increased with relative humidity (RH). SA formed on supermicron aerosols reached comparable levels with that on submicron particles during evolutionary stages of haze episodes. These results suggested that dust particles served as a medium for rapid secondary organic and inorganic aerosol formation under favorable photochemical and RH conditions in a highly polluted environment. Further analysis indicated that SA formation pathways differed among distinct size ranges. Overall, our study highlights the importance of dust in SA formation during non-dust storm periods and the urgent need to perform size-resolved aerosol chemical and physical property measurements in future SA formation investigations that are extended to the coarse mode because the large amount of SA formed thereon might have significant impacts on ice nucleation, radiative forcing, and human health.

Characterization of Aerosol Types by Columnar Optical Properties Analysis at a Coastal Site on Eastern Seaboard of India

The coastal stretch of the northwestern Bay of Bengal experiences a mixture of aerosol types from a variety of sources attributed to spatial heterogeneity. The present study aims to identify aerosol types from ground-based spectral measurements of Aerosol Optical Depth (AOD) at Gopalpur (19.30° N, 84.88° E), a coastal site on the eastern seaboard of India. The spectral variation of AOD exhibits four distinct types viz. strong wavelength-dependent with a high loading of fine and coarse aerosol (type 1), strong wavelength-dependent with low loading of fine and coarse aerosol (type 2), weak wavelength-dependent with a high loading of fine and coarse aerosol (type 3) and weak wavelength-dependent with low loading of fine but high loading of coarse aerosol (type 4).The seasonal variation of aerosol size characterization revealed the prevalence of coarser mode particles during pre-monsoon (March–June) and monsoon (July–October) period. The coarse mode aerosols during premonsoon were attributed to transport of dust, minerals, and local sea spray aerosols, whereas during monsoon due to wet removal of finer aerosol particles. In contrast, the dominance of fine mode particles was observed during postmonsoon (November–February) possibly due to strong local anthropogenic emission and advection of finer particles. In addition, AOD variations were strongly associated with daily fluctuations in wind speed. The relatively higher wind speed during premonsoon acts conducive to the genesis of relatively larger aerosol particles such as sea-salt. The present study complements the information on daily and seasonal variability of aerosol size characteristics that could be critical in filling gaps for global-level assessment. Additionally, this study strengthens the existing knowledge of the aerosol influence on the regional weather pattern, contributing to a more comprehensive understanding of the regional climate system in response to anthropogenic forcing.

Climatological aspects of size-resolved column aerosol optical properties over a rural site in the southern peninsular India

We present the long-term (2008–2018) climatological characteristics of column aerosol optical properties from the Sky Radiometer at Gadanki (13.48∘ N, 79.18∘ E), a typical rural site in southern peninsular India. A method to separate the fine and coarse mode spectral aerosol optical properties was presented. The climatological aspects related to the diurnal, monthly, and intra-annual variations of observed aerosol optical properties exhibited both seasonal and spectral dependence. While the size-resolved aerosol optical depth (AOD), absorption aerosol optical depth, and asymmetry parameter exhibited a decreasing trend, single scattering albedo (SSA) showed an increasing trend from shorter to longer wavelengths in all seasons with distinct magnitudes. Seasonal asymmetry in the diurnal variation of AOD and Angstrom exponent was observed. Intra-annual variability clearly indicated distinct fine and coarse mode dominance during winter and monsoon, respectively, while mixed contributions were found in other seasons. This is attributed to the influence of prevailing air masses on the column aerosol properties. Annual mean SSA values indicated the dominance of moderately absorbing (0.91) to weakly absorbing (0.95) aerosols. On evaluating the size-resolved AOD (at 500 nm) with those from extended Spectral Deconvolution Algorithm (SDA+), the observed differences are found to be remarkable for coarse mode AOD although the correlations for both fine and coarse AODs were observed to be 0.96. Lastly, the Dark Target and Deep Blue merged AOD (at 550 nm) from MODIS Terra/Aqua are evaluated to examine the consistency with ground measurements and both have shown a systematic underestimation with an increase in the magnitude of AOD strongly dependent on the seasons. These results form a basis for the comprehensive characterization of aerosol types and associated radiative impacts that are vital for understanding the regional climate change.

Exploring the oxidative potential and respiratory deposition of size-segregated particulate matter at an urban site

According to the current understanding, the adverse health effects of particulate matter (PM) are produced by oxidative stress processes. Hence, the oxidative potential (OP) can be used as an exposure metric for more accurate approaches to the health response to ambient PM. This work explores the relationship between respiratory deposition of size-segregated PM and OP in the city of Santiago, Chile. Size-segregated PM samples were collected at an urban site using a Micro-Orifice Uniform Deposit Impactor (MOUDI) sampler. Dithiothreitol (DTT) loss rates were measured in aqueous PM extracts to estimate the OP for each sample. Size-segregated OP showed a bi-modal distribution with a maximum centered in the accumulation mode (0.18–3.2 μm) and other in the coarse mode (3.2–18 μm). The magnitude of the OP estimated in the accumulation mode was 2.9 ± 0.8 and 2.3 ± 0.6 times the magnitude of the OP estimated in the coarse and quasi-ultrafine mode, respectively. Respiratory deposition calculations indicated that PM3.2 (with diameter less than 3.2 μm) dominated the OP lung deposition with contributions ranging from 65 to 94% in different areas of the human respiratory tract. This was due to the deposition efficiency and greater surface area of these fractions that allowed more efficient adsorption of active redox chemical species. These results may account for the greater negative impact on the lung health of the fine fractions of PM. The characterization of the OP respiratory deposition related to different fractions of PM, can result in adequate metrics to evaluate human exposure to PM and its possible health impacts.

Dust Impacts of Rapid Agricultural Expansion on the Great Plains

AbstractClimate change and land use are altering the landscape of the U.S. Great Plains, producing increases in windblown dust. These increases are investigated by combining coarse mode aerosol observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor in addition to the Aerosol Robotic Network (AERONET) and Interagency Monitoring of Protected Visual Environments (IMPROVE) aerosol monitoring networks. Increasing trends of up to 5%/year in MODIS aerosol optical depth for dust observations are observed throughout the Great Plains (2000–2018). Cropland coverage has increased 5–10% over the majority of the Great Plains (2008–2018), and positive monthly trends in IMPROVE (1988–2018) and AERONET (1995–2018) coarse mode 90th percentile observations coincide with planting and harvesting seasons of predominant crops. Presently, results suggest increased dust due to agricultural expansion is negatively influencing human health and visibility in the Great Plains. Furthermore, results foreshadow a future where desertification becomes an increasing risk in the Great Plains.

The interaction of ozone and aerosol in a semi-arid region in the Middle East: Ozone formation and radiative forcing implications

Investigations on the variations in ozone concentration, aerosol properties, and ozone-aerosol interactions can help to better understand the atmospheric radiation budget and their effects on climate. In this study, the interaction of ozone and aerosol was analyzed over Zanjan, Iran. The seasonal cycle of total column ozone demonstrated that high ozone concentrations often occur in early spring and winter. However, the seasonal cycle of tropospheric ozone showed that high concentrations of tropospheric ozone often happen in the spring and summer. The high values of Aerosol Optical Depth (AOD), Particulate Matter (PM10) and Absorption Aerosol Optical Depth (AAOD) were observed in the spring and summer, whereas low values in the fall and winter. The results showed that tropospheric ozone and AOD were negatively correlated during the spring and summer seasons. Especially on dusty days, the aerosol has reduced the concentration of ozone in the atmosphere. The correlation coefficients between PM10 and ozone were negative during dusty days/months. Moreover, the relation of ozone with a fine mode fraction of AOD depicted negative correlations in the spring and summer and positive correlation in the fall and winter. A comprehensive study on the effects of the aerosol types on ozone concentration showed that anthropogenic fine mode aerosols such as urban/industrial had a positive impact on ozone production, contrary to the effect of dust, which reduced ozone. This study showed that coarse mode aerosols such as dust with higher AOD, reduced the sunlight to reach the earth's surface. As a result, aerosol radiative forcing (ARF) values were found to be more negative at the surface; thus, the photolysis rate decreased considerably, and the concentration of surface ozone was reduced significantly.

A synergy of linear model and wavelet analysis towards space-time characterization of aerosol optical depth (AOD) during pre-monsoon season (2007–2016) over Indian sub-continent

The pre-monsoon aerosol concentration plays a significant role in modifying precipitation amount over the Indian sub-continent. A large variety of aerosol from different sources produces a complex radiative and climate response through the interaction with the hydrometeorological parameters. In this study, we analyzed the space-time dynamics of aerosol optical depth (AOD) in relation to the meteorological and surface parameters over Indian sub-continent during pre-monsoon season from 2007 to 2016. The level-3 daily aerosol products from Moderate Resolution Imaging Spectroradiometer (MODIS) and Multi-angle Imaging Spectroradiometer (MISR) were used. The IMD gridded rainfall and temperature, ECMWF derived RH and wind velocity, and GLADS derived soil moisture data were also used at daily time scale. For the space-time model, the Mann-Kendall trend test and a pixel-based multiple linear regression were used, while, wavelet transformation was used on these daily observations for analyzing periodicities of AOD. The time-series average shows moderate to high AOD (0.4–0.8), including a consistent positive anomaly in the Indo-Gangetic basin (IGP) in north India. A significant inter-annual variation is also observed both in MODIS and MISR datasets. The trend statistics shows an increasing trend of aerosol concentration in the eastern and southern India. The linear regression shows a variable response of AOD with changing magnitude of meteorological factors. However, a substantial spatial coverage of significantly decreased AOD is observed with increasing soil moisture content (β < −0.04). The wavelet analysis manifests the abundances of 32–128 days of cycle with a periodic interjection of 8–32 days of cycle suggesting occurrence of fine and coarse mode aerosols events, respectively. The coherency of time series AOD and other covariates shows varying leading and lagging dynamics in these two principal periodicities. The findings, however, evidenced a notable difference in the space-time patterns of AOD in MODIS and MISR datasets. The analyzed AOD cycles are coincided with the Madden-Julian-Oscillation (MJO) that recurs every 30–60 days interval. The findings also support the theoretical proposition of elevated heat pump theory (EHP) driven by fine mode aerosols for occurring pre-monsoon and monsoon precipitation over the Indian sub-continent. The analyzed periodicity of AOD can provide useful insights in studying the short/long term variability of precipitation over polluted environments during the pre-monsoon season.

Variations in physicochemical properties of airborne particles during a heavy haze-to-dust episode in Beijing

The variations in physicochemical properties of airborne particles collected during a typical transition from haze to dust were investigated using single particle analysis with transmission and scanning electron microscopes combined with online measurement of chemical compositions of airborne particles in Beijing in February 2013. The transition was divided into three phases based on the weather condition. During haze pollution (Phase 1), gaseous and particle pollutants enhanced gradually. Results from single particle analysis showed that more coatings and more anthropogenic elements (e.g., S) appeared on the surface of fine and coarse particles, which was probably caused by efficient aqueous-phase reactions under high humidity (70%) condition. Phase 2 was dust intrusion episode. PM10 reached over 1000 μg m−3. Larger fractions of mineral particles and bare-like soot particles were observed in fine particles, while the fraction of secondary particles with coatings decreased. The proportion of black carbon in submicron particles also increased. Photochemical oxidation in gas phase likely dominated in secondary formation under high O3 concentration. After the dust episode (Phase 3), secondary formation enhanced obviously. Soot aged quickly and had a larger mode of 0.45 μm than the other phases. The size modes of airborne fine particles during Phases 1 and 3 were 0.35 μm, which were a bit larger than that during Phase 2 (0.24 μm). These results indicate that dust plumes accompanied with strong wind brought mineral particles in both fine and coarse modes and freshly emitted particles with smaller sizes, and swept away pre-presence air pollutants. This study could provide detailed information on the physicochemical properties of airborne particles during typical severe pollution processes in a short time. Such short-term change should be taken into account in order to more accurately assess the environmental, climatic and health-related effects of airborne particles.

Geochemical characteristics of trace elements in size-resolved coastal urban aerosols associated with distinct air masses over tropical peninsular India: Size distributions and source apportionment

Trace elements in atmospheric particulate matter play a significant role in air quality, health and biogeochemical cycles. The present study reports on geochemical characteristics of size-resolved trace elements in PM10 aerosols collected under different air masses over a coastal urban location in peninsular India. A contrast in elemental distribution was observed for the particle size above 7.0 μm and below 1.1 μm under the influence of northeasterly air masses as characterized by Al > Fe > Zn and Fe > Al > Zn, respectively. The concentrations of the crustal elements (Al, Fe, Ti, P, Ba, Co) were high and illustrated by a unimodal size distribution with a peak in coarse mode (>2.0 μm) during northwesterly air masses. On the other hand, combustion-derived metals (Cu, Zn, Cd, Sb, and Pb) were maximized under northeasterly air masses, characterized by unimodal size distribution with a peak in fine mode (<2.0 μm). The enrichment factor (EF) analysis reveals the contribution of anthropogenic emissions to Cd, Sb, Pb, Zn, Cu, Cr, Ni, As, and Sn metals, particularly to the high enrichment of trace metals in fine mode. These results suggest that crustal emissions are major sources of trace metals in coarse mode aerosols; whereas combustion derived anthropogenic emissions contribute to the fine mode aerosols. The positive matrix factorization (PMF) analysis revealed that crustal sources (52–90%) were most abundant for particles >7.0 μm, whereas combustion related emissions such as vehicular and traffic sources are predominant for particles <1.1 μm. The present study demonstrates that trace metals in coastal urban aerosols are affected by changes in emission sources/strengths and regional transport of air masses originated from the northeasterly and northwesterly parts of the tropical Indian subcontinent.

Interannual and seasonal variations in the aerosol optical depth of the atmosphere in two regions of Spitsbergen (2002–2018)

Abstract. In this work, hourly averaged sun photometer data from Barentsburg and Ny-Ålesund, both located on Spitsbergen in the European Arctic, are compared. Our data set comprises the years from 2002 to 2018 with overlapping measurements from both sites during the period from 2011 to 2018. For more turbid periods (aerosol optical depth, AOD, τ0.5&gt;0.1), we found that Barentsburg is typically more polluted than Ny-Ålesund, especially in the shortwave spectrum. However, the diurnal variation in the AOD is highly correlated. Next, τ was divided into a fine and coarse mode. It was found that the fine-mode aerosol optical depth generally dominates and also shows a larger interannual than seasonal variation. The fine-mode optical depth is in fact largest in spring during the Arctic haze period. Overall the aerosol optical depth seems to decrease (at 500 nm the fine-mode optical depth decreased by 0.016 over 10 years), although this is hardly statistically significant.

Size distribution and formation processes of aerosol water-soluble organic carbon during winter and summer in urban Beijing

Size-resolved aerosol organic carbon (OC) and water-soluble OC (WSOC) was determined weekly in urban Beijing between 2013 and 2018. The results showed that in winter, the concentrations of WSOC and OC in fine (<2.1 μm) and coarse (>2.1 μm) mode were highest in 2013. During the following years, the concentrations of WSOC and OC were lower but an increasing trend was found from 2014 to 2016, which might be due to the increasing precursor concentrations. From summer 2013 to 2018, the concentrations of OC and WSOC in both fine and coarse mode decreased, which might be affected by decreasing emission intensity and photochemical oxidation. To characterize the seasonality of WSOC, two intensive field observations were further performed in winter 2013 and summer 2018. In winter of 2013, WSOC was mainly enriched in PM1 (<1 μm) on clean days and lightly polluted days. In addition, the fine mode maximum shifted from 0.43-0.65 μm to 1.1–2.1 μm with the haze development due to the hygroscopic growth of particles. In summer of 2018, the WSOC was mainly concentrated in PM1, but in a few samples, the maximum size appeared in coarse mode. This difference might due to the different relative humidity and air mass sources. While the formation process of WSOC in winter was affected by photochemical oxidation below 2.1 μm, in 1.1–2.1 μm, it tended to change from photochemical oxidation to aqueous-phase oxidation. For the coarse mode WSOC formation in winter, however, it was mainly affected by heterogeneous processes. The findings advanced our understanding of the annual trends, size distribution and formation processes of WSOC, with significant implications for air pollution control in Beijing and surrounding regions.

Revisiting particle dry deposition and its role in radiative effect estimates

Wet and dry deposition remove aerosols from the atmosphere, and these processes control aerosol lifetime and thus impact climate and air quality. Dry deposition is a significant source of aerosol uncertainty in global chemical transport and climate models. Dry deposition parameterizations in most global models were developed when few particle deposition measurements were available. However, new measurement techniques have enabled more size-resolved particle flux observations. We combined literature measurements with data that we collected over a grassland in Oklahoma and a pine forest in Colorado to develop a dry deposition parameterization. We find that relative to observations, previous parameterizations overestimated deposition of the accumulation and Aitken mode particles, and underestimated in the coarse mode. These systematic differences in observed and modeled accumulation mode particle deposition velocities are as large as an order of magnitude over terrestrial ecosystems. As accumulation mode particles form most of the cloud condensation nuclei (CCN) that influence the indirect radiative effect, this model-measurement discrepancy in dry deposition alters modeled CCN and radiative forcing. We present a revised observationally driven parameterization for regional and global aerosol models. Using this revised dry deposition scheme in the Goddard Earth Observing System (GEOS)-Chem chemical transport model, we find that global surface accumulation-mode number concentrations increase by 62% and enhance the global combined anthropogenic and natural aerosol indirect effect by -0.63 W m-2 Our observationally constrained approach should reduce the uncertainty of particle dry deposition in global chemical transport models.

Multi-year ACSM measurements at the central European research station Melpitz (Germany) – Part 1: Instrument robustness, quality assurance, and impact of upper size cutoff diameter

Abstract. The aerosol chemical speciation monitor (ACSM) is nowadays widely used to identify and quantify the main components of fine particles in ambient air. As such, its deployment at observatory platforms is fully incorporated within the European Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS). Regular intercomparisons are organized at the Aerosol Chemical Monitoring Calibration Center (ACMCC; part of the European Center for Aerosol Calibration, Paris, France) to ensure the consistency of the dataset, as well as instrumental performance and variability. However, in situ quality assurance remains a fundamental aspect of the instrument's stability. Here, we present and discuss the main outputs of long-term quality assurance efforts achieved for ACSM measurements at the research station Melpitz (Germany) since 2012 onwards. In order to validate the ACSM measurements over the years and to characterize seasonal variations, nitrate, sulfate, ammonium, organic, and particle mass concentrations were systematically compared against the collocated measurements of daily offline high-volume PM1 and PM2.5 filter samples and particle number size distribution (PNSD) measurements. Mass closure analysis was made by comparing the total particle mass (PM) concentration obtained by adding the mass concentration of equivalent black carbon (eBC) from the multi-angle absorption photometer (MAAP) to the ACSM chemical composition, to that of PM1 and PM2.5 during filter weighing, as well as to the derived mass concentration of PNSD. A combination of PM1 and PM2.5 filter samples helped identifying the critical importance of the upper size cutoff of the ACSM during such exercises. The ACSM–MAAP-derived mass concentrations systematically deviated from the PM1 mass when the mass concentration of the latter represented less than 60 % of PM2.5, which was linked to the transmission efficiency of the aerodynamic lenses of the ACSM. The best correlations are obtained for sulfate (slope =0.96; R2=0.77) and total PM (slope =1.02; R2=0.90). Although, sulfate did not exhibit a seasonal dependency, total PM mass concentration revealed a small seasonal variability linked to the increase in non-water-soluble fractions. The nitrate suffers from a loss of ammonium nitrate during filter collection, and the contribution of organo-nitrate compounds to the ACSM nitrate signal make it difficult to directly compare the two methods. The contribution of m∕z 44 (f44) to the total organic mass concentration was used to convert the ACSM organic mass (OM) to organic carbon (OC) by using a similar approach as for the aerosol mass spectrometer (AMS). The resulting estimated OCACSM was compared with the measured OCPM1 (slope =0.74; R2=0.77), indicating that the f44 signal was relatively free of interferences during this period. The PM2.5 filter samples use for the ACSM data quality might suffer from a systematic bias due to a size truncation effect as well as to the presence of chemical species that cannot be detected by the ACSM in coarse mode (e.g., sodium nitrate and sodium sulfate). This may lead to a systematic underestimation of the ACSM particle mass concentration and/or a positive artifact that artificially decreases the discrepancies between the two methods. Consequently, ACSM data validation using PM2.5 filters has to be interpreted with extreme care. The particle mass closure with the PNSD was satisfying (slope =0.77; R2=0.90 over the entire period), with a slight overestimation of the mobility particle size spectrometer (MPSS)-derived mass concentration in winter. This seasonal variability was related to a change on the PNSD and a larger contribution of the supermicrometer particles in winter. This long-term analysis between the ACSM and other collocated instruments confirms the robustness of the ACSM and its suitability for long-term measurements. Particle mass closure with the PNSD is strongly recommended to ensure the stability of the ACSM. A near-real-time mass closure procedure within the entire ACTRIS–ACSM network certainly represents an optimal quality control and assurance of both warranting the quality assurance of the ACSM measurements as well as identifying cross-instrumental biases.

Inter-annual and seasonal variations in optical and physical characteristics of columnar aerosols over the Pokhara Valley in the Himalayan foothills

ABSTRACT This study reports comprehensive analysis of seasonal and inter-annual variations of aerosol optical and physical properties over the Pokhara Valley in the foothills of central Himalayas in Nepal utilizing the high-quality multi-year columnar aerosol data observed recently from January 2010 to December 2017. The influence of forest fires and agro-residue fires on aerosol properties is also investigated. Seasonal mean aerosol optical depth (AOD) over Pokhara is ≥0.3 during the year, highest in pre-monsoon and lowest in monsoon season. AOD exhibits a consistent seasonal cycle every year with a peak in pre-monsoon. Fine mode fraction contributing to AOD and Angstrom exponent (α) corroborate seasonal pattern of AODs. AODs show good correlation with fire counts in the Pokhara Valley and surrounding region. The aerosol volume size distributions are bimodal. The peaks in volume size distribution and volume concentrations obtained in 2016 pre-monsoon are the highest during the 8-year observation period which coincided with peaks in the occurrence of highest number of fires in the Pokhara Valley and surrounding region. Effective radius of coarse mode aerosols is an order of magnitude larger than fine mode aerosols. The analysis of optical and physical properties of aerosols over the Pokhara Valley suggest that the aerosols observed over the Himalayan foothills are of urban/industrial, biomass burning and dust origin with proportional contributions varying in different seasons. The study provides observational constraints on aerosol physical and optical properties that can serve as inputs for model simulation of aerosol physiochemical processes and quantification of impacts on air quality, climate and sensitive ecosystems in this data-sparse region.

Fundamental investigation into characteristics of particulate matter produced from rapid pyrolysis of biochar in a drop-tube furnace at 1300 °C

This paper reports the emission characteristics of leaf and wood biochar (LC500 and WC500) pyrolysis in a drop tube furnace at 1300 °C in argon atmosphere. The char yields at 1300 °C are ∼ 65% and ∼ 73% respectively for LC500 and WC500. Over 60% Mg, Ca, S, Al, Fe and Si are retained in char after pyrolysis at 1300 °C. The retentions of Na and K in the char from LC500 pyrolysis are lower than those in the char from WC500 pyrolysis due to release via enhanced chlorination as a result of much higher Cl content in LC500. Particulate matter (PM) with aerodynamic diameter of < 10 µm (i.e. PM10) from LC500 and WC500 pyrolysis exhibits a bimodal distribution with a fine mode diameter of 0.011 µm and a coarse mode diameter of 4.087 µm. The PM10 yield for LC500 pyrolysis is ∼ 8.2 mg/g, higher than that of WC500 pyrolysis (∼2.1 mg/g). Samples in PM1-10 (i.e. PM with aerodynamic diameter 1 µm – 10 µm) are char fragments that have irregular shapes and similar molar ratio of (Na+K + 2Mg+2Ca)/(Cl+2S+3P) as the char collected in the cyclone. In PM1 (i.e. PM with aerodynamic diameter < 1 µm), the main components in sample are inorganic species, and carbon only contributes to ∼5% and ∼8% the PM1 produced from rapid pyrolysis of LC500 and WC500, respectively. Na, K and Cl are main inorganic species in PM1, contributing ∼ 98.8% and ∼ 97.5% to all inorganic species. Na, K and Cl from rapid pyrolysis of biochar have a unimodal distribution with a mode diameter of 0.011 µm. In PM1–10, Ca is the main inorganic specie, contributing to ∼71.2% and ∼65.3% to all inorganic species in PM1–10 from pyrolysis of LC500 and WC500, respectively.

Seasonal Evolution of Size-Segregated Particulate Mercury in the Atmospheric Aerosol Over Terra Nova Bay, Antarctica.

Size-fractionated particulate mercury (PHg) measurements were performed from November 2017 to January 2018 at Terra Nova Bay (Antarctica) for the first time. Samples were collected every 10 days by a six-stage high-volume cascade impactor with size classes between 10 μm and 0.49 μm. Total PHg concentrations were maxima (87 ± 8 pg m−3) in November, then decreased to values ~40% lower and remained almost constant until the end of the sampling period (~30 pg m−3). The trimodal aerosol mass distribution reveals that from 30% to 90% of the total PHg came in the size > 1.0 μm. Hg in the two coarse fractions was probably produced by the adsorption of oxidized Hg species transported by air masses from the Antarctic plateau or produced locally by sea ice edges. PHg in accumulation mode seemed to be related to gas–particle partitioning with sea salt aerosol. Finally, average dry deposition fluxes of PHg were calculated to be 0.36 ± 0.21 ng m−2 d−1 in the accumulation mode, 47 ± 44 ng m−2 d−1 in the first coarse mode, and 37 ± 31 ng m−2 d−1 in the second coarse mode. The present work contributed to the comprehension of the Hg biogeochemical cycle, but further research studies are needed.

Impact assessment of continental and marine air-mass on size-resolved aerosol chemical composition over coastal atmosphere: Significant organic contribution in coarse mode fraction

Size-segregated aerosols (five stages from <1.1 to >7 μm in diameter) collected from a coastal location in south-west peninsular India were analysed for water-soluble ionic species, carbonaceous aerosols and metallic species on a seasonal basis for two years (2017–19). The annual mean concentration of the components Na+, Ca2+, Mg2+, Cl−, NO3−, Al, Fe exhibited coarse mode (> 2 μm) enhancement; while NH4+, K+, SO42−, MSA−, oxalate, Organic carbon (OC), Elemental carbon (EC) peaked in fine mode (< 2 μm). The contribution of fine mode aerosols dominated under the influence of continental air-mass and a relative shift towards coarse mode size regime with respect to marine air-mass was observed. The relative contribution of analysed species showed the dominance of sea-salt fraction in coarse mode during monsoon, anthropogenic ions in fine mode during post-monsoon, crustal species in coarse mode during pre-monsoon and carbonaceous species in fine mode during winter. While nss-SO42− was found to be a major component of fine mode aerosol fraction, OC contributed significantly for both coarse and fine mode. Distinct seasonal variation in OC size-distribution was observed. Fine mode OC dominated during post-monsoon and winter while coarse mode OC peaked during pre-monsoon and monsoon, indicating its multiple sources and formation pathways. The water-soluble fraction of OC predominantly existed in fine mode and was modulated by changing synoptic air-masses. The water insoluble OC fraction was found to be modified by sea-land breeze activities and exhibited coarse mode nature. The significant contribution of OC in coarse mode size regime over this coastal atmosphere has been attributed to; the condensation of gaseous precursors on pre-existing coarse particles leading to the formation of coarse mode WSOC and the primary emission from natural terrestrial and marine organic matter resulting in the emission of coarse mode WIOC.