Aerosol Size Distribution and Gaseous Species in April at Barrow
Y. Zaizen, K. Okada, M. Ikegami, T. Aoki, and Y. Sawa
Meteorological Research Institute, 1-1 Nagamine, Tsukuba, 305-0052 Japan
Hokkaido University of Education, 1-15-55 Shiroyama, Kushiro 085-0826 Japan
Research Institute of Civilization, Tokai University, 1117 Kitakaname, Hiratsuka 259-1207 Japan
Atmospheric aerosols are one of the major components for climate through direct radiative forcing and changing radiative characteristics of clouds by acting as cloud condensation nuclei (CCN). The most effective particles for these processes are accumulation mode particles that originate from the growth of nuclei mode particles formed by homogeneous nucleation. The growth process is considered to be coagulation, heterogeneous condensation, and interaction with clouds.
In arctic regions, tropospheric aerosols are studied in relation to the arctic haze phenomena. Aerosol particles in the cases of arctic haze are mostly in the accumulation mode [Pacyna et al., 1984; Shaw, 1985; Lewis, 1985]. A great majority of the arctic aerosols are in or near the accumulation mode [Heintzenberg, 1980; Shaw, 1984]. The aim of this study is to obtain the characteristics of aerosol size distributions, especially those in the smaller size range, in Barrow during April 1997 together with the concentrations of gaseous species concentrations.
Observation Site and Instruments
This observation was conducted from April 13 to 27, 1997, at the NOAA CMDL Barrow Observatory, Alaska (BRW). Aerosol size distribution in the smaller size ranges (0.004 £ r £ 0.13 mm) was measured with a differential mobility analyzer (DMA), TSI, model 3071, and a condensation nucleus counter (CNC), TSI, model 3025. The size distribution in the larger size ranges (0.15 £ r < 5 mm) was measured with an optical particle counter (OPC) (Dan Industry Co. Ltd., model PM-730-NS15P). Concentrations of gaseous species (SO2, NO, and NO2) were measured with an SO2 analyzer (Thermo Electron, model 43S) and a NOx analyzer (Thermo Electron, model 42S). The principal of these two instruments is chemiluminescence. Individual aerosol particles of 0.05-0.5 mm radius were collected on a carbon-covered nitrocellulose (collodion) film with a cascade impactor and examined with an electron microscope.
According to meteorological data and back-trajectory analyses, a polar airmass covered northern Alaska in the former part of the observational period (April 14-19, 1997) and an airmass from lower latitudes was dominant in the latter part (April 20-28, 1997).
Figure 1 shows the averaged number-size distributions during three periods. In each distribution a maximum is found in the radius range of 0.1 0.2 mm. The existence of this peak was reported by former investigators in Arctic areas [e.g., Covert, 1993]. This maximum possibly exists constantly over the arctic air in spring and is supposed to consist of well-aged particles. On the other hand, the concentrations of particles smaller than 0.005 mm radius are elevated in each size distribution, suggesting the occurrence of nucleation from gas phase material. In period A (April 13-18), the concentrations in the small size ranges (less than 0.1 mm radius) are relatively low, suggesting that new particle formation is passive in the polar airmass. Besides, higher concentrations of smaller particles in period C (April 21-25) might suggest relatively active nucleation in airmass from lower latitudes. The concentrations in the larger size range (r ³ 0.15 mm) are higher in period A than period C. The higher concentration of large particles in period A might be due to a long residence time of aerosols in association with small exchange rates of airmass and small amounts of precipitation in polar regions. This high concentration of larger particles intends to restrict new particle formation by reducing the vapor pressure of sulfuric acid by heterogeneous condensation on the surface of pre-existing particles. The size distribution in period B is different from others. This shape of the size distribution, with a maximum around 0.02 mm, is typical in background air of middle or low latitude [Zaizen et. al., 1996].
Fig. 1. Averaged number-size distributions of aerosols during period A (April 13-18), period B (April 19) and period C (April 21-25).
The concentrations of precursor gases (SO2, NO, and NO2) were less than 50 pptv except for locally polluted air. The SO2 concentration showed a diurnal variation with the maximum in the daytime during the latter part of the observational period. Since this type of diurnal variation is contradicted with the variation of mixing layer thickness, the variation may imply the dimethylsulfide (DMS) emission from the cracks of sea ice.
According to the morphological features of individual particles collected on April 21 and 22, most of the particles are estimated to consist of sulfuric acid or ammonium sulfate. Therefore, sulfate is considered to be one of the dominant components of accumulation mode particles in arctic regions in this season.
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