Air pollution “holiday effect” resulting from the Chinese New Year
Introduction
An anthropogenic weekend effect in ambient air quality, due to human habits associated with a weekly cycle, has been reported in various studies. Gaseous pollutants such as nitrogen oxides (NOx), carbon monoxide (CO), volatile organic compounds (VOCs) or non-methane hydrocarbon (NMHC), and sulfur dioxide (SO2), are measured to be higher on weekdays than on weekends (e.g., Bronnimann and Neu, 1997, Beirle et al., 2003, Qin et al., 2004, Riga-Karandinos and Saitanis, 2005, Steinbacher et al., 2005, Riga-Karandinos et al., 2006). Particulate pollutants, such as PM10 (particles smaller than 10 μm) and black carbon, show also a similar pattern (Morawska et al., 2002, Madhavi Latha and Badarinath, 2003). Not only in outdoor but also in indoor environments like classrooms, PM10, PM2.5 and PM1 are all higher during weekdays than during weekends (Branis et al., 2005). On the contrary, concentrations of ozone (O3), a gaseous pollutant and major secondary photochemical oxidant, reveal a more complex distribution: either lower on weekdays than on weekends (e.g., Cleveland et al., 1974, Lebron, 1975, Bronnimann and Neu, 1997, Diem, 2000, Qin et al., 2004, Riga-Karandinos and Saitanis, 2005, Tsai, 2005, Riga-Karandinos et al., 2006), or vice versa—higher on weekdays than on weekends (Bronnimann and Neu, 1997, Diem, 2000). Although the former is more commonly seen, especially over urban areas, it is odd that O3 precursor substances, such as NOx and NMHC, are decreased on weekends, while O3 is simultaneously increased.
Several hypotheses have been proposed to explain this paradox: (1) a nonlinear relation between O3 and its precursors NOx and NMHC—lower NOx on weekends resulting in a faster ozone production rate in NOx-saturated (VOC-limited) areas (e.g., Liu et al., 1987, Sillman, 1999, Blanchard and Fairley, 2001, Qin et al., 2004, Riga-Karandinos and Saitanis, 2005, Murphy et al., 2007); (2) an NO titration effect – lower NOx leading to less ozone destruction on weekends (Qin et al., 2004, Chou et al., 2006, Moreno et al., 2006, Murphy et al., 2007); (3) aerosol and UV radiation—a reduced aerosol burden on weekends allowing more sunshine to initiate more ozone production (California Air Resources Board, 2003, Murphy et al., 2007); (4) NOx timing—a pairing of the timing between delayed weekend traffic patterns and stronger sunshine giving more efficient ozone production (California Air Resources Board, 2003, Murphy et al., 2007); and (5) carry-over emissions: higher traffic volumes on Friday evenings than on weekends, causing more O3 precursors on weekends (California Air Resources Board, 2003, Murphy et al., 2007).
The changes of most gaseous pollutant emissions during weekdays and weekends are directly or indirectly linked to changes in traffic (Pont and Fontan, 2001, Beaney and Gough, 2002, Morawska et al., 2002, Beirle et al., 2003, Riga-Karandinos and Saitanis, 2005) and to industrial emissions (Beirle et al., 2003, Steinbacher et al., 2005). In general, the dominant anthropogenic emissions of NOx, CO, and NMHC are from vehicle exhausts (e.g., Seinfeld and Pandis, 1998, Morawska et al., 2002, Streets et al., 2003, Yang et al., 2005, Kato and Akimoto, 2007). SO2 can be produced by coal-fired power plants (e.g., Brasseur et al., 1999, Streets et al., 2003, Moreno et al., 2006, Kato and Akimoto, 2007). Tropospheric ozone is mainly produced from photochemical reactions (e.g., Chameides et al., 1992, Sillman, 1999, Dodge, 2000, Hidy, 2000). Ozone has also been reported to be affected by meteorological conditions (Bronnimann and Neu, 1997, Diem, 2000) and the light scattering of fine particles (Qin et al., 2004). PM10 has more diversified sources, such as vehicle emissions, industrial activity, wind-blown soil and dust, construction dust, sea spray and biomass burning (e.g.; Streets et al., 2003, Madhavi Latha and Highwood, 2006, Moreno et al., 2006). The weekend–weekday differences of particulate pollutant emissions are associated with outdoor traffic conditions (Bhugwant et al., 2000, Morawska et al., 2002, Madhavi Latha and Badarinath, 2003, Madhavi Latha and Highwood, 2006), and indoor population density (Branis et al., 2005).
Besides local emissions, pollution sources can be the long-range transport of aerosol and air pollutants (e.g., Rao et al., 1997, Pochanart et al., 1999, Huser et al.,, Lin et al., 2005, Chou et al., 2006, Moreno et al., 2006). Compared to NOx, other pollutants, such as SO2, PM10 and CO, which have a lifetime of about 2 days, 1–2 weeks and 1–2 months, respectively, are relatively long-lived air pollutants and could be indicators of the long-range transport of air pollutants (e.g., Liu et al., 1987, Jaffe et al., 1997, Seinfeld and Pandis, 1998, Kato et al., 2004, Lin et al., 2005).
Weekend–weekday differences have also been reported in meteorological parameters, such as diurnal temperature range (Forster and Solomon, 2003, Gong et al., 2006a), daily minimum temperature (Forster and Solomon, 2003), daily maximum temperature (Gong et al., 2006a), daily precipitation frequency (Gong et al., 2006b), annual precipitation and tropical cyclonic mean maximum wind speed (Cerveny and Balling, 1998), seasonal rainfall, seasonal maximum and minimum temperature (Simmonds and Keay, 1997), and visibility (Tsai, 2005). Most differences between weekdays and weekends are statistically significant, but their sign might be opposite for different regions or seasons (Forster and Solomon, 2003, Gong et al., 2006a). This “weekend effect” observed in meteorological parameters has been attributed to anthropogenic influences (Cerveny and Balling, 1998, Beaney and Gough, 2002, Forster and Solomon, 2003).
The weekend effect of air pollutant concentrations and meteorological parameters is a useful tool to detect the influence of human-related activities on the environment and climate system. This effect depends an individual country's working and non-working lifestyle, degree of industrialization, or religious and cultural background (Forster and Solomon, 2003, Beirle et al., 2003), so it may not be consistently observed everywhere in the world. Thus, other similar effects, such as a “holiday effect”, defined as differences in air pollutant concentrations or meteorological parameters between holiday and non-holiday periods, provide a possibility to study such phenomena for regions not showing a clear weekend effect. The holiday effect has not been explored as much as the weekend effect. Some researchers studied the impact of the Christmas holiday on air quality, but primarily focused on PM10, or black carbon, with data spanning one or two years (Bhugwant et al., 2000, Madhavi Latha and Highwood, 2006).
Our study is an attempt to conduct a comprehensive and systematical examination of the holiday effect. Six major air pollutants were analyzed, with an emphasis on Chinese New Year (CNY) in Taipei for the past thirteen years. The differences in air pollutant concentrations between the CNY and non-Chinese New Year (NCNY) periods in the daily (24-h) mean, diurnal cycle and interannual variation are given respectively in Section 3, followed by conclusions.
Section snippets
Description of the Chinese New Year holiday
The Chinese New Year or Spring Festival is the most important holiday for the Chinese people, who heavily emphasize family values, making family reunions an important tradition during the CNY period. Right before the CNY period, many residents and non-residents are used to travel back to their hometowns for family reunions. After the CNY period, they return to the city for school and work. Thus, we usually observe a significant reduction of motor vehicles in our study area during the CNY period
The CNY–NCNY differences in pollutant concentrations
Daily (24-h) mean concentrations of the monitored pollutants are presented in Table 2. The differences in air pollutants between the CNY and NCNY periods, individually in the majority of the stations but also averaged over all stations, were statistically significant (p < 0.05).
Table 2 shows that the daily means of NOx, CO, NMHC, SO2 and PM10 were lower in the CNY than in the NCNY period, which implies that local emissions were the major sources for these air pollutants. The reductions of those
Conclusions
Hourly and daily surface measurements of six air pollutants from thirteen TEPA air quality monitoring stations in 1994–2006 were used to study the holiday effect, especially that of Chinese New Year, over the Taipei metropolitan area. The main conclusions from this study are as follows:
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An air pollution “holiday effect”, the differences between holiday and non-holiday periods, was found in the daily (24-h) mean, the diurnal cycle and the interannual variation. Five air pollutants, NOx, CO, NMHC,
Acknowledgments
We extend special thanks to the Taiwan Environmental Protection Administration for providing useful surface measurements of air pollutants. This work was supported by the National Science Council Grants 97-2111-M-415-001 and 97-2628-M-001-002. Comments from two anonymous reviewers were very helpful for improving the quality of this paper.
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