enum MeasurementUnitEnumMeasurement units.
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                  Parts per million. DeclarationSwift case ppm
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                  Parts per billion. DeclarationSwift case ppb
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                  Parts per trillion. DeclarationSwift case ppt
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                  Milligrams per cubic meter. DeclarationSwift case mgm3
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                  Micrograms per cubic meter. DeclarationSwift case ugm3
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                  Nanogram per cubic meter. DeclarationSwift case ngm3
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                  Particles per cubic meter. DeclarationSwift case pm3
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                  Becquerels per cubic meter. DeclarationSwift case bqm3
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                  Creates a new instance with the specified raw value. If there is no value of the type that corresponds with the specified raw value, this initializer returns nil. For example:enum PaperSize: String { case A4, A5, Letter, Legal } print(PaperSize(rawValue: "Legal")) // Prints "Optional(PaperSize.Legal)" print(PaperSize(rawValue: "Tabloid")) // Prints "nil"DeclarationSwift init?(rawValue: UInt64)ParametersrawValueThe raw value to use for the new instance.