Primary is a direct emission source and forms a significant portion of the PM pollution, comprising of metals, elemental carbon (also known as black carbon), and organic carbon. Other primary pollutants include sulfur dioxide, nitrogen oxides, volatile hydrocarbons, methane, carbon monoxide, and carbon dioxide.
Secondary is due to chemical transformation of primary gaseous emissions. In PM, the secondary aerosol components include sulfates from sulfur dioxide emissions, nitrates from nitrogen oxide emissions, organic aerosols from hydrocarbon emissions, and ammonium from ammonia emissions. The path and the quantity of chemical transformation depends on the strength of the pollutant emissions and the mix of the emissions. In an atmospheric chemical mechanism, these interlinkages can run into 300 equations. Other secondary pollutants include ozone – a result of a mix of reactions between nitrogen oxides and hydrocarbon emissions in the presence of sunlight.
More such questions are answered in SIM-series # 44-2021 - 10 frequently asked questions on particulate matter (PM)
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