The mixture process between air and fuel  in gasoline engines is done homogeneously before ignition at near stoichiometric( chemically correct) conditions and so release of soot is less. In diesel engines fuel is injected into the air and combustion may occur while fuel continue to be injected and so regions of high fuel concentrations are found where soot can easily form specially at high loads and speeds.

Thank you very much sir for your kind and valuable reply.

The fuel is injected over the compressed air which makes it completely heterogeneous mixture leading to incomplete combustion. This attributes to the formation of particulate matter and soot.  But in case of gasoline engine the mixture is either partially or completely homogeneous and the combustion propagates in the forma of flames from the tip of the spark plug to entire combustion chamber volume so no soot occurs in case of gasoline engine.

The attached figure will clearly explains you about the combustion process.... please find the attachment..

Unburnt carbon particles are referred as soot. In Diesel engine the ignition delay is very less since diesel has high cetane number and low self ignition temperature (210 deg Celsius). There will not be sufficient time for diesel to form uniform mixture which results in soot. 

Heterogeneous mixture formation in diesel engines pays way to more soot emissions compared to petrol engine.

Diesel fuel is heavier than gasoline , its distillation range is 180-360 deg C comparing to that of gasoline is 40-180 deg.C.  Difference in combustion characteristics of the SI engines and of the CI engines contribute to types of exhaust components.   In a conventional SI engines, fuel is mixed with air in the intake system, entering the cylinder through the intake valve, and compressed.  Under normal normal operating conditions, combustion takes place towards the end of compression stroke by an electric discharge at the spark plug.  Gasoline engines typically operates with rich air-fuel mixture.  The CI  or diesel engines, the liquid fuel is usually injected at high velocity through nozzles, atomizes into small droplets, vaporizes and mixes with the compressed air  (its temperature and pressure are above fuel's ignition point) inside the cylinder.  Combustion of the fuel-air mixture occurs and fuel injection continues until the desired amount of fuel has entered the cylinder.  As the fuel injected amount per cycle is increased, problems with air-utilization during combustion lead to the formation of excessive amounts of soot which cannot be burned up before exhaust.  This excessive soot or black smoke in the exhaust constrains the fuel/air ratio at the maximum engine power to 20% (or more) lean of stoichiometric air-fuel ratio.  Diesel engines always operate with lean fuel-air ratio and at part load with very lean fuel-air ratio.  Achieving sufficiently rapid mixing of the injected fuel and compressed inside the cylinder to complete combustion in the appropriate crank angle interval close to top dead center is the major problem in diesel combustion chamber design.  (referred to J.B. Heywood, Internal Combustion  Engine Fundamentals, Ch. 9-10)

Soot emission is produced in low temperatures and local fuel-rich mixture spots.

In general, for stratified mode operation soot emission are usually higher, on similar lines, pre-mixed fuel/air operation i.e. stoichiometric or rich operation will produce low Soot quantity.

Particulate is expressed in terms of number and mass.

Particulate mass (PM) emission is directly proportional to soot emission. They are the primary contributors to accumulation mode emissions. Soot is basically formed during combustion. Their size and weight increase as they combine among themselves to formed bigger soot agglomerates during expansion, till exhaust stroke. Through the tailpipe of the engine, where exhaust gas is diluted by the atmosphere, the secondary source of accumulation mode, and contributor to PM emission, is unburnt hydrocarbon (uHC's), they are deposited over soot particle and converted from gas phase to solid phase to form particulate of higher diameter, process is known as surface condensation of HC's.

Particulate Number (PN) emission is created from uHC's directly (Nucleation Mode), those that are not interacting in the tailpipe with soot created through combustion. They are very small in size, so they are light in weight, hence contribute minimum to PM, but their number intensity is significantly higher. The fact that they can possibly reach to the human lung through breathing system, makes them quite a health hazard. That's emission norms are specifying the PN limit in legislations.

uHC's sources are quite similar in most engine operation:

A: The HC's absorbed by lubrication oil during compression stroke due to high cylinder pressure, they are desorbed in expansion stroke as pressure decreases.

B: Flame quenching effect that happens near the cylinder wall, due to a relatively cooler cylinder surface, creates uHC's.

C: Slow combustion reaction, create uHC's (Lean mixture or Rich), the higher charge increases the heat capacity of the overall mixture, hence slows down the rate of combustion, leaving unburnt charge.

D: The charge escaped in crevices, due to high surface to volume ratio of crevices, it becomes almost impossible for the flame to react with crevice charge. Charge escapes in crevices during compression stroke at high pressure. It escapes back into the cylinder during expansion stroke when pressure lowers down.

E: uHC's generated by wall/piston film, case of diffusion-based combustion, common for direct injection systems.

"The rich mixture will contribute to PN mostly, assuming we are using pre-mixed most, as the charge will escape, creating high uHC's, which will contribute to nucleation mode."

Refer to Heywood: Chapter: Pollutant formation and control.

because of the difference in the nature of combustion in diesel and petrol engines