About NOx

Nitrogen oxides, herein referred to as NOx include nitric oxide and nitrogen dioxide. These oxides result primarily from the combustion of fossil fuels by vehicles and various stationary sources, though they are by products of all combustion-based systems and of all high temperature industrial processes.

NO2 is produced by the combination of elemental nitrogen and oxygen in air within the burner/chamber in which combustion is carried out as a result of the high temperature from the highly exothermic combustion reaction.

NO2 is partly produced and emitted with NO, but its origin is mostly due to peroxyl radicals and therefore to the complex oxidation chain fueled in the atmosphere after emission. NOX is a deleterious air pollutant which can be poisonous at high concentration levels. Nitrogen oxides (NOx) emissions,  and in particular NO2 are a known precursor to the formation of ozone and of secondary aerosol largely contributing to PM 10 and to PM2.5.

NOX therefore is not only criteria pollutant on their own, but they contribute sensibly to other pollutants of major health and environmental concern. In addition, NOx can react with volatile organic compounds to form photo-chemical smog, a phenomenology mainly connected with high temperatures and insolation concerning areas and seasons characterized by mild to hot conditions such as for example the Mediterranean region, at least in the summer season

About So2

What are the effects of sulphur oxides on human health and ecosystems?
95% of the SOx emitted from the combustion of fossil fuel is sulphur dioxide. SO2 is a toxic gas, which is directly harmful to human health. It is heavier than air and has a suffocating odour at an atmospheric concentration of around 500 parts per billion (ppb), at which level it can be fatal.  At lower levels, chest pains, breathing problems, eye irritation and a lowered resistance to heart and lung diseases can be experienced. At 20 ppb or lower there should be no ill effects to a healthy person. The normal atmospheric background concentration of SO2 is generally less than 10 ppb. 
A secondary effect is the formation of sulphates (and nitrates), in the form of aerosols or very fine airborne particles, which can comprise a significant proportion of the particulate matter and have been linked to increased asthma attacks, heart and lung disease and respiratory problems in susceptible population groups. 
A third effect can occur further away from the emission source where the sulphur oxides will have converted to acids by aqueous phase reactions in the atmosphere. These acidic aerosols are eventually precipitated as acid rain, snow, sleet or fog but only when they encounter the right meteorological conditions. In the absence of man made pollution rain water would be slightly acidic, around pH 5, due to the presence of carbonic acid from the interaction of water vapour and naturally occurring levels of CO2. Acid rain on the other hand has been measured with pH levels below 3 corresponding to vinegar.]
The effects of acid deposition depend on the fragility of the materials, plants, soils and waters involved. In those instances where there is not the natural alkalinity to neutralise this acidity or alternatively the capability to withstand such attack, acid deposition has been linked with the acidification of ground and surface water, deforestation, reduction – even elimination – of aquatic life and building decay. The exposed surface of limestone (CaCO3) used for the fabric of many historical buildings turns to gypsum (CaSO4), which has a lower density and is more water soluble, hence suffers from spalling). The impacts on the natural environment can cause further problems for example, acidic water leeches out heavy metals. Soil no longer bound by tree roots, may be washed away, leaving a denuded landscape and potentially adding to the flood risk downstream where this soil is subsequently deposited as silt.