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Analysis of PM10 in urban and rural environment in Sumatra Island over the past half-decade

Particulate Matter 10 (PM10) is an inhalable particle with a diameter smaller than 10 μm that disrupt the health of human respiratory. The distribution of PM10 varies depending on the region. This study aims to analyze the temporal distribution of PM10 in Sumatra Island, Indonesia, as one of the forest fires prone areas that affect neighboring countries, over the past half-decade. Study location was devided into two, Pekanbaru, Medan, Batam, and Palembang represent urban; and Jambi and Aceh represent rural areas, as we want to see the difference of variation of PM10 between two different locations of pollution source. In general, the results showed that the average PM10 concentration throughout the past half-decade was below the value threshold in all regions. The monthly PM10 concentration in rural areas is below the threshold except in Jambi, a monthly concentration above the threshold is in October. While the monthly PM10 concentration in urban areas tends to be above the threshold except for Batam and Pekanbaru. The monthly concentration of PM10 above the threshold occurs in Medan and Palembang in October. For trends in the rural and urban area, it shows different. Pekanbaru and Palembang which are located in urban area has significant decrease trend of concentration. For diurnal, the hourly average value tends to have different pattern in both urban and rural areas. PM10 concentrations in urban areas tend to be low in the morning and high during the day and rise again at night. The frequency of PM10 concentration above the threshold which is >10% occurrences is only in Medan and Palembang. Others have less than 10% occurrences of the frequency of PM10 concentration below the threshold.


Introduction
Air is one of the important factors in life. But the increasing of the development of cities and industrial centers, the quality of our air has changed. The air that used to be fresh is now dry and dirty. Until 2018, many of the major cities in the world have experienced poor air quality. PM or particulate matter represents a complex mixture of organic and inorganic substances in the form of solid or liquid suspended in the air. PM has a variation of particle size. It could be large or dark enough to be seen with the naked eye. Others are so small they can only be detected using an electron microscope [1]. Particulate matter was grouped according to particle diameter size, Total Suspended Particulate (TSP) <100 µm, PM10 <10 µm in diameter, PM2.5 <2.5 µm [2] and PM1 <1 µm in diameter [3]. The number of particles in the air is expressed as a measure of concentration (µg/m 3 ). Inhaling particulate pollution can have a detrimental effect on health [4]. In general, air pollution coincides with an increase in human activity, where more human activity can reduce air quality both spatially and temporally [5], related to the number of population and time of occurrence.
Air quality in urban areas is dominated by a large number of vehicles and industrial machinery, for example large cities in Java, in contrast to the air quality in rural areas which are dominated by seasons, for example the regions of Sumatra and Kalimantan where in general dry seasons can cause forest and land fires which increase high air pollution [6]. The physical and chemical composition of airborne particulate matter varies in time and space and is influenced by meteorological factors and their precursors. These physical and chemical properties, then, will govern PM concentration, composition, deposition, removal, and transport [7]. Rapid economic growth consequently will increase emissions and pollutants concentration in Indonesia. Without control and increased awareness, biomass burning and urban air pollution that occur, consecutively, in several wildfire prone provinces [8] [9] and big cities in Indonesia will likely severe in the future. This study aims to analyze the temporal distribution of PM10 in Sumatra Island, Indonesia, as one of the forest fires prone areas that affect neighboring countries, over the past half-decade. Until now, there has been no study in comparing variations of PM10 in Sumatera regions. Whereas, information of how the temporal distribution of PM10, particularly in forest fire prone areas, which also has the potential to spread the smoke to neighboring country, is needed.

Data and location
Data series of PM10 concentration from 6 (six) locations of air quality observation which are laid along Sumatera island were collected for this study. Those 6 (six) locations are Pekanbaru, Medan, Palembang, Jambi, Batam, and Aceh. In terms of population density, a number of urban facilities, formal education facilities, public health facilities, it's categorized that Pekanbaru, Medan, Batam, and Palembang are areas that represent urban. Batam is known as its industrial area, also Medan, Palembang, and Pekanbaru are well-known for their overcrowding and their city development that is still ongoing. While Jambi and Aceh are categorized as an area that represents rural (BPS, 2010).
Hourly and monthly observation data of PM10 concentration from each location were taken over the last five years, from 2014 to 2018, aim to see the current conditions of the PM10. The data series were taken from the BMKG (Meteorology/Climatology Station) station which is located in those six areas (Picture 1).

Analysis methods
Time series data of PM10 concentration for each study location were analyzed using trend analysis to find out how the trend of PM10 concentration in the long term. The trend is an up or down movement (tendency) which is obtained from the average change over time. Its changes average can increase and also can decrease. In some researches, linier trends helped to study the extreme precipitation events (Zhang et al, 2001) and changes in air temperature (Easterling et all, 1997). The statistical significance of the trends was assessed by the Mann-Kendall test. The Mann-Kendall (MK) test is a nonparametric test used to analyze the null hypothesis of no change in the mean of a series (no trend) against the alternative hypothesis of an increase or decrease overtime (existence of a trend) [10].
Times series data are averaged per month to see whether PM10 concentrations in the study areas exceed the monthly threshold value. Indonesian government regulation already specified the threshold value for PM10 per day which is 150 µgram/m 3 . PM10 concentration considered normal if the concentration value is equal or less than 150 µgram/m 3 per day, which in this paper we assumed that equal or less than 150 µgram/m 3 is also categorized as normal for a monthly period. Besides the monthly average, the data series are also averaged per hour. Hourly average for 24 hours in the last 5 (five) years period can be used to find out at what time the concentration value has increased and also decreased in each location.

Results and discussions
The monthly average of PM10 concentration in urban and rural areas in Sumatra Island shows different results. In general, almost every month in both urban and rural areas, the concentration value is below the threshold (150 µgram/m 3 ) (Figure 2). In Jambi, the monthly average of the PM10 concentration reached above the threshold in October, reaching 164.2 µgram/m 3 . In the other months, the monthly average of the concentration is below the threshold, with a year-round average concentration of 54.5 µgram/m 3 . The high monthly average concentration in October in Jambi was influenced by El Nino which was happened in 2015. It caused a lot of forest fires in 2015 so that it became high on the average during the study period.
Even though it is a rural area, Jambi has a high concentration of PM10 in the dry season. It was influenced by Jambi as one of the prone areas to forest fires. The average PM10 concentrations in 2015 in Jambi were 485.1 µgram/m 3 . While the annual average in other years was below the threshold. The average concentration throughout the year in Aceh is 23.5 µgram/m 3 , and the highest PM10 concentration occurred in March, which is 33.8 µgram/m 3 . March happens to be dry season in Aceh. Rainy and dry season in Aceh is not influenced by El Nino in general. NASA recorded that Jambi had 3702 hotspots in 2015 and Aceh only had 100 hotspots. The high number of hotspot in Jambi in 2015 effected the increase of PM10 concentration. As result, the monthly concentration in October was above the threshold in the study period. Comparing to the rural area, concentration of PM10 in urban area in rainy season (Oct-Mar) was different. PM10 concentration in urban area was slightly higher than in rural in the rainy season. Figure  3 shows hourly concentration of the PM10 in both rural and urban locations have different variation, but it has the same pattern in general.

Figure 3. Hourly PM10 concentration in rural areas (left) and urban areas (right)
Trend in rural locations shows different results (Figure 4). The significance of the trends in both location tested by Mann-Kendall shows that the trends is not significant. The decrease of the trends of the concentration in rural location means that the concentration during the period of the research in the rural decrease. The increase of the trends of the concentration in rural location means that the concentration during the period of the research in the rural increase. Slope value for Aceh is -0.001 means that the trend is decreasing and tested by Mann-Kendall test, p-value for Aceh is 0.546, more than the significant level (%) 5 means that the trend is not significant (p-value > 0.05). Meanwhile Jambi has uptrend with p-value > 0.05, indicates that the trend is not significant. The upward trend in Jambi can be happened because of the increase of the fire forest fires happened in Jambi, considering that Jambi is categorized as forest fire prone area in Indonesia. The same with rural areas, urban areas also shows different results (Picture 5). Trends of concentration in Pekanbaru and Pelambang tend to decrease, meanwhile in Batam and Medan tend to increase. Slope value shows -0.06 and -0.07 for Pekanbaru and Palembang respectively. The significance of the trends in both location tested by Mann-Kendall shows that the trend is significantly decrease. As for Medan and Batam, the slope value are 0.02 and 0.01 resepectively. It shows that the trend for both location are increased. P-value for those locations is more that the significant level (%) 5, means that the trends in those locations are not increase significantly. This potential of increasing concentration of PM10 needs to be concerned. As there are different results between the trends of concentration in urban and rural area, both locations should prepare for the worst condition in the future. The decrease of the PM10 concentration in some locations can indicates that the pollution source in these areas also decrease. But it also can mean that the particle which is under 10 µm in diameter turns into smaller particle, namely PM2.5. Therefore, further study about PM2.5 concentration in Sumatera regions needs to be done. The frequency of PM10 concentration above the threshold which is >10% occurrences is only in Medan and Palembang (Table 1)

Conclusions
The results showed that the average PM10 concentration throughout the past half-decade was below the value threshold in all regions. The monthly PM10 concentration in rural areas is below the threshold except in Jambi, a monthly concentration above the threshold is in October. High concentration of PM10 in rural area was caused by forest fire happened during dry season, as we know that Jambi is forest fire prones are. The trend is non-significantly decreasing in Aceh and increasing in Jambi for rural locations.
While the monthly PM10 concentration in urban areas tends to be above the threshold except for Batam and Pekanbaru. The monthly concentration of PM10 above the threshold occurs in Medan and Palembang in October. For trends in the urban area, tends to increase in Medan and Batam, but the trends are not significant. And as for Pekanbaru and Palembang, the trends tends to decrease significantly. The decreasing trend of PM10 in these areas, indicates that the concentration of PM10 contained in the air is reduced. Nevertheless, we still need to beware since the reduction of PM10 can also means that the particles are getting smaller, namely PM2.5, which is more harmful for human health. For diurnal, the hourly average value tends to have different pattern in both urban and rural areas. PM10 concentrations in urban areas tend to be low in the morning and high during the day and rise again at night. The frequency of PM10 concentration above the threshold which is >10% occurrences is only in Medan and Palembang. Others have less than 10% occurrences of the frequency of PM10 concentration below the threshold.