The conversion of sulfur dioxide to sulfur trioxide in the absence of a catalyst is a slow process. However,this process occurs easily in the atmosphere. Describe how this happens and write the chemical equations for the formation of $SO_3$ from $SO_2$.

(N/A) The conversion of $SO_2$ to $SO_3$ in the atmosphere occurs through photochemical or non-photochemical processes.
$1$. Photochemical process: $SO_2$ molecules absorb light in the near-ultraviolet region and react with ozone $(O_3)$:
$SO_2 + O_3 \xrightarrow{h\nu} SO_3 + O_2$
$2SO_2 + O_2 \xrightarrow{h\nu} 2SO_3$
$2$. Non-photochemical process: In the presence of particulate matter (dust),$SO_2$ reacts with atmospheric oxygen:
$2SO_2 + O_2 \rightarrow 2SO_3$
$3$. Role of $NO_2$: In the presence of sunlight,$NO_2$ undergoes photolysis to produce $NO$ and an active oxygen atom:
$NO_2 \xrightarrow{h\nu} NO + [O]$
This highly reactive oxygen atom reacts with $O_2$ to form ozone $(O_3)$:
$O_2 + [O] + M \rightarrow O_3 + M$
(where $M$ is a third body like $N_2$ that absorbs excess energy).