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Figure 12. Computed capacitance (diamonds) of rosettes as a function of number of lobes. Thick solid curve represents power fit by Eq. (15). Also shown are the capacitances of a sphere (thin solid line), a circular plate (dashed), and two prolate spheroids with semi-axis ratio a/b = 2 and 5, respectively. The rosettes, the sphere and circular plate all have a radius a. The unit of capacitance is a (the radius of the crystal) (adopted from Chiruta and Wang, 2003).

Figure 13. The capacitances of hexagonal ice columns, prolate and oblate (the one indicated by OB) spheroids, and circular cylinders as a function of the aspect ratio R. Experimental results are taken from Podzimek (1966). The unit of capacitance is a (the radius of the crystal) (adopted from Chiruta and Wang, 2005).

Figure 13. The capacitances of hexagonal ice columns, prolate and oblate (the one indicated by OB) spheroids, and circular cylinders as a function of the aspect ratio R. Experimental results are taken from Podzimek (1966). The unit of capacitance is a (the radius of the crystal) (adopted from Chiruta and Wang, 2005).

Acknowledgements

We are grateful for the support of NSF Grants ATM-0234744 and ATM-0244505. The lead author would also like to acknowledge the support of the National Science Council, Taiwan, for a summer visit to the National Taiwan University in 2005.

[Received 28 December 2005; Revised 22 August 2007; Accepted 27 September 2007.]

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A Review of Ozone Formation in Megacities of East Asia and Its Potential Impact on the Ozone Trends

Shaw Chen Liu

Research Center for Environmental Changes, Academia Sinica and Department of Atmospheric Sciences, National Taiwan University, Taipei,, Taiwan

[email protected]. tw

In this work, we review the formation of high levels of ozone in megacities of East Asia and their potential impact on the increasing trends of tropospheric ozone concentration. A series of intensive observation experiments were conducted in Kaohsiung, Taipei and Taichung by scientists of the Research Center for Environmental Changes (RCEC) of Academia Sinica, in collaboration with colleagues from the National Taiwan University and the National Central University, to study the ozone formation and the strategy for its control in 2003-2005. In addition, RCEC scientists participated in three large-scale international experiments led by Peking University in the Pearl River Delta (PRD) and in Beijing in 2004 and 2006. A one-dimensional model and an observation-based method (OBM) were used to analyze data from these experiments to examine the photochemical processes of ozone formation and the relationship of ozone to its precursors. We found that the O3 production rate was NMHCs-limited, and that controlling NMHCs was more efficient than controlling NOz in reducing ozone levels in all the megacities studied.

The increasing trends of ozone formation at the background stations in Taiwan, Hong Kong and Japan from the 1980s to 2005 strongly suggest that photochemical production of ozone in Asia has been increasing due to anthropogenic emissions of ozone precursors. In addition, we believe that the increasing trend of ozone concentration in Mauna Loa (4.1% per decade between 1973 and 2004) could be a good indicator/measure and be useful for inverse-modeling the trend of the background ozone level of the entire Asia and even the Northern Hemisphere.

1. Introduction

Tropospheric ozone is an important gas in many aspects. Near the surface, ozone is a major air pollutant because at high concentrations it is hazardous to human health and can damage vegetation. In addition, tropospheric ozone plays a critical role in atmospheric chemistry because it controls the oxidation capacity of the atmosphere, both directly and indirectly through the generation of hydroxyl radicals (OH). It is well known that OH is responsible for the oxidation of the majority of reduced gases in the atmosphere. For example, CO, H2 and CH4 and most nonmethane hydrocarbons (NMHCs) are all oxidized by OH radicals. Ozone is also an important greenhouse gas because it absorbs infrared radiation near 9.6 ¡m, in the window of carbon dioxide and water vapor. According to the IPCC Report (2001), changes in tropospheric ozone since the industrial revolution (i.e. 1750-2000) have made it the third-most-important greenhouse gas, behind carbon dioxide and methane.

Tropospheric ozone was thought to come mostly from the stratosphere and to be deposited at the surface (Regener, 1949) until the early 1970s, when Crutzen (1973) and Chameides and Walker (1973) proposed that photochemical production of ozone can be important. In the presence of OH, CO (or NMHC) and NOK, O3 can be produced via the following simplified catalytic reaction scheme:

OH + CO(NMHC) + O2 ^ HO2 + CO2 + RO2 HO2 (RO2) + NO ^ NO2 + OH(RO) NO2 + hu ^ NO + O O + O2 + M ^ O3 + M Net : CO(NMHC) + 2O2 + hv ^ CO2 + O3 + (RO)

NMHC, CO and NOK are called precursors of O3. The OH radicals needed to initiate the ozone production reactions are generated through ozone's photolysis by solar ultraviolet radiation at a wavelength of less than 320 nm:

The electronically excited atomic oxygen O(1D) is highly reactive. It can react with water vapor and form the hydroxyl radicals:

The key step in the simplified ozone production scheme is the HO2(RO2) + NO reaction, which splits the O2 bond. The HO2 (RO2) + NO reaction is the so-called rate-limiting process of O3 production in most of the troposphere, because NOK has a shorter lifetime than the average lifetime of hydrocarbons and CO. The lifetimes of CH4 and CO are so long that there is plenty of CO and CH4 in the background troposphere to produce O3. As a result, NOK is the rate-limiting precursor of O3 (i.e. it controls the production rate of O3) in most parts of the troposphere. In fact, NMHC becomes the rate-limiting precursor only in some urban and suburban areas where concentrations of NOT are high and the ratios of NMHC to NOK are low

(e.g. Seinfeld and Pandis, 1998; Blanchard et al., 1999).

There is a significant diffused source of NOK from lightning, such that the concentration of NOk in the background troposphere is often greater than the nominal breakeven level of NOK of about 10pptv, at which the gross photochemical O3 production equals the gross photochemical O3 loss. In addition to the lightning source, there is a small but significant stratospheric source of NOK (and NOy) in the upper troposphere, making the gross O3 production frequently comparable to the gross O3 loss in the middle and the upper troposphere. As a result, the "apparent lifetime" of ozone, which is defined as O3/(gross production — gross loss), is usually very long. It can last for many months in the middle and the upper troposphere. Once O3 is produced or transported into the middle and the upper troposphere, it can be transported like an inert species over a long distance.

The gross loss of ozone can be calculated from the following reactions:

OH + CO + O2 ^ HO2 + CO2 H + O2 + HO2 +M HO2 +O3 ^ OH + 2O2 Net: CO + O3 ^ CO2 + 2O2

The estimated value of the gross loss of ozone for the Northern Hemisphere is about 50 x 1028 S_1. This is about four times the downward flux of ozone from the stratosphere, and is thus a key basis for arguing that photochemical processes play a controlling role in the tropospheric budget of ozone (Fehsenfeld and Liu, 1993).

Changes of ozone ranging from the local to the regional and even the global scale have been reported extensively (e.g. Lee et al., 1998; Oltmans et al., 1998; Jacob et al., 1999; Akimoto, 2003; Parrish et al., 2004). The various scales of changes occur because of the relatively long photochemical lifetime of ozone in the atmosphere which ranges from a few days in the boundary layer to a few months in the upper troposphere. For example, Jacob et al.

(1999) suggested that tripling Asian anthropogenic emissions from 1985 to 2010 would lead to an increase in the monthly mean O3 concentration by 2-6 ppbv in the western US and by 1-3 ppbv in the eastern US. Akimoto et al. (2003) discussed that intercontinental transport of O3 and hemispheric ozone pollution jeopardized agriculture and ecosystems worldwide and had a strong effect on climate. Both studies emphasize that international initiatives to mitigate global air pollution from both developed and developing countries are necessary. Observational evidence of a regional increase of O3 over East Asia is estimated to be 7-10 ppb from the 1970s to the 1980s (Lee et al., 1998). Also, the increase in Asian continental outflows is suggested to account for the increase of O3 in the northern Atlantic Ocean region (Parrish et al., 2004).

Rapid economic development in Asia in the last few decades has brought about an unprecedented level of prosperity. On the other hand, the environmental price of prosperity is also overwhelmingly high. As a result Asian cities are among the most-polluted metropolitan areas in the world. High levels of surface O3 in major metropolitan areas are of particular concern because of their impact on a large segment of the population. For example, in the Beijing-Tianjin area, the Pearl River Delta (PRD) and the Yangtze Delta, pollution episodes with O3 concentration of over 100 ppb have been frequently observed (Shao et al., 2006; Wang et al., 2003; Zhang et al., 1998). In addition, satellite images of high levels of anthropogenic aerosols, CO, NO2 and HCHO are strikingly visible over large areas in Asia (Kunhikrishnan et al., 2004). Because NO2, CO and HCHO are the precursors of ozone and both CO and HCHO are surrogates of NMHCs, it is not surprising to see a large regional scale of high concentrations of ozone detected from the satellite (Ziemke et al., 2006). These regional scale high O3 concentrations eventually link up with the previously isolated high levels of O3 in other megacities of

East Asia, such as Tokyo, Osaka and Taipei (Wakamatsu et al., 1996; Chou et al., 2006), and form the East Asian ozone plume, which, in combination with the ozone in South Asia, can have a potential impact on the entire Northern Hemisphere. Therefore it is extremely important to understand the ozone formation processes and determine the rate-limiting precursor in the megacities of East Asia.

In this work, we review the formation of high levels of ozone in major metropolitan areas or megacities of East Asia and explore their possible impacts on the long term increasing trends of tropospheric ozone concentration in East Asia. The review is based on recent research efforts of scientists at the Research Center for Environmental Changes (RCEC) of Academia Sinica, in collaboration with colleagues from the National Taiwan University (NTU) and the National Central University (NCU), as well as a large group of international scientists coordinated by Peking University in the last few years. A series of intensive observation experiments were conducted in Taiwan centered in Kaohsiung, Taipei and Taichung by scientists of the RCEC, NTU and NCU to study the ozone formation and its strategy for control in 20032005. In addition, RCEC scientists participated in three large scale international experiments led by Peking University in the Pearl River Delta and in Beijing in 2005 and 2006. In particular, the experiment in Beijing in August 2006 was designed to address air pollution problems in preparation for the 2008 Olympics. We will use field experiments conducted in southern Taiwan, a one-dimensional model and an observation-based method (OBM) as an example to examine the relationship between O3 and its precursors (Shiu et al., 2007). The OBM is then applied to similar field experiments conducted in the Pearl River Delta and in Beijing (Chang et al., 2008a,b). The results are important for understanding the budget of ozone and developing an effective ozone control strategy for megacities in East Asia as well as for the whole region.

On the long term trends of tropospheric ozone concentration, we will first examine the trends of ozone formation observed in Taiwan and compare them to trends observed in other areas of East Asia. We will show that long range transport of ozone from the Asian continent plays a key role in the ozone distribution over East Asia. Furthermore, we have noticed that the increasing trend of ozone formation observed in Mauna Loa, Hawaii, from 1973 to 2004 is consistent with the trends in East Asia. Given the long lifetime of ozone in the free troposphere, the trend of ozone formation observed in Mauna Loa may have an important implication not only for East Asia but also for the entire Northern Hemisphere.

2. Ozone Formation in Megacities

Southern Taiwan at the Kaohsiung urban center is a multitown "megacity" spanning over 30 km x 50 km, with more than 6 million inhabitants. Oxidants such as ozone and PAN have become a serious environmental problem. In addition to large vehicle emissions, there are significant emission sources from four industrial parks nearby. During autumn and winter when high pressure conditions prevail, severe deteriorations of air quality with O3 over 120 ppbv frequently occur in this region.

2.1. Field measurements

Two intensive field experiments were conducted by the RCEC in the Kaohsiung-Pingtung (KaoPing) area in southern Taiwan during 2629 September 2003 and 23-28 October 2003 (hereafter referred to as the RCEC 2003 Experiment). NMHCs were collected daily with stainless canisters side by side with the air quality instruments at 12 Taiwan EPA (TEPA) air quality monitoring stations in two periods, namely 9-10 a.m. and 11 a.m. to 12 noon, during the 10-day field campaign. The measurements between 11 a.m. and 12 a.m. were intended to examine the NMHCs' composition just before the peak of ozone, which usually occurs between 12 noon and 1 p.m. Measurements between 9 a.m. and 10 a.m. were designed to study the composition of NMHCs near the start of the daily photochemical cycle to get a measure of their ozone production potential for that day. The EPA monitoring stations have routine hourly measurements of O3, CO, NOK, SO2, PM10 and meteorological parameters. Detailed sampling strategy and geographical information can be found in Chang et al. (2005).

Ozone is measured using an ultraviolet photometric instrument with a precision of 1 ppbv. The oxides of nitrogen (NO and NO2) are measured using a chemiluminescence instrument. The manufacturer's suggested value of precision is 0.4 ppbv. However, our independent evaluation of the accuracy showed that the signal-to-noise level was about 1 at 3 ppbv for NO, significantly worse than the manufacturer's suggested value (Chou et al., 2006). Similar accuracy was found for NO2. This can cause serious problems for NO measurements, as NO values frequently dip below 5 ppbv during pho-tochemically active hours. It is not a serious problem for NO2, because its values are usually greater than 10 ppbv. On the other hand, it is well known that the catalytic converter converting NO2 to NO suffers interference from non-NO2 compounds such as PAN and even HNO3. This problem will be elaborated later. A total of 56 C2-C10 NMHCs were analyzed by a GC/MS system. It is estimated that species not resolved by the GC/MS system are about 30-60% of the 56 NMHCs in terms of reactivity toward OH (Chang et al., 2003). A detailed description regarding the accuracy of the GC/MS instrument can be found in Chang et al. (2003).

Meteorological parameters observed by the Central Weather Bureau station in Kaohsiung showed that the weather was fair during the two field experiments and there was a typical land-sea breeze with westerly to northwesterly winds in the daytime, and easterly to northeasterly winds at night. The diurnal variation of wind speed was consistent with the influence of the land-sea breeze. The daytime maximum temperature during the first period of the experiment (26-29 September 2003) was around 32°C, and 28°C during the second period (23-28 October 2003). The atmospheric moisture contents for the second period were lower than those for the first period.

Hourly time series of concentrations of O3, NO, NO2, CO and PM10 for three EPA stations are shown in Fig. 1. Two urban stations, Lernwu and Zhaoin, are located in the Kaohsiung city, while Chaochou is a suburban station in Pingtung County. The day-to-day variations of O3 for different stations show a similar pattern, but the primary pollutants like

CO, NO and NO2 distributions change greatly from station to station. Concentrations of NO, NO2 and CO for Chaochou are usually smaller than those for Lernwu and Zhaoin because of a low level of industrial activities and a smaller population density.

2.2. One-dimensional model

A one-dimensional model is developed to help with the analysis and interpretation of measurements. It is an updated version of the 1D photochemical model developed by Trainer et al. (1987, 1991). Photochemical mechanisms and reaction rate constants are updated following those in Wang et al. (2000). The vertical

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