Direct observations of fine primary particles from residential coal burning: insights into their morphology, composition, and hygroscopicity

Emissions of residential coal burning are an important contributor to air pollution in developing countries, but few studies have yet comprehensively characterized the physicochemical properties of individual primary particles from residential coal burning. Fine primary particles emitted from eight types of coal with low, medium, and high maturity were collected in the flaming and burn‐out stages in a typical residential stove. Based on morphology and composition of individual particles, they were divided into six types: organic matter (OM), OM‐S, soot‐OM, S‐rich, metal, and mineral particles. Low‐maturity coals (e.g., lignite) dominantly emitted soot‐OM particles in the flaming stage, the medium‐maturity coals (e.g., medium‐maturity bituminous coals) emitted abundant OM particles, and high‐maturity coals (e.g., anthracite) emitted abundant OM‐S particles. We found that carbonaceous particles from coal burning significantly decreased with an increase of coal maturity and that soot particles were mainly formed in the flaming stage of low‐maturity coals under higher burning temperatures. We concluded that coal maturity and burning temperature both determine particulate properties in coal emissions. In addition, OM and soot particles from residential coal burning displayed extremely weak hygroscopicity, while inorganic salts within individual particles determined particle hygroscopic growth. Understanding the characteristics of particulate matter emitted from residential coal burning is helpful to trace sources of ambient particles and clarify their possible aging mechanism in air influenced by coal burning emissions. Our results suggest that air quality improvements can benefit substantially from the replacement of low‐ and medium‐maturity coals with high‐maturity coals, natural gas, or electricity in rural areas.