Because of the chemical differences between non-exhaust and exhaust emissions, they result in different secondary PM. Secondary PM is formed in the atmosphere through chemical reactions, rather than being directly emitted by a source. The volatile organic compounds in exhaust gases react with sunlight in the atmosphere to form secondary organic aerosols (SOAs) whereas non-exhaust emissions are mainly inorganic and therefore form secondary inorganic aerosols (SIAs). However, it is exceedingly difficult to model SOAs and SIAs emissions