A survey of atmospheric aerosols in a suburban area near Tokyo, Japanwas conducted using an Andersen sampler. Significant amounts of Na +and Cl - collected were considered to be derived from sea salt. The difference between the Na +/Cl -ratio in the area and that in sea salt indicated a considerable loss of Cl -. This is assumed to be caused by the formation of NaNO 3, which is one of the main nitrate species present. Most of the sulfate in the sample was found to be (NH 4) 2SO 4 in the form of fine particles, which is different from the sulfate derived from sea salt and soil. The size distributions of K and Mg are also discussed in relation to particular sources.
Air pollutants emitted into the atmosphere from various sources can be classified as either gaseous or particulate matter (PM). These types of pollutants are closely related to each other through gas-to-particle conversion, called condensation, and particle-to-gas conversion, called evaporation [
From the viewpoint of aerosol science, a particle is described as primary when it is emitted directly from a source into the atmosphere. On the other hand, a particle that is initially emitted as gaseous matter and then converted to PM through ambient chemical and physical processes is described as a secondary particle [
Understanding the behaviors of PM and the complex influences it has on human beings, animals, and plants requires analysis of size distribution, mass concentration, and chemical composition [4,5]. Size distribution and chemical composition analysis detail basic characteristics of atmospheric PM. These characteristics are closely related to the effects PM has on human health [6-8].
This study investigates the size distribution of anion species (, , and Cl-) and cation species (, Ca2+, Mg2+, Na+, and K+) of PM in the atmosphere. The sources of the anion and cation species with respect to their contribution to ambient aerosol are discussed.
Aerosols in ambient air were collected at the Center for Environmental Science at Saitama (CESS), Kazo, Saitama Prefecture, Japan. CESS is located in a suburban area surrounded by paddy fields, vegetable gardens, and roads, as illustrated by
Ambient aerosols were collected continuously for one
week from October 23 to 30, 2009 using a 47 mm quartz filter and an eight-stage Andersen impactor (Model AN- 200) with a backup filter at an air flow rate of 28.3 L min−1. This system was used to obtain information on size-segregated particles and to classify particles into nine size ranges (0.08 to 30 μm). The Andersen sampler can selectively trap different sized particles according to their momentum.
After ambient air was collected, the backup filter and the quartz filters from the eight stages were removed from the Andersen sampler and soaked in a controlled environment at 35˚C and 50% relative humidity for 24 h. After soaking, the PM mass concentrations of each stage were determined gravimetrically using an electronic microbalance in a temperature and humidity controlled room. An experiment was conducted to evaluate ion composition. The quartz filter from each stage was cut into a piece of a quarter (9 pieces in total). Cations and anions were extracted from the pieces of quartz filter using ultrapure water in an ultrasonic bath for 20 min. The cations (, Na+, K+, Ca2+, and Mg2+) and anions (, , and Cl−) were analyzed by ion chromatography (DionexIC-20) using the following materials and conditions. Anion chromatography was performed with an AS12A column using 2.7 mM Na2CO3 and 0.3 mM NaHCO3 as eluents at 35˚C, a flow rate of 1.2 mL/min, and an injection volume of 25 µL. Cation chromatography was performed with a CS12A column using 20 mM methane sulfuric acid as the eluent at 35˚C, a flow rate of 1.0 mL/min, and an injection volume of 50 µL. The detection limits in millimolar were 0.003 for Na+, 0.004 for, 0.004 for K+, 0.004 for Mg2+, 0.015 for Ca2+, 0.021 for Cl−, 0.028 for, and 0.019 for. The calibration curves of the anions and cations were determined using standard solutions. The blank filters were also extracted and analyzed for operation blanks.
The size distribution of PM relative to mass is shown in
Typically, aerosols in the coarse particle range (>2μm) originate from natural sources, while those in the fine
particle range (<2 μm) are derived from anthropogenic sources [1-5]. The size distribution shown in
Moreover, a small peak of Cl− was observed in the fine particle range. This peak is assumed to be derived from HCl emitted from the incineration of garbage that included polychlorinated hydrocarbons. Prior to the implementation and enforcement of strict controls, HCl emissions in Japan were significant. This study confirms that HCl remains at a noticeable level. The counter ion of Cl− is considered to be after the neutralization of HCl with ammonia gas in ambient air.
Nitrate can be formed as a secondary pollutant through photochemical reactions in the atmosphere [13-16]. Nitric acid may react with ammonia gas to form fine particles and may also react with NaCl to form coarse particles. In this study, nitrate is predominantly classified in the coarse particle range. Therefore, it can be concluded that NaNO3 was the major species in the sample, and NH4NO3 was present in lower amounts. Moreover, NH4NO3 and NH4Cl are known to be volatile under ambient temperature [10,14].
The dominance of NaNO3 could be caused by the evaporation of NH4NO3 due to the moderate temperature (20˚C - 25˚C during the day) at the time of sample collection.
a strong peak in the fine particle range accompanied by a slight shoulder in the coarse particle range. Sulfate is commonly formed by a slight shoulder in the coarse particle range. Sulfate is commonly formed by the gas-toparticle conversion of SO2 through photochemical reactions [17,18]. Thus, the main peak falls in the fine particle range. The slight shoulder in the coarse particle range could be explained if some sulfate is derived from sea salt and soil [
The monomodal size distribution of shown in
Mg2+ and K+ distributions are shown in
Some K+ may be produced by the incineration of plant derivatives, such as paper, wood, and vegetable garbage, which would cause fine particle aerosols. Some studies have shown that K+ in aerosols can result from biomass burning [22,23]. Potassium can also originate from sea salt and soil and would fall into the coarse particle range. These two sources result in two size distribution peaks for atmospheric potassium, as shown in
rich trend was observed; however, this is considered an acceptable coincidence between ion balances in both particle ranges. In the coarse particle range, the contribution of unevaluated carbonate ions may be a factor in the cation rich calculation. Ionic balances would be improved by evaluating organic hydrocarbon acids, which were not analyzed in the fine particle range.
The orders of the ionic compounds were > > in the fine particle range and > > Na+ in the coarse particle range. Sulfate was the predominant ion and occupied approximately 45% of the total mass concentration of the eight ionic compounds, followed by nitrate (22%) and ammonium (19%).
Overall, the results of this study are acceptable in comparison with those of previous research [18-23].
Size distributions of atmospheric aerosol components in a suburban area of Tokyo, Japan were investigated using an Andersen sampler.
Significant amounts of Na+ and Cl− were considered to be derived from sea salt. The difference between the Na+/Cl− ratio in the coarse particles and that in sea salt indicated a considerable loss of Cl−. We concluded that this was caused by the formation of NaNO3, which is the main nitrate species present. Moreover, NH4NO3 was estimated to be in the fine particle range.
The size distribution of had a sharp monomodal peak in the fine particle range, which is similar to that of sulfate. Most of the sulfate is considered to be ammonium sulfate or ammonium bisulfate. The size distribution of Ca was found to have a strong peak in the coarse particle range. Ca is considered to be a main counter ion of in the coarse particle range.
Mg is considered to be derived primarily from sea salt and soil and has a peak in the coarse particle range. Some K will originate from sea salt and will also be produced by the incineration of plant derivatives. These two sources most likely caused the two peaks in the size distribution of K.
We greatly appreciate the research members of the Center for Environmental Science at Saitama for their enthusiastic supporting our research activities with respect to the collection, the analysis and the interpretation of ambient aerosols in Saitama, Japan.
This report is an outcome of the program supported by 2005 World Exposition, Aichi, Japan and implemented by International Center for Environmental Technology Transfer (ICETT). We are also thankful to their foundations.