Which chemicals in smog obscure visibility?
Winter and Summer smog

There are two types of smog: 


winter smog: 


high emissions of soot and of $SO_2$ through heating with wood, coal, etc.
$SO_2$ reacts to sulfuric acid => formation of particulate matter => reduced visibility

summer smog (photochemical smog):


strong sunlight and emissions of $NO_X$, $CO$ and VOCs
emissions of primary particulate matter not necessary
reaction of these air pollutants to other gaseous air pollutants (PAN, $O_3$, $HNO_3$) through photo-oxidation
photo-oxidation of various VOCs => lump together to ultra-fine particles (secondary PM) => particles grow 



Species that impact visibility

We 'need' the gaseous air pollutants to form secondary aerosol particles. The reduced visibility during winter and summer smog events is caused by these secondary aerosol particles (they scatter the light and, by this process, reduce visibility). The gaseous air pollutants might contribute to reduced visibility but their contribution is negligible compared to the aerosol impact. 

Contribution of different aerosol species

The composition as well as the size distribution of the aerosol particles is important to determine the aerosol's optical properties. If you look into mass concentrations, a given mass of fine particles extincts more light than the same mass of coarse particles. If you look into composition (inorganic components), nitrate extincts more light than sea salt and sulfate. The extinction of organic aerosol depends on their composition (I have no idea which factors/species are relevant there).

If you are interested in further literature on the impact of the particle composition on the light extinction you might look into Pitchford et al. (2007). I didn't find a publication which quantifies the contribution of different particulate compositions/species to light extinction for smog. However, Pitchford et al. (2007) deals with light extinction for calculating the optical thickness from model data.