Supermicron Size Range Research Articles

Coal Mine Dust Size Distributions, Chemical Compositions, and Source Apportionment

Current regulations mandate the monitoring of respirable coal mine dust (RCMD) mass and crystalline silica in underground coal mines to safeguard miner health. However, other RCMD characteristics, such as particle size and chemical composition, may also influence health outcomes. This study collected RCMD samples from two underground coal mines and performed detailed chemical speciation. Source apportionment was used to estimate RCMD and silica contributions from various sources, including intake air, fire suppression limestone dust, coal dust, diesel engine exhaust, and rock strata. The mine dust mass-based size distributions were comparable to those recorded over a decade ago, with a peak around 10 μm and the majority of the mass in the supermicron size range. The current mine conditions and mining practices do not appear to have significantly increased the generation of smaller particles. Limestone rock dust was prevalent in many locations and, along with coal dust, was the main contributor to RCMD at high-concentration locations. Silica accounted for over 10% of RCMD mass at several active mining locations, primarily from limestone and rock strata dust. Reducing the concentration of limestone dust and its silica content could reduce RCMD and silica levels. Further cleaning of the intake air could also improve the overall mine air quality.

Secondary ice production – no evidence of efficient rime-splintering mechanism

Abstract. Mixed-phase clouds are essential for Earth’s weather and climate system. Ice multiplication via secondary ice production (SIP) is thought to be responsible for the observed strong increase in ice particle number concentration in mixed-phase clouds. In this study, we focus on the rime splintering also known as the Hallett–Mossop (HM) process, which still lacks physical and quantitative understanding. We report on an experimental study of rime splintering conducted in a newly developed setup under conditions representing convective mixed-phase clouds in the temperature range of −4 to −10 °C. The riming process was observed with high-speed video microscopy and infrared thermography, while potential secondary ice (SI) particles in the super-micron size range were detected by a custom-built ice counter. Contrary to earlier HM experiments, where up to several hundreds of SI particles per milligram of rime were found at −5 °C, we found no evidence of productive SIP, which fundamentally questions the importance of rime splintering. Further, we could exclude two potential mechanisms suggested to be the explanation for rime splintering: the freezing of droplets upon glancing contact with the rimer and the fragmentation of spherically freezing droplets on the rimer surface. The break-off of sublimating fragile rime spires was observed to produce very few SI particles, which is insufficient to explain the large numbers of ice particles reported in earlier studies. In the transition regime between wet and dry growth, in analogy to phenomena of the deformation of drizzle droplets upon freezing, we also observed the formation of spikes on the rimer surface, which might be a source of SIP.

Enhanced secondary aerosol formation driven by excess ammonia during fog episodes in Delhi, India

The Indo-Gangetic Plain (IGP) has high wintertime fine aerosol loadings that significantly modulate the widespread fog formation and sustenance. Here, we investigate the potential formation of secondary inorganic aerosol driven by excess ammonia during winter fog. Physicochemical properties of fine aerosols (PM1 and PM2.5) and trace gases (HCl, HONO, HNO3, SO2, and NH3) were simultaneously monitored at hourly resolution using Monitor for AeRosols and Gases in Ambient air (MARGA-2S) for the first time in India. Results showed that four major ions, i.e., Cl−, NO3−, SO42−, and NH4+ contributed approximately 97% of the total measured inorganic ionic mass. The atmosphere was ammonia-rich in winter and ammonium was the dominant neutralizer with aerosol neutralization ratio (ANR) close to unity. The correlation between ammonium and chloride was ≥0.8, implying the significant formation of ammonium chloride during fog in Delhi. Thermodynamical model ISORROPIA-II showed the predicted PM1 and PM2.5 pH to be 4.49 ± 0.53, and 4.58 ± 0.48 respectively which were in good agreement with measurements. The ALWC increased from non-foggy to foggy periods and a considerable fraction of fine aerosol mass existed in the supermicron size range of 1–2.5 μm. The sulfur oxidation ratio (SOR) of PM1, PM2.5 reached up to 0.60, 0.75 in dense fog and 0.74, 0.87 when ambient RH crossed a threshold of 95%, much higher than non-foggy periods (with confidence level of ≥95%) pointing to enhanced formation of secondary aerosol in fog.

Characterization of aerosol particles at Cabo Verde close to sea level and at the cloud level – Part 2: Ice-nucleating particles in air, cloud and seawater

Abstract. Ice-nucleating particles (INPs) in the troposphere can form ice in clouds via heterogeneous ice nucleation. Yet, atmospheric number concentrations of INPs (NINP) are not well characterized, and, although there is some understanding of their sources, it is still unclear to what extend different sources contribute or if all sources are known. In this work, we examined properties of INPs at Cabo Verde (a.k.a. Cape Verde) from different environmental compartments: the oceanic sea surface microlayer (SML), underlying water (ULW), cloud water and the atmosphere close to both sea level and cloud level. Both enrichment and depletion of NINP in SML compared to ULW were observed. The enrichment factor (EF) varied from roughly 0.4 to 11, and there was no clear trend in EF with ice-nucleation temperature. NINP values in PM10 sampled at Cape Verde Atmospheric Observatory (CVAO) at any particular ice-nucleation temperature spanned around 1 order of magnitude below −15 ∘C, and about 2 orders of magnitude at warmer temperatures (>-12 ∘C). Among the 17 PM10 samples at CVAO, three PM10 filters showed elevated NINP at warm temperatures, e.g., above 0.01 L−1 at −10 ∘C. After heating samples at 95 ∘C for 1 h, the elevated NINP at the warm temperatures disappeared, indicating that these highly ice active INPs were most likely biological particles. INP number concentrations in PM1 were generally lower than those in PM10 at CVAO. About 83±22 %, 67±18 % and 77±14 % (median±standard deviation) of INPs had a diameter >1 µm at ice-nucleation temperatures of −12, −15 and −18 ∘C, respectively. PM1 at CVAO did not show such elevated NINP at warm temperatures. Consequently, the difference in NINP between PM1 and PM10 at CVAO suggests that biological ice-active particles were present in the supermicron size range. NINP in PM10 at CVAO was found to be similar to that on Monte Verde (MV, at 744 m a.s.l.) during noncloud events. During cloud events, most INPs on MV were activated to cloud droplets. When highly ice active particles were present in PM10 filters at CVAO, they were not observed in PM10 filters on MV but in cloud water samples instead. This is direct evidence that these INPs, which are likely biological, are activated to cloud droplets during cloud events. For the observed air masses, atmospheric NINP values in air fit well to the concentrations observed in cloud water. When comparing concentrations of both sea salt and INPs in both seawater and PM10 filters, it can be concluded that sea spray aerosol (SSA) only contributed a minor fraction to the atmospheric NINP. This latter conclusion still holds when accounting for an enrichment of organic carbon in supermicron particles during sea spray generation as reported in literature.

Aerosol arriving on the Caribbean island of Barbados: physical properties and origin

Abstract. The marine aerosol arriving at Barbados (Ragged Point) was characterized during two 3-week long measurement periods in November 2010 and April 2011, in the context of the measurement campaign CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over BArbados). Through a comparison between ground-based and airborne measurements it was shown that the former are representative of the marine boundary layer at least up to cloud base. In general, total particle number concentrations (Ntotal) ranged from as low as 100 up to 800 cm−3, while number concentrations for cloud condensation nuclei (NCCN) at a supersaturation of 0.26 % ranged from some 10 to 600 cm−3. Ntotal and NCCN depended on the air mass origin. Three distinct types of air masses were found. One type showed elevated values for both Ntotal and NCCN and could be attributed to long-range transport from Africa, by which biomass burning particles from the Sahel region and/or mineral dust particles from the Sahara were advected. The second and third type both had values for NCCN below 200 cm−3 and a clear minimum in the particle number size distribution (NSD) around 70 to 80 nm (Hoppel minimum). While for one of these two types the accumulation mode was dominating (albeit less so than for air masses advected from Africa), the Aitken mode dominated the other and contributed more than 50 % of all particles. These Aitken mode particles likely were formed by new particle formation no more than 3 days prior to the measurements. Hygroscopicity of particles in the CCN size range was determined from CCN measurements to be κ = 0.66 on average, which suggests that these particles contain mainly sulfate and do not show a strong influence from organic material, which might generally be the case for the months during which measurements were made. The average κ could be used to derive NCCN from measured number size distributions, showing that this is a valid approach to obtain NCCN. Although the total particulate mass sampled on filters was found to be dominated by Na+ and Cl−, this was found to be contributed by a small number of large particles ( > 500 nm, mostly even in the super-micron size range). Based on a three-modal fit, a sea spray mode observed in the NSDs was found to contribute 90 % to the total particulate mass but only 4 to 10 % to Ntotal and up to 15 % to NCCN. This is in accordance with finding no correlation between Ntotal and wind speed.

Ground‐based measurement of fluorescent aerosol particles in Tokyo in the spring of 2013: Potential impacts of nonbiological materials on autofluorescence measurements of airborne particles

AbstractWe present results from the observation of fluorescent (FL) aerosols in Tokyo in the spring of 2013. Ambient aerosol particles were analyzed using a single‐particle laser‐induced fluorescence instrument. The fluorescent emission from a single particle excited by a pulsed UV laser (excitation wavelength of 263 nm) was detected by three photomultiplier tubes with band‐pass filters (fluorescence wavelength of 335–379, 420–500, and 500–600 nm). Fluorescence patterns from single particles were analyzed for investigating fluorophores and their sources using backward trajectory analysis, factor analysis, and a comparison with other tracer observations. The backward trajectory analysis suggested possible influences of long‐range transport from northern China in some cases. Observed supermicron FL aerosols were classified into two types: combustion‐ and dust‐type particles. The detection of combustion‐type FL aerosols, which showed temporal variations similar to nitrogen oxides and elemental carbon, implied the presence of polyaromatic hydrocarbons (PAHs) and their derivatives. Dust‐type FL aerosols, which were well correlated with the lidar‐derived dust‐aerosol optical depth near the surface (<1 km) and with mineral ions, were dominant in the supermicron size range. The number concentrations of coarse FL aerosols (≥2 µm), with which nonnegligible levels of fluorescence were recorded in all three channels, increased during Asian dust events (relative to a local dust event). It is suggested that PAHs and/or biological materials mixed with dust particles were transported from the North China Plain to the outflow region in East Asia during the observation period. Moreover, the autofluorescence measurement of airborne particles can be substantially affected by nonbiological materials in this region.

Assessment of African desert dust episodes over the southwest Spain at sea level using in situ aerosol optical and microphysical properties

Desert dust (DD) aerosols reach the El Arenosillo observatory (southwest Spain) following two characteristic pathways at sea level, each showing significant differences in its aerosol microphysical and optical properties. These differences, in turn, determine the influence on the radiative forcing over the region. For these events, the meteorological scenarios show a depression located over North Africa at ground level. A Mediterranean pathway occurs when: (1) the depression is located over North Africa and the Mediterranean Basin or (2) it is coupled to a high pressure in higher latitudes. A North Africa pathway is observed when the depression is located only over North Africa. In our inventory, there are clear DD episodes under the Mediterranean flow, whereas other specific DD events take place under a mixture of Mediterranean and North African flows. The pure Mediterranean flow is associated with a higher increase of particle volume and scattering coefficient within the sub-micron than the super-micron size ranges. This result indicates that the contribution to the radiative forcing through the scattering processes over the region for particles with D<1 µm is larger than for particles with D>1 µm. In contrast, the episodes with a mixture of Mediterranean and North African flows show a similar effect of sub- and super-micron size ranges on radiative forcing. The size range with the largest impact on the scattering processes is 0.3 µm<D<0.6 µm. Similar temporal variability of in situ and columnar-integrated aerosol properties on episodes with mixed flow suggests that the amounts of aerosols within the super-micron size range at the surface and aloft are correlated. During the episodes with pure Mediterranean flow, the amounts of aerosol vary independently within all size ranges, but the particle size distributions at surface and aloft are similar.

Detection and characterization of biological and other organic-carbon aerosol particles in atmosphere using fluorescence

This paper offers a brief review on the detection and characterization of biological and other organic-carbon (OC) aerosol particles in atmosphere using laser-induced-fluorescence (LIF) signatures. It focuses on single individual particles or aggregates in the micron and super-micron size range when they are successively drawn through the interrogation volume of a point detection system. Related technologies for these systems that have been developed in last two decades are also discussed. These results should provide a complementary view for studying atmospheric aerosol particles, particularly bioaerosol and OC aerosol particles from other analytical technologies.

Seasonal comparisons of single-particle chemical mixing state in Riverside, CA

The Study of Organic Aerosols in Riverside, CA (SOAR) field campaign, conducted in the summer and fall of 2005, focused on developing a stronger understanding of seasonal influences on the sources and processes contributing to some of the highest levels of particulate matter in the United States. On-line single-particle mass spectrometry measurements showed that carbonaceous particles generally dominated the submicron size range (>75% by number), compared to primarily aged sea salt and dust in the supermicron size range. During periods with high PM2.5 (particulate matter <2.5 μm) mass and number concentrations, hygroscopic water-containing carbonaceous particles internally mixed with sulfate and nitrate extended up into the supermicron size mode. The mixing state of carbonaceous particles changed with season. In the summer, carbonaceous particles were primarily mixed with secondary (oxidized) organic carbon and sulfate; whereas in the fall, they were mixed mostly with ammonium nitrate. During each season, different processes contributed to the highest pollution levels. In the summer, photochemical production of highly aged secondary organic carbon resulted in daily peaks in both number and mass particle concentrations, followed by removal in the afternoon by the arrival of the clean marine air masses from the coast. In contrast, cooler, more stagnant meteorological conditions in the fall season resulted in the buildup over several days of submicron carbonaceous particles mixed with semivolatile ammonium, nitrate, and amines. In addition, Santa Ana wind events in the fall led to dust and biomass burning particles dominating both the submicron and supermicron size ranges.

Emissions of particles and trace elements from coal gasification

Some of the trace elements in coal are easily emitted with the syngas obtained from coal gasification in the gaseous phase and condensed as fine particles. Such impurities in syngas could result in corrosion and/or degradation of electrode materials in fuel cells if the coal gasification process is applied to Integrated Gasification Fuel Cell (IGFC). Thus, to realize the coal gasification process combined with fuel cells, it is extremely important to control the emission behavior of hazardous trace elements in coal gasification. In this study, particles in the coal gasification gas were sampled and analyzed in detail to determine their particle size distributions and elemental compositions. A drop tube type furnace was utilized for the coal gasification experiment. Pb and Se were evaluated in terms of their distribution behavior to particles. Pb is one of the most typical volatile heavy metals, and Se is known as a corrosive substance for Solid Oxide Fuel Cells (SOFC). Thermal equilibrium calculations were also conducted to derive the chemical forms of Pb and Se during their emissions. As a result, particles entrained in syngas from the coal gasification process were confirmed to have bimodal peaks in their particle size distributions. The peak larger than 7.8μm in the super-micron size range consists of ash with unburnt carbon, while the smaller peak at 0.5μm in the sub-micron range consists of soot with mineral deposits. Se and Pb were found to react together during coal gasification process. In addition, almost all of them were vaporized in the reactor and deposited on soot particles in the sub-micron range during the gas cooling process.

Concentration and size measurements of fly ash particles from the clean gas side of a pressurised pulverised coal combustion test facility

One of the most significant problems related to pressurised pulverised coal combustion (PPCC) is the separation of the fly ash at operating conditions (gas temperatures of 1400 °C and a pressure of (up to) 16 bar). To determine the efficiency of an advanced multi-stage separation device, which was made to deliver clean gas (upstream of a turbine) with a required maximum particle size smaller than 3 μm and a particle mass concentration of less than 3 mg/Nm 3, a novel scattered light particle sizer (A 150-HDT) was developed. The device allows for measurements of a sample flow at a pressure of up to 16 bar and (to date) at temperatures of up to 450 °C (however, with further enhancements, up to 600 °C to 800 °C is expected). Several series of measurements using this device have been performed successfully at a PPCC test site, and results of two series – as well as results gained from LPI measurements – will be presented, which show a sufficiently high efficiency of the multi-stage separator in the supermicron size range for the last measurement series. The novel measurement device proved to be capable of measuring the required low particle concentrations with sufficient precision even under these demanding conditions, and the detected clean gas concentrations indicate a successful removal of the fly ash particles, which fulfils the requirements for feed gases entering a gas turbine.

Size-segregated characterization of atmospheric aerosols in Taipei during Asian outflow episodes

To investigate the size resolved physico-chemical properties of aerosols transported from the eastern Asian Continent following the winter monsoons, an intensive field experiment was conducted in Taipei, Taiwan in January 2003. Mass concentration and the abundance of major functional groups in size-segregated aerosol samples were measured. The size distribution parameters of the aerosol mass and the associated functional groups were analyzed. The results of the infrared spectroscopic analysis showed that the contents of sulfate and ammonium in Taipei's sub-micron aerosols were significantly increased due to continental pollution. In addition, the Asian outflow aerosols contained higher levels of silicate and nitrate in the super-micron size range. The size-resolved functionalities of the aerosol samples collected during Asian outflow episodes were compared with those of local pollution samples. This comparison revealed that ammonium and sulfate generally coexisted in sub-micron aerosols, suggesting that there are substantial emissions of ammonia to neutralize the acidity of sulfate in the region from eastern China to northern Taiwan. The nitrate had a bimodal distribution over the size range between 0.1 and 10 μm. A significant enhancement was found in coarse nitrate during the Asian outflow episodes, implying heterogeneous reactions of nitric acid on the surface of dust and sea salt particles during the transport processes. High concentrations of sub-micron ammonium nitrate aerosols were observed prior to the frontal passage and served as an important indicator of local photochemical pollution. The aliphatic and carbonyl carbon in the aerosols were suggested as being mostly due to local pollution in Taipei. Nevertheless, our analysis found that the aerosols of Asian outflows had a higher infrared absorbance ratio of carbonyl to aliphatic carbon than those of the local pollution. The enrichment of carbonyl carbon in the Asian outflow aerosols is most likely due to the emissions of biomass burning aerosols in the eastern China.

Dust particles in the free atmosphere over desert areas on the Asian continent: Measurements from summer 2001 to summer 2002 with balloon‐borne optical particle counter and lidar, Dunhuang, China

Vertical changes of aerosol concentration and size in the free troposphere over the Asian desert areas were first observed using a balloon‐borne optical particle counter (17 August and 17 October 2001 and 11 January, 30 April, and 27 August 2002) at Dunhuang (40°00′N, 94°30′E), China. The concentration of particles with diameter larger than 0.1 μm was more than 1 particle/cm3 in the free troposphere in all seasons. The particle number‐size distribution in this free troposphere shows the possible effect of diffusion of soil particles from the lower atmosphere to the free troposphere, and a noticeable peak of particle concentration is found in the supermicron size range of the number‐size distribution pattern not only in the boundary mixing layer but also in the free troposphere. Trajectory analysis of air masses corresponding to aerosol layers in the free troposphere shows that the background aerosols could be transported far away. The scattering ratio and depolarization ratio measured with a lidar in May and August 2002 suggest the existence of nonspherical particles, possibly mineral particles, in the high free troposphere about 5–6 km over the Dunhuang area and showed good agreement, concerning distribution of particulate matter, with the balloon‐borne measurements.

Pool of dust particles over the Asian continent: balloon-borne optical particle counter and ground-based lidar measurements at Dunhuang, China.

Measurements of aerosols were made in 2001 and 2002 at Dunhuang (40 degrees 00'N, 94 degrees 30'E), China to understand the nature of atmospheric particles over the desert areas in the Asian continent. Balloon-borne measurements with an optical particle counter suggested that particle size and concentration had noticeable peaks in super micron size range not only in the boundary mixing layer but also in the free troposphere. Super-micron particle concentration largely decreased in the mid tropopause (from 5 to 10 km; above sea level, a.s.l.). Lidar measurements made during August 2002 at Dunhuang suggested the possibility that mixing of dust particles occurred from near the ground to about 6 km even under calm weather conditions, and a large depolarization ratio of particulate matter was found in the aerosol layer. The top of the aerosol layer was found at heights of nearly 6 km (a.s.l.). It is strongly suggested that nonspherical dust particles (Kosa particles) frequently diffused in the free atmosphere over the Taklamakan desert through small-scale turbulences and are possible sources of dust particles of weak Kosa events that have been identified in the free troposphere not only in spring but also in summer over Japanese archipelago. Electron microscopic experiments of the particles collected in the free troposphere confirmed that coarse and nonspherical particles observed by the mineral particle were major components of coarse mode (diameter larger than 1 microm) below about 5 km over Dunhuang, China.

Marine and anthropogenic aerosols at Punta Del Hidalgo, Tenerife, and the aerosol nitrate number paradox

Results from analysis of aerosol ion composition and condensation nuclei (CN) concentration measurements carried out at the second Aerosol Characterization Experiment (ACE‐2) Tenerife site, in the eastern North Atlantic subtropical region between July 1995 and May 1997, are described. Sea‐salt derived Na+ dominated the samples having an average concentration of 2.4 μg m−3. Extensive anthropogenic influences are evident with average non‐sea‐salt sulfate (nssSO42−) and nitrate (NO3−) concentration values of 2.1 μg m−3 and 1.3 μg m−3 respectively. High levels of these species are linked to air mass transport from Europe: observed as short‐term (2–3 days) pollution peaks. The pollution peaks overlie a summertime maximum in background levels. From comparison with Southern Hemisphere data, it is estimated that ∼80% of the observed nssSO42− originated from anthropogenic sources. Regression analysis shows a high degree of linearity between the NO3− and CN concentration, which is stronger than found with between NssSO42− and CN concentration. This is considered paradoxical as in the maritime atmosphere NO3− has primarily been found in supermicron size range linked to sea‐salt, while nssSO42− and CN are primarily considered to be submicron aerosol. The CN:NO3− linearity is considered to arise from NO3− formation and transport processes over ocean regions.

A 3‐year record of simultaneously measured aerosol chemical and optical properties at Barrow, Alaska

Results are presented from 3 years of simultaneous measurements of aerosol chemical composition and light scattering and absorption at Barrow, Alaska. All results are reported at the measurement relative humidity of ≤ 40%. Reported are the annual cycles of the concentration of aerosol mass, sea salt, non‐sea‐salt (nss) sulfate, methanesulfonate or MSA−, NH4+, and nss K+, Mg+2, and Ca+2 for the submicron and supermicron size ranges. Submicron nss SO4=, NH4+, and nss K+, Mg+2, and Ca+2 peak in winter and early spring corresponding to the arrival and persistence of Arctic Haze. Submicron sea salt displays a similar annual cycle presumably due to long‐range transport from the northern Pacific Ocean. Supermicron sea salt peaks in summer corresponding to a decrease in sea ice extent. Submicron and supermicron MSA− peak in the summer due to a seasonal increase in the flux of dimethylsulfide from the ocean to the atmosphere. A correlation of MSA− and particle number concentrations suggests that summertime particle production is associated with this biogenic sulfur. Mass fractions of the dominant chemical species were calculated from the concentrations of aerosol mass and chemical species. For the submicron size range the ionic mass and associated water make up 80 to 90% of the aerosol mass from November to May. Of this ionic mass, sea salt and nss SO4= are the dominant species. The residual mass fraction, or fraction of mass that is chemically unanalyzed, is equivalent to the ionic mass fraction in June through October. For the supermicron size range the ionic mass and associated water make up 60 to 80% of the aerosol mass throughout the year with sea salt being the dominant species. Also reported for the submicron size range are the annual cycles of aerosol light scattering and absorption at 550 nm, Ångström exponent for the 450 and 700 nm wavelength pair, and single scattering albedo at 550 nm. On the basis of linear regressions between the concentrations of sea salt and nss SO4= and the light scattering coefficient, sea salt has a dominant role in controlling light scattering during the winter, nss SO4= is dominant in the spring, and both components contribute to scattering in the summer. Submicron mass scattering efficiencies of the dominant aerosol chemical components (nss SO4=, sea salt, and residual mass) were calculated from a multiple linear regression of the measured light scattering versus the component concentrations. Submicron nss SO4= mass scattering efficiencies were relatively constant throughout the year with seasonal averages ranging from 4.1 ± 2.9 to 5.8 ± 1.0 m2 g−1. Seasonal averages for submicron sea salt ranged from 1.8 ± 0.37 to 5.1 ± 0.97 m2 g−1 and for the residual mass ranged from 0.21 ± 0.31 to 1.5 ± 1.0 m2 g−1. Finally, concentrations of nss SO4= measured at Barrow were compared to those measured at Poker Flat Rocket Range, Denali National Park, and Homer for the 1997/1998 and 1998/1999 Arctic Haze seasons. Concentrations were highest at Barrow and decreased with latitude from Poker Flat to Denali to Homer revealing a north to south gradient in the extent of the haze.

Ion balances of size-resolved tropospheric aerosol samples: implications for the acidity and atmospheric processing of aerosols

A large set of size-resolved aerosol samples was inspected with regard to their ion balance to shed light on how the aerosol acidity changes with particle size in the lower troposphere and what implications this might have for the atmospheric processing of aerosols. Quite different behaviour between the remote and more polluted environments could be observed. At the remote sites, practically the whole accumulation mode had cation-to-anion ratios clearly below unity, indicating that these particles were quite acidic. The supermicron size range was considerably less acidic and may in some cases have been close to neutral or even alkaline. An interesting feature common to the remote sites was a clear jump in the cation-to-anion ratio when going from the accumulation to the Aitken mode. The most likely reason for this was cloud processing which, via in-cloud sulphate production, makes the smallest accumulation-mode particles more acidic than the non-activated Aitken-mode particles. A direct consequence of the less acidic nature of the Aitken mode is that it can take up semi-volatile, water-soluble gases much easier than the accumulation mode. This feature may have significant implications for atmospheric cloud condensation nuclei production in remote environments. In rural and urban locations, the cation-to-anion ratio was close to unity over most of the accumulation mode, but increased significantly when going to either larger or smaller particle sizes. The high cation-to-anion ratios in the supermicron size range were ascribed to carbonate associated with mineral dust. The ubiquitous presence of carbonate in these particles indicates that they were neutral or alkaline, making them good sites for heterogeneous reactions involving acidic trace gases. The high cation-to-anion ratios in the Aitken mode suggest that these particles contained some water-soluble anions not detected by our chemical analysis. This is worth keeping in mind when investigating the hygroscopic properties or potential health effects of ultrafine particles in polluted environments.

A study of size‐segregated aerosol chemistry in the Antarctic atmosphere

During austral summer 1998, aerosol particles were sampled at the Finnish station Aboa in continental Antarctica. The collected aerosol samples were analyzed for major inorganic ions, methane sulphonate, and dicarboxylates. Measured ions composed on average about one third of the total gravimetric mass in fine particulate matter (particle diameter &lt; 2 μm). Particulate sulphate, ammonium, and methane sulphonate were internally mixed and found predominantly in the submicron size range. The submicron mass size distribution of these three ions had a dominant mode peaking at ∼ 0.3 μm of particle aerodynamic diameter, another mode peaking at 0.6 μm, and an identifiable Aitken mode below 0.2 μm. The mass size distribution of sodium, a tracer for sea salt particles, peaked in most samples slightly below 2 μm; in addition, it had a submicron mode between ∼0.5 and 1.0 μm and a mode with great variability between 3 and 10 μm. The overall concentration of sea salt was 1–2 orders of magnitude lower than concentrations measured in coastal Antarctica. Particulate nitrate was found almost entirely in the supermicron size range, the most likely formation pathway being the interaction of nitric acid or some other reactive nitrogen compound with sea‐salt particles in the Antarctic atmosphere. The only dicarboxylate above the analytical limit of detection was oxalate, which was distributed in about equal quantities between the submicron and supermicron size ranges. The average total particle number concentration varied in the range 235–955 cm−3 between the samples and showed no obvious correlation with any aerosol species. The great majority of these particles (&gt; 80–90%) were shown to be smaller than ∼ 0.1 μm in diameter.

Chemistry of sea-salt particles in the summer Antarctic atmosphere

The chemistry of sea-salt particles was investigated in summer Antarctica at a site about 150 km from the open ocean. Aerosol samples were collected using a low-pressure impactor which divides particles into 12 size fractions over the aerodynamic particle diameter range 0.045–15 μm. Measured sea-salt particle concentrations were clearly lower than concentrations typically observed at coastal Antarctica. The mass size distribution of sea salt was tri-modal with a submicron mode centering at 0.5–1 μm and two supermicron modes centering slightly below 2 μm and somewhere between 2 and 10 μm, respectively. On average more than 70% of sea salt was found in the supermicron size range, the lower supermicron mode being usually the dominant. Sea-salt particles displayed a large chloride loss with respect to the bulk sea water. The average loss percentage was more than 90% for submicron particles and decreased to about 50% for particles larger than 3 μm in diameter. The primary ions causing the chloride loss were sulfate, nitrate, and methanesulfonate (MSA). The aerosol MSA to non-sea-salt sulfate weight ratio seemed to have been increased by the presence of sea-salt particles. Particulate nitrate was associated strongly with sea salt, the most likely formation pathway being the interaction of nitric acid or some other gaseous nitrogen compounds with these particles in the Antarctic atmosphere.

Substitution of chloride in sea-salt particles by inorganic and organic anions

Depletion of chloride in sea-salt particles was studied at a site near the Arctic Ocean. The investigation was based on size-segregated particle sampling using a Berner low-pressure impactor. According to the impactor measurements, average chloride losses were close to 100% for submicron particles. However, this is successively less for increasing particle size in the supermicron size range. The main constituents replacing chloride from supermicron sea-salt particles were sulfate and nitrate followed by MSA - and oxalate, and with malonate and succinate giving a minor contribution. Anions of organic dicarboxylic acids became more important for air spending a longer time over the continent. Our analysis suggests that principal mechanisms accumulating sulfate into sea-salt particles are cloud processing and, to a lesser degree, heterogeneous reactions taking place in deliquescent sea-salt particles. Mechanisms for the chloride replacement by nitrate are less clear. The distributions of MSA - and oxalate over the sea-salt particle size range were similar to each other, whereas other organic anions analyzed here had a peak concentration at a somewhat larger particle size. Better understanding on the chemistry associated with sea-salt particles requires investigating not only reactions in deliquescent sea-salt particles, but also the interactions between these particles and clouds.