Explanation:

Because of longer carbon chains, diesel packs more energy per unit volume compared to petrol. Higher energy and better combustion efficiency means diesel delivers greater power and ensures better mileage. Longer the carbon chain, higher is the probability of finding other elements like sulphur and nitrogen attached to it. Petrol is thus a more uniform carbon compound than diesel. Crucially, diesel has higher sulphur content.
CO2 is formed when the fuel is completely burnt. Generally speaking diesel is long chain compound than petrol and they are heavier (hexanes). It means that they have more number of carbon atoms. More the carbon atoms more is CO2 emission. If you burn equal volumes of fuel, the amount of CO2 released will be higher for diesel owing to it’s much higher energy (read carbon) content. But Diesel engine emits lesser CO2 per Km travelled.
This is so because diesel powered vehicles are usually 20% - 30% more fuel efficient than petrol the per Km CO2 emission is lower .(Studies point this difference to be anywhere b/w 5% to 20%).
Particulate matter is the solid form of fuel which is left behind aster combustion. Petrol is the more refined hence they don't have particulate emission. While diesel are heavier oils which large number of C bonds which are tough to break completely. This incomplete breaking appears as particulate matter. Diesel engines produce a lot of particulate matter, that's the reason why you see thick smoke emitted by diesel engines.
But CO emissions are less compared to petrol engines, because diesel undergoes better combustion than petrol.