# Long-Term and Seasonal Changes in Sources of Urban Atmospheric Particulates in the Western Pacific
## 1 Introduction  
Air pollution kills millions of people every year, especially in African, south-east Asian, eastern Mediterranean, and Western Pacific regions [1]. Among airborne particulate matter (PM) of various sizes, finer inhalable particles with diameters that are generally 2.5 um and smaller (PM\({}_{\rm 2.5}\)) have attracted research attention, as they are implicated in respiratory and cardiovascular diseases. The incomplete combustion of organic materials produces many types of polycyclic aromatic hydrocarbons (PAHs). In the air, PAHs with four or more rings, such as the carcinogenic benzo[\(a\)]pyrene, exist mostly as being adsorbed to PM. Combustion also produces nitropolycyclic aromatic hydrocarbons (NPAHs), formed by the nitration of PAHs. Although combustion is the primary source for NPAHs, several NPAHs are formed secondarily in the atmosphere. NPAHs with four or more rings exist mostly as being adsorbed to PM [2, 3]. NPAHs in particulates warrant research attention since they are more mutagenic than their parent PAHs [4, 5]. PAHs and NPAHs are emitted from many combustion sources, including vehicles, motorcycles, factories, power stations, incinerators, heaters and ovens in homes, post-harvest field burning, and forest fires [6].  
In general, atmospheric concentrations of NPAHs are at least two orders of magnitude lower than those of PAHs [7, 8, 9]. To clarify the current trends in air pollution caused byPAHs and NPAHs in the Western Pacific, the total suspended particulates (TSP) in 10 cities in Japan, China, Russia, and Korea in both summer and winter have been collected since 1997 [10; 11; 12]. In Japan, urban air pollution was serious in the 1980s and 1990s due to the enormous increase in vehicular traffic. This resulted in an increase in respiratory disease patients [13]. To improve air quality, the Japanese government lowered allowable PM and nitrogen oxide (NO\({}_{\text{x}}\)) emissions from new vehicles. Consequently, the urban atmospheric PAH concentrations and especially NPAH concentrations decreased in many Japanese cities in the 2000s [14; 15; 16; 17]. Several cities in central and northern China and far-eastern Russia showed PAH concentrations that were an order or more higher, although the concentration ratios of NPAHs to PAHs were smaller. This suggests that coal combustion was a major pollution source in those regions. PAH concentrations in Chinese cities did not decrease as rapidly as they did in Japanese cities [11]. Thus, vehicle emissions and coal combustion products were major contributors to atmospheric pollution in the Western Pacific countries. However, the fraction in TSP derived from combustion and the contributions of vehicle emissions and coal combustion were not known.  
Since the nitration of PAHs is dependent on combustion temperature, ratios of NPAHs to PAHs differ between high-temperature combustion sources, such as vehicles, and low-temperature combustion sources, such as coal combustion. However, contributions of these sources to atmospheric concentrations of PM, PAHs, and NPAHs cannot be obtained by simple NPAH:PAH ratios. Recently, we developed a method, named the NP method, to calculate the contributions of major sources to atmospheric combustion-derived PM, using 1-nitropyrene (1-NP) and pyrene (Pyr), and found its usefulness by using several TSP samples collected in the Western Pacific [18; 19]. Here, the method is applied to atmospheric TSP samples collected in 10 cities in this region from 1997 to 2018. This is the first study to estimate the long-term and seasonal changes in contributions from vehicles and coal combustion to combustion-derived particulates in this region.