Ozone and Its Precursors in the Atmosphere of Mexico City

The relative apportionment of hydrocarbons (HCs) coming from mobile, fixed, and other sources (not correlated either to carbon monoxide [CO] or sulfur dioxide [SO₂] emissions as solvent evaporation and biogenic sources) is calculated as previously proposed by Riveros et al.¹ through the linear approximation [HC]_tol = [HC]₀ + m₁ [CO] + m₂ [SO₂], where m₁ and m₂ are fitted constants. The obtained apportionment with air samples taken in 1993 is consistent with the earlier published apportionment with air samples taken in 1992, validating the previous procedure. This analysis suggests that 75% of HC originate from mobile sources, 5-18% from fixed sources, and 7-20% from other sources (mainly solvents and bio-genic sources). A similar analysis was employed to estimate the relative contribution of HCs and nitric oxides (NO₂) to ozone (O₃) formation, the most important air pollutant in Mexico City. In this way, through a local lineation of O₃ isopleths, simultaneous measurements of HC and NO₂ in the atmosphere were fitted to the equation-[O₃]_peak = [O₃]₀ + m_a [HC] + m_b [NO₂]-to predict O₃ peak. With these data, the adjusted parameters show that NO₂, not HC as was proposed previously, is the most important contributor to O₃ formation.