Natural emission sources are not influenced by human activities. A variety of air pollutants is generated during a range of processes taking place in terrestrial and aquatic ecosystems or in Earth’s crust.
Various carbon and nitrogen species are emitted to the atmosphere during microbiological and plant physiological processes. Natural sources of carbon compounds, such as carbon dioxide (CO2)and methane (CH4) can be aerobic and anaerobic respiration. Nitrogen species such as nitrogen (N2), nitrous oxide (N2O) and nitric oxide (NO) are produced by biological oxidations and reductions of inorganic nitrogen in soils and waters. The most important factor controlling the rate of the nitrogen emission is the soil temperature.
The soil fluxes of N2O in a Norway spruce forest in Mátra Mountain were determined by small static chambers. Accumulation of nitrous oxide was measured by a gas chromatography-mass spectrometry system. N2O emission fluxes were calculated from the accumulation of nitrous oxide gas in the chamber (Figure 2.4).
Due to metabolic processes in forest trees, grasslands or scrublands several types of hydrocarbons are also emitted to the atmosphere.
Several physical, chemical and biological processes occur at surface of wetlands and oceans, which influence the global cycles on a number of species, e.g. carbon, nitrogen, sulphur and trace metals. The most important emission process in wetlands and anaerobic waterlogged soils is the bacterial production of methane. Oceans play an important role in the global biochemical sulphur cycle. Dimethyl sulphide (DMS) is produced during biological activity of phytoplankton then enters the atmosphere by the exchange between see and the atmosphere. Dimethyl sulphide is the major precursor of sulphate particles (). Large amount of water droplets and sea salts are emitted into the maritime atmosphere. After the evaporation of water aerosol particles, such us sodium chloride (NaCl), magnesium sulphate (MgSO4) are formed. Various halogenated organic compounds are also emitted to the atmosphere from the oceans. Among them, methyl chloride (CH3Cl) is the major natural source of atmospheric chlorine.
Almost all forest fires are human induced. Naturally occurring biomass burnings are usually the consequence of lightning. The major products of biomass burning are carbon dioxide (CO2) and water vapour. However, a large number of particulates and trace gases are produced, including the products of incomplete combustion of CO, NMVOCs, nitrogen and sulphur species.
Volcanoes release significant amount of gases and ash particles into the atmosphere. Volcanic emissions occur both during occasionally eruption and long term non-eruptive activities. Most prevailing volcanic gas is water vapour, which can reach 50–90% of total volcanic emission. However, this amount of water vapour is negligible in comparison to its atmospheric concentration. Other important gases emitted from volcanic activities are carbon dioxide (CO2), sulphur dioxide (SO2), hydrogen sulphur (H2S) and some other sulphur species, hydrogen chloride (HCl) and other halogen compounds. Besides the gases, considerable emissions of aerosols are present in most volcanic plumes.
Lightning during thunderstorm events creates plasma channels in the atmosphere characterized by the high fraction of ionic loads and high temperatures. High temperature cause chemical reactions. Major compounds of the atmosphere (nitrogen, oxygen, and water vapour) are undergoing chemical transformation and various species containing nitrogen, oxygen and hydrogen atoms are formed. A major species produced during lightning is nitric oxide (NO).