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Particulate matter (PM) is important for its human health concern, its role in atmospheric physico-chemistry and that for aesthetics. Past few decades have seen raised understanding, awareness and concern on the impacts of PM on human health, air quality and climate globally. However, knowledge gaps related to its nature, formation, transport and impact continue to exist; particularly for those related to smaller PM (mainly ‘submicron PM’, also referred to as ‘PM₁’) and their contributions to various processes and impacts.

Adverse impact of particulates on human health has been long established, however, some of the recent findings paint a much grimmer picture. Specific PM sources like traffic, coal, oil and biomass combustion, soil or road dust have shown a positive correlation with mortality or adverse health effects.^{1,2,3,4, 5}Fine PM can penetrate deep into the lungs⁵and cause an increased risk of respiratory morbidity, emergency hospitalization and death risk.^{5, 6}PM_2.5is associated with acute lower respiratory infections and mortality,⁷and submicron PM can influence blood and different organs.⁸Evidence even support that ultrafine PM can quickly enter the circulatory system,⁹and can cause major structural damage of epithelial cells when localized in cell mitochondria.¹⁰Further, it has been reported that iron-bearing nanoparticles can get transported directly into the brain posing serious health hazard.¹¹PM, therefore, pose unprecedented human health concerns; particularly with respect to submicron PM.

In the atmosphere, PM is intricately linked to a various atmospheric phenomenon including climate change. Lately, several works have reported important aspects of atmospheric processes and climate change, where the role of the submicron PM is considered to be increasingly significant. Particles scatter and absorb light, and they serve condensation surfaces for water vapour, affecting cloud formation, precipitation rates and indirect climatic effects due to clouds.¹²Some of the particles are semi-volatile, and they can even modify trace-gas composition, and impact gas phase reaction pathways through heterogeneous reaction in the atmosphere.⁵PM are responsible for a range of striking visual and aesthetic features of our skylines, including stunning sunsets, morning haze, blue haze over forests, and complex cloud formations.¹³These features arise from their direct interaction with solar radiation and their roles in cloud formation.¹³As a consequence, they have both direct and indirect effects on the global radiation budget.¹³The predominantly cooling effect on PM, at least temporarily, acts to mask warming by greenhouse gases. The submicron PM have been identified to play an influential role in many of these atmospheric processes.

Particulate size, composition and source decide the impacts from both air-pollution as well as climate perspectives. A thorough understanding of particle concentration, size distribution, chemical composition, state of mixing and morphology are therefore essential to address its radiative, ecological and human health aspects. A lot has been known on these aspects for PM₁₀and PM_2.5. However, lately, the submicron PM has been dominating the environmental concern related to atmospheric PM worldwide. Understanding the share and contribution of submicron PM in the overall PM mass-loading and its physico-chemistry has become highly important. Some of the recent work on size-segregated aerosol characteristics shows that knowledge of patterns and features of the submicron PM can be very helpful for impact studies.

Mass concentration of particulates represents its atmospheric concentration in terms of the mass of particulates present per unit vol of air (e.g. µg/m³). Numerous studies from different parts of the world representing size-segregated mass concentrations of particulates, mainly as PM₁₀, PM_2.5and PM₁, reflect the rising importance of PM₁. Studies from different environmental setting in China^{14,15, 16}have shown that PM₁dominates the share and variation patterns of PM_2.5and PM₁₀. The high share of PM₁(in total dust) have been found from different forms of combustion in Germany.¹⁷Studies from India (New Delhi),¹⁸Greece (Athens),¹⁹Switzerland (High Alpine)²⁰and Italy (Po Valley)²¹show that share and/or trend of PM₁mass significantly influence/dominate the mass of larger PM. Similarly, non-refractive PM₁mass has dominated PM_2.5in Finland (Helsinki)⁵and smaller biomass burning particles have been found to dominate summer aerosol in USA (Colorado).²²Many other such studies show that the mass concentration of PM is significantly influenced by that of submicron PM throughout the world. Therefore, submicron PM mass is now turning out to be a relatively bigger concern worldwide.

The chemical nature of particulates have been found to vary widely with source, age, size, atmospheric processing and environmental condition globally. The chemical characteristics of PM₁, PM_2.5and PM₁₀are interesting to analyze, and very commonly the chemistry of PM₁are found to influence/dominate that of the overall PM. Numerous studies have shown that the spectrum of chemical species found is more often distinct between submicron and larger PM. For example, different studies from Switzerland,^20,23South Africa,²⁴Hong Kong,²⁵Finland (Helsinki)⁵, USA (Sacramento Valley),^23,26Africa (Senegal),²³China,²³France²³, Italy,²³the Netherlands,²³Spain²³and ‘Greater European Region²⁷support dominance of one or more of such species like organics, sulfate, black carbon, nitrate, ammonium, chloride and iron in submicron PM, whereas, by other species like calcium in larger PM. Further, some of the major anions and cations have been majorly found to be part of submicron PM in Greece (Athens).¹⁹Similarly, certain物种被发现更丰富的罚款标准ticles at an urban air pollution site in China (Beijing), where the contributions of pollution sources was found to decrease with increasing PM size.²⁸Another global study²⁹showed that ‘dust’ is primarily found in the coarse mode whereas carbonaceous components in fine and ultrafine mode PM, close to emission sources. Thus submicron PM is expected to significantly influence/dominate the overall chemistry of atmospheric PM.

PM size largely govern its physical behaviour from various impact perspectives. There has been a large amount of data on the size distribution of PM, however, in the past few decades there has been a significant advancement in measurement facilities and there is now an appreciable spectrum of data also on submicron PMand its physical properties from many regions of the world. PM physical properties like those related to light scattering and absorption, hygroscopicity, cloud formation capabilities, etc are being widely studied, also giving striking features of submicron PM; thereby increasing its significance in understanding overall PM impacts. Apart from mass share, other physical properties of PM also show a significant contribution from PM₁in different parts of the world. In Central Himalayas, Nainital, India the scattering and absorption coefficients, scattering and absorption Angstrom exponents, backscatter fraction and single scattering albedo share and trends of PM₁₀were seen to be highly influenced by those of PM₁.³⁰In Sacramento, California, USA similar measurements showed significant influence of PM₁on similar optical properties of PM₁₀.^31,32In Granada, Spain PM₁was found to influence the scattering and absorption efficiencies in visible wavelengths.³³Similar observations were also recorded in Southwestern³⁴and Arizona,³⁵USA. Several other workers have also considered that the scattering process was mainly due to particles in the fine mode. An important finding in this respect was reported at a rural site in northeast Spain in the Western Mediterranean Basin, which showed that polluted air masses were linked with an increase in PM₁concentration and consequent increase in the values of scattering, backscattering and absorption coefficients and angstrom exponents. Thus, it is important to note that the physical behaviour of PM can be significantly influenced by the presence of submicron PM.

Thus, several data-set and studies under varying environmental settings from different parts of the world point towards the raised significance of submicron PMin larger PM (like PM_2.5and PM₁₀). The contribution of submicron PM in terms of share and variation pattern is often dominating the mass of larger PM in different regions of the world. The chemistry of larger PM are also in many cases influenced by the chemical nature of submicron PM; as some chemical groups/species largely remain concentrated in the latter. The physical nature and behaviour of larger PM, like its optical properties, also in many instances remain highly influenced by those of submicron PM. In this context, it is important to note that rain may not be able to wash out all of submicron PM, as observed in Wuhan, China.¹⁶Therefore, the significance of submicron PM highlights the need for global efforts to include submicron PM as a major pollutant in all atmospheric PM pollution initiatives and impact studies, and work towards early development and proper utilization of air quality standards for submicron PM.

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