Tai-Yi Yu1, How-Ran Chao This email address is being protected from spambots. You need JavaScript enabled to view it.2,3,4,5, Ming-Hsien Tsai6, Chih-Chung Lin2, I-Cheng Lu2, Wei-Hsiang Chang7, Chih-Cheng Chen8, Liang-Jen Wang9, En-Tzu Lin2, Ching-Tzu Chang3, Chunneng Chen10, Cheng-Chih Kao11, Wan Nurdiyana Wan Mansor12,13, Kwong-Leung J. Yu11,14 

1 Department of Risk Management and Insurance, Ming Chuan University, Shilin District, Taipei 111, Taiwan
2 Department of Environmental Science and Engineering, College of Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan
3 Institute of Food Safety Management, College of Agriculture, National Pingtung University of Science and Technology, Pingtung 912, Taiwan
4 Emerging Compounds Research Center, General Research Service Center, National Pingtung University of Science and Technology, Pingtung 912, Taiwan
5 School of Dentistry, College of Dental Medicine, Kaohsiung Medical University, Kaohsiung 80708, Taiwan
6 Department of Child Care, College of Humanities and Social Sciences, National Pingtung University of Science and Technology, Pingtung 912, Taiwan
7 Department of Food Safety/Hygiene and Risk Management, National Cheng Kung University, Tainan 70101, Taiwan
8 Section of Neonatology, Department of Pediatrics, Kaohsiung Chang Gung Memorial Hospital and Chang Gung University College of Medicine, Kaohsiung 83347, Taiwan
9 Department of Child and Adolescent Psychiatry, Kaohsiung Chang Gung Memorial Hospital and Chang Gung University College of Medicine, Kaohsiung 83347, Taiwan
10 JS Environmental Technology and Energy Saving Co. Ltd., Kaohsiung 806, Taiwan
11 Superintendent Office, Pingtung Christian Hospital, Pingtung 90053, Taiwan
12 Faculty of Ocean Engineering Technology & Informatics, Universiti Malaysia Terengganu, 21300, Malaysia
13 Air Quality and Environment Research Group, Universiti Malaysia Terengganu, 21300, K. Nerus, Malaysia
14 Department of Anesthesiology, College of Medicine, Kaohsiung Medical University, Kaohsiung 80708, Taiwan


Received: January 30, 2021
Revised: April 21, 2021
Accepted: April 24, 2021

 Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.


Download Citation: ||https://doi.org/10.4209/aaqr.210020  


Cite this article:

Yu, T.Y., Chao, H.R., Tsai, M.H., Lin, C.C., Lu, I.C., Chang, W.H., Chen, C.C., Wang, L.J., Lin, E.T., Chang, C.T., Chen, C., Kao, C.C., Wan Mansor, W.N., Yu, K.L.J. (2021). Big Data Analysis for Effects of the COVID-19 Outbreak on Ambient PM2.5 in Areas that Were Not Locked Down. Aerosol Air Qual. Res. 21, 210020. https://doi.org/10.4209/aaqr.210020


HIGHLIGHTS

  • Big data analysis used to examine PM2.5 during pre-COVID-19 and post-COVID-19.
  • Low-cost PM2.5 sensors investigating PM2.5 patterns during pre- and post-COVID-19 situation.
  • A slight reduction of PM2.5 from January to March in 2020 compared with 2019.
  • Similar PM2.5 patterns observed in the industrial areas in north and south Taiwan in 2019 and 2020.
  • PM2.5 decline during COVID-19 due to decreased domestic emissions of PM2.5 and its precursors.
 

ABSTRACT


COVID-19, which is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), first broke out at the end of 2019. Despite rapidly spreading around the world during the first half of 2020, it remained well controlled in Taiwan without the implementation of a nationwide lockdown. This study aimed to evaluate the PM2.5 concentrations in this country during the 2020 COVID-19 pandemic and compare them with those during the corresponding period from 2019. We obtained measurements (taken every minute or every 3 minutes) from approximately 1,500 PM2.5 sensors deployed in industrial areas of northern and southern Taiwan for the first quarters (January–March) of both years. Our big data analysis revealed that the median hourly PM2.5 levels decreased by 3.70% (from 16.3 to 15.7 µg m–3) and 10.6% (from 32.4 to 29.3 µg m–3) in the north and south, respectively, between these periods owing to lower domestic emissions of PM2.5 precursors (viz., nitrogen dioxide and sulfur dioxide) and, to a lesser degree, smaller transported emissions of PM2.5, e.g., from China. Additionally, the spatial patterns of the PM2.5 in both northern and southern Taiwan during 2020 resembled those from the previous year. Finally, controlling local PM2.5 emission sources critically contributes to reducing the number of COVID-19 cases.


Keywords: COVID-19, SAS-cov-2, PM2.5, Low-cost sensors, Domestic emission, Big data


1 INTRODUCTION


Since the sudden outbreak of the COVID-19 airborne disease, which is caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus that initially emerged in December 2019, the World Health Organization (WHO) has reported approximately 142,057,939 confirmed cases and 3,034,210 deaths worldwide as of April 19, 2021 (Li et al., 2020d; WHO, 2021). Taiwan has experienced two waves of COVID-19 cases. The Taiwan Center for Disease Control (TCDC) announced 1,076 confirmed cases and 11 deaths by April 19, 2021 (TCDC, 2020). At that time, the TCDC announced guidelines, including wearing a mask and social distancing to help stop the spread of SARS-CoV-2 virus and cellular broadcast services to advise threat-risk population and reduce potential exposure to SARS-CoV-2 virus in densely populated areas starting in early February 2020. Even though the highly infectious SARS-CoV-2 virus continues to spread around the world, Taiwan has successfully controlled the pandemic and minimized the impact on Taiwanese daily life through a policy of transparency and honesty about COVID-19. Unlike other regions, Taiwan is available for human activities, including industrial production without any lockdown process excluding overseas visitors.

Several reports indicated that during normal talking and breathing, individuals infected with COVID-19 could easily generate small droplets of SARS-CoV-2, which are possibly transported by aerosol and remain on suspended particulate matter (PM) including PM2.5 in the air for hours and travel over social distancing (1.5 or 2 m or 6 feet) (Anderson et al., 2020; Banik and Ulrich, 2020; Doremalen et al., 2020; Setti et al., 2020a, c; Wang and Du, 2020). Experimental results supported the existing evidence of accelerated transmission of the SARS-CoV-2 virus by PM2.5, particularly in northern Italy and in Wuhan in China (Sharma and Balyan, 2020). These small particulates carry the SARS-CoV-2 virus in indoor environments and diffuse it more than 10 m from the source (Setti et al., 2020b). Especially in poor indoor environments with habitual indoor burning of incense, such as in Middle Eastern countries (Amoatey et al., 2020), it is possible to spread the COVID-19 disease through heavy PM2.5 and PM10 pollution. Based on the current global data, air pollution, such as that caused by PM2.5 and PM10, may be associated with COVID-19 mortality rates or incidence if population density, racial segregation, socioeconomic position, and meteorological variables, such as temperature and humidity, are not considered (Brandt et al., 2020; Jiang et al., 2020). Wu et al. (2020) revealed the positive association of PM2.5 with COVID-19 deaths based on a 1 µg m–3 increase in PM2.5, leading to a 15% increase in mortality in the United States. Sharma and Balyan (2020) indicated that high PM2.5 might accelerate COVID-19-related mortality based on experience with increases in air-pollution-related comorbidities after humans were exposed to high levels of PM2.5. Scientists have warned that future works need to consider whether air pollution, such as PM2.5, directly affects COVID-19 mortality independent of population density, racial and socioeconomic segregation, cardiopulmonary disease, and meteorological conditions (Brandt et al., 2020; Jiang et al., 2020; Saadat et al., 2020).

Several reports have indicated that air quality was improved after lockdown related to COVID-19 (Bao and Zhang, 2020; Collivignarelli et al., 2020; Dantas et al., 2020; Dutheil et al., 2020; Kerimray et al., 2020; Li et al., 2020a; Li and Tartarini, 2020; Mahato et al., 2020; Nakada and Urban, 2020; Otmani et al., 2020; Saadat et al., 2020; Sharma et al., 2020; Tobías et al., 2020; Wang and Su, 2020; Wang et al., 2020a, c). Notable reductions in PM2.5, PM10, SO2, NO2, NO, and CO were observed. A study from Ecuador showed that a large drop in NO2 (68%), SO2 (48%), CO (38%), and PM2.5 (19%) was found in Quito during the first month of quarantine after COVID-19 outbreak compared with pre-COVID-19 period (Zalakeviciute et al., 2020). Ma and Kang (2020) indicated that levels of PM2.5 and NO2 were obviously reduced in Wuhan by 29.9% and 53.2%, respectively, after lockdown and decreased in Daegu by 20.9% and 19.0%, respectively, and Tokyo by 3.6% and 10.4%, respectively, after self-quarantine between January 23 and April 30 in 2020. Agarwal et al. (2020) revealed the average drop of PM2.5 and NO2 in six cities (Xiangyang, Jingzhou, Huanggang, Xiaogan, Wuhan, and Yichang) in Hubei Province, China, and six megacities (Delhi, Lucknow, Kolkata, Mumbai, Chennai, and Jaipur) in India by 11.3% and 48.6% for PM2.5 and NO2 in China as well as by 20.2% and 59.3% in India after a week of lockdown for COVID-19 issues. The Singaporean report showed the large reduction of PM2.5, PM10, SO2, NO2, and CO by 29%, 23%, 52%, 54%, and 6%, respectively, while the increment fold of ozone (O3) was 1.18 times after lockdown compared with pre-lockdown periods from April 2016 to May 2020 (Li and Tartarini, 2020). The decline of O3 in the ambient air after lockdown implementation might be mainly due to lower titration O3 by NOx (Bedi et al., 2020; Brimblecombe et al., 2020; Kumari et al., 2020b). At the same time, O3 was significantly increased due to reductions in human and economic activities, traffic restrictions, and decreases in energy consumption during lockdowns for COVID-19 in various countries in comparison to the pre-lockdown period (Collivignarelli et al., 2020; Dantas et al., 2020; Kerimray et al., 2020; Li et al., 2020a, b; Mahato et al., 2020; Nakada and Urban, 2020; Otmani et al., 2020; Sharma et al., 2020; Wang et al., 2020c). The improvement of air quality may be associated with the reduced emissions from transportation, which is related to NO2 reductions, and the manufacturing industry, which has been linked to reductions of PM2.5 and CO in China during the COVID-19 outbreak (Wang et al., 2020c). The effects of lockdowns due to COVID-19 on O3 production in southern European cities (Turin, Valencia, Rome and Nice), as well as in Wuhan, China, may be a reasonable explanation for the unpredicted decrease in NOx emissions due to lowering ozone generation by NO (Sicard et al., 2020). Generally, the mechanism of ground-level O3 formation and depletion is complicated to be associated with the secondary pollutants which are formed by photochemical reaction of NOx and volatile organic compounds (VOCs) due to natural sources or human activities (Bedi et al., 2020). The possible explanation for the generation of O3 related to COVID-19 is the lower PM2.5 loading, which decreases the sink for hydroperoxy radicals (HO2) (Wang et al., 2020b). Meteorological interference or variations should also be taken into account in terms of their air quality impacts during this outbreak (Dantas et al., 2020; Kerimray et al., 2020; Li et al., 2020b). Kerimray et al. (2020) indicated that benzene and toluene levels in the COVID-19 period had 2–3-fold higher magnitudes compared with those in the same seasons from 2015 to 2019. These sources contribute substantially from various non-traffic-related sources, such as combined coal-fired heat and power plants and household heating systems (Kerimray et al., 2020). Long-range transport aerosols contribute to air pollution, particularly in the case of PM2.5 and PM10 in receptor areas, and in turn decrease the benefits of local emission reductions related to COVID-19 (Li et al., 2020a; Otmani et al., 2020). Obvious reductions in PM, such as emission reductions in transportation and slight reductions in industrial sources, in cities in central or southern China, did not lead to avoiding the PM loadings due to severe air pollution in northern China via long-range transport during the lockdown period (Griffith et al., 2020; Li et al., 2020a). To summarize the current global data, lockdowns or quarantining for COVID-19 have contributed to a positive impact on improvements in air quality (Nakada and Urban, 2020; Wang et al., 2020c), recovery from climate change (Rosenbloom and Markard, 2020), and attenuation of environmental impacts (Saadat et al., 2020).

The present study was focused on changes in PM2.5 from January to March in 2019 and 2020 in selected industrial areas in northern and southern Taiwan, where no lockdown or quarantine was implemented during the COVID-19 outbreak. A huge amount of data on PM2.5 was recorded and provided by the PM2.5 sensors in the selected areas to further examine PM2.5 concentrations in the first quarter (from January to March) of 2019 and 2020 and evaluate the effects of PM2.5 loading from domestic sources and transboundary transportation during the COVID-19 outbreak.

 
2 METHODS


 
2.1 Data Collection from PM2.5 Sensors

The Taiwanese Environmental Protection Administration (TEPA) established 10,200 air quality sensors in Taiwan before 2020. More than 3,200 air quality micro-stations are located in northern and southern Taiwan (Fig. 1; Civil IoT Taiwan, 2020). An air quality micro-station to assess PM2.5 is set up at the height of 3 m from the ground in industrial areas to gather data following the TEPA standard. The PM2.5 data from PM2.5 sensors were freely announced to the public on the Civil IoT Taiwan Data Service Platform website. PM2.5 data from air quality micro-stations are generated every 1 or 3 minutes. The PM2.5 from the network for air quality micro-stations developed by the TEPA was aimed at investigating PM2.5 emissions and levels in hotspot areas to identify and trace PM2.5 emission sources in these regions. The hourly average PM2.5 concentrations are calculated based on acceptable PM2.5 data. More than 75% of all hourly average concentrations in a sensor are accepted as effective PM2.5 data over an hour based on the TEPA standard. Based on the previous criteria, PM2.5 micro-sensor stations were selected at 609 and 891 stations for northern and southern Taiwan, respectively, from January to March in 2019 and 549 and 872 stations for northern and southern Taiwan, respectively, from January to March in 2020 (Table 1). The hourly average PM2.5 levels were collected between January and March in 2019 and 2020. For standardization of PM2.5 data quality and assurance of the reliability of PM2.5 sensors, the TEPA regulates whether the data meet the standard through certified laboratory certification, field certification, and parallel comparisons between a TEPA air pollution monitor site and an air sensor. The PM2.5 data taken from air sensors were collected daily between January and March in 2019 and 2020 for further statistical analyses. The PM2.5 data in the cloud system were gathered and transferred to statistical software, including Statistical Analysis System (SAS) and Microsoft Excel, based on time series before the big data analysis. More than 63,000,000 and 59,000,000 units of data on PM2.5 from the air quality micro-stations were gathered in the first quarter of 2019 and 2020, respectively.

Fig. 1. The location and distribution of PM2.5 sensors in the industrial areas from north and south Taiwan (DY: Dayuan, GS: Guishan, KI: Kuangin, KD: Kaohsiung downtown, LK: Linkou, NK: north Kaohsiung, PZ: Pingzhen, PN: Pingnan (southern Pingtung)).Fig. 1. The location and distribution of PM2.5 sensors in the industrial areas from north and south Taiwan (DY: Dayuan, GS: Guishan, KI: Kuangin, KD: Kaohsiung downtown, LK: Linkou, NK: north Kaohsiung, PZ: Pingzhen, PN: Pingnan (southern Pingtung)).

Table 1. Hourly average levels of PM2.5 from air quality micro stations between January and March in 2019 and 2020 located in the industrial areas from northern and southern Taiwan.

 
2.2 Statistical Analysis

A descriptive analysis, including mean, standard deviation (SD), median, mode, and standard error (SE), was used on the PM2.5 data in the first stage. The probability density distribution of PM2.5 in the first quarter of 2019 and 2020 and the difference between the probability density distribution in 2019 and 2020 were compared, where the median PM2.5 in 2019 minus the median PM2.5 in the corresponding period of 2020 (PM2.5/2019–PM2.5/2020) were calculated. An unrotated principal component analysis (PCA) was used to determine the initial PM2.5 conditions by replacing the original complex variables with linear combinations of major components. The unrotated PCA maximizes the sums of the root mean square (RMS) of the correlation coefficient (CV), whereas the PCA (varimax method) maximizes the variance of the squared correlation coefficients in the rotated principal components (RPCs) (Kaiser, 1958; Horel, 1981). In order to interpret PM2.5 dependence or independence of RPCs between 2019 (pre-COVID-19 periods) and 2020 (COVID-19 outbreak), thousands of PM2.5 data from the sensors in the industrial areas from northern and southern Taiwan were used. The maximum of the CV second-order moment causes the CVs to generate a wide distribution of RPCs and measurable variables. However, the factor loadings of most measurable variables were zero, and few of them presented the high loading magnitudes among the RPCs that would easily simplify the explanation of the relationship between the RPCs and the measurable variables. A time series normalization for the PM2.5 data was thus needed before the RPCs could be determined (Eq. (1)).

where Zik represents the Z score of kth time series PM2.5 value from air quality micro-station i; Cik represents the kth time series PM2.5 concentrations from air quality micro-station i; µi represents the mean PM2.5 concentrations at air quality micro-station i; and Si represents the standard deviation of PM2.5 concentrations at air quality micro-station i. The association of the RPCs and Z scores could be expressed as in Eq. (2):

 

where Lij represents the factor loadings of the jth RPCs from air quality micro-station i, and Pjk represents the kth observation value of the jth RPC.

All statistical analyses were tested using SAS JMP (NC, USA). The figures were drawn using SigmaPlot 14.0 software (Systat Software Inc., CA, USA).


3 RESULTS


 
3.1 Hourly PM2.5 in Air Quality Micro-stations Located in Northern and Southern Taiwan

As shown in Table 1, the median hourly PM2.5 concentrations from the air quality micro-stations (air sensors) in the industrial areas were 16.3 and 32.4 µg m–3 between January and March of 2019 in northern (2019N) and southern (2019S) Taiwan, respectively, and were 15.7 and 29.3 µg m–3 in 2020 in northern (2020N) and southern (2020S) Taiwan, respectively. Compared with the corresponding period in 2019, the median PM2.5 was reduced by 3.70% and 10.6% in northern and southern Taiwan, respectively, in 2020 during the pre- and post-COVID-19 outbreak. When considering the total hourly average PM2.5 data in the big data comprising more than 5,000,000 measurements, the reduction in PM2.5 from 2019N to 2020N was 1.70%, and that from 2019S to 2020S was 4.42%, respectively.

 
3.2 Probability Density Distribution of Hourly PM2.5 in Northern and Southern Taiwan

The changes in the probability density distribution for hourly PM2.5 data in the industrial areas of northern and southern Taiwan from January to March in 2019 and 2020 are shown in Fig. 2. A unimodal distribution was found in these four probability density distribution modes, including 2019N, 2019S, 2020N, and 2020S (Fig. 2(A)). The median and CV of the hourly PM2.5 values in northern Taiwan were lower than those in southern Taiwan. The mode for hourly PM2.5 decreased from 14 µg m–3 in 2019N to 12 µg m–3 in 2020N, and from 34 µg m–3 in 2019S to 26 µg m–3 in 2020S, respectively. The modes were consistent: 9.2% (2019N) and 10.2% (2020N) of the total PM2.5 in northern Taiwan, and 4.2% (2019S) and 5.1% (2020S) of the total PM2.5 in southern Taiwan. According to the air quality guidelines of PM2.5 established by the WHO, PM2.5 was not to exceed 10 and 25 µg m–3 for the annual and 24-hour means, respectively. If WHO’s guideline for PM2.5 24-hour mean was used as the cutoff point in the hourly PM2.5 data in the present study, the 25.1% (2019N), 23.7% (2020N), 65.3% (2019S), and 60.6% (2020S) PM2.5 data exceeded the WHO’s guideline. Evaluating the impact of COVID-19 on outdoor air, the differences in the median for hourly PM2.5 between 2019 and 2020 (e.g., PM2.5 on January 22, 2019, minus PM2.5 on January 22, 2020) are shown in Fig. 2(B). Although the differences in the median for hourly PM2.5 between 2019 and 2020 (2019N–2020N and 2019S–2020S) were not consistently either positive or negative, most figures were positive and ranged from 0.0 to 5.0 µg m–3 (2019N–2020N) in northern Taiwan and between 1.0 and 9.0 µg m–3 (2019S–2020N) in southern Taiwan (Fig. 3).

Fig. 2. (A) Distribution of probability density from hourly average PM2.5 concentrations in PM2.5 sensors of industrial areas was shown. The 2020S and 2019S was presented as PM2.5 data from southern Taiwan between January and March in 2020 and 2019, respectively. The 2020N and 2019N was expressed as PM2.5 data from northern Taiwan between January and March in 2020 and 2019, respectively. (B) Probability density distribution for difference of median PM2.5 from January to March between 2019 and 2020 (median PM2.5 in 2019 minus median PM2.5 in 2020 in the corresponding period).Fig. 2. (A) Distribution of probability density from hourly average PM2.5 concentrations in PM2.5 sensors of industrial areas was shown. The 2020S and 2019S was presented as PM2.5 data from southern Taiwan between January and March in 2020 and 2019, respectively. The 2020N and 2019N was expressed as PM2.5 data from northern Taiwan between January and March in 2020 and 2019, respectively. (B) Probability density distribution for difference of median PM2.5 from January to March between 2019 and 2020 (median PM2.5 in 2019 minus median PM2.5 in 2020 in the corresponding period).

 Fig. 3. Hourly PM2.5 concentrations in northern and southern Taiwan (A) median hourly PM2.5 from northern Taiwan in 2019 (2019N); (B) difference of median hourly PM2.5 between 2019 and 2020 from northern Taiwan (2019N–2020N); (C) median hourly PM2.5 from southern Taiwan in 2019 (2019S); (D) difference of median hourly PM2.5 between 2019 and 2020 from northern Taiwan (2019S–2020S).Fig. 3. Hourly PM2.5 concentrations in northern and southern Taiwan (A) median hourly PM2.5 from northern Taiwan in 2019 (2019N); (B) difference of median hourly PM2.5 between 2019 and 2020 from northern Taiwan (2019N–2020N); (C) median hourly PM2.5 from southern Taiwan in 2019 (2019S); (D) difference of median hourly PM2.5 between 2019 and 2020 from northern Taiwan (2019S–2020S).


3.3 Rate at Which Hourly PM2.5 Levels Exceeded the PM2.5 Daily Standard

Three ambient air quality standards for daily PM2.5 (24-hour mean), 35 µg m–3 for Taiwan, 25 µg m–3 for the WHO, and 55.5 µg m–3 for sensitive groups (air quality index [AQI] = 150) on the U.S. Environmental Protection Agency (U.S. EPA), were chosen as the evaluation and comparison criteria to demonstrate the hourly PM2.5 concentrations. A Pareto analysis (Cao et al., 2019) is a technique used for decision making based on the 80/20 rule, which statistically indicates that a limited number of input factors (counted hours) have the most significant impact on an outcome (exceedance rate for PM2.5 daily standard). If the proportion of limited hours can capture the proportion of most hours during which PM2.5 concentrations are higher than the legal standard, decision makers can capture the majority of high PM2.5 pollution events with a few hours or days. This study was concerned with the cumulative ratios for the number of effective hours as the input, with the cumulative ratios for the numbers of hours exceeding the daily PM2.5 standards.

For the hourly average PM2.5 levels in 2019N (Fig. 4(A)), the top 10% of the hours with high concentration values explained 16.8%, 25.5%, and 37.1% of the total hours exceeding the PM2.5 standards of 25, 35, and 55.5 µg m–3, respectively, and the top 20% of the hours explained 30.6%, 38.7%, and 56.7% of the total hours, respectively. The top 20% hours in 2020N explained 31.5%, 38.2%, and 53.6% of the total hours exceeding the PM2.5 standards of 25, 35, and 55.5 µg m–3, respectively. The top 20% hours explained 56.7% and 53.6% of the total hours exceeding the PM2.5 standards of 55.5 µg m–3 in 2019N and 2020N, where the cumulative rate exceeding the standard in northern Taiwan decreased by 3.10% during the first quarter of 2020.

Fig. 4. Pareto analysis for exceeding rates of PM2.5 daily standards in TEPA (35 µg m–3; L35), WHO guideline (25 µg m–3; L25), and the sensitive groups (55.5 µg m–3; L55.5) were performed (A) in north Taiwan; (B) in south Taiwan.Fig. 4. Pareto analysis for exceeding rates of PM2.5 daily standards in TEPA (35 µg m3; L35), WHO guideline (25 µg m3; L25), and the sensitive groups (55.5 µg m3; L55.5) were performed (A) in north Taiwan; (B) in south Taiwan.

For the hourly PM2.5 levels in 2019S (Fig. 4(B)), the top 10% hours explained 11.8%, 14.4%, and 25.8% of the total hours exceeding the PM2.5 standards of 25, 35, and 55.5 µg m–3, respectively, and the top 20% hours explained 23.1%, 27.1%, and 44.4% of the total hours exceeding that standard, respectively. The top 20% hours in 2020 explained 25.8%, 33.6%, and 55.7% of the total hours exceeding the PM2.5 standards of 25, 35, and 55.5 µg m–3, respectively. The top 20% hours explained 44.4% and 55.7% of the total hours higher than the PM2.5 standards at 55.5 µg m–3 in 2019S and 2020S, where the cumulative rate at which the standards were exceeded in southern Taiwan decreased by 11.3% during the first quarter of 2020.

The rates at which the hourly PM2.5 levels exceeded distinct standards demonstrated the concentration profiles of high PM2.5 levels over northern and southern Taiwan. The rates at which the standards of 25.0, 35.0, and 55.5 µg m–3 were exceeded in northern Taiwan (Table 2) were 25.8%, 9.45%, and 1.05% in 2019 and 23.6%, 9.79% and 1.56% in 2020, respectively. The rates at which the standards of 25.0, 35.0 and 55.5 µg m–3 were exceeded in southern Taiwan were 64.5%, 40.2%, and 13.6% in 2019 and 60.6% and 36.9%, and 11.7% in 2020, respectively. Compared to 2019, there was a decreasing trend at which the PM2.5 concentrations exceeded the standards for most stations during the COVID-19 period in northern Taiwan; however, sporadic stations experienced an upward trend and resulted in the rates beyond the standard increasing 0.34% and 0.51% for the standard of 35 and 55.5 µg m–3. The PM2.5 concentrations showed a decreasing trend for most stations during COVID-19 period in southern Taiwan, where the rate at which the standards of 25.0, 35.0 and 55.5 µg m–3 were exceeded decreased by 3.88%, 3.34%, and 1.91%.

Table 2. Exceeding rate (%) for varied PM2.5 daily standards.

 
3.4 Spatial Changes in PM2.5 between January and March in 2019 and 2020

The descriptive statistics for PM2.5 and the regional changes in PM2.5 based on the geographical statistics could not be used in the present study due to collection from more than 2,000 air quality micro-stations that generate thousands of units of PM2.5 data in each station. Therefore, the RPCs were used to explain the spatial characteristics of the median hourly average PM2.5 in our selected areas. Three factors, including eigenvalues, explained variances, and factor loadings, were mainly used to determine the number of principal components. The RPCs of the hourly average PM2.5 data were determined as shown in Table 3. The number of principal components was first determined, based on having an eigenvalue over 1.00, to be 28 and 19 RPCs in 2019 and 2020, respectively, from PM2.5 data of northern Taiwan as well as 34 and 37 RPCs in 2019 and 2020 from southern Taiwan. Secondly, the number of principal components was chosen based on the explained variances being over 1% and was reduced to 7 and 8 RPCs in 2019 and 2020, respectively, from southern Taiwan. In northern Taiwan, both had 6 RPCs in the two years under consideration. Therefore, the number of principal components was finally determined to be 6 PM2.5 RPCs in northern Taiwan. For the hourly average PM2.5 data in southern Taiwan, the station numbers for the 7th, 8th, and 9th RPCs were 11, 11, and 2 in 2019 and 0, 12, and 0 in 2020, respectively, based on factor loadings over 0.5 (data not shown). The 7th PM2.5 RPC in 2020 from the PM2.5 data for southern Taiwan did not reveal better correlations between the principal components and the air quality micro-stations as compared with the other RPCs from the 1st to 9th RPCs. Finally, the predominant number of RPCs in the present study was determined to be 6 in the PM2.5 stations in both northern and southern Taiwan after the eigenvalues, explained variances, and factor loadings were taken into consideration (Figs. 5 and 6).

Table 3. Eigenvalues and explained variance of rotated components for hourly average PM2.5 levels in PM2.5 sensors from January to March in 2019 and 2020.

Fig. 5. Loading values of 6 rotated principal components in PM2.5 levels from PM2.5 sensors of the industrial areas from northern Taiwan between January and March in 2019 and 2020.Fig. 5. Loading values of 6 rotated principal components in PM2.5 levels from PM2.5 sensors of the industrial areas from northern Taiwan between January and March in 2019 and 2020.

Fig. 6. Loading values of 6 rotated principal components in PM2.5 levels from PM2.5 sensors of the industrial areas from southern Taiwan between January and March in 2019 and 2020. Fig. 6. Loading values of 6 rotated principal components in PM2.5 levels from PM2.5 sensors of the industrial areas from southern Taiwan between January and March in 2019 and 2020.

Fig. 5 shows the contour maps of factor loadings for distinct RPCs in 2019 and 2020 from northern Taiwan, and those from southern Taiwan are shown in Fig. 6 to present the main PM2.5 contour maps. In Fig. 5(A), the main PM2.5 factor loadings in the industrial areas in 2019 were as follows: (1) The 1st RPC was in Dayuan (DY) and Kuangin (KI), (2) the 2nd RPC was focused on Taipei City and New Taipei City, (3) the 3rd RPC was located at Pingzhen (PZ), (4) the 4th RPC was in Linkou (LK), and (5) the 5th and 6th RPCs were in Guishan (GS). The PM2.5 factor-loading map of the 6 main RPCs in 2020 was consistent with those in 2019 except for the 5th RPC in GS due to a lower factor loading in 2020 compared with that in 2019 (Fig. 5(B)). As shown in Fig. 6(A), the 1st RPC of the hourly average PM2.5 factor loading in 2019 was located at downtown Kaohsiung City, the 2nd RPC was distributed in the industrial areas of Pingtung County, the 3rd RPC was in the northern part of Kaohsiung City, the 4th RPC was focused on the Pingnan (PN) industrial area, and the 5th and 6th RPCs were scattered in sporadic points in industrial areas in Kaohsiung City. For the duration of the COVID-19 outbreak in 2020, the main factor loadings for the PM2.5 for the 1st to 4th RPCs are shown to be consistent in terms of the factor-loading map with those in 2019 from the industrial areas of southern Taiwan (Fig. 6(B)). Various PM2.5 factor loadings were found in the 5th and 6th RPCs between 2019 and 2020 in southern Taiwan.


4 DISCUSSION


Compared with the global pandemic outbreak of COVID-19, the spread of the SARS-CoV-2 virus was prevented and controlled effectively in Taiwan by the TCDC due to encouraging the Taiwanese population to wear masks in public, practice social distancing, reduce outdoor activities, and stay away from crowded areas. Taiwan was one of the few countries in the world without a lockdown required during the COVID-19 outbreak. Even with the absence of a lockdown in Taiwan during the COVID-19 outbreak, anthropogenic activity was limited, which led to a reduction in air pollution based on the results of the present study. In Table 1, it can be seen that the median hourly average PM2.5 levels were slightly decreased from 16.3 (2019N) to 15.7 (2020N) µg m–3 in northern Taiwan and from 32.4 (2019S) to 29.3 (2020S) µg m–3 in southern Taiwan. The reduction in PM2.5 between January and March of 2019 and the corresponding duration in 2020 was 1.70% and 4.42% in northern and southern Taiwan, respectively. Our findings were consistent with those from the previous report announced by the TEPA (Liang and Tsai, 2020) indicating that nationwide average PM2.5 collected at the national-scale air pollution monitoring sites was reduced from 21.4 µg m–3 between October 2018 and March 2019 to 19.8 µg m–3 in the corresponding period from 2019–2020 (PM2.5 attenuated by 7.1%) in Taiwan. Han et al. (2020) assessed improvements in the PM2.5 pollution in Seoul, Korea, which did not execute a lockdown, before and after implementing social distancing during the COVID-19 outbreak and found a 10.4% reduction. Ma and Kang (2020) also revealed that the decline in daily PM2.5 in the month before and after social distancing implementation without a lockdown was 20.9% in Daegu and 3.6% in Tokyo, respectively. According to the two previous reports for Korea and the results of the present study (Han et al., 2020; Ma and Kang, 2020), implementation of social distancing and self-quarantining by the government legislation resulted in significantly obvious PM2.5 decline in the regions and cities being investigated, including Taiwan, Korea, and Japan, without a mandatory lockdown. This was probably due to lower PM2.5 emissions in lockdown-free regions due to limited anthropogenic activities, such as industrial production, transportation, and economic activities. The reduction could also have been related to the meteorological conditions discussed earlier (Lolli et al., 2020). Due to the global spread of the SARS-CoV-2 virus, most countries were under mandatory lockdown conditions to introduce precautionary measures including limitations on travel, public gatherings, and a mandatory 14-day quarantine period for suspected or confirmed patients and overseas travelers, to help prevent the spread of COVID-19. The precautionary measures taken during lockdown facilitated temporary improvements in air quality particularly in highly polluted countries (e.g., China, India, and Italy), where PM2.5 levels were significantly and dramatically decreased by at least 25% (Bedi et al., 2020; Li et al., 2020a; Li et al., 2020c; Kumar et al., 2020a; Şahin, 2020; Shakoor et al., 2020; Srivastava et al., 2020; Zoran et al., 2020).

According to Figs. 5 and 6, the spatial patterns of 6 main PCs in 2019 and 2020 were overlapped. The intensity of PCs in 2019 was slightly higher than in 2020 from northern and southern Taiwan, as shown in Table 1 and Figs. 2 and 3, indicating that PM2.5 levels between January and March in 2019 had significantly and slightly higher magnitudes than those in the corresponding periods of 2020 in northern and southern Taiwan. Liang and Tsai (2020) revealed that PM2.5 in Taiwan is associated with transboundary transportation of PM2.5, weather type, meteorological conditions, and precursors of PM2.5, but PM2.5 levels from transboundary transportation in 2020 were reduced by 0.31 µg m–3 compared with those in the corresponding period in 2019 (Fig. S1). This finding indicated that the main reason for the improvement in air quality due to PM2.5 in 2020 was related to the decline of PM2.5 precursors, nitrogen dioxide (NO2) and sulfur dioxide (SO2), probably due to limitations on artificial and economic activities, but the improvement was slightly linked to transboundary transportation of PM2.5, weather type, and meteorological conditions (Figs. S2 and S3; Liang and Tsai, 2020). Therefore, the long-range PM2.5 transport from China (transboundary transportation) had minor effects on reductions in PM2.5 during the COVID-19 outbreak in Taiwan. Reductions in PM2.5 from local emissions were mainly associated with the decline in PM2.5 during the COVID-19 situation (Figs. S1–S3). Inversely, Han et al. (2020) indicated that synoptic conditions and the decline in aerosol optical depth from eastern China to the Korean Peninsula led to improvements in air quality, including decreased PM2.5, in Seoul during the COVID-19 outbreak, which might have been due to declines in domestic emissions and reductions in long-range transboundary transportation of air pollutants. According to our findings, PM2.5-related air quality was temporarily improved during the COVID-19 outbreak mainly from reduced levels of NO2 and SO2 from local emissions via limitations placed on human activities. PM2.5 levels have a significantly positive association with the number of cases and mortalities associated with the SARS-CoV-2 virus. It is reiterated that PM2.5 control and reductions in PM2.5 emissions, especially those from local sources, are very important to reducing COVID-19 case numbers.

 
5 CONCLUSIONS


By implementing a multifaceted policy that includes community mitigation, border restrictions, travel quarantines, and a coordinated medical response, the TCDC has effectively limited the spread of COVID-19 in Taiwan from January 20, 2020, through the present without mandating a lockdown. Based on our big data analysis, the PM2.5 concentrations in the industrial areas of northern and southern Taiwan decreased by 1.70% and 4.42%, respectively, between the first quarter of 2019 and the corresponding period in 2020. Furthermore, this pollutant exhibited similar spatial patterns during both years, indicating that social distancing and other COVID-19-related restrictions drove the observed changes. Although lower domestic emissions were primarily responsible for the decline in PM2.5 during the outbreak, reduced transported emissions also played a minor role.

 
DECLARATION OF COMPETING INTEREST


The authors declare no conflict of interest including financial interests and personal relationships.

 
ACKNOWLEDGEMENTS


This study was financially and partially supported by a grant from the Ministry of Science and Technology (MOST 109-2221-E-020-009-MY3). We thank the staff of JS Environmental Technology and Energy Saving Co., Ltd., Kaohsiung, Taiwan for assisting with the acquisition of PM2.5 big data.


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