To elucidate the differences in the dry deposition process between sulfate and nitrate in PM_2.5, vertical profiles of the inorganic components were measured in a forest in suburban Tokyo (FM Tama site) for one year from December, 2012 to November, 2013. We carried out weekly samplings of PM_2.5 at 4 heights (2 heights above canopy, 2 heights below canopy) on a tower built in the forest. The major inorganic aerosols consisted of NH₄NO₃ and (NH₄)₂SO₄ at the 4 heights through out the year. Significant differences between the SO₄^2- and NO₃^- profiles were observed. NO₃^- relatively decreased compared to SO₄^2- toward the forest floor through out the year. In the winter and spring, the air temperature of the forest floor was higher than that of the other heights. On the other hand, in the summer and autumn, the air temperature of the forest floor was lower than that of the other heights. The profiles of the concentrations and the air temperatures indicated that volatilization of NH₄NO₃ by the higher temperature on the floor caused the rapid decrease of NO₃^- in the winter and spring, and the volatilization of NH₄NO₃ by the low concentration of HNO₃ due to its significant removal by the leaves caused the rapid decrease of NO₃^- in the summer and autumn. These volatilization processes indicated that the dry deposition of ammonium nitrate to forests was more efficient than that of ammonium sulfate.

View full abstract

The chemical components of daily sampled PM_2.5 were monitored in Nagano city in the summers of 2012 and 2013. We conducted multiple linear regression analyses with sulfate as the dependent variable and with an oil combustion tracer element (V or Ni) and coal combustion tracer element (As, Pb, or Cd) as explanatory variables taking into account the existence of an outlier. As a result of the regression analyses using V as the oil combustion tracer element, the regression equations explained 90% of the sulfate variation. However, the sulfate concentrations from 26 to 29 July, 2012 were unusually high, and were excluded as outliers. The intercept of the equations were nearly equal to zero, and most of the observed sulfates was considered to have originated from the oil and coal combustion. The average ratio of sulfate from the oil combustion to that from the coal combustion was estimated to be 54:46. For the backward trajectories from the area around Mainland China, the East China Sea, and Japan, the sulfate concentrations from oil combustion were estimated to be 2.7, 2.5, and 2.3 μg/m³, respectively, and those from coal combustion were estimated to be 3.8, 2.1, and 1.3 μg/m³, respectively. Therefore, as for the sulfate concentrations from coal combustion, they were higher for the trajectories nearer to Mainland China, and the contributions from the area around Mainland China were suggested to be high. The influence of volcanoes in Kyushu, such as Sakurajima, was considered the cause of the high sulfate concentrations from 26 to 29 July, 2012. The volcanic sulfates were estimated to be 5 to 6 tenths of the observed sulfates during the period.

View full abstract

Particle size classifiers must be used for analyzing PM_2.5. Particle rebound may occur when using a traditional impactor or cyclones. The use of virtual impactors would prevent particle rebounds. However, typical virtual impactors have complicated structures, and are operated at a relatively low flow rate in many cases. In this study, a simple single-nozzle middle-volume PM_2.5 virtual impactor was developed, and its performance evaluation was carried out. The classification characteristics of the VI developed in this study were similar to those of WINS, which was a USEPA-approved PM_2.5 inlet, but the 50% cut-point of the VI was lower than that of WINS. Changes in the minor flow rate and the distance between the inlet and collection nozzles did not significantly affect the separation characteristics of the VI. On the contrary, a wider inlet nozzle diameter produced higher 50% cut-points of the VI. The mass and chemical compositions of PM_2.5 collected using the VI were compared to those collected using WINS. The concentrations of several elements in the particles collected using the VI were different from those collected using WINS due to the difference in the classification characteristics of the VI and WINS. However, the PM_2.5 mass obtained using these two devices were similar to each other.

View full abstract

A method of acid digestion using microwaves and measurement by inductively coupled plasma-mass spectrometry (ICP-MS) was modified for the simultaneous determination of 30 elements including silicon in PM_2.5. In this method, hydrofluoric acid was not removed in order to prevent volatilization losses of silicon in the digested solution. An instrumental measurement was then carried out by hydrofluoric acid resisting ICP-MS. The recoveries of 30 elements ranged from 96.0–110%. The results determined by the modified method for Na, Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Rb, Mo, Sb, Ba, La, Ce, Th and Pb agreed well with the certified values of NIES CRM No. 28 and NIST SRM2783; their values of Measured/Certified of NIES CRM No. 28 and NIST SRM2783 were 0.92–1.10 for 24 elements and 0.80–1.21 for 20 elements, respectively. The result showed that this method can be used for the rapid digestion of 30 elements in atmospheric particle matter which would be quite useful for monitoring PM_2.5.

View full abstract