Ambient air quality and emissions both represent the amount of pollution in units of weight per volume. The difference lies in the source of that pollution. In case of emissions, it is the amount of air pollutants emitted by a particular (known) source – a chimney of a power plant, exhaust pipe of a particular vehicle etc. In contrast, ambient air quality represents the amount of pollutants at a particular place at a particular time. This pollution, however, comes from various air pollution sources, which can be close to that place or very far. In addition, there are other important factors that affect the concentration values (it is therefore a combination of sources, contribution of which can never be exactly determined).
The factors that come into play are for example secondary reactions in the air (chemical reactions that can give rise to new substances, meaning different substances are detected than emitted) or the effect of dispersion and meteorological conditions (for example wind speed and direction, which determines the extent and direction of transport of the pollutants in the air from its source). Another important factor is the long-range transport – air pollutants can travel thousands of kilometers from its source.