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Factors influencing the seasonal variation in particulate nitrate at Cheju Island, South Korea
PII:
Atmospheric Environment Vol. 32, No. 8, pp. 1427—1434, 1998 ( 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S1352–2318(97)00323–3 1352—2310/98 $19.00#0.00
FACTORS INFLUENCING THE SEASONAL VARIATION IN PARTICULATE NITRATE AT CHEJU ISLAND, SOUTH KOREA HIROSHI HAYAMI* and GREGORY R. CARMICHAEL Center for Global and Regional Environmental Research, IATL, The University of Iowa, Iowa City, IA 52242-1000, U.S.A. (First recieved 24 October 1996 and in final form 7 July 1997. Published April 1998) Abstract—The seasonal variation in measured particulate nitrate at Cheju island, South Korea, is analyzed and shown to exhibit high concentrations from February to June and in October, and low values from July to September. Total nitrate concentrations, which are not monitored, are estimated with a gas—aerosol equilibrium model with two-size bins. The total nitrate concentrations are shown to be maximum in June and minimum in August, and these differences are associated with air—mass trajectories. The fraction of particulate to total nitrate is estimated and shown to be sensitive to the total nitrate concentrations and the coarse-mode composition that is dominated by non-volatile species. The particulate nitrate fractions are found to vary within a relatively narrow range around the annual-mean value, indicating that the particulate nitrate concentrations behave similarly to the total nitrate concentrations. Exceptions are found for April, June and July. For April the largest particulate nitrate fraction is found in association with very high concentrations of nss-calcium, an indicator of mineral aerosol. The smallest particulate nitrate fractions occur in June and July and are the result of a combination of low concentrations of nss-calcium, sea salt and total nitrate, and high concentration of nss-sulfate. ( 1998 Elsevier Science Ltd. All rights reserved. Keyword index: Partitioning of nitrate, gas—aerosol equilibrium, coarse particles, East Asia.
INTRODUCTION
Cheju Island, South Korea (Fig. 1) is regarded as an ideal site for air quality monitoring in East Asia. The island has no large industrial emissions but is surrounded by large source areas in China, Korea and Japan, located only a few hundred kilometers away. Daily aerosol compositions have been measured with a tape sampler since March 1992 in cooperation among Korea Institute of Science and Technology and Ajou University, Korea; Kyusyu University and National Institute for Environmental Studies, Japan; and the University of Iowa, U.S.A. The sampling method and chemical analysis are described in detail in Carmichael et al. (1997). Chen et al. (1997) discuss the typical aerosol composition and the major features of the seasonal cycles in aerosol components collected at Cheju (Fig. 2). They found that sea-salt components represented by sodium showed peak values in the winter corresponding to the period of
* Author to whom correspondence should be addressed. Present address: Central Research Institute of Electric Power Industry, 2-11-1 Iowado-kita, Komae, Tokyo 201, Japan.
maximum wind speed. Calcium and nitrate, and to a lesser extent, sulfate and ammonium, showed high concentrations in the spring. All species showed a pronounced minimum value in the summer. Understanding the behavior of nitrate at Cheju is important since nitrate represents a significant fraction of the total oxides of nitrogen (NO ) in the y troposphere, and particulate nitrate contributes to a variety of environmental problems in the region including reduced visibility and acid deposition. Particulate nitrate at Cheju shows a seasonal variation with high concentrations from February to June and in October, and low values in July to September, as shown in Fig. 2. Chen et al. (1997) showed that particulate nitrate concentration has relatively high correlation with nss-calcium concentration, but little correlation with ammonium concentration. Model analysis by Hayami and Carmichael (1997) suggested that ammonium at Cheju mostly coexists with nsssulfate in the fine- (or accumulation-) mode aerosol, and that particulate nitrate is associated with the nonvolatile cation species (sodium, potassium, calcium, and magnesium) in the coarse-mode aerosol. Factors other than the ammonium nitrate relationship,
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NH NO (aerosol) ¢ NH (gas)#HNO (gas) 4 3 3 3
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H. HAYAMI and G. R. CARMICHAEL
nitrate. Unfortunately, there are no continuous measurements of total or gaseous nitrate available at Cheju. In this paper, we perform a modeling analysis to estimate the total nitrate and the particulate nitrate fraction at Cheju. The model is then used to study the seasonal variation of nitrates and to examine which factors control the particulate nitrate fraction at Cheju.
ESTIMATION OF PARTICULATE NITRATE FRACTION AND TOTAL NITRATE CONCENTRATION
Fig. 1. Map around Cheju Island.
Fig. 2. Monthly-mean concentrations of particulate nitrate, sodium, nss-calcium and nss-sulfate of aerosol measured at Cheju.
appear to be important for the seasonal variation in particulate nitrate at Cheju. Nitrate is formed via both heterogeneous and homogeneous chemical reactions in the atmosphere. Because of its volatility, it is distributed between the gas and aerosol phases. Nitrate heterogeneously formed on the aerosol surface may evaporate into the gas phase, while nitrate homogeneously formed in the gas phase may condense onto particles. The distribution of nitrate between the gas and particle phases depends on various parameters including gaseous and particulate chemical composition, and temperature and relative humidity. Therefore, the variation in particulate nitrate concentration needs to be discussed along with total (gaseous#particulate)
Hayami and Carmichael (1997) applied a gas—aerosol equilibrium model with two-size bins (fine and coarse modes) to the analysis of the mean composition of the aerosol at Cheju. The analysis assumed that particles in each mode are internally mixed, and that particulate volatile species (ammonium, nitrate and chloride) in each mode are in equilibrium with their respective gaseous components. In this paper, we use this approach to analyze the behavior of particulate nitrate. The partitioning of the volatile species between the gas and aerosol phases is calculated using SCAPE (Kim et al., 1993a, b; Kim and Seinfeld, 1995). The SCAPE model gives the partitioning and condition of sodium, potassium, calcium, magnesium, ammonium, sulfate, nitrate and chloride from their total concentrations, temperature and relative humidity. In this analysis the total concentrations for the non-volatile species are specified using the observed values (see Carmichael et al. (1997) and Chen et al. (1997) for details on the measurements and analysis). Ammonium is also volatile, but measurements of particulate ammonium are regarded as total ammonium concentrations based on our previous study where we found total ammonium to be mostly distributed in the aerosol phase at Cheju (Hayami and Carmichael, 1997). Total chloride concentrations are assumed to be supplied from sea salt, and are estimated from the sodium measurements. Particulate nitrate is assumed to coexist with particulate chloride in the same aqueous phase of the aerosol. With these assumptions the equilibria can be written as K c2 [NO~] P 3 # HNO3" HC- HNO3,# P K c2 [Cl~] HCHNO3 HC-,# # K c2 [NO~] 3 & " HC- HNO3,& K c2 [Cl~] HNO3 HC-,& &
(1)
where P, K and c indicate partial pressure, equilibrium coefficient and activity coefficient, and subscripts f and c indicate values of the fine and coarse modes. While the equilibrium coefficients are functions of temperature, the activity coefficients vary with both temperature and composition. Preliminary calculations for monthly mean aerosol compositions at Cheju were implemented to see the
Seasonal variation of particulate nitrate
behavior of c2 /c2 , using the gas—aerosol equilibHNO3 HCrium model using the Pitzer method for the activity coefficient calculations. The calculated mean ratio was 0.65 with a standard deviation of 0.087 for the coarse aerosols where the mean molalities of HNO 3 and HCl were 1.09 and 2.83, respectively. This value is very close to the mean ratio, 0.63 with a standard deviation of 0.089, for the fine aerosols where the molalities of HNO and HCl were 0.01 and 0.04, 3 eventhough the compositions are quite different between the modes. Since most of the particulate nitrate is distributed in the coarse aerosols, we fixed the ratio as a constant at 0.65. Equation (1) can be rearranged to provide estimates of total nitrate concentration from measurements of sodium, chloride and nitrate in the aerosols, [Total nitrate]"
A
1#0.65
B
K 1.79[Na`]![Cl~] HC[NO~] (2) 3 K [Cl~] HNO3
where concentrations are in kg m~3. The gas—aerosol equilibrium model with SCAPE and the above assumptions were evaluated for the monthly-mean aerosol compositions at Cheju. Estimates of particulate concentrations of the volatile species are presented in comparison with their measurements in Fig. 3. The model provides a good representation of the measurements with a tendency of !3% underestimation for ammonium, #12% overestimation for nitrate, and !1% underestimation for chloride. Furthermore, the correlations are very high, more than 0.90. The largest four overestimates of particulate nitrate are found for March, April, October and November. The former two months have high concentrations of nss-calcium associated with dust aerosol. The partitioning ratio of fine to total nss-sulfate, which was fixed to 0.8 in the
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calculations, probably varies with season. According to the modeling study of Dentener et al. (1996) the value is smaller during high dust periods because the dust aerosol offers a reactive surface for heterogeneous production of sulfate. As discussed in more detail later, the fraction of particulate to total nitrate decreases with the partitioning ratio for nss-sulfate. The latter two months are the only two cases where the relative humidities of the months are lower than the deliquescence point of NaNO (0.7379 at 3 298.15 K, Kim et al., 1993b) and where some of the particulate nitrate in the coarse mode is present in the form of solid NaNO . The existence of solid nitrate is 3 not assumed in equation (2). For particulate chloride, no solid was predicted, and the concentrations predicted by equation (2) were closer to the measurements than those for the particulate nitrate concentrations. Including the treatment of solid nitrate would improve the estimates of total nitrate. In the remainder of the paper, equation (2) is used to estimate the total nitrate concentrations. The model estimates might be improved by changing the assumptions and parameter values such as the ratio of the activity coefficients or the nss-sulfate partitioning ratio between the size modes. However, rather than performing such a tuning, we instead present a series of sensitivity calculations.
VARIATION IN ESTIMATION TOTAL NITRATE CONCENTRATION AND PARTICULATE NITRATE FRACTION
Total nitrate concentrations and particulate nitrate fractions (calculated as the ratio of the estimated particulate nitrate and the estimated total nitrate) were estimated from the monthly-mean measurements of aerosol composition, temperature and relative humidity using equation (2). The results are shown in Fig. 4
Fig. 3. Calculated concentrations [kg m~3] of particulate ammonium, nitrate and chloride to the measurements. The lines indicate the relationship of 1 on 1. Some months with overestimates of particulate nitrate are indicated.
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H. HAYAMI and G. R. CARMICHAEL
Fig. 4. Annual-mean and monthly concentrations of total (gray bars) and particulate (black bars) nitrate and its fraction (triangles) estimated with the gas—aerosol equilibrium model.
Fig. 5. Backward trajectories at 850 hPa arriving at Cheju Island in (a) April, (b) June, and (c) August 1993. Each trajectory is drawn every 12 h with a time step of 3 h.
along with the estimates of particulate nitrate concentration. Most of the particulate nitrate fractions vary within $0.1 from the annual mean of 0.74. The exceptions are 0.85 in April, 0.45 in June and 0.22 in July. The particulate nitrate fractions show higher values compared to the annual mean from October to April and lower values from May to September. The higher fractions vary in a relatively narrow band between 0.75 and 0.85. Because of this stable variation in the particulate nitrate fraction, the particulate nitrate concentrations behave in a similar manner as the total nitrate concentrations during this period. From April to June, the particulate nitrate fraction decreases, while the total nitrate concentration increases up to the annual maximum of 3.15 kg m~3 in June. In July, the total nitrate concentration is about half of that in June, and the particulate nitrate fraction shows the annual minimum. In August and September, the total nitrate concentrations are at a minimum, but the particulate nitrate fractions are above 0.6. The particulate nitrate concentrations are nearly twice the minimum in July. Meteorological features play an important role in the seasonal cycle of total nitrate at Cheju. Fig. 5 shows isobaric backward trajectories at 850 hPa for
April, June and August in 1993. The trajectories show that the flows in June are from the west, bringing high concentrations of NO from source regions in eastern x China. Photochemical formation of nitrate becomes active under the summer condition with high temperature and sufficient solar radiation, leading to high concentration of total nitrate. Dominant flows in August are southerly, bringing clean, marine air masses with low concentrations of the precursors. Therefore, the total nitrate concentration is low in August. The flows in April are from northwestern China, a major source region of mineral aerosol. Mineral aerosol provide an efficient surface for the heterogeneous formation of nitrate (Dentener et al., 1996) and helps to retain nitrate in the aerosol phase. Typical winter flows are strong and northwesterly, represented by cold outbreaks. Sea-salt components at Cheju show high concentrations in winter as shown in Fig. 2, reflecting these strong winds over the sea and the high emissions of sea-salt aerosols. Nitrate also may deposit onto sea-salt particles. Carmichael et al. (1997) took another approach to estimate particulate nitrate fractions at Cheju. Aerosol composition has also been measured at Cheju on cellulose filters placed in a Hi-Vol sampler operated
Seasonal variation of particulate nitrate
by the University of Miami. These measurements are co-located next to the tape—filter sampler, and have been performed since 1992. Carmichael et al. (1997) estimated the fraction of particulate nitrate by regarding the nitrate on the UM filters as total nitrate and the nitrate on the tape sampler as particulate nitrate. The values ranged from 0.3 to 0.4 in the spring and winter, to &0.1—0.2 in the summer and early fall, with an annual average of 0.23. Dentener et al. (1996), who used a three-dimensional global chemical model with dust calculated the annual and FebruaryMarch-April average fractions around Cheju Island to be &0.2 and &0.4, respectively. Although these values are significantly smaller compared to the values estimated in this paper, a consistent seasonal tendency is found in all of these estimates with higher fractions in winter and spring than in summer. The only direct estimate based on measurements of both gas and aerosol nitrate at Cheju are from three daily samples in March 1994 (Kim et al., 1995). Based on these measurements the average particulate nitrate fraction is estimated to be 0.84, a value close to the estimates presented in this paper. Clearly, more extensive and long-term monitoring of gaseous and particulate nitrate is required to evaluate our approach and the contribution of nitrate to acidification of the atmosphere in this region.
FACTORS INFLUENCING THE PARTICULATE NITRATE FRACTION
To understand the seasonal variation in the particulate nitrate fraction, we investigated the sensitivity of particulate nitrate to various initial conditions using the gas—aerosol equilibrium model. The calculations were based on the annual-mean aerosol composition at Cheju (Table 1). Results for the sensitivity of the particulate nitrate fraction to nine sets of parameters: temperature; relative humidity; nss-sulfate concentration; total nitrate concentration; total ammonium concentration; ammonium and nitrate concentrations; ammonium, nitrate and nss-sulfate concentrations; nss-calcium concentration; sea-salt component concentrations; and partitioning ratio of fine-mode to total (fine- and coarse-mode) nss-sulfate are discussed. The partitioning ratios for nss-sulfate and non-volatile cations were fixed to the base-case values (0.8 and 0.2, respectively), except when these were subject to the sensitivity study. Sea salt was defined to have mass ratios relative to sodium of 0.04 for potassium, 0.04 for calcium, 0.12 for magnesium, 1.79 for chloride and
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0.25 for sulfate. Simulations were performed for conditions where the base values were multiplied by factors of 0.5, 2 and 4. The results are summarized in Fig. 6. One important result is that the nss-sulfate concentration greatly changes the nitrate fraction. As the nss-sulfate concentration is simultaneously increased in both the fine and coarse modes (the 0.5] to 4] nssSO cases in 4 the figure), the nitrate fraction changes from nearly 0.88 to 0. As the fraction of nss-sulfate in the coarse mode is decreased (or, p(nssSO ) is increased in the 4 figure), the particulate nitrate fraction increases. These results show that the amount of nitrate in the particle phase is very sensitive to the coarse-mode nss-sulfate concentration. A general tendency is found from these results that the particulate nitrate fraction increases with increasing concentration of cations, e.g., the particulate nitrate fraction increases with nss-calcium. However, the partitioning ratio for nss-sulfate may be reduced at high nss-calcium concentration since coarse-mode nss-sulfate may be produced on the reactive surface of coarse-mode particles (Dentener et al., 1996). Therefore, the increase in particulate nitrate fraction with nss-calcium can somewhat be canceled by the opposite effect that the partitioning ratio for nss-sulfate has on the particulate nitrate fraction. According to Chen et al. (1997), the sea-salt components comprise approximately one-quarter of the mass of water soluble inorganic species in the aerosols at Cheju. The particulate nitrate fraction is found to be sensitive to the sea-salt concentration and to increase as the sea-salt mass increases. Sea salt is only slightly cation-rich (1.1 neq (kg Na)~1) but behaves like a cation (stronger than nss-calcium) in retaining nitrate in the aerosol phase. This is due to the volatile nature of sea salt chloride. Particulate nitrate competes with the sea-salt chloride to share non-volatile cations in the coarse mode. As the total nitrate concentration increases, the concentrations of both particulate and gaseous nitrate increase, but the particulate nitrate fraction decreases. Most of the increase in total nitrate stays in the gas phase. The fine-mode nitrate is still at a trace level even at the 4] total nitrate concentration, and the changes in the particulate nitrate fraction induced by the total nitrate concentration occur between the gas and coarse-aerosol phases. Ammonia plays an important role. The particulate nitrate fraction is insensitive to decreasing ammonia values, but is quite sensitive to increasing values. At higher ammonia values the particulate nitrate fraction
Table 1. The annual-mean composition measured at Cheju Component
Na`
K`
Ca2`
Mg2`
NH` 4
SO2~ 4
NO~ 3
Cl~
Concentration
1.74
0.43
0.46
0.26
1.26
7.30
1.14
1.75
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H. HAYAMI and G. R. CARMICHAEL
Fig. 6. Sensitivity of particulate nitrate fraction to various initial conditions. The base initial concentrations of nss-sulfate (shown as nssSO ), nss-calcium (nssCa), sea salt, nitrate (NO ), and ammonium (NH ), 4 3 4 which are shown in Table 1, were individually or simultaneously changed by multiplying factors of 0.5, 2 and 4. The partitioning ratio of fine-mode to total nss-sulfate was changed from the base value of 0.8 to 0.6, 0.7, 0.9 and 1.0. Temperature was changed from the annual mean value of 15.5°C to 5 and 25°C, and relative humidity from 78% to 60 and 90%.
increases as more nitrate can be retained in the fine mode. As ammonia and nitrate are increased together the fine-mode nitrate concentration increases, implying the formation of ammonium nitrate. However, even at 4] the initial concentrations of the two species the fine-nitrate fraction is only 0.11. The existence of little ammonium nitrate is also indicated by the fact that the particulate nitrate fraction is insensitive to temperature and relative humidity. These results suggest that the volatilization of nitrate captured on the Teflon filters does not appear to be a serious artificial error for the aerosol at Cheju. It is likely that sulfate, nitrate and ammonium loadings will double in the next decade or two in East Asia (Foel et al., 1995). As a result the aerosol composition at Cheju will also change. If anthropogenic emissions, or total concentrations, of ammonium, nitrate and nss-sulfate are doubled, the sensitivity results suggest that most of the nitrate would be found in the gas
phase. These results show that large changes in the aerosol composition in this region can be expected, and that continued measurements are needed to detect such changes. Such data are also needed to address acidification on ecosystem and human health in East Asia. To summarize, the results indicate that the particulate nitrate fraction at Cheju is sensitive to the coarse-aerosol composition. Since particulate nitrate is mostly distributed in the coarse mode, the amount of non-volatile cation species that are free from neutralizing sulfate in the coarse mode is a key parameter to retain nitrate in the aerosol phase. The total nitrate in excess to the non-volatile cation species remains in the gas phase, which results in reducing the nitrate fraction. An increase in the initial concentrations of nss-calcium, sea salt and ammonium raises the nitrate fraction. Sea salt plays a role like a cation species by releasing some sea-salt chloride into the gas
Seasonal variation of particulate nitrate
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Fig. 7. Nitrate fractions of the annual-mean composition with concentrations of various species of specific months.
phase. Sea-salt chloride in the aerosol phase shares the non-volatile cation species in the coarse mode with nitrate. It is previously shown in Fig. 4 that the particulate nitrate fractions vary within $0.1 from the annual mean of 0.74, except for April, June and July. Since the free non-volatile cations are important for the nitrate fraction, the stability of the nitrate fraction appears to be maintained by high concentrations of nss-calcium in the spring and sea salt in the winter, and low concentrations of nss-sulfate in the fall (see Fig. 2). For the months where the particulate nitrate fraction is very high or low additional simulations were performed with the gas—aerosol equilibrium model. In these simulations the particulate nitrate fraction was calculated using combinations of the annual-mean and monthly average composition of sea salt, nsscalcium, nss-sulfate and total nitrate in each of the months. For example, to see the effect of sea salt on the April particulate nitrate fraction, the model was re-run using the sea-salt concentration in April, keeping the annual-mean composition for the other species. The results are shown in Fig. 7. For April the largest effect is found by changing the initial concentration of nss-calcium. Using the April nss-calcium concentration results in a particulate nitrate fraction of 0.92, which is very close to the monthly-mean composition in April. Varying the other species concentrations does not cause large changes, but tends to reduce the particulate nitrate fraction. The high nsscalcium concentration in April appears to cause the annual maximum value of the particulate nitrate fraction. Note that nss-calcium considered here is in the form of CaCO (calcite) or CaO. According to the 3 trajectories in April shown in Fig. 5, the nss-calcium in April is associated with dust-aerosol represented by
yellow sand. Yellow sand contains large amounts of CaCO , and calcium in natural soils is in the form of 3 CaO (Tsuruta, 1991). If the nss-calcium is supplied as CaSO , the nitrate fraction becomes smaller because 4 the nss-calcium is equivalently neutralized by the sulfate from CaSO . 4 In June the total nitrate concentration is estimated to be remarkably high. When the annual-mean aerosol composition is used, a particulate nitrate fraction of 0.62 is predicted. When the species concentrations in June are individually varied the particulate nitrate fraction decreases, but a large distance still remains from the estimated nitrate fraciton in June. However, when the concentrations of sea salt, nss-calcium and total nitrate in June are used together, the annualmean particulate nitrate fraction becomes 0.51, very close to the nitrate fraction in June. The same thing is found for July, where using the July concentrations of sea salt, nss-calcium and nss-sulfate, the predicted particulate nitrate fraction is reduced to 0.37. Therefore, the lowest nitrate fractions found in June and July appear to be controlled by the coarse-aerosol composition and the combination of sea salt, nsscalcium, nss-sulfate, and total nitrate.
SUMMARY
Nitrate is a key compound in the study of tropospheric chemistry in East Asia. It is the major component of the oxidation of NO arising from x anthropogenic emissions, which are rapidly growing in East Asia. In addition, it is an important acidic component of the aerosol and precipitation which can contribute to such adverse environmental problems as reduced visibility and acid deposition.
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H. HAYAMI and G. R. CARMICHAEL
In this paper we analyzed the seasonal variation of particulate nitrate observed at Cheju island, South Korea, which exhibits high concentrations from February to June and in October, and low values from July to September. Since the total nitrate concentrations are not monitored, a gas—aerosol equilibrium model with two-size bins was used to estimate these concentrations. The estimated total nitrate concentrations showed maximum in June and the minimum in August. These variations are associated with changes in air—mass trajectories. Air flows pass over East Asian continent in April and June, but come from the south bringing clean, marine air in August. A sensitivity study was performed to investigate which factors influence the fraction of particulate nitrate. The particulate nitrate fractions were found to be sensitive to total nitrate concentrations and the coarse-mode composition that consists of non-volatile species. The particulate nitrate fraction was found to vary within a relatively narrow range around the annual-mean value of 0.736, which means that the particulate nitrate concentrations behave similarly to the variation in the total nitrate concentrations. Exceptions were found for April, June and July. For April the largest particulate nitrate fraction is the result of very high concentrations of nss-calcium induced associated with dust aerosol. The lowest fractions of June and July are due to combinations of low concentration of nss-calcium, sea salt and total nitrate, and high concentrations of nss-sulfate. Finally, understanding the chemical composition of the various size fractions is important in the assessment of the how these aerosols effect regional climate, visibility and acidic deposition. There is a clear need for continued measurements at Cheju, augmented with measurements of gaseous components and sizeresolved aerosol composition. This is especially true in light of the fact that sulfur, nitrogen, and ammonia emissions are increasing dramatically in the region, and as a result the aerosol composition will also change. Furthermore, the growth in nitrate and ammonium is anticipated to be more rapid than that for sulfate. Particulate nitrate is mainly distributed in the coarse aerosol at the present. However, ammonium nitrate associated with the fine mode may become important.
Acknowledgements—This research was supported in part by NASA (grant d NAGW-2428). Special thanks to Professor Min-Sun Hong and Dr Shang-Gyoo Shim for their assistance in obtaining the data. Hiroshi Hayami would like to thank Li-Ling Chen and Chul-Han Song for their support after his leaving from Iowa.
REFERENCES
Carmichael, G. R., Hong, M.-S., Ueda, H., Chen, L.-L., Murano, K., Park J. K., Lee, H., Kim, Y., Kang, C. and Shim, S. (1997) Aerosol composition at Cheju Island, Korea. Journal of Geophysical Research 102, 6047—6061. Chen, L.-L., Carmichael, G. R., Hong, M.-S., Ueda, H. Shim, S.-G., Song, C.-H., Kim, Y.-P., Murano, K., Park, J.-K., Lee, H., and Kang, C.-H. (1997) Analysis of ground-based measurements at Cheju Island, South Korea. Journal of Geophysical Research (submitted). Dentener, F. J., Carmichael, G. R. and Zhang, Y. (1996) Role of mineral aerosol as a reactive surface in the global troposphere. Journal of Geophysical Research 101, 22 869—22 889. Foell, W., Green, C., Amann, M., Bhattacharya, S., Carmichael, G., Chadwick, M., Cinderby, S., Haugland T., Hettelingh, J.-P., Hordijk, L., Kuylenstierna, J., Shah, J., Shrestha, R., Streets, D. and Zhao, D. (1995) Energy use, emissions, and air pollution reduction strategies in Asia. Journal of ¼ater, Air and Soil Pollution 85, 2277—2282. Hayami, H. and Carmichael, G. R. (1997) Analysis of aerosol composition at Cheju Island, Korea, using a two-bin gas—aerosol equilibrium model. Atmospheric Environment 31, 3429—3439. Kim,Y.-P, Seinfeld, J. H. and Saxena, P. (1993a) Atmospheric gas—aerosol equilibrium I. Thermodynamic model. Aerosol Science and ¹echnology 19, 157—181. Kim, Y.-P, Seinfeld, J. H. and Saxena, P. (1993b) Atmospheric gas—aerosol equilibrium II. Analysis of common approximations and activity coefficient calculation methods. Aerosol Science and ¹echnology 19, 182—198. Kim, Y.-P and Seinfeld, J. H. (1995) Atmospheric gas—aerosol equilibrium: III. Thermodynamics of crustal elements Ca2`, K` and Mg2`. Aerosol Science and ¹echnology 22, 93—110. Kim, Y.-P., Shim, S.-G., Moon, K.-C., Baik, N.-J., Kim, S.-J., Hu, C.-G. and Kang, C.-H. (1995) Characteristics of particles at Kosan, Cheju Island: intensive study results during March 11—17 1994. Journal of Korea Air Pollution Research Association 11, 263—272 (in Korean). Tsuruta, H. (1991) Chemical composition of kosa and rain. In Kosa, Kokon-shoin Publishers, Tokyo (in Japanese).
Atmospheric Environment Vol. 32, No. 8, pp. 1427—1434, 1998 ( 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S1352–2318(97)00323–3 1352—2310/98 $19.00#0.00
FACTORS INFLUENCING THE SEASONAL VARIATION IN PARTICULATE NITRATE AT CHEJU ISLAND, SOUTH KOREA HIROSHI HAYAMI* and GREGORY R. CARMICHAEL Center for Global and Regional Environmental Research, IATL, The University of Iowa, Iowa City, IA 52242-1000, U.S.A. (First recieved 24 October 1996 and in final form 7 July 1997. Published April 1998) Abstract—The seasonal variation in measured particulate nitrate at Cheju island, South Korea, is analyzed and shown to exhibit high concentrations from February to June and in October, and low values from July to September. Total nitrate concentrations, which are not monitored, are estimated with a gas—aerosol equilibrium model with two-size bins. The total nitrate concentrations are shown to be maximum in June and minimum in August, and these differences are associated with air—mass trajectories. The fraction of particulate to total nitrate is estimated and shown to be sensitive to the total nitrate concentrations and the coarse-mode composition that is dominated by non-volatile species. The particulate nitrate fractions are found to vary within a relatively narrow range around the annual-mean value, indicating that the particulate nitrate concentrations behave similarly to the total nitrate concentrations. Exceptions are found for April, June and July. For April the largest particulate nitrate fraction is found in association with very high concentrations of nss-calcium, an indicator of mineral aerosol. The smallest particulate nitrate fractions occur in June and July and are the result of a combination of low concentrations of nss-calcium, sea salt and total nitrate, and high concentration of nss-sulfate. ( 1998 Elsevier Science Ltd. All rights reserved. Keyword index: Partitioning of nitrate, gas—aerosol equilibrium, coarse particles, East Asia.
INTRODUCTION
Cheju Island, South Korea (Fig. 1) is regarded as an ideal site for air quality monitoring in East Asia. The island has no large industrial emissions but is surrounded by large source areas in China, Korea and Japan, located only a few hundred kilometers away. Daily aerosol compositions have been measured with a tape sampler since March 1992 in cooperation among Korea Institute of Science and Technology and Ajou University, Korea; Kyusyu University and National Institute for Environmental Studies, Japan; and the University of Iowa, U.S.A. The sampling method and chemical analysis are described in detail in Carmichael et al. (1997). Chen et al. (1997) discuss the typical aerosol composition and the major features of the seasonal cycles in aerosol components collected at Cheju (Fig. 2). They found that sea-salt components represented by sodium showed peak values in the winter corresponding to the period of
* Author to whom correspondence should be addressed. Present address: Central Research Institute of Electric Power Industry, 2-11-1 Iowado-kita, Komae, Tokyo 201, Japan.
maximum wind speed. Calcium and nitrate, and to a lesser extent, sulfate and ammonium, showed high concentrations in the spring. All species showed a pronounced minimum value in the summer. Understanding the behavior of nitrate at Cheju is important since nitrate represents a significant fraction of the total oxides of nitrogen (NO ) in the y troposphere, and particulate nitrate contributes to a variety of environmental problems in the region including reduced visibility and acid deposition. Particulate nitrate at Cheju shows a seasonal variation with high concentrations from February to June and in October, and low values in July to September, as shown in Fig. 2. Chen et al. (1997) showed that particulate nitrate concentration has relatively high correlation with nss-calcium concentration, but little correlation with ammonium concentration. Model analysis by Hayami and Carmichael (1997) suggested that ammonium at Cheju mostly coexists with nsssulfate in the fine- (or accumulation-) mode aerosol, and that particulate nitrate is associated with the nonvolatile cation species (sodium, potassium, calcium, and magnesium) in the coarse-mode aerosol. Factors other than the ammonium nitrate relationship,
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NH NO (aerosol) ¢ NH (gas)#HNO (gas) 4 3 3 3
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H. HAYAMI and G. R. CARMICHAEL
nitrate. Unfortunately, there are no continuous measurements of total or gaseous nitrate available at Cheju. In this paper, we perform a modeling analysis to estimate the total nitrate and the particulate nitrate fraction at Cheju. The model is then used to study the seasonal variation of nitrates and to examine which factors control the particulate nitrate fraction at Cheju.
ESTIMATION OF PARTICULATE NITRATE FRACTION AND TOTAL NITRATE CONCENTRATION
Fig. 1. Map around Cheju Island.
Fig. 2. Monthly-mean concentrations of particulate nitrate, sodium, nss-calcium and nss-sulfate of aerosol measured at Cheju.
appear to be important for the seasonal variation in particulate nitrate at Cheju. Nitrate is formed via both heterogeneous and homogeneous chemical reactions in the atmosphere. Because of its volatility, it is distributed between the gas and aerosol phases. Nitrate heterogeneously formed on the aerosol surface may evaporate into the gas phase, while nitrate homogeneously formed in the gas phase may condense onto particles. The distribution of nitrate between the gas and particle phases depends on various parameters including gaseous and particulate chemical composition, and temperature and relative humidity. Therefore, the variation in particulate nitrate concentration needs to be discussed along with total (gaseous#particulate)
Hayami and Carmichael (1997) applied a gas—aerosol equilibrium model with two-size bins (fine and coarse modes) to the analysis of the mean composition of the aerosol at Cheju. The analysis assumed that particles in each mode are internally mixed, and that particulate volatile species (ammonium, nitrate and chloride) in each mode are in equilibrium with their respective gaseous components. In this paper, we use this approach to analyze the behavior of particulate nitrate. The partitioning of the volatile species between the gas and aerosol phases is calculated using SCAPE (Kim et al., 1993a, b; Kim and Seinfeld, 1995). The SCAPE model gives the partitioning and condition of sodium, potassium, calcium, magnesium, ammonium, sulfate, nitrate and chloride from their total concentrations, temperature and relative humidity. In this analysis the total concentrations for the non-volatile species are specified using the observed values (see Carmichael et al. (1997) and Chen et al. (1997) for details on the measurements and analysis). Ammonium is also volatile, but measurements of particulate ammonium are regarded as total ammonium concentrations based on our previous study where we found total ammonium to be mostly distributed in the aerosol phase at Cheju (Hayami and Carmichael, 1997). Total chloride concentrations are assumed to be supplied from sea salt, and are estimated from the sodium measurements. Particulate nitrate is assumed to coexist with particulate chloride in the same aqueous phase of the aerosol. With these assumptions the equilibria can be written as K c2 [NO~] P 3 # HNO3" HC- HNO3,# P K c2 [Cl~] HCHNO3 HC-,# # K c2 [NO~] 3 & " HC- HNO3,& K c2 [Cl~] HNO3 HC-,& &
(1)
where P, K and c indicate partial pressure, equilibrium coefficient and activity coefficient, and subscripts f and c indicate values of the fine and coarse modes. While the equilibrium coefficients are functions of temperature, the activity coefficients vary with both temperature and composition. Preliminary calculations for monthly mean aerosol compositions at Cheju were implemented to see the
Seasonal variation of particulate nitrate
behavior of c2 /c2 , using the gas—aerosol equilibHNO3 HCrium model using the Pitzer method for the activity coefficient calculations. The calculated mean ratio was 0.65 with a standard deviation of 0.087 for the coarse aerosols where the mean molalities of HNO 3 and HCl were 1.09 and 2.83, respectively. This value is very close to the mean ratio, 0.63 with a standard deviation of 0.089, for the fine aerosols where the molalities of HNO and HCl were 0.01 and 0.04, 3 eventhough the compositions are quite different between the modes. Since most of the particulate nitrate is distributed in the coarse aerosols, we fixed the ratio as a constant at 0.65. Equation (1) can be rearranged to provide estimates of total nitrate concentration from measurements of sodium, chloride and nitrate in the aerosols, [Total nitrate]"
A
1#0.65
B
K 1.79[Na`]![Cl~] HC[NO~] (2) 3 K [Cl~] HNO3
where concentrations are in kg m~3. The gas—aerosol equilibrium model with SCAPE and the above assumptions were evaluated for the monthly-mean aerosol compositions at Cheju. Estimates of particulate concentrations of the volatile species are presented in comparison with their measurements in Fig. 3. The model provides a good representation of the measurements with a tendency of !3% underestimation for ammonium, #12% overestimation for nitrate, and !1% underestimation for chloride. Furthermore, the correlations are very high, more than 0.90. The largest four overestimates of particulate nitrate are found for March, April, October and November. The former two months have high concentrations of nss-calcium associated with dust aerosol. The partitioning ratio of fine to total nss-sulfate, which was fixed to 0.8 in the
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calculations, probably varies with season. According to the modeling study of Dentener et al. (1996) the value is smaller during high dust periods because the dust aerosol offers a reactive surface for heterogeneous production of sulfate. As discussed in more detail later, the fraction of particulate to total nitrate decreases with the partitioning ratio for nss-sulfate. The latter two months are the only two cases where the relative humidities of the months are lower than the deliquescence point of NaNO (0.7379 at 3 298.15 K, Kim et al., 1993b) and where some of the particulate nitrate in the coarse mode is present in the form of solid NaNO . The existence of solid nitrate is 3 not assumed in equation (2). For particulate chloride, no solid was predicted, and the concentrations predicted by equation (2) were closer to the measurements than those for the particulate nitrate concentrations. Including the treatment of solid nitrate would improve the estimates of total nitrate. In the remainder of the paper, equation (2) is used to estimate the total nitrate concentrations. The model estimates might be improved by changing the assumptions and parameter values such as the ratio of the activity coefficients or the nss-sulfate partitioning ratio between the size modes. However, rather than performing such a tuning, we instead present a series of sensitivity calculations.
VARIATION IN ESTIMATION TOTAL NITRATE CONCENTRATION AND PARTICULATE NITRATE FRACTION
Total nitrate concentrations and particulate nitrate fractions (calculated as the ratio of the estimated particulate nitrate and the estimated total nitrate) were estimated from the monthly-mean measurements of aerosol composition, temperature and relative humidity using equation (2). The results are shown in Fig. 4
Fig. 3. Calculated concentrations [kg m~3] of particulate ammonium, nitrate and chloride to the measurements. The lines indicate the relationship of 1 on 1. Some months with overestimates of particulate nitrate are indicated.
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H. HAYAMI and G. R. CARMICHAEL
Fig. 4. Annual-mean and monthly concentrations of total (gray bars) and particulate (black bars) nitrate and its fraction (triangles) estimated with the gas—aerosol equilibrium model.
Fig. 5. Backward trajectories at 850 hPa arriving at Cheju Island in (a) April, (b) June, and (c) August 1993. Each trajectory is drawn every 12 h with a time step of 3 h.
along with the estimates of particulate nitrate concentration. Most of the particulate nitrate fractions vary within $0.1 from the annual mean of 0.74. The exceptions are 0.85 in April, 0.45 in June and 0.22 in July. The particulate nitrate fractions show higher values compared to the annual mean from October to April and lower values from May to September. The higher fractions vary in a relatively narrow band between 0.75 and 0.85. Because of this stable variation in the particulate nitrate fraction, the particulate nitrate concentrations behave in a similar manner as the total nitrate concentrations during this period. From April to June, the particulate nitrate fraction decreases, while the total nitrate concentration increases up to the annual maximum of 3.15 kg m~3 in June. In July, the total nitrate concentration is about half of that in June, and the particulate nitrate fraction shows the annual minimum. In August and September, the total nitrate concentrations are at a minimum, but the particulate nitrate fractions are above 0.6. The particulate nitrate concentrations are nearly twice the minimum in July. Meteorological features play an important role in the seasonal cycle of total nitrate at Cheju. Fig. 5 shows isobaric backward trajectories at 850 hPa for
April, June and August in 1993. The trajectories show that the flows in June are from the west, bringing high concentrations of NO from source regions in eastern x China. Photochemical formation of nitrate becomes active under the summer condition with high temperature and sufficient solar radiation, leading to high concentration of total nitrate. Dominant flows in August are southerly, bringing clean, marine air masses with low concentrations of the precursors. Therefore, the total nitrate concentration is low in August. The flows in April are from northwestern China, a major source region of mineral aerosol. Mineral aerosol provide an efficient surface for the heterogeneous formation of nitrate (Dentener et al., 1996) and helps to retain nitrate in the aerosol phase. Typical winter flows are strong and northwesterly, represented by cold outbreaks. Sea-salt components at Cheju show high concentrations in winter as shown in Fig. 2, reflecting these strong winds over the sea and the high emissions of sea-salt aerosols. Nitrate also may deposit onto sea-salt particles. Carmichael et al. (1997) took another approach to estimate particulate nitrate fractions at Cheju. Aerosol composition has also been measured at Cheju on cellulose filters placed in a Hi-Vol sampler operated
Seasonal variation of particulate nitrate
by the University of Miami. These measurements are co-located next to the tape—filter sampler, and have been performed since 1992. Carmichael et al. (1997) estimated the fraction of particulate nitrate by regarding the nitrate on the UM filters as total nitrate and the nitrate on the tape sampler as particulate nitrate. The values ranged from 0.3 to 0.4 in the spring and winter, to &0.1—0.2 in the summer and early fall, with an annual average of 0.23. Dentener et al. (1996), who used a three-dimensional global chemical model with dust calculated the annual and FebruaryMarch-April average fractions around Cheju Island to be &0.2 and &0.4, respectively. Although these values are significantly smaller compared to the values estimated in this paper, a consistent seasonal tendency is found in all of these estimates with higher fractions in winter and spring than in summer. The only direct estimate based on measurements of both gas and aerosol nitrate at Cheju are from three daily samples in March 1994 (Kim et al., 1995). Based on these measurements the average particulate nitrate fraction is estimated to be 0.84, a value close to the estimates presented in this paper. Clearly, more extensive and long-term monitoring of gaseous and particulate nitrate is required to evaluate our approach and the contribution of nitrate to acidification of the atmosphere in this region.
FACTORS INFLUENCING THE PARTICULATE NITRATE FRACTION
To understand the seasonal variation in the particulate nitrate fraction, we investigated the sensitivity of particulate nitrate to various initial conditions using the gas—aerosol equilibrium model. The calculations were based on the annual-mean aerosol composition at Cheju (Table 1). Results for the sensitivity of the particulate nitrate fraction to nine sets of parameters: temperature; relative humidity; nss-sulfate concentration; total nitrate concentration; total ammonium concentration; ammonium and nitrate concentrations; ammonium, nitrate and nss-sulfate concentrations; nss-calcium concentration; sea-salt component concentrations; and partitioning ratio of fine-mode to total (fine- and coarse-mode) nss-sulfate are discussed. The partitioning ratios for nss-sulfate and non-volatile cations were fixed to the base-case values (0.8 and 0.2, respectively), except when these were subject to the sensitivity study. Sea salt was defined to have mass ratios relative to sodium of 0.04 for potassium, 0.04 for calcium, 0.12 for magnesium, 1.79 for chloride and
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0.25 for sulfate. Simulations were performed for conditions where the base values were multiplied by factors of 0.5, 2 and 4. The results are summarized in Fig. 6. One important result is that the nss-sulfate concentration greatly changes the nitrate fraction. As the nss-sulfate concentration is simultaneously increased in both the fine and coarse modes (the 0.5] to 4] nssSO cases in 4 the figure), the nitrate fraction changes from nearly 0.88 to 0. As the fraction of nss-sulfate in the coarse mode is decreased (or, p(nssSO ) is increased in the 4 figure), the particulate nitrate fraction increases. These results show that the amount of nitrate in the particle phase is very sensitive to the coarse-mode nss-sulfate concentration. A general tendency is found from these results that the particulate nitrate fraction increases with increasing concentration of cations, e.g., the particulate nitrate fraction increases with nss-calcium. However, the partitioning ratio for nss-sulfate may be reduced at high nss-calcium concentration since coarse-mode nss-sulfate may be produced on the reactive surface of coarse-mode particles (Dentener et al., 1996). Therefore, the increase in particulate nitrate fraction with nss-calcium can somewhat be canceled by the opposite effect that the partitioning ratio for nss-sulfate has on the particulate nitrate fraction. According to Chen et al. (1997), the sea-salt components comprise approximately one-quarter of the mass of water soluble inorganic species in the aerosols at Cheju. The particulate nitrate fraction is found to be sensitive to the sea-salt concentration and to increase as the sea-salt mass increases. Sea salt is only slightly cation-rich (1.1 neq (kg Na)~1) but behaves like a cation (stronger than nss-calcium) in retaining nitrate in the aerosol phase. This is due to the volatile nature of sea salt chloride. Particulate nitrate competes with the sea-salt chloride to share non-volatile cations in the coarse mode. As the total nitrate concentration increases, the concentrations of both particulate and gaseous nitrate increase, but the particulate nitrate fraction decreases. Most of the increase in total nitrate stays in the gas phase. The fine-mode nitrate is still at a trace level even at the 4] total nitrate concentration, and the changes in the particulate nitrate fraction induced by the total nitrate concentration occur between the gas and coarse-aerosol phases. Ammonia plays an important role. The particulate nitrate fraction is insensitive to decreasing ammonia values, but is quite sensitive to increasing values. At higher ammonia values the particulate nitrate fraction
Table 1. The annual-mean composition measured at Cheju Component
Na`
K`
Ca2`
Mg2`
NH` 4
SO2~ 4
NO~ 3
Cl~
Concentration
1.74
0.43
0.46
0.26
1.26
7.30
1.14
1.75
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H. HAYAMI and G. R. CARMICHAEL
Fig. 6. Sensitivity of particulate nitrate fraction to various initial conditions. The base initial concentrations of nss-sulfate (shown as nssSO ), nss-calcium (nssCa), sea salt, nitrate (NO ), and ammonium (NH ), 4 3 4 which are shown in Table 1, were individually or simultaneously changed by multiplying factors of 0.5, 2 and 4. The partitioning ratio of fine-mode to total nss-sulfate was changed from the base value of 0.8 to 0.6, 0.7, 0.9 and 1.0. Temperature was changed from the annual mean value of 15.5°C to 5 and 25°C, and relative humidity from 78% to 60 and 90%.
increases as more nitrate can be retained in the fine mode. As ammonia and nitrate are increased together the fine-mode nitrate concentration increases, implying the formation of ammonium nitrate. However, even at 4] the initial concentrations of the two species the fine-nitrate fraction is only 0.11. The existence of little ammonium nitrate is also indicated by the fact that the particulate nitrate fraction is insensitive to temperature and relative humidity. These results suggest that the volatilization of nitrate captured on the Teflon filters does not appear to be a serious artificial error for the aerosol at Cheju. It is likely that sulfate, nitrate and ammonium loadings will double in the next decade or two in East Asia (Foel et al., 1995). As a result the aerosol composition at Cheju will also change. If anthropogenic emissions, or total concentrations, of ammonium, nitrate and nss-sulfate are doubled, the sensitivity results suggest that most of the nitrate would be found in the gas
phase. These results show that large changes in the aerosol composition in this region can be expected, and that continued measurements are needed to detect such changes. Such data are also needed to address acidification on ecosystem and human health in East Asia. To summarize, the results indicate that the particulate nitrate fraction at Cheju is sensitive to the coarse-aerosol composition. Since particulate nitrate is mostly distributed in the coarse mode, the amount of non-volatile cation species that are free from neutralizing sulfate in the coarse mode is a key parameter to retain nitrate in the aerosol phase. The total nitrate in excess to the non-volatile cation species remains in the gas phase, which results in reducing the nitrate fraction. An increase in the initial concentrations of nss-calcium, sea salt and ammonium raises the nitrate fraction. Sea salt plays a role like a cation species by releasing some sea-salt chloride into the gas
Seasonal variation of particulate nitrate
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Fig. 7. Nitrate fractions of the annual-mean composition with concentrations of various species of specific months.
phase. Sea-salt chloride in the aerosol phase shares the non-volatile cation species in the coarse mode with nitrate. It is previously shown in Fig. 4 that the particulate nitrate fractions vary within $0.1 from the annual mean of 0.74, except for April, June and July. Since the free non-volatile cations are important for the nitrate fraction, the stability of the nitrate fraction appears to be maintained by high concentrations of nss-calcium in the spring and sea salt in the winter, and low concentrations of nss-sulfate in the fall (see Fig. 2). For the months where the particulate nitrate fraction is very high or low additional simulations were performed with the gas—aerosol equilibrium model. In these simulations the particulate nitrate fraction was calculated using combinations of the annual-mean and monthly average composition of sea salt, nsscalcium, nss-sulfate and total nitrate in each of the months. For example, to see the effect of sea salt on the April particulate nitrate fraction, the model was re-run using the sea-salt concentration in April, keeping the annual-mean composition for the other species. The results are shown in Fig. 7. For April the largest effect is found by changing the initial concentration of nss-calcium. Using the April nss-calcium concentration results in a particulate nitrate fraction of 0.92, which is very close to the monthly-mean composition in April. Varying the other species concentrations does not cause large changes, but tends to reduce the particulate nitrate fraction. The high nsscalcium concentration in April appears to cause the annual maximum value of the particulate nitrate fraction. Note that nss-calcium considered here is in the form of CaCO (calcite) or CaO. According to the 3 trajectories in April shown in Fig. 5, the nss-calcium in April is associated with dust-aerosol represented by
yellow sand. Yellow sand contains large amounts of CaCO , and calcium in natural soils is in the form of 3 CaO (Tsuruta, 1991). If the nss-calcium is supplied as CaSO , the nitrate fraction becomes smaller because 4 the nss-calcium is equivalently neutralized by the sulfate from CaSO . 4 In June the total nitrate concentration is estimated to be remarkably high. When the annual-mean aerosol composition is used, a particulate nitrate fraction of 0.62 is predicted. When the species concentrations in June are individually varied the particulate nitrate fraction decreases, but a large distance still remains from the estimated nitrate fraciton in June. However, when the concentrations of sea salt, nss-calcium and total nitrate in June are used together, the annualmean particulate nitrate fraction becomes 0.51, very close to the nitrate fraction in June. The same thing is found for July, where using the July concentrations of sea salt, nss-calcium and nss-sulfate, the predicted particulate nitrate fraction is reduced to 0.37. Therefore, the lowest nitrate fractions found in June and July appear to be controlled by the coarse-aerosol composition and the combination of sea salt, nsscalcium, nss-sulfate, and total nitrate.
SUMMARY
Nitrate is a key compound in the study of tropospheric chemistry in East Asia. It is the major component of the oxidation of NO arising from x anthropogenic emissions, which are rapidly growing in East Asia. In addition, it is an important acidic component of the aerosol and precipitation which can contribute to such adverse environmental problems as reduced visibility and acid deposition.
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H. HAYAMI and G. R. CARMICHAEL
In this paper we analyzed the seasonal variation of particulate nitrate observed at Cheju island, South Korea, which exhibits high concentrations from February to June and in October, and low values from July to September. Since the total nitrate concentrations are not monitored, a gas—aerosol equilibrium model with two-size bins was used to estimate these concentrations. The estimated total nitrate concentrations showed maximum in June and the minimum in August. These variations are associated with changes in air—mass trajectories. Air flows pass over East Asian continent in April and June, but come from the south bringing clean, marine air in August. A sensitivity study was performed to investigate which factors influence the fraction of particulate nitrate. The particulate nitrate fractions were found to be sensitive to total nitrate concentrations and the coarse-mode composition that consists of non-volatile species. The particulate nitrate fraction was found to vary within a relatively narrow range around the annual-mean value of 0.736, which means that the particulate nitrate concentrations behave similarly to the variation in the total nitrate concentrations. Exceptions were found for April, June and July. For April the largest particulate nitrate fraction is the result of very high concentrations of nss-calcium induced associated with dust aerosol. The lowest fractions of June and July are due to combinations of low concentration of nss-calcium, sea salt and total nitrate, and high concentrations of nss-sulfate. Finally, understanding the chemical composition of the various size fractions is important in the assessment of the how these aerosols effect regional climate, visibility and acidic deposition. There is a clear need for continued measurements at Cheju, augmented with measurements of gaseous components and sizeresolved aerosol composition. This is especially true in light of the fact that sulfur, nitrogen, and ammonia emissions are increasing dramatically in the region, and as a result the aerosol composition will also change. Furthermore, the growth in nitrate and ammonium is anticipated to be more rapid than that for sulfate. Particulate nitrate is mainly distributed in the coarse aerosol at the present. However, ammonium nitrate associated with the fine mode may become important.
Acknowledgements—This research was supported in part by NASA (grant d NAGW-2428). Special thanks to Professor Min-Sun Hong and Dr Shang-Gyoo Shim for their assistance in obtaining the data. Hiroshi Hayami would like to thank Li-Ling Chen and Chul-Han Song for their support after his leaving from Iowa.
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Carmichael, G. R., Hong, M.-S., Ueda, H., Chen, L.-L., Murano, K., Park J. K., Lee, H., Kim, Y., Kang, C. and Shim, S. (1997) Aerosol composition at Cheju Island, Korea. Journal of Geophysical Research 102, 6047—6061. Chen, L.-L., Carmichael, G. R., Hong, M.-S., Ueda, H. Shim, S.-G., Song, C.-H., Kim, Y.-P., Murano, K., Park, J.-K., Lee, H., and Kang, C.-H. (1997) Analysis of ground-based measurements at Cheju Island, South Korea. Journal of Geophysical Research (submitted). Dentener, F. J., Carmichael, G. R. and Zhang, Y. (1996) Role of mineral aerosol as a reactive surface in the global troposphere. Journal of Geophysical Research 101, 22 869—22 889. Foell, W., Green, C., Amann, M., Bhattacharya, S., Carmichael, G., Chadwick, M., Cinderby, S., Haugland T., Hettelingh, J.-P., Hordijk, L., Kuylenstierna, J., Shah, J., Shrestha, R., Streets, D. and Zhao, D. (1995) Energy use, emissions, and air pollution reduction strategies in Asia. Journal of ¼ater, Air and Soil Pollution 85, 2277—2282. Hayami, H. and Carmichael, G. R. (1997) Analysis of aerosol composition at Cheju Island, Korea, using a two-bin gas—aerosol equilibrium model. Atmospheric Environment 31, 3429—3439. Kim,Y.-P, Seinfeld, J. H. and Saxena, P. (1993a) Atmospheric gas—aerosol equilibrium I. Thermodynamic model. Aerosol Science and ¹echnology 19, 157—181. Kim, Y.-P, Seinfeld, J. H. and Saxena, P. (1993b) Atmospheric gas—aerosol equilibrium II. Analysis of common approximations and activity coefficient calculation methods. Aerosol Science and ¹echnology 19, 182—198. Kim, Y.-P and Seinfeld, J. H. (1995) Atmospheric gas—aerosol equilibrium: III. Thermodynamics of crustal elements Ca2`, K` and Mg2`. Aerosol Science and ¹echnology 22, 93—110. Kim, Y.-P., Shim, S.-G., Moon, K.-C., Baik, N.-J., Kim, S.-J., Hu, C.-G. and Kang, C.-H. (1995) Characteristics of particles at Kosan, Cheju Island: intensive study results during March 11—17 1994. Journal of Korea Air Pollution Research Association 11, 263—272 (in Korean). Tsuruta, H. (1991) Chemical composition of kosa and rain. In Kosa, Kokon-shoin Publishers, Tokyo (in Japanese).