1. Introduction

According to the State of Global Air Report 2019, air pollution is the fifth leading risk factor for mortality worldwide with the Philippines ranking number 10 in terms of the highest mortality burden attributable to air pollution [1].  Exposure to short-term and long-term air pollution is associated with respiratory and cardiovascular diseases [2].  In highly urbanized places like the National Capital Region (NCR) in the Philippines, popularly known as Metro Manila, air pollution aside from urban heating is one of the region’s major challenges [3].  In the latest National Emissions Inventory by source conducted in 2015, 88% of air pollution in Metro Manila comes from mobile sources, 10% from stationary sources, and a mere 2% from area sources. The increase in the number of vehicles is attributable to the increase in population to cater to the transport needs of the people [4]. In Metro Manila alone, there was an average increase of 7.77% in vehicular registration the year 2016-2018 (NCA, 2018). One of the 16 highly urbanized cities (HUCs) in Metro Manila is the City of Manila, the capital of the Philippines. It was the most densely populated among the HUCs with 71,263 persons per square kilometer in 2015 [5].  Criteria pollutants such as Nitrogen Dioxide

Figure 1: The location of the sampling sites relative to each other.  Yellow and white lines indicate roads for vehicular traffic

Table 1: Location, coordinates, and addresses of the sampling sites

Location	Coordinates	Address
Halang (Background site)	13°57’26.9″N 121°04’55.8″E	Halang, Lipa City, Batangas
Arroceros Forest Park	13°57’26.9″N 121°04’55.8″E	Antonio Villegas St, 659 A Ermita, Manila, 1000 Metro Manila
Japanese Garden (in Luneta Park)	14°35’3″ N 120°58’44” E	Maria Orosa Street corner Padre Burgos Street, Manila, Metro Manila 1004, Philippines
Manila Zoological and Botanical Garden	14°33’50” N 120°59’18” E	Adriatico St, Malate, Manila, 1004 Metro Manila

Table 2: Summary of the sampling dates for each site

Site	May 2018	Jun 2018	July 2018	Aug 2018	Sept 2018	Oct 2018	Nov 2018	Dec 2018	Jan 2019	Feb 2019	Mar 2019	Apr 2019
Halang	13	17	15	19	23	28	30	16	27	25	17	14
Arroceros	17	27	19	24	27	22	21	14	31	27	20	29
Luneta	18	25	30	29	26	24	23	13	30	28	21	11
Mla Zoo	25	16	31	30	25	23	22	12	24	26	27	30

(NO₂), Ozone (O₃), and fine particulate matter (PM_2.5) mostly come from mobile sources [4].

The increasing population and vehicles make Manila an example of unsustainable urbanization [6].  Hence, the presence of urban parks and green spaces in the city can play a role in maintaining urban residents’ quality of life. Urban green space, a category of land cover that includes public parks and other (public or private) vegetated areas in densely populated places plays a great role in improving not only human health and but also the air quality  [7–11]. To date, there were no available data on the level of air pollution in urban parks in Manila. People come to public parks to exercise, sometimes have family picnics, and just for relaxation. Some urban poor and street-dwellers make parks as their temporary home. Measurement of air pollution level is important especially if the parks are located or surrounded by main thoroughfares where traffic congestions are always present.  These can increase the potential of park users to be exposed to pollutants associated with vehicular emissions. Lam et al made a study in Hongkong urban parks, and they have shown that the air quality in urban parks is better compared to the roadside but not significantly different from that in the ambient conditions [12].  Three of the popular parks in Metro Manila are located within the City of Manila. These are Manila Zoo, Luneta or Rizal Park, and Arroceros Forest Park.  These three parks are located near the main thoroughfares, hence the proximity to vehicular traffic, which is the main source of air pollution in the city. There have been no air quality measurements in these parks as most static reference standard instruments are usually positioned near the roadside.  The use of low-cost and portable air quality monitoring systems has become very popular as there is a need to provide a more accurate assessment of human air pollution exposure assessments [13].  In this study, the ambient concentrations of criteria pollutants such as nitrogen dioxide (NO₂), fine particulate (PM_2.5), and ozone (O₃) at these three public parks were measured once a month for one year using portable sensors.  The measured ambient concentrations at the three parks are then compared to a background site in Halang, Batangas which is far from any vehicular traffic. The comparison between the urban parks and the background site will demonstrate the effect of vehicular traffic on the ambient concentrations of PM_2.5, NO₂, and O₃.  Furthermore, this study will explore the usefulness of portable sensors to provide real-time determination and assessment of air quality in public areas so that people will know the quality of the air they are breathing and, once the air quality drops to unhealthy levels, people can be requested to vacate the area.   This will also provide information to authorities if there is a need to place static reference standard air quality monitoring system in such public areas.

2. Materials and Methods

2.1.  Sampling Sites

      The study was conducted at the three urban parks located in the city of Manila. These are Arroceros Forest Park, Luneta or Rizal Park, and Manila Zoo. For comparison, a background sampling site was included, and this was located in Halang, Batangas which is about 70 km south of Manila.  The location and addresses of the sampling sites are given in Table 1 and Figure1 shows the relative location of each sampling site.  Yellow and white lines indicate roads for vehicular traffic.   Permits were secured with the park administrators for us to conduct the measurements, which should be done only during office hours.  Sampling Site 1 is the Arroceros Forest Park (AFP), also called as the “Last Lung of Manila”. It is a 2.2-hectare (5.4-acre) manmade urban forest along Pasig River, at the foot of Quezon Bridge. The second sampling site is Rizal Park also known as Luneta Park or simply Luneta is a historical urban park in the Philippines and one of the major tourist attractions of Manila. It is approximately 16.24 hectares (40.01) acre and located along Roxas Boulevard, Manila, and also adjacent to Taft Avenue, Manila.  The last urban park is the Manila Zoological and Botanical Garden or Manila Zoo.  It is a 5.5-hectare (14-acre) zoo located in Malate, Manila, Philippines.

• Measurement

      The criteria pollutants measured in this study are PM_2.5, NO₂, and O₃. A DustTrak™DRX aerosol monitor (Model 8533, TSI incorporated) that used a light scattering technique to infer the mass concentration of particles was used for PM_2.5 measurement [14]. It was set on a 1-s time resolution at 3L/minutes. Before the start of each measurement, zero calibration was performed using a TSI 800663 zero filter. NO₂ and O₃ concentrations were measured by a real-time portable battery-operated gas sensor monitor AEROQUAL Series 500 with NO₂ and O₃ sensor heads.  The sensor heads used gas-sensitive electrochemical (GSE) technology where it measures the concentration of a target gas by using oxidation or reduction reactions to generate a positive or negative current flow through an external circuit and the magnitude of this current is proportional to the gas concentration. The operating parameters of the sensor heads are as follows; O₃ (detection range 0–10 ppm; resolution and minimum detection limits 0.01 ppm; response time 60 secs; temperature 0 – 40^oC; relative humidity 15 – 90%), NO₂ (detection range 0–1 ppm; resolution 0.001 ppm; minimum detection limits 0.005 ppm; response time 30 secs; temperature 0 – 40^oC; relative humidity 15 – 90%).  A 1-min time resolution was used during the measurement.  Although the DustTrak and Aeroqual are not regulatory-grade monitors they are widely used in prior air quality research studies [13,15–24].  Measurements done for this paper were performed during the warranty period of the instruments. Being under warranty, instrument calibration and performance was within factory specifications. During the measurements, researchers observed the performance of the instruments, and temperature and humidity on the site were within their specified operating conditions. Simultaneous measurement on all sampling sites was not possible because only one set of instruments was available. Background measurement in Halang, Batangas was done on a Sunday, and the measurements at the three parks in Manila were conducted on the following weekdays if weather permits. The sampling dates are summarized in Table 2.  To observe the same atmospheric and vehicular traffic conditions in the site, measurements were done on successive days in the morning between 8 am to 11 am local time for one hour, once a month for each sampling site for one year.  Measurement was not conducted during bad weather conditions and sampling time was chosen to also avoid the effect of sunlight on O₃ concentration.

3. Results

One Way Analysis of Variance (ANOVA) was the statistical method used for identifying if there are any significant differences in the measured concentration of a particular criteria pollutant at the different sampling sites.  If there’s a significant difference, a Tukey honestly significant difference (HSD) test is then carried out to identify what sampling locations are significantly different, and then based on the mean concentration, the locations are ranked from highest to lowest concentration.  RealStat (http://www.real-statistics.com/) Microsoft Excel Add-In was used to carry out the statistical calculation.  An alpha value of 0.01 was used.  To illustrate the variation of air pollution concentrations at each sampling site, Box and Whisker plots were used in terms of the lower quartile, upper quartile, median, mean, minimum, and maximum in each of the four study locations.

• Ozone (O₃) Measurement

Shown in Figure 2 is the box plot of the O₃ concentrations from the four different locations from May 2018 to April 2019.  Table 3a shows the result of the statistical analysis for O₃.  In all the sampling months, there was a significant difference in O₃ concentration between each sampling site.  The O₃ concentrations at the background site always have the lowest concentration on all sampling months.   The DENR EMB National Ambient Air Quality Guideline Values (NAAQGV)  for 1-Hour Averaging is  0.07 ppm [4], [25].   As shown in Figure 3, in some months, the measured  O₃  concentration  was 0  ppm  at  the  background  site

Table 3: One way Anova P-value between each sampling sites per month for (a) Ozone, (b) Nitrogen Dioxide, (c) PM_2.5.

Figure 2: Box plot of monthly concentration of O₃ at the four sampling sites from May 2018-April 2019

which indicates that the concentration was below the detection limit of the instrument.  For the whole year, as shown in Figure 3, Luneta had the highest concentration of 0.071 ppm and Arroceros was a close second at 0.067 ppm, followed by Manila Zoo at 0.063 ppm, and the background site at 0.050 ppm.   A high concentration of O₃ exceeding the NAAQGV was observed for all sites except the background site in January 2019 which is one of the coldest months in the Philippines.

Figure 3: Box plot of the one year mean O3 concentration at the four sampling sites from May 2018-April 2019.

• Nitrogen Dioxide (NO₂) Measurement

      Shown in Figure 4 is the box plot of the NO₂ concentration from the four different locations from May 2018 to April 2019. Table 3b shows the result of the statistical analysis. In all the sampling months, there was a significant difference in NO₂ concentration between each sampling site. The NO₂ concentrations at the background site have the lowest concentration on all sampling months. On some months, the NO₂ concentration was 0 ppm indicating that the NO₂  concentration was below the detection limit of the NO₂ sensor.  This was true most especially in the case of the background site where the lower quartile value was equal to the minimum value measured by the NO₂ sensor.  For the whole year, as shown in Figure 5, there is a significant difference in all sampling locations.  Luneta having the highest concentration of 0.032 ppm and Arroceros is a close second at 0.025 ppm, followed by Manila Zoo at 0.022 ppm and the background site at 0.017 ppm..  The National guideline value for NO₂ is 150 µg/Ncm (0.08 ppm) maximum for a 24-hour exposure [25]. As in the case of O_3, the coldest month of January and February 2019 registered higher NO₂ concentrations for all the urban parks except the background site.

Figure 4: Box plot of monthly concentration of NO2 at the four sampling sites from May 2018-April 2019.

Figure 5: Box plot of the one year mean NO₂ concentration at the four sampling sites from May 2018-April 2019.

• Particulate Matter (PM₅) Measurement

      For Particulate Matter measurement, TSI DUSTTRAK DRX Aerosol Monitor 8533 was used and this can measure PM₁₀, PM_2.5, and PM_1.0 but only PM_2.5 will be presented.  For statistical analysis, only PM_2.5 measurement will be used since all the other PM sizes follow the same trend.  Furthermore, in terms of Air Quality Guidelines, PM_2.5 is the one that is reported.  No air quality guidelines yet for PM_1.0.  Shown in Figure 6 are the box plots of the PM_2.5 concentration measured by the DUSTTRAK.  The lowest concentration of PM is in Halang, Batangas while Arroceros always gives the highest concentration.  The result of One-way ANOVA indicates a significant difference in the PM concentration for all months between each sampling location. Arroceros always ranks the highest while Halang is the lowest.  The WHO Air Quality Guideline values for PM_2.5 is 10 μg/m³ annual mean and 0.025 mg/m³ 24-hour mean [4].  NAAQGV are 0.025 mg/m³ annual mean and 0.050 mg/m³ 24-hour mean [4].  It can be seen from Figure 7 that the PM_2.5 concentration measured from TSI DUSTTRAK at the sampling locations in Manila exceeds the annual mean guideline values.  However, please note that the instrument is not the reference instrument.  Like O₃ and NO₂, it was also observed that in January 2019, high values of PM_2.5 concentration were measured at the urban parks exceeding the 24-hour mean NAAQGV.

Figure 6: Box plot of monthly concentration of PM2.5 at the four sampling sites from May 2018-April 2019

Figure 7: Box plot of the one year mean PM_2.5 concentration at the four the four sampling sites from May 2018-April 2019

4. Conclusion

      With the unsustainable urbanization in Manila, the presence of urban parks can provide residence with quality of life.  However, the location of these parks affects the air quality within the park.  The measured values of O₃, NO₂, and PM_2.5 at the three well known urban parks in Manila are higher compared with a background site in Halang, Batangas.  Using portable sensors, the ambient concentration of O₃, NO₂, and PM_2.5 was measured once a month between 8 am to 11 am local time from May 2018 to April 2019 at three known urban parks in Manila, namely, Arroceros Forest Park, Rizal or Luneta Park, and Manila Zoo.  Mean concentrations of O₃ and NO₂ are highest in Luneta Park at 0.071 ppm and 0.032 ppm, respectively.  Among the three parks, Luneta is the one surrounded by major roads.  For mean concentration of PM_2.5, Luneta, and Arroceros Park are the same at 0.070 mg/m³. One of the main reasons is the proximity of the three parks to vehicular traffic.  It was also observed that measured concentrations of the three criteria pollutants were relatively higher during the cool dry months from October to February.  Although the instruments used were not the reference instruments, the result provides information on the necessity of monitoring the air quality within the parks.  Many residents most especially people who cannot afford, and don’t have time to go out of the city need these urban parks for relaxation and recreational activities.

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgment

The authors would like to acknowledge the support from Commission on Higher Education (CHED) project entitled UDM CHED DARETO “Using wireless environmental monitoring sensors in assessing the impact of megacity environmental pollution and local climate on butterfly diversity in Manila, Philippines” and CHED –Grant-In-Aid Project “Evaluation of Human Skin Analogues” and we also acknowledge ARCHERS, CENSER through EARTH lab and De La Salle University Research Coordination Office (URCO) 47 F U 2TAY18-2TAY19 for partially supporting this project.

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