Ozone variations through vehicle emissions reductions based on air quality monitoring data in Taipei City, Taiwan, from 1994 to 2003

doi:10.1016/j.atmosenv.2006.01.038

Atmospheric Environment

Volume 40, Issue 19, June 2006, Pages 3513-3526

https://doi.org/10.1016/j.atmosenv.2006.01.038 Get rights and content

Abstract

The ozone (O₃) ambient air quality is evaluated using two indices in Taiwan: the daily maximum O₃ concentration (O_3, max) greater than 120 ppb and the daily 8-h O₃ moving average (O_3, 8 h) greater than 60 ppb. Results of air quality monitoring data from 1994 to 2003 in Taipei City indicate that the 99th percentile O_3, max was reduced, but those below the 90th percentile O_3, max obviously increased and days with O_3, 8 h greater than 60 ppb were doubled after the reduction of O₃ precursor emissions. Because of Taipei's unique vehicle emissions pattern in having more motorcycles than cars, Sunday, as compared to weekdays, has more reductions of non-methane hydrocarbons (NMHCs) than NO_X. This results in Sunday O_3, max being lower than weekday O_3, max average for those days with O_3, max exceeding 80 ppb. On the contrary, if O_3, max is lower than 60 ppb, Sunday O_3, max is relatively higher than weekday O_3, max average, due to the reduced NO titration effect. In addition, our evaluations of O₃ levels with respect to the ambient air quality standards show that the number of Sundays with O_3, max greater than 120 ppb is fewer than the number average of weekdays with O_3, max exceeding 120 ppb for every month. Concerning O_3, 8 h, only in June, July, and August, when the meteorological conditions favor O₃ production, does the reduction of Sunday vehicle exhaust reduce the number of days with O_3,8h greater than 60 ppb. However, in September and October, the reduction of Sunday vehicle exhaust will increase the number of days. The results presented in this paper indicate that the O₃ pollution in Taipei City is VOC-limited. Thus, for a greater reduction in NMHCs, like the Sunday vehicle reduction in Taipei City, the upper percentile O_3, max is reduced more and those below the moderately high percentile O_3, max are increased less.

Introduction

Taipei City has 700,000 cars and more than 1 million motorcycles, in addition to the vehicles used by the residents commuting to Taipei City from neighboring Taipei County (Taiwan EPA, 2004). This high vehicle density—6536 motor vehicles per square km—leads to severe air pollution. The government of Taiwan revised its air quality standards in 1992 and implemented multiple control strategies to reduce air pollution. For many years, the air quality in Taipei City showed obvious improvement; the ratio of pollutant standards index (PSI) greater than 100 dropped from 4.01% in 1994 to 1.96% in 2003. However, the pollution levels after emissions reduction still exceed the air quality standards in some instances. Among those criteria pollutants, ozone (O₃) is the most frequently occurring pollutant, followed by particles with an aerodynamic diameter less than or equal to 10 μm (PM₁₀), to cause poor air quality with PSI greater than 100 (Taiwan EPA, 2004).

According to the source inventory from the Taiwan Environmental Protection Administration (TEPA), carbon monoxide (CO) and nitrogen oxides (NO_X) emitted from motor vehicles constitute 98% and 77% of the total CO and NO_X emissions in Taipei City, respectively (Taiwan EPA, 2004). Concentrations of ambient pollutants show maximum levels during morning rush hours and also demonstrate seasonal variations. Various primary pollutants tend to accumulate when the meteorological conditions in March and April are unfavorable to dispersion, thus causing high pollution concentrations. The O₃ concentration caused by photochemical reactions often exceeds the air quality standards of both 120 ppb for the daily maximum O₃ concentration (O_3, max) and 60 ppb for the daily maximum 8-h O₃ moving average (O_3, 8 h) from May to August (Taiwan EPA, 2004).

The variation of O₃ concentration is influenced by the sources of O₃ precursor emissions such as NO_X and non-methane hydrocarbons (NMHCs), as well as by meteorological conditions. The US Environmental Protection Agency (US EPA) has implemented “ozone action days” to encourage the public to share a ride, take public transportation, walk, or ride a bike when meteorological conditions become favorable for O₃ formation. The purpose is to reduce O₃ precursor emissions so as to keep the O₃ concentration below the unhealthy level. However, since O₃ formation reactions are non-linear (Brönnimann and Neu, 1997), even though the Sunday emission is only 60% of Friday emission, 85% of days show no obvious difference in O_3, max; the remaining 15% of the days have less than 20% difference in O_3, max. Therefore, the effectiveness of reducing local traffic volume intermittently—like the typical reduction that occurs between Fridays and Sundays—during an O₃ episode in order to lower O₃ formation is limited (Pont and Fontan, 2001). Unlike primary pollutants, the O₃ concentration does not show obvious weekly cycles (Jenkin et al., 2002). However, the presence of the weekend vehicle reduction provides a simple clue to whether an area is NO_X- or VOC-limited with respect to O₃ formation (Altshuler et al., 1995).

The objectives of this work are to study the characteristics of O₃ pollution from 1994 to 2003 in Taipei City using the drastic difference between Sunday and weekday concentrations of precursor pollutants and the unique vehicle emissions pattern in having more motorcycles than cars, and to analyze the seasonal variations of days with O₃ concentrations exceeding either the O_3, max or O_3, 8 h standard. The results will provide the necessary information for drafting an O₃ control strategy in Taipei City.

Section snippets

Area of study and measurements

TEPA established the Taiwan Air Quality Monitoring Network (TAQMN) in September 1993, which consists of 74 monitoring stations around the island. Five ambient automated air quality monitoring stations are set up in Taipei City on top of school buildings, at least 15 m above ground level. Taipei City, a basin located in northern Taiwan (Fig. 1), with a population of 2.62 million in 272 km², is the area of study. It has 6536 motorized vehicles per square km, with 700,000 cars and more than 1

Emission characteristics of Taipei's motor vehicles

According to TEPA's year 2000 source inventory, the total NMHCs emissions from motorcycles are 11,042 tons year⁻¹, twice the total from small passenger cars (5546 tons year⁻¹) and even greater than the total (9238 tons year⁻¹) from all other motor vehicles, including small passenger cars, buses, taxis, and trucks (Taiwan EPA, 2004). The contributions of NO_X and NMHCs from mobile, fugitive, and stationary sources are 77% and 41%, 14% and 58%, 9% and 1%, respectively. Since the contributions from

O₃ exceedance of air quality standards

Taipei air quality has been obviously improved in the last decade (Chang and Lee, 2006). However, unlike the decreasing primary pollutant concentrations, the O_3, max concentrations—apart from the trend of the extremely high values—show an increasing trend in the last decade. Statistic results show that although the extremely high values such as the 99th and the 98th percentiles of O_3, max appear unchanged or slightly decreasing, the medium as well as moderately high O₃ concentrations such as the

NO_X reduction on O₃ diurnal variation over years

In order to understand the influence of decreasing NO titration effect from the reduction of vehicle emissions on the increasing O₃ concentration in Taipei City, the data are classified into three periods—1994–1996, 1997–2000, and 2001–2003—for analyzing the annual changes in the diurnal variations of NO_X and O₃, as shown in Figs. 4(a) and (b). Fig. 4(a) shows that NO_X comes mainly from vehicle emissions in Taipei City; its concentration is the highest during the morning rush hours. At noon,

Discussion

The northern part of Taiwan is frequently under the influence of a northeasterly monsoon (from winter to spring), which not only brings cold air but also transports O₃ and dust occasionally from China over a long distance to Taiwan (Lin et al., 2005; Lin, 2001). However, the long-range transport dominated by the northeasterly monsoon is characterized by a strong wind and a low temperature, which is unfavorable for the formation and accumulation of high local O₃ concentrations.

It is our

Conclusions

Results of this work show that the number of days with O_3, max greater than 120 ppb has not obviously increased in Taipei City for the last 10 years, but days with O_3, 8 h exceeding 60 ppb have doubled. This indicates that O₃ pollution control should emphasize the long-term-exposure health effects in addition to the acute health effects of the maximum O₃ concentration that exceeds the standard. The Sunday pollution reduction may lead to a variety of O₃ reductions at different levels of photochemical

Acknowledgments

We are grateful for the monitoring data provided by the Taiwan Environmental Protection Administration (TEPA). Although the data were taken from TEPA's official monitoring network, the results of this paper are not peer-reviewed by TEPA, and the mention of instrument trade names does not constitute the endorsement of TEPA. We would also like to show our gratitude for the contributions from two anonymous reviewers and Dr. Junker.

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Ozone variations through vehicle emissions reductions based on air quality monitoring data in Taipei City, Taiwan, from 1994 to 2003

Abstract

Introduction

Section snippets

Area of study and measurements

Emission characteristics of Taipei's motor vehicles

O3 exceedance of air quality standards

NOX reduction on O3 diurnal variation over years

Discussion

Conclusions

Acknowledgments

Atmospheric Environment

Atmospheric Environment

Atmospheric Environment

Atmospheric Environment

Atmospheric Environment

Atmospheric Environment

Atmospheric Environment

Atmospheric Environment

Atmospheric Environment

Atmospheric Environment

O₃ exceedance of air quality standards

NO_X reduction on O₃ diurnal variation over years