Air quality and associated atmospheric deposition in the high-elevation urban centers of Mexico City and Denver were compared. The Mexico City Metropolitan Zone (MCMZ) is located in a closed basin of the Sierra Madre Mountains at 2,240 meters (m) elevation with more than 20 million inhabitants, 15 electrical generating units (EGUs, 1,416 MW), and 5 million vehicles.  The Denver-Boulder, Metropolitan Area (DBMA) is also located in a basin, on the eastern slope of the Front Range of the Rocky Mountains at 1,700 m elevation with approximately 3.2 million residents, 11 EGUs (4,600 MW), and 2 million vehicles. 

In the MCMZ, particulates and sulfur dioxide (SO₂) were identified as the main atmospheric pollutants three decades ago. Replacement of fuel oil with natural gas inside the MCMZ resulted in reduction of both particulates and SO₂ in ambient air. Although SO₂ no longer exceeds its ambient air quality standard, acid rain remains significant in the MCMZ with median pH<5.6 and sulfate (SO₄^-2) and nitrate (NO₃^- ) concentrations of approximately 60 (µeq/L) and 40 µeq/L, respectively in wet deposition. The SO₄^2- : NO₃^- wet-deposition concentration ratio was homogeneous (1.5) at MCMZ sampling sites. A fuel oil power plant (1600 MW) is located in the northern outskirts of the MCMZ, and samples were consistently more acidic in the southern versus the northern stations with prevailing northerly winds that transport acid-rain precursor emissions from north to south.  Annually, MCMZ precipitation had a median ammonium (NH₄⁺) concentration of 100 mEq/L. Inorganic reactive nitrogen (Nr) was contributed predominantly by NH₄⁺ (71%) compared to NO₃^- (29%).

Continuous urban wet-deposition monitoring, which began in Denver in 2017, indicated that Nr deposition loads (4.0 kilograms N per hectare (kg/ha)) exceeded SO₄^-2 loads (1.6 – 2.6 kg/ha) across the DBMA. Nr was contributed predominantly by NH₄⁺ (74%) compared to NO₃^- (26%).  Annually, DBMA precipitation had median pH>6.3, and concentrations of SO₄ ^2- (2 – 5 mEq/L), NO₃^- (24 – 28 mEq/L), and NH₄⁺ (60 – 78 mEq/L). The SO₄^2- : NO₃^- ratio was homogeneous across the DBM at 0.1 – 0.2.  Easterly winds transport urban and agricultural Nr pollution upslope into the mountainous terrain located west of the DBMA.

Sulfate concentrations in MCMZ wet atmospheric deposition are 10 times higher than in the DBMA.  The NH₄⁺: NO₃^- concentration ratio in MCMZ and DBMA is similar: 2.5 and 2.8, respectively.