Tropospheric or ground level ozone (O3) is the ozone present in the lowest portion of the atmosphere (up to 10-15km above the ground). It is responsible for a large part of the human enhancement of the global greenhouse effect and has a lifetime of a few days to a few weeks. It is not directly emitted but formed by sunlight-driven oxidation of other agents, called ozone precursors, in particular methane (CH4) but also carbon monoxide (CO), non-methane volatile organic compounds (NMVOCs) and nitrogen oxides (NOX). Tropospheric ozone is a harmful pollutant that has detrimental impacts on human health and plants and is responsible for important reductions in crop yields. 

O3 also reduces the ability of plants to absorb CO2, altering their growth and variety. It damages ecosystem structures and functions, as well as the health and productivity of crops, thus threatening food security.

Tropospheric ozone is a major component of urban photochemical smog, and a highly reactive oxidant which, when inhaled, can worsen bronchitis and emphysema, trigger asthma, and permanently damage lung tissue. Tropospheric ozone exposure is responsible for an estimated 150,000 premature  deaths every year (Lim S. et al. 2012). Children, older adults and people with lung or cardiovascular diseases are particularly at risk of adverse health effects.

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