Ambient Nanoparticles Mapping in Thailand: Comparison
Please note this is a comparison between Version 2 by Worradorn Phairuang and Version 3 by Jason Zhu.

Nanoparticles (NPs), nanoaerosols (NAs), ultrafine particles (UFPs), and PM0.1 (diameters ≤ 0.1 µm or 100 nm) are used interchangeably in the field of atmospheric studies. PM0.1 mainly originate from combustion processes such as in motor vehicles. The highest mass concentration of PM0.1 occurs during the dry season, in which open fires occur in some regions of Thailand. The northern area of the country has higher PM0.1 mass concentrations, followed by the central and southern areas. Carbonaceous nanoaerosols are produced during normal periods, and the proportions of organic to elemental carbon and char to soot suggest that these originate from motor vehicles. However, in haze periods, biomass fires can also produce carbon-containing particles. PM0.1 pollution from local and cross-border countries also needs to be considered. The overall conclusions reached will likely have a beneficial long-term impact on achieving a blue sky over Thailand through the development of coherent policies and managing new air pollution challenges and sharing knowledge with a broader audience.

  • biomass burning
  • motor vehicles
  • nanoaerosols
  • nanoparticles

1. Recent Studies of PM0.1 in Thailand

1.1. PM0.1 Particle Mass Concentration and Particle Number Concentration

The PM0.1 levels in ambient air are usually extensively measured by particle number concentration (PNC) due to their minuscule size in addition to mass concentration [1][40]. No standards for airborne PM0.1 have been adopted in Thailand. Thailand’s National Ambient Air Quality Standards recently established parameters for six air pollutants that are deemed the highest priority to protect public health, including PM (TSP, PM10, PM2.5), O3, CO, SO2, NO2, and lead (Pb). The six criteria for pollutants are classified into health risk levels based on criteria defined by Thailand’s Air Quality and Noise Management Bureau, Pollution Control Department, and Ministry of Natural Resources and Environment. This is the current standard as of 2022; particulate pollution is a severe and increasing problem for Thailand. The Pollution Control Department announced in 2022 [2][41] that it will decrease Thailand’s National Ambient Air Quality of 24 h PM2.5 concentration to 37.5 µg/m3 in 2023. This is because of human health concerns about smaller particles in the recent decade.
Moreover, according to the new guidelines on air quality by the World Health Organization (WHO) (2021) [3][42], the suggested mean annual concentration for PM10 was 200 µg/m3 in 2005 and the mass concentration for 2021 moved to 150 µg/m3. The 24 h concentration was updated from 50 µg/m3 in 2005 to 45 µg/m3. Furthermore, in 2005, the highest recommended average PM2.5 annual mass concentration was 10 µg/m3; the 2021 revision reduced that number by half, to just 5 µg/m3. The 24 h level changed from 25 µg/m3 in 2005 to 15 µg/m3. The WHO was confident that there was insufficient information to provide guidelines for other types of PM, including elemental and black carbon, sand and dust storm particles, and PM0.1 particles. The WHO does not create a set of best practices for managing those pollutants, even though it recommends further study into their risks and methods for mitigation.
In European countries, the Condensation Particle Counter (CPC) is a standard method for measuring nanoparticles [4][43]. However, the ambient nanoparticle standard for all emission types is still limited. Only the gasoline and diesel emission standard representing the non-volatile particle of diameter >23 nm has been defined (the Solid Particle Number > 23 nm method (SPN23)) [5][44]. Surface area and particle number are appropriate for measuring minor mass concentrations of PM0.1 in most atmospheres [6][16]. NPs are commonly measured as particle number concentration (PNC), representing more than 85% of the total PM2.5 particle number [7][45]. In contrast, it contributes only slightly (10–20%) to the total PM concentration.
In the BMR, the PM0.1/PM2.5 ratio is around 0.23 [8][32]. Motor vehicles account for smaller particles in this area, and the ratio slightly increases to 0.26 during the dry season, indicating that some biomass burning episodes produce PM0.1 [9][24]. Hat Yai, Songkhla province, is an economic city in the south of Thailand. A previous study showed that the primary particulate pollutants in Hat Yai are caused by biomass combustion from the rubber industry [10][56] because southern Thailand is different from the other regions of Thailand. The economic crop in the region is oil palm and para-rubber, which are produced in plantations in the south of Thailand [11][12][57,58]. However, PM0.1 in the southern part of Thailand is lower than in other parts due to less frequent open biomass burning fires in the area. The PM0.1/PM2.5 ranges from 0.15 to 0.19 depending on the transboundary particulate effects that increase the mass concentration [13][14][26,55].

1.2. Carbonaceous Nanoaerosol

The most significant portion of airborne PM is carbon-containing materials with various physical and chemical characteristics, which account for around 20–50% of the mass concentration of PMs [15][16][59,60]. The PM-bound total carbon (TC) can be divided into two types, including organic carbon (OC) and black carbon (BC) or elemental carbon (EC). BC and EC are used interchangeably depending on the analytical method being used [17][18][61,62]. Brown carbon (BrC) was recently discovered with light absorption characteristics similar to atmospheric aerosols [19][63]. BrC is a non-soot organic carbon aerosol that is produced from bioaerosols, tar, and humic-like substances (HULIS) [20][21][64,65]. BC is mainly emitted by high-temperature combustion processes (diesel and gasoline exhausts, coal combustion, and biomass burning) [22][23][66,67]. BrC is primarily emitted by biomass burning. BC and BrC are the two most crucial light-absorbing substances in atmospheric aerosols [24][68]. In contrast, OC is a light-scattering material that is mainly generated from biomass fires, coal combustion, motor vehicles, and secondary chemical processes in the atmosphere [25][26][69,70]. The Intergovernmental Panel on Climate Change (IPCC) predicted that EC would lead to a direct global radiative forcing of around +0.2 Wm−2 [27][71]. In contrast, OC was produced at around the same magnitude [28][72]. Therefore, the primary emissions of BC clearly have global warming potential and can influence the hydrological cycle [29][73]. Primary pollutants, including NOx, BC and OC, include an atmospheric photochemical activity and can produce secondary organic aerosols (SOA) and ozone (O3), creating an even more complicated effect [30][74].
Information concerning OC and EC is crucial in estimating the impact of PMs and the understanding of the source and strength of these pollutants. EC can be divided into char-EC and soot-EC [31][75]. Char consists of the residue remaining after burning solid residue. Soot, however, is different from the physical and chemical properties of the source materials after the high-temperature condensation of hot gases during the combustion process [32][76]. The ratio of Char-EC and Soot-EC varies depending on the main sources and can be used to categorize the origin of this material. Only a small number of studies have reported on the pattern for Thailand’s carbonaceous nanoaerosols (OC and EC) [8][9][14][33][24,25,32,55]. Brown carbon (BrC) in nanoaerosols, which affects the splitting between OC and EC via a thermal-optical protocol, has not been studied so far in Thailand. A reliable method for detecting BrC plays a vital role in accurately estimating carbonaceous nanoaerosols [34][77]. The effect on regional and global warming is highly uncertain due to carbonaceous aerosols that are emitted into the atmosphere. This is because the distribution of carbon fractions varies with the time and location, which basically contributes to the chemical, physical, and optical characteristics of carbon components in PMs. Accordingly, information on carbonaceous nanoaerosols is vital in terms of assessing their radiative effects on global warming. Only limited studies of carbon components and spatial and temporal variations in Thailand have appeared, particularly of the nano-scale ambient particles related to carbon components.

1.3. Carbon Characteristics of OC, EC, Char-EC, and Soot-EC

The ratios of OC/EC can be used to classify the exact emission sources of carbonaceous particulate matter. Ratios for diesel exhaust, coal burning, and biomass combustion are different. Biomass burning has a higher ratio, while the OC/EC ratios for fossil burning results are lower [35][78]. The ratio of OC to EC for biomass combustion is higher (~6–8) [36][79] and that from fossil fuel is lower (<1) [37][80]. The characteristics of emission sources of carbon fractions include diesel exhaust (OC/EC ~0.1–0.8) [26][70], biomass combustion (OC/EC ~4–6) [38][39][33,81], and long-range transport/aged aerosol (OC/EC ~12) [40][82]. On the other hand, OC/EC depends on three factors for appropriately categorizing the source of the emission. The three factors include the primary emission source, the deposition rate, and secondary organic aerosols (SOA) [14][26][55,70]
Unlike the OC/EC ratio, the char-EC/soot-EC ratio differs from each source; it is frequently possible to identify the sources [41][83]. Only two factors can affect the char/soot ratio: the primary emission source and particle deposition by scavenging. A higher proportion of char/soot (generally >1.0) is suggestive of biomass fires; char contributes to the total EC levels. In contrast, char/soot <1.0 suggests that emissions from diesel engines are an essential contributor to the total EC concentrations [8][42][32,84]. The Char-EC/Soot-EC ratios in nanoparticles in Thailand are almost constant and less than 1.0 in both the wet and dry seasons, suggesting that motor vehicles are a key source of PM0.1 particles in Thailand. However, only in Chiang Mai during the dry season, the Char-EC content and Char-EC/Soot-EC were increased higher than 1.0 because of open biomass burning to smaller particles [8][14][26][32,55,70]. Therefore, the PM0.1 particles represent diesel engine emissions, although sensitive to biomass emissions in Thailand, e.g., the Chiang Mai area, which is recognized to have airborne particulate pollution from biomass burning for a long time [43][44][85,86]. Moreover, the increased Char-EC content and Char-EC/Soot-EC ratio should be studied in detail in future studies for the accuracy of carbonaceous nanoaerosols in Thailand and elsewhere.

1.4. PM0.1 Derived from Biomass Burning

In SEA, haze has occurred nearly every year during the dry season [43][44][85,86]. These haze episodes generated PM that was derived from biomass combustion in the past decade [9][38][24,33]. Forest fires and slash and burn in agricultural areas are typical methods for removing biomass residues in SEA [45][87]. Research reports addressed the high PM concentration that is released from open biomass fires in Thailand [44][46][47][86,88,89]. Hata et al. (2014) [48][51] reported, based on chamber experiments, that biomass fuel combustion releases around 80% of all sub-micron particles and nanoparticles of approximately 30% of the total particles. Similarly, open biomass fires during a haze episode in northern Thailand revealed that more than 60% of the total PM is smaller than PM1.0 [8][32]. The size distribution of PM released from open fires depends on fuel type, moisture content, and excess air during combustion [49][50][90,91].
Biomass burning is a significant contributor to the production of ambient particles. As reported by Hata et al. (2014) [48][51], in chamber experiments, biomass solid fuel combustion accounted for more than 30% of the biomass burning and that the particle mass concentration was smaller than <100 nm. However, in the atmospheric environment, PM0.1 particles are contained in the ambient atmosphere due to anthropogenic activities and natural sources or chemical processes. Therefore, determining the actual emission sources under ambient conditions is not an easy task. Phairaung et al. (2021) [8][32] reported on the source apportionment of PM0.1 particles in Bangkok. They found that around 10% of the ambient nanoparticles in Bangkok during haze episodes came from biomass fires. However, PM0.1 particles, primarily derived from motor vehicle emissions, are also strongly affected by forest fires in the north of Thailand [8][32]. Hence, this activity has an important influence on the quality of ambient air during the dry season. As a result, the main emission sources of PM0.1 are both natural and anthropogenic.

2. Health Concerns of PM0.1 in Thailand

Smaller particles, especially nano-size particles, are recognized as being detrimental to human health due to their small size, chemical makeup, and the fact that they accumulate in ambient conditions [51][8]. Evidence collected in the past decade makes it clear that PM0.1 affects public health. The Health Effects Institute (HEI) [52][14] suggests that the PM0.1 data on health risk assessment are still an ongoing study and it cannot conclude or decide on policy making for the control of ambient PM0.1 worldwide. However, health risks, such as oxidative stress and inflammatory damage, may result from human exposure to atmospheric PM0.1 through inhalation [51][53][8,10].
In the same manner, studies of PM0.1 in Thailand make it clear that there are health effects from these particles. Only a few studies have appeared on health risk assessment from PM0.1 as related to the chemical composition of these particles. Chomanee et al. (2020) [13][26] reported on a health risk assessment of nanoparticle-bound PAHs in southern Thailand during a period of transboundary particulate pollution. It is known that the lower SEA suffers from the effects of large peat-land fires during the dry season, around July–September, almost every year.
Similarly, Phairuang et al. (2022) [54][28] reported on the year-long health effects of PM0.1-bound trace elements in southern Thailand in 2018. They found that the health risk from hazardous components is generally highly recognized during the pre-monsoon season. Toxic elements from peat-land fires that are transported from other sources to southern Thailand depend on the speed and direction of the wind. Cross-border particulate pollution must be investigated in more detail, with emphasis on the origin and health concerns during haze episodes in this region. During the normal period, the primary emission sources of PM0.1 are land transportation [33][25].
In other parts of Thailand, the knowledge of the health risks from PM0.1 related to the chemical components remains limited. Phairuang et al. (2021) [55][53] reported that the health risk assessment from PM0.1-bound metals in Bangkok, Thailand, was substantial during a smog haze period. PM0.1-bound elements in Bangkok differ with the season but are generally related to road transport emissions. It is well known that in the Bangkok Metropolitan Region air quality worsens during periods of heavy traffic congestion [8][56][57][32,47,49]. There is general agreement that the production of PM0.1 worldwide is derived from motor vehicles in urban areas [1][58][40,50]. Diesel and benzene engines are the primary sources of ambient nanoparticles in mega-cities [59][60][92,93]. However, open biomass burning, e.g., forest fires, crop waste, and grass burning, significantly contribute to PM0.1 during intense haze episodes in many countries [8][61][5,32]. Most studies have concluded that inhaled airborne PM0.1 has adverse effects on human health. Data of relationships between PM0.1 and sickness are limited.

3. Challenges in Studies of PM0.1 in Thailand

In the past decade, Thailand has been faced with particulate pollution almost yearly. In particular, in the dry season, emissions from open fires and meteorological conditions can temporarily affect the particle concentration [43][62][85,94]. Phairuang et al. (2019) [8][32] examined the influence of biomass fires on air quality in Thailand, i.e., Bangkok and Chiang Mai, in a case study of size-fractionated particulate matter ranging from small to nano-sized. The influence of biomass burning strongly affects ambient PM0.1 in Bangkok, although many reports have suggested that the main contribution of PM2.5 in BMR is from motor vehicles [56][57][47,49]. On the other hand, PM0.1 is ubiquitous in the atmospheric environment in the northern part of Thailand during the dry season, as in Chiang Mai, the economic city in the northern part of Thailand. This is a new challenge in the studies of biomass burning, especially crop waste burning and woodland fires in agricultural countries, to understand the production of ambient nanoparticles [8][14][61][5,32,55]. The apportionment of the sources of PM0.1 is very limited in Thailand due to the small amount of mass and chemical composition. Moreover, a recent study of particle size distribution in Bangkok by Panyametheekul et al. (2022) [63][95] found that the particle number concentration of samples collected from three locations in Bangkok revealed that up to 90% of the PM0.1 was produced in comparison with other sizes. Consequently, in the case of PM0.1, both the number and mass particle concentration are subjects that need to be examined in terms of air quality management in Thailand’s land based on heavy particulate pollution in the past decade.
It is generally assumed that PM0.1 particles are highly toxic substances compared to larger particles because they have a vast surface area per volume that can carry and absorb hazardous chemicals such as heavy metals, carbon components, and carcinogenic PAHs [51][8]. In the past decade, strong evidence has appeared to suggest that PM2.5 and PM10 induce human illness, including respiratory symptoms, cardiovascular effects, and chronic obstructive pulmonary disease (COPD), which contribute to mortality and morbidity [64][65][66][67][96,97,98,99]. This is especially true in northern Thailand, which experiences particulate pollution almost yearly. Many reports have revealed that the smoke haze episodes induce more people to visit hospitals in the north of Thailand [68][69][100,101]. However, there is no evidence of risks to health from nanoaerosols. Although the northern part of Thailand, during the dry season, has a high mass concentration of PM0.1 particles [8][32], the relationship and epidemiological survey of ultrafine particles and health effects still underestimate human public health due to insufficient information concerning the source, characteristics, and abundance of such small particles.
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