Fifty-Year Change in Air Pollution in Kaohsiung, Taiwan

doi:10.21203/rs.3.rs-1206643/v1

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Research Article

Fifty-Year Change in Air Pollution in Kaohsiung, Taiwan

https://doi.org/10.21203/rs.3.rs-1206643/v1

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The change in air quality in cities can be the product of regulation and emissions. Regulations require enforcement of emission reduction, but it is often a shifting economic and societal structures that influence pollutant emissions. This study examines the long-term record of air pollutants in Kaohsiung, where post-war industrialisation increased pollution dramatically, although improvements are observed in recent decades as the city moved to a more mixed economy. The study tracks both gases and particles across a period of significant change in pollution sources in the city. Concentrations of SO₂ and aerosol SO₄^2- were especially high ~1970, but these gradually declined, although the SO₄^2- to a lesser extent than its precursor, SO₂. While 21st century emissions of SO₂ and NOx have declined, this has been less so for NH₃, because it arises from predominantly agricultural sources. The atmosphere in Kaohsiung continues to have high concentrations of O₃, and these have risen in the city, likely a product of less titration by NO. The changes have meant that ozone has become an increasing threat to health and agriculture. Despite a potential for the production of (NH₄)₂SO₄ and NH₄NO₃aerosols, a product of a relatively constant supply of NH₃, visibility has improved in recent years. The emissions of SO₂ and NOx should continue to be reduced, as these strongly affect the amount of fine secondary aerosol. However, the key problem may be ozone, which is difficult to control as it needs careful consideration of the balance of NOx and hydrocarbons so important to its production.

aerosols

economic change

health effects

agricultural effects

visibility

Kaohsiung emissions and primary pollutants decline, but NOx/SO₂ andO₃ increases
Air quality change reflects regulation and a move from an industrial to mixed economy
The increasing O₃ threatens on health and agriculture
Visibility has improved despite agricultural contributions to (NH₄)₂SO₄ and NH₄NO₃

Air quality in many cities has improved in line with changes in their economy and the regulation of emissions. Declining concentrations of air pollutants, most notably SO₂, but later NOx can be ascribed to changes in industries and their control, and a more modern vehicle fleet (Brimblecombe, 2005; Power & Worsley, 2018). However, the link between emission control and reduced air pollutant concentrations is weakened because of a mediating atmospheric chemistry, best characterised by O₃ formation. Its production is affected by hydrocarbons and nitrogen oxides. Ozone concentrations can increase when nitrogen oxide emissions decrease, so air pollution regulation needs to go beyond simple emission control and requires the application of air quality management (Elsom, 1992). Particulate matter is an important contributor to air pollution, yet significant fraction of the urban aerosol is again produced through reactions of primary pollutants in the atmosphere that lead to both inorganic components (Ravishankara, 1997), such as sulfates and nitrates. Secondary sulfate aerosol was probably more abundant in the past when SO₂ levels were high. Secondary organic components best represented by the carboxylic acids, perhaps most notably low volatility dicarboxylic acids (Kawamura et al., 2001).

Taiwan (Fig. 1) experienced a rapid transformation from the largely agrarian colony left by the Japanese after World War 2, when it saw innovative expansion, such as that in the semiconductor industry. During the colonial period, Kaohsiung’s harbour became a focus for shipping and rail transport that allowed the city's development as a major hub for Taiwan’s south, with an industrial base in steel, cement, petrochemicals, paper making etc. The Taiwan Economic Miracle (Tsai, 1999), saw rapid growth of industrial infrastructure (~1960-1990), which contributed to pollutant emissions that paralleled the strengthening economy. Today, Kaohsiung remains an industrial city, though with an increasing shift in the local economy (Fig. 1c) towards financial services, tourism and the arts, with plans for the waterfront to become a landscape resource (KEC, 2021).

Post war industrialisation led to regulations needed to improve air quality, initially under the Air Pollution Control Act of 1975. The relaxation of Martial Law (late 1980s) saw newly democratised systems, and though electoral politics can stifle environmental debate, Taiwan established a cabinet-level Environmental Protection Administration (TEPA) in 1987. However, the 1975 Act was only effective after 1992, when stricter rules were implemented (Tang, 1993). Under democratisation this “credit for the improvement has been given to the air emission fee program that was first implemented” in 1995 (Tang and Tang, 2000). Before then, “the traditional command-and-control program and tax-allowance subsidy were the two major instruments used for air pollution control… ” (Shaw and Hung, 2001). Emission standards were established for power facilities 1994-05-04, while 1995-07-01 saw the introduction of an air pollution control fee for SOx emissions, and subsequent regulations to reduce volatile organic emissions (see supplementary table in Chen et al., 2014).

Postwar Kaohsiung expanded with an urban population of 168 008 in 1947 to 1 512 798 in 2017, where there were 2 776 912 in the municipal area. In parallel there was a growth of heavy industry (1976-86) and a mature stage for the heavy chemical industry (1986-96). Industrial zones were established in Fengshan (1974), Yongan and Linyuan (1974-75). Although the service industry has surpassed manufacturing in terms of value, steel, petrochemical, cement, shipbreaking, and processing exports remain substantial, though characterised by pollution. Challenge 2008: National Development Key Project encouraged investments in infrastructure such as High Speed Rail, Kaohsiung MRT, but with newer less polluting developments emerging from 2009: (i) biotechnology (ii) tourism (iii) green energy (iv) medical care (v) low intensity agriculture and (vi) cultural creativity (KCG, 2010).

The long history of industrial emissions has promoted many studies of air pollution in Kaohsiung, with deposit gauge measurements from the 1960s (Hsu and Wei, 1971; Selya, 1975; Wei, 1966), and more modern measurements from the 1970s (Chow et al., 1983). In Europe and North America there are estimates of long-term air pollutant loads in urban air using modelling (Brimblecombe, 1977), observations of smoke-days (Davidson, 1979) or pollutant deposit (Brimblecombe, 1982), but fewer from Asia (Ishikawa & Hara, 1997). Taiwan’s National Health Administration planned an optimal monitoring network for SO₂ in Kaohsiung from 1975-1977 (Liang & Lee, 1980). There is a long series of studies of health (Yang et al., 1998), visibility (Lee et al., 2005; Lee, 2006) and PAHs (Lai et al., 2017). Early problems seem dominated by primary pollutants, but the situation with ozone has caused concern across recent years (Hung-Lung et al., 2007; Shiu at al., 2007; Tsai et al., 2012). Although there are also contemporary claims that for two decades, atmospheric visibility in Kaohsiung has worsened (Lee and Lai, 2018), they are not supported by Maurer at al. (2019), who suggest 2000-2015 saw improvement in Kaohsiung, paralleling change elsewhere in Taiwan, but not fully explained by lower RH or PM₁₀.

Ammonia has not often been measured in Taiwan, but it may be important, given the density of pigs and poultry (Cheng et al., 2011). Additionally, Hsieh & Chen (2010) measured NH₃ at industrial parks in southern Taiwan at Neipu, Pingtung and Pingnan over two consecutive days each (2003-09/2004-12), finding means of 90.4 ppb, 72.8 ppb and 84.9 ppb; while at the National Pingtung University of Science and Technology campus dormitory and a bamboo grove near the village of Laopi in Pingtung County concentrations were 52.2 ppb and 4.6 ppb showing the significance in the region. Vehicles represent a potential source of NH₃ in urban areas, e.g., in urban Guangzhou vehicles produce 19% of the ammonia (Liu et al., 2014), although 2006 estimates across the Pearl River Delta suggest vehicles accounted for just 2.5% (Zheng et al., 2012).

This paper explores a fifty-year history of air pollution in Kaohsiung to understand the changes and assess the relevance of shifts in the economy and regulatory activity. The city represents an interesting and somewhat isolated location, where there have been great changes as it moved from an unregulated industrial centre for manufacturing, steel-making, oil refining and shipbuilding to a place aspiring to be noted for international exhibitions, tourism and the arts. This transition has required more stringent urban planning and concerns over air quality and visibility. Although coastal cities have been well studied, the difficulty of maintaining sites may limit measurement duration (e.g. Alastuey et al., 2004; Galindo et al., 2020), but the longer record used here means we can explore how urban transformation is reflected through a half century of development. We give special attention to changes in the threat to health, agricultural production and visibility.

2.1 Economic, air pollution and meteorological data

The project used economic data from the Kaohsiung City Government, Department of Budget, Accounting and Statistics as plotted in Fig. 1c (KCG, 2021; KCGDG, 2021) and population (DHR, 2021). A network of sites (Taiwan Air Quality Monitoring Network, TAQMN) is maintained by the Taiwan Environmental Protection Administration (TEPA) to measure air pollutants (https://airtw.epa.gov.tw). In Kaohsiung, it began with sites at Sanmin, Fengshan, Fuxing and Qianjin providing data, though incomplete, from 1984. Nanzih came two years later, and a widening network added observations from Qianzhen, Daliao, Renwu, Xiaogang, Meinong, Linyuan, Qiaotou and Zuoying, 1993 onwards (Fig. 1). Early monitoring in Kaohsiung was weighted towards crowded industrial areas of the city, but became more widespread over time, with a site placed at Meinong, a Hakka farming community on the Laonong River, 40 km from the centre of Kaohsiung. Corrections to PM_2.5 from 2014 adopted the USEPA Non-Federal Reference Method, which led to a reduction in average values (~25%), but has little effect on our work as we largely avoid using the fine particulate data. Emission estimates (https://teds.epa.gov.tw/) are tuned to meet the boundaries of current Kaohsiung City, now a county-sized special municipality (area ~2950 km²). These along with concentration (c) are plotted in Fig. 2. Aerosol composition is less frequently measured in the region, and much has been done as a part of research projects, rather than regular monitoring, though the most relevant is given in the supplement. Daily visibility data for Kaohsiung (WMO_ID:467440) were extracted as daily observations from the historical record (https://e-service.cwb.gov.tw/HistoryDataQuery/index.jsp) on the CODiS of Taiwan's Central Weather Bureau (CWB).

The number of data points was often large, so we used parametric methods (e.g. Welch’s t-test), but where small and the distribution undefined, non-parametric techniques were preferred along with the median and quartile ranges. The Wilcoxon signed rank test (rather than a t-test) was used where the data set was small and occasionally Theil Sen slopes were determined as these are more robust against outliers than a classical linear regression (Vannest et al., 2016).

The record of pollutants PM₁₀ PM_2.5, SO₂, NOx, NO₂ and O₃ as measured by the TAQMN are shown in Fig. 2.

3.1 Decadal change in primary pollutants

Monthly average pollutant concentrations from the TAQMN site in Kaohsiung and estimated emissions for the region 2002-2020 are shown in Fig. 2. There are distinct annual cycles to the primary pollutants (Fig. 2a,c,e,g), higher values occurring each winter (Lee et al, 2018; Tsai et al., 2013); seasonal cycles 2000-2020 appear as insets. The trends suggest continuous improvement to the primary pollutants, and often, though most notably for SO₂, low values are evident at the Meinong rural site (Fig. 2c). There are hints of a weaker annual cycle for rural NOx, which can be noticed mid-cycle (Fig. 2e). The SO₂ concentrations were especially high before 1994, when measurements were made at crowded urban locations: Sanmin, Fengshan, Fuixing, Qianjin and Nanzih. The concentrations typically continued to be higher than at other sites added in the record beyond 1994. However, even at these crowded sites, levels declined over time; continuing improvement perhaps a result of charges for sulfur emissions from 1995.

Although concentrations of CO, NOx and non-methane hydrocarbons (NMHCs) decreased 1995-2012, oxidants have increased (Chen et al., 2014). Ozone concentrations have risen (Fig. 2i), although the precursor NMHCs declined from ~2002 (Fig. 2j), but this would be easily understood if Kaohsiung's hydrocarbons (Chang et al., 2005) were more reactive. In the eastern parts of Kaohsiung toluene from paint and solvent industries plays an important role in O₃ production as in inland areas production is often limited by the NMHCs i.e. volatile organic compounds (Hung-Lung et al., 2007). Ozone production in the air aloft, often reflecting long range transport, can be NOx limited (Hung-Lung et al., 2007). The seasonal cycle of ozone, with a bimodal structure, is more complex than the primary pollutants (inset to Fig. 2i).

Estimated emissions from the Kaohsiung area for a range of pollutants from 2002 onward (Fig. 2b,d,f,h,j) reflect emission reduction policies (Chen et al., 2014). However, the sudden change in PM₁₀ in 2010 relates to altered assessments of industrial emissions. Some 24 kt a⁻¹ ammonia was emitted from poultry farms in Taiwan (Cheng et al., 2011), which makes Kaohsiung’s estimated emissions substantial at 18.74 kt a⁻¹ in 2002.

Selya (1975) listed the two-year average for total suspended matter as 371 µg m⁻³ and SO₄²⁻ at 21.4 µg m⁻³. Although this early SO₄²⁻ concentration is high, it seems compatible with later measurements for 1994/1995: 11.5 µg m⁻³ (Yang et al., 1998) and 1998/1999: 14.34±5.10 µg m⁻³ (Lin, 2002) as tabulated in the supplement. It is supported by the trends in SO₂ concentrations in the early part of the record (Fig. 2c) and the suggestions of high levels from Chow et al (1983).

Figure 3a shows the mole ratio of nitrogen to sulfur in the gas phase (i.e., n_NOx/n_SO2 as points and a shaded interquartile range). Measurements from the late 1960s (Selya, 1975) would suggest that n_NO3/n_SO4 was lower than at present (~0.24) in a sulfur dominated atmosphere with uncontrolled industrial use of soft coal; a cheap fuel widely used in factories, hotels, dwellings, schools etc. From the late 1960s soft coal was banned in Taipei, so some entrepreneurs in Kaohsiung may have for a short time increased its use (Selya, 1975). Such changes are attributed to the shift from coal to petroleum, and in Kaohsiung are reflected in the change in n_NOx/n_SO2 from 1.5 in the 1980s to 4.5 over the last decade.

3.2 Change in secondary pollutants

The changing volatile organic components as non-methane hydrocarbons (Fig. 2j) and the increasing dominance of NOx (Fig. 3a) encourages the formation of secondary pollutants. Toluene has proved particularly relevant to the production of ozone (Hung-Lung et al., 2007), although Kuo et al (2015) suggested that VOC did not significantly correlate with ozone fluctuations in the few episodes examined in Kaohsiung. Over the years 1994 to 2003, Shui et al (2007) noticed that the mixing ratios of NO₂ in Southern Taiwan decreased, while those for ozone increased, which could be accounted for by the reduction in NO₂, because of decreased NO titration. This is illustrated in Fig. 3b, which shows the increasing fraction of NOx present as NO₂ at urban Fuxing and in rural Meinong. Despite being in an urban area, Fuxing has become increasingly less industrial over the decades, now focussed on commercial and residential activities. Over time decreasing amounts of NOx have allowed the available O₃ to oxidise larger fractions of NO to NO₂. A quarter century back, the five-year average O₃ at the urban site of Xiaogang was 19.9 ± 6.9 ppb (1993-08/1998-07), but much higher at rural Meinong 29.4 ± 6.6 ppb. More recently (2016-01/2020-12) the differences had narrowed to 26.4 ± 7.7 ppb and 27.4 ± 6.2 ppb. The increases in urban areas are typical of Southern China where titration of O₃ by NO has decreased with declining emissions of NOx (Li et al., 2022a).

These changes are likely accompanied by the formation of secondary inorganic aerosols that have been easy to trace in Hong Kong as the record of aerosol is detailed since 1995 (Brimblecombe, 2022). The record of aerosol composition is less complete in Kaohsiung and fails to reveal a satisfying and coherent picture of change (see supplement Fig. S2), although it is likely that in the 1970s the sulfate was high.

3.3 Changing health risk

Air pollution poses both long- and short-term risks to health. The risk of daily hospital admissions from air pollution can be calculated on the basis of exposure to the pollutants and here we have adopted the method used in calculating Hong Kong’s Air Quality Health Index (GovHK, 2014). The risk is determined as the sum of percentage added health risk (R_AHR) for daily hospital admissions attributable to the 3-hour moving average concentrations of NO₂, SO₂, O₃ and particulate matter (here taken as PM₁₀). These risk factors were derived from Hong Kong health statistics and air pollution data, so will not be exact for Kaohsiung, but a reasonable proportionality is likely to exist given a similar population activity and climate. The R_AHR,i for each pollutant i, as:

R _AHR,i = 100[exp(β_ic_i)–1] …. (eq. 1)

and c_i the 3-hour moving average concentration of pollutants (µg m⁻³), with the factors: β_NO2 = 0.0004462559; β_SO2 = 0.0001393235; β_O3 = 0.0005116328 and β_PM10 = 0.0002821751 (Wong et al., 2012). Although it would make more sense to use PM_2.5 in these calculations PM₁₀ was used as the record was more complete, but the PM₁₀ can make a reasonable estimate of the health risk, and it includes PM_2.5 (Brimblecombe, 2021).

Figure 4 shows the added daily health risk averaged for each month at (a) Fuxing (b) Xiaogang and (c) Meinong. The risk is much higher at the urban site in Xiaogang, but declines over time, in much the same way as the risk in rural Meinong. The risk from PM₁₀ is relatively constant across the sites reflecting the broad distribution that arises from a multiplicity of sources. The balance of risk arises differently at the sites, so at Meinong a larger proportion comes from O₃, while the effect of SO₂ on health risk is very much lower compared with Fuxing, especially in the earlier years at this site. The proportions are clearer in the ternary plot of Fig. 4d, which shows monthly risk across the five-year period 2016-2020. It reveals the contemporary situation where the urban sites of Fuxing and Xiaogang are distinct from that in Meinong. The time trends for the years 1993-2020 in the ternary diagram of Fig. 4e. This illustrates the transition to lower risk from particulate matter, but a greater proportion of risk that arises from O₃, especially at the rural site, but the change has also been evident in the urban areas.

3.4 Agriculture

Agricultural crops are sensitive to O₃ (Fuhrer, et al., 1997), so this places pressure on food security (Wang et al., 2017). The hinterland to Kaohsiung is important in the production of a range of crops (ABKMG, 2021): fruit (banana 57 900 t a⁻¹, guava 71 468 t a⁻¹ and pineapple 56 971 t a⁻¹) and vegetables (bamboo shoots 20 497 t a⁻¹, green soybeans 19 665 t a⁻¹, tomato 12 902 t a⁻¹ and radish 11 496 t a⁻¹).

Crops accumulate damage over time, so it is usual to express the risk to vegetation in terms of an AOT40 (Accumulated Ozone exposure over a Threshold of daytime 40 ppb as ppb h). The summation is typically made over daylight hours over the growing season for the crop, although sometimes it is expressed each month. In Europe the target value is 9000 ppb h considered over five years. The long-term objective is 3000 ppb h. In Europe the growing season is typically May to July. As Taiwan is a tropical country the growing season is more difficult to define as crops tend to be grown throughout the year. The summer days are not especially long either, so daylight hours are taken as being shorter in our calculations: 07:00 to 17:00. There are two periods (Fig. 2i) of high O₃ levels, March-May and September to November (Chen et al., 2004). The three-month long AOT40 for the two urban sites and the rural site at Meinong are shown in Fig. 5ab for each O₃ season. The O₃ has increased dramatically over the late parts of the 20th century, but it is a little uncertain as records don't easily overlap so the cross checking is poor. We can see that Meinong is typically the highest and Wilcoxon signed rank test shows it to be higher (p<.0001) than both Xiaogang (1994 to 2020) and Fuxing (2004 to 2020). Despite increases in O₃ in general in the Kaohsiung area (Fig. 2i), there are hints the AOR40 is in decline particularly late in the year. The number of hours each year where late season O₃ exceeds 40 ppb is plotted in Fig. 5c, which suggests that although AOT40 might be decreasing, the number of hours above 40 ppb are relatively stable over recent years. The decline in AOT40 is mostly caused by a decrease in hours with high O₃ (i.e. >80 ppb). These have declined recently especially in the late part of the year, but since 2001, in both ozone seasons, high O₃ periods have become been less common.

There are only a few studies of O₃ and crop damage in Taiwan (e.g. Sheu & Liu, 2003). Additionally, there has been little research on major crops found in the Kaohsiung area except for some studies of soybean and tomato. Soybean shows visual damage after accruing several thousand ppb h across the growing season (Gosselin et al., 2020), conditions certainly exceeded at the Meinong site (Fig. 5a,b). Over a period of weeks, tomatoes (Lycopersicon esculentum Mill. H-11) exposed to O₃ at 200 and 350 ppb for 2.5 h at 3 days a week showed extensive foliar injury, defoliation, reduction in biomass, although fruit yield was only lower at the higher concentration (Oshima et al., 1975. However, such high values are not experienced at Meinong, and even an hourly concentration >150 ppb is found less than 40 times since 1993. Nevertheless, most years exceed the European guideline value of 9000 ppb h, especially during the September to November period. The long-term objective 3000 ppb h is always exceeded. However, the experiments of Reinert et al (1997) show that the Tiny Tim cherry tomato (L. esculentum L. cv. Tiny Tim) shows a 20% reduction in vegetative dry weight after 13 weeks exposure to just 80 ppb.

3.5 Visibility

Visibility is important as a publicly perceptible marker of air pollution over long periods (Brimblecombe, 2021), although studies from southern Taiwan do not always use the most recent data (Lee & Lai, 2018). Maurer et al (2019) were able to use the record up to 2016, which hints at the influence of PM₁₀ on visibility and supports the notion that it has generally improved in Taiwan over recent decades. Yuan et al (2006) suggest an empirical equation for the light scattering coefficient, b_sp/km⁻¹ as:

b _sp = 0.0046c_(NH)42SO4 + 0.0067c_NH4NO3 + 0.0033c_TC + 0.0032(c_PM2.5 - c₀) …. (eq. 2)

where c is the concentration of aerosol components and c₀ a remainder term. Yang et al (2005) determined the amount of (NH₄)₂SO₄ and NH₄NO₃ assuming that all SO₄ and NO₃ was present as the ammonium salt, which requires that NH₄ be in excess, a reasonable assumption in Kaohsiung, and increasingly so given the decline in SO₂ and NOx. The calculated visibility from aerosol measurements listed in the supplementary materials can be compared with observed visibility in Kaohsiung (Fig. 6). Improvement is not always dramatic, and as Li et al (2022b) show for Beijing the secondary aerosol can make contributions to visibility that can outweigh the effects of primary emissions.

The long-term change in air pollutants in Kaohsiung, shows a pattern among primary pollutants similar to other cities along with social change that has shifted fuel use (e.g., London in Brimblecombe, 2006). Pressure for regulation in Taiwan, led to reductions in concentrations of SO₂ first, with NOx and PM seeming to reach a maximum in the last decade of the 20th century. However as dramatic as many of the improvements have been, the changing economy of the city has made a significant contribution to reductions. The NO₃⁻/SO₄²⁻ ratio was probably low in the 1970s and grew after that. The NOx/SO₂ ratio in the atmosphere of Kaohsiung has increased, which follows early reduction of sulfur emissions, and enhanced by NOx from a growing vehicle fleet that has been difficult to keep in check. In Taiwan vehicle registrations are increasing at 0.24 million a year (https://tradingeconomics.com/taiwan/car-registrations), but there are additionally 0.9 million polluting motor-scooters (Everington, 2018). Despite this. the overall concentrations on NOx have declined in line with emissions (Fig. 2ef) suggesting some regulatory success in responding to an enlarged automobile fleet, i.e. private vehicles 1994: 432 228 to 2020: 763 975 (KCGDG, 2021). The Kaohsiung special municipality is agricultural, so the hinterland provides NH₃ to neutralise acidity and these emissions have changed only a little over time (Fig. 2h). Aerosol NH₄⁺ is probably insensitive to the total NH₃, but highly sensitive to total H₂SO₄ and HNO₃ (Cheng & Wang-Li, 2019). Nevertheless, the special municipality has not been able to greatly reduce its agricultural NH₃, but it is probably more critical to ensure that emissions of SO₂ and NOx continue to be reduced as these strongly affect the amount of fine secondary aerosol.

Concentrations of O₃, as a secondary pollutant, have remained high for the last quarter century and represent a risk to health and a threat to agriculture. Secondary pollutants are more difficult to control, separated as they are from their sources through a mediating chemistry. This difficulty was recognised with the discovery of photochemical smog 70 years ago (Brimblecombe, 2014). Chemical transformations mean that particulate SO₄²⁻ and NO₃⁻ concentrations might not necessarily follow regulatory improvements to their precursors. However, in Kaohsiung it is likely that over longer timescales, particulate sulfate has declined in parallel with SO₂. The Theil-Sen slope for the medians of the SO₄²⁻ suggests a decline of ~0.3 µg m⁻³ a⁻¹ from 1970, which would accumulate to ~80% over time. Since the mid-1990s it decreased from 11.5-14.3 µg m⁻³ (Lin, 2002; Yang et al., 1998) to 3.9-4.4 µg m⁻³ at present (Shen et al., 2020), i.e., ~65% decline. This is proportionally less than the decrease in SO₂, from ~25 µg m⁻³ in the 1990s to ~3 µg m⁻³ at present (~90% decrease).

This study has revealed long-term reductions in air pollution as a city transformed from an industrial base to a broader economy, with industry increasingly around sites such as Xiaogang. On a day-to-day basis, concentrations of precursors and secondary pollutants might not correlate well, but it is likely that the effects of pollution chemistry and meteorology are smoothed out over the years, so primary and secondary pollutants seem to follow similar patterns. However, the response is non-linear, so the reduction in secondary SO₄²⁻ in Kaohsiung has not been as large as the reduction in SO₂ over the last decade. In parallel, the N/S ratio has increased with the decline in sulfur emissions. Both regulation and economic changes have made improvements in air quality possible over recent decades, yet O₃ remains a problem. This secondary pollutant difficult to control as it needs careful consideration of the balance of NOx and hydrocarbons, especially as the NMHC emissions are no longer in sharp decline.

Future work could compare primary and secondary pollutant concentrations with emissions via modelling, although data for spatially resolved emission over long periods can be difficult to assemble. However, economic records could reveal fuel imports and farming statistics which would suggest the magnitude and distribution of emissions. Changes in secondary organic compounds have been neglected in this study, though it would be interesting topic for further research. Assessing the effects of emission reduction on visibility change or health outcomes is important when formulating regulatory policy, and while modelling is available to link emissions to concentrations, nonlinear effects on exposure or health outcomes can be more difficult to represent.

Ethical Approval: no human or animal subjects were used in this research

Consent to Participate: all authors agreed with being involved in the research project.

Consent to Publish: all authors agreed with the content and that all gave explicit consent to submit and that they obtained consent from the responsible authorities at the institute/organization where the work has been carried out, before the work was submitted.

Author Contributions: Conceptualisation of the investigation (CLL, PB), methodology (PB), supervision (CLL); undertaking investigation (CHL), preparing data (CHL), statistical analysis (CHL, PB), original draft (CHL, PB), figures (PB) and final editing (PB). All authors have read and agreed to the published version of the manuscript.

Funding:

Ministry of Science and Technology: MOST 109-2611-M-110-012, MOST 109-2621-M-110-005 and MOST 110-2621-M-110-009.

Competing Interests: The authors have no conflicts of interest.

Availability of data and materials: The data is publicly available as denoted by URLs in the text

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Editorial decision: Major Revision
13 Apr, 2022
Reviews received at journal
14 Mar, 2022
Reviewers invited by journal
25 Jan, 2022
Editor invited by journal
14 Jan, 2022
Editor assigned by journal
03 Jan, 2022
First submitted to journal
26 Dec, 2021

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Fifty-Year Change in Air Pollution in Kaohsiung, Taiwan

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Abstract

Figures

Highlights

1. Introduction

2. Method

2.1 Economic, air pollution and meteorological data

3. Results

3.1 Decadal change in primary pollutants

3.2 Change in secondary pollutants

3.3 Changing health risk

3.4 Agriculture

3.5 Visibility

4. Discussion

5. Conclusion

Declarations

References

Supplementary Files

Status:

Version 1