Daizhou Zhang

Asian dust particles converted into aqueous droplets under remote marine atmospheric conditions

The chemical history of dust particles in the atmosphere is crucial for assessing their impact on both the Earth’s climate and ecosystem. So far, a number of studies have shown that, in the vicinity of strong anthropogenic emission sources, Ca-rich dust particles can be converted into aqueous droplets mainly by the reaction with gaseous HNO3 to form Ca(NO3)2. Here we show that other similar processes have the potential to be activated under typical remote marine atmospheric conditions. Based on field measurements at several sites in East Asia and thermodynamic predictions, we examined the possibility for the formation of two highly soluble calcium salts, Ca(NO3)2 and CaCl2, which can deliquesce at low relative humidity. According to the results, the conversion of insoluble CaCO3 to Ca(NO3)2 tends to be dominated over urban and industrialized areas of the Asian continent, where the concentrations of HNO3 exceed those of HCl ([HNO3/HCl] > ∼ 1). In this regime, CaCl2 is hardly detected from dust particles. However, the generation of CaCl2 becomes detectable around the Japan Islands, where the concentrations of HCl are much higher than those of HNO3 ([HNO3/HCl] < ∼ 0.3). We suggest that elevated concentrations of HCl in the remote marine boundary layer are sufficient to modify Ca-rich particles in dust storms and can play a more important role in forming a deliquescent layer on the particle surfaces as they are transported toward remote ocean regions.

Chlorine deposition on dust particles in marine atmosphere

A scanning electron microscope and an energy dispersive X-ray spectrometer were applied to analyze the particles collected during dust-storm events at southwestern Japan in spring 2000. Particle morphology and elemental composition were investigated and the weight ratios of different elements in individual particles were obtained. The results indicated that, besides the coagulation of sea-salt and dust particles, chlorine could deposit onto dust particles through the absorption of chlorine-containing gases when the particles passed through the marine atmosphere between China and Japan. The quantitative estimation revealed that the chlorine deposition on many particles was not negligible compared to sulfur deposition, suggesting that dust particles could remarkably modify the chlorine chemistry in marine atmosphere and possibly vise versa.

Soot particles and their impacts on the mass cycle in the Tibetan atmosphere

Atmospheric aerosol particles in urban and mountain areas around Lhasa city (29.65°N, 91.13°E) in the Tibetan Plateau were collected in the summers of 1998 and 1999. The particles were analyzed with electron microscopes and an energy dispersive X-ray spectrometer. Individual particle morphology, elemental composition and mixture of sulfate and nitrate were investigated. In the urban area, soot particles emitted from vegetation burning were dominant. These particles were characterized by chain or aggregate morphologies, and an elemental composition of potassium and sulfur. Such particles were frequently detected in mountain areas out of the city, where they formed droplets acting as condensation nuclei. Quantitative estimation indicated that sulfur was accumulated onto the soot particles during their dispersion from the urban area to mountain areas. Sulfate and nitrate detections indicated that soot particles collected in the urban area did not contain nitrate and BaCl2-reactive sulfate, which revealed that the combination of sulfur and potassium in the particles was not K2SO4. In contrast, the particles dispersed to mountain areas contained BaCl2-reactive sulfate and some contained nitrate, suggesting that soot particles emitted from the urban area could increase the buffering capacity of aerosol particles and enhance the formation of particulate sulfate through heterogeneous conversion in the Tibetan atmosphere.

Air pollution–aerosol interactions produce more bioavailable iron for ocean ecosystems

Acidic air pollutants dissolve iron in aerosols and fertilize the ocean. It has long been hypothesized that acids formed from anthropogenic pollutants and natural emissions dissolve iron (Fe) in airborne particles, enhancing the supply of bioavailable Fe to the oceans. However, field observations have yet to provide indisputable evidence to confirm this hypothesis. Single-particle chemical analysis for hundreds of individual atmospheric particles collected over the East China Sea shows that Fe-rich particles from coal combustion and steel industries were coated with thick layers of sulfate after 1 to 2 days of atmospheric residence. The Fe in aged particles was present as a “hotspot” of (insoluble) iron oxides and throughout the acidic sulfate coating in the form of (soluble) Fe sulfate, which increases with degree of aging (thickness of coating). This provides the “smoking gun” for acid iron dissolution, because iron sulfate was not detected in the freshly emitted particles and there is no other source or mechanism of iron sulfate formation in the atmosphere.

Nature of Atmospheric Aerosols over the Desert Areas in the Asian Continent: Chemical State and Number Concentration of Particles Measured at Dunhuang, China

Measurements of aerosol were made in August and October 2001, and January 2002, at Dunhuang, China (40°00′N, 94°30′E), to understand the nature of atmosphericparticles over the desert areas in the Asian continent. Balloon-borne measurements with an optical particle counter suggested that particle size and concentration had a noticeable peak in size range of super micron in not only the boundary mixing layer but also the free troposphere. Thickness of the boundary mixing layer, from distributions of particle concentration, was about 4 km in summer (17 August 2001), about2.5 km in fall (17 October 2001), and about 3 km in winter (11 January 2002), which suggest active mixing of particles near the boundary in summer. Number-size distribution of particleshowed a noticeable peak in the super micron particles size range inthe mixing boundary layer: 0.4–2 particles cm-3 at diameter>1.2 μm in summer, 0.05–4 particles cm-3 at diameter >1.2 μm in fall, and 0.1–5 particles cm-3 at diameter>1.2 μm in winter. In winter strong inversion of atmospherictemperature was found in the height range from the boundary to about 3 km and vertical distribution of particle concentration well corresponded with the temperature distribution. Chemical elements of individual aerosols, which were collectedin the boundary layer atmosphere at Dunhuang (18 October 2001) were analyzed with an electron microscope equipped with EDX. Thosesingle particle analysis suggested that most of the particles with supermicron size were soil particles, and those particles had littlesulfate on its surface. This is a very important different point,comparing with the chemical state of soil particles, which weretransported from the desert area of China to Japan, and showed frequentlythe existence of sulfate on the particle surface. Therefore, it isstrongly suggested that dust particles can be chemically modifiedduring their long-range transport from desert areas to Japan.

Comparison of the Chemical Composition of Mineral Particles Collected in Dunhuang, China and those Collected in the Free Troposphere over Japan: Possible Chemical Modification during Long-Range Transport

The Asian continent is recognized as one of the most important sources of mineral (or soil) particles. These particles have a large potential to effect global changes through the biogeochemical cycle of particulates and through radiative balance (IPCC Third Assessment Report, 2001). Therefore, comparison of particle compositions near the source region and those after long-range transport is important in understanding the long-range particle transport phenomenon. Individual aerosolparticles were collected in Dunhuang (40°09′N; 94°41′E), China. Particles were collected at the campus of the Meteorological Bureau of Dunhuang City (17 August 2001) and near the Mogao Grots, located approximately 30 km from Dunhuang (18 August 2001, 18 October 2001 and 13 January 2002) using a two-stage low-volume impactor. The morphology of individual aerosol particles and their elemental compositions were examined via a scanning electron microscope (Hitachi, S-3000N) equipped with an energy dispersive X-ray (EDX) analyzer (Horiba, EMAX-500). The particles collected at these locations were comprised primarily of minerals, with the exception of sulphateparticles in the submicron range that were contained in thesample collected on 18 October 2001 (likely, ammonium sulphate). The most abundant elements were found to be Si and Al. Approximately 46–77% of the collected particles were Si-richparticles (composed primarily of quartz and aluminosilicate),and 13–41% of the collected particles were Ca-rich particles,such as calcite (CaCO3), dolomite (CaMg(CO3)2),and gypsum (CaSO4ċ2H2O). The fractions of Fe-rich, Mg-rich, Ti-rich, K-rich, and Cl-rich were 3–10, 0–7, 0–3, 0–1, and 0–1%, respectively. Similar types of mineral particles were found in the free troposphere over Japan(Trochkine et al., 2002). A number of differences were found to exist between the particles collected in China and thosecollected in Japan, and these differences can be explained bychemical modification of the particles during transport fromChina to Japan.

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°00′N, 94°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 6km 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 6km (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 μm) below about 5 km over Dunhuang, China.

Sea salt shifts the range sizes of Asian dust

A noticeable phenomenon of Asian dust particles during their trans-Pacific transport is that in cases where there is no precipitation, the mode diameters at different surface sites are approximately the same, about 3–4 microns for number distributions (particle number via particle size in per cubic unit of air) or a little larger for mass and volume distributions. This can be confirmed by comparing published data obtained in coastal areas of China, the Korean peninsula, Japanese islands, and the west coast of the United States. While significant progress in the study of mineral dust particles in the atmosphere has been made, this phenomenon is not yet understood. Recent studies [e.g.,Zhang and Iwasaka, 2004] suggest that the interaction between dust particles and sea salt is a significant process involved in the changes of dust particles in size and composition in the marine boundary layer. This process can lead to the growth of dust particles and is likely responsible for the consistency of the mode diameters at different sites.

Roles of regional transport and heterogeneous reactions in the PM 2.5 increase during winter haze episodes in Beijing

Regional transport and chemical conversions are two major processes that lead to the severe haze pollution in China. Our observations during five haze episodes in Beijing between February 19 and March 12 of 2014 show that the two processes played different roles as PM2.5 increased from the clean (<75μgm-3) to the light-medium pollution level (75-150μg m-3) and to levels of heavy (150-250μgm-3) and severe (>250μgm-3) pollution. In the initial twelve hours of each episode, the PM2.5 reached the light-medium level with an increase of approximately 120μgm-3. At the same time, the particle (~10-700nm) number concentration also showed a distinct increase accompanied by a rapid increase in the mean diameter. A light-medium PM2.5 occurred in the south areas prior to the haze occurrence in Beijing and the southerly winds were predominant, indicating the rapid increase of PM2.5 in the initial stage was caused by the regional transport from the south. Subsequently, PM2.5 elevated to the heavy and severe levels when the wind was weak, relative humidity was high and ozone concentration was low. The increase of PM2.5 in the elevated stages was characterized by a high percentage (45% for the heavy level and 55% for the severe level) of secondary inorganic components, indicating the substantial contribution of the formation of secondary aerosols. In addition, the increases of the mean diameter (from 108nm to 120nm) and the total volume concentration (by 67%) are regarded as a consequence of heterogeneous reactions on the surfaces of aerosol particles because the particle number concentration remained nearly constant in these two stages. Our results indicate that, during the five winter haze episodes, the regional transport from the south was the major reason for the initial-stage PM2.5 increase, while heterogeneous reactions dominated the later elevation.

Effect of sea salt on dust settling to the ocean

Dust particles frequently become mixtures of mineral dust and sea salt during their transport in the marine boundary layer, consequently growing in size, which causes changes in their settling velocities. In this study, the effect of sea salt on the gravitational settling of dust particles is investigated. Results show that the adhering of sea salt to dust particles can dramatically increase the gravitational settling of the particles, in particular if the particles become larger than 3-4 μm. Estimates with the observational data from six dust events in southwestern Japan revealed that, due to sea salt adhering, the gravitational settling flux of mineral dust increased approximately 14-17% in well-mixed events and 4-6% in less-mixed events, indicating a potential significant effect of sea salt on dust settling and the importance of considering this effect in the schemata of particle gravitational settling when mapping dust flux to the ocean.