The Bajío's air: three problems, three maps

Guanajuato runs 23 automatic monitoring stations in 11 municipalities. What they record is not one air-quality problem but three, with different sources, different geographies and different solutions. Reading them together, as “the Bajío is polluted”, leads to poor policy and poor investment decisions.

The 2024 data

Days above the standard, according to the state environmental authority:

Municipality	SO₂	Ozone	PM10	PM2.5
Salamanca	172	52	41	2
Irapuato	14	84	84	14
Celaya	0	—	108	27
Silao	0	—	68	8
León	0	24	67	3

— : no published figure for that pollutant.

Three patterns stand out:

1. Sulphur dioxide: a single point. Salamanca accounts for about 78% of the state’s SO₂, according to Guanajuato’s air-quality directorate, which attributes it to refining and power generation. No other municipality comes close.
2. Particulates: a diffuse problem. Celaya, Irapuato, Silao and León exceed the PM10 standard on 67 to 108 days a year. There is no single stack to shut here: agricultural burning (10,400 recorded fires in 2024, against 9,300 in 2023), brick kilns, traffic, tanneries and dust.
3. Ozone: the one that is growing. Irapuato went from 39 to 84 days between 2023 and 2024; León from 0 to 24; Salamanca from 36 to 52. Ozone is not emitted: it forms from nitrogen oxides and volatile organic compounds in sunlight. It grows with the vehicle fleet and industrial activity of the whole corridor, not with one plant. The car’s role in that corridor is covered in Cars and the air we breathe in the Bajío.

What the combined cycle fixed, and what it did not

On 14 February 2025 CFE put the 927.1 MW Salamanca Combined Cycle Plant into commercial operation, replacing the fuel-oil thermal plant that ran for 55 years. CFE estimates it avoids 3.5 million tonnes of CO₂ a year.

The change was necessary. It was not enough for sulphur. In 2025 Salamanca had logged 73 days above the SO₂ standard by the end of July and 87 by the end of September, and the authority declared pre-contingency alerts in July and November; the one on 11 November recorded an hourly average of 196 ppb at the Cruz Roja station.

The remaining source is the Ingeniero Antonio M. Amor refinery. Based on the sulphur content Pemex reports, Energía a Debate estimated its SO₂ emissions at about 98 tonnes a day in 2023. That is the lowest figure among the country’s six refineries, and it is still enough to keep a city of about 270,000 people above the standard almost one day in three. The figure excludes the fuel oil the refinery itself burns for its own operations.

The lesson for the Bajío is simple: switching the fuel of power generation was the easy part. The sulphur comes from the refining process and depends on sulphur-recovery units and the crude slate Pemex runs, not on the power grid.

Where the investment is

Each pollutant points to a different kind of project:

• Crop residue → bioenergy. The 10,400 fires of 2024 are biomass currently released into the air. Sorghum, maize and wheat stubble can become pellets, biogas or fuel for industrial boilers. The challenge is not the technology but collection logistics and the contract with the grower.
• Process heat → fuel switching or electrification. Industry along the corridor still burns diesel, fuel oil and LPG in boilers and furnaces. Moving to natural gas, heat pumps or solar heat cuts both emissions and cost. It is the same market we described in the Bajío’s hidden energy market.
• Transport → public e-mobility. Ozone and fine particulates grow with traffic. Urban and staff buses are the fleet with the best economic case, as we explained in the bus before the car.
• Brick kilns → efficient kilns. Small, informal and scattered, but with real weight in León and Celaya. It is a micro-enterprise financing problem more than an engineering one.
• Monitoring → data. 23 stations in 11 of 46 municipalities leave most of the state unmeasured. Low-cost sensor networks, calibrated against the official network, make it possible to locate sources and measure the effect of each project.

What it means for investors

Air quality is now a siting variable. In airsheds where the authority frequently declares pre-contingencies, it is reasonable to expect more scrutiny of environmental permits, more questions from customers auditing their supply chains and higher occupational-health costs.

San Luis Potosí confirms this is not only a Guanajuato problem: researchers at UASLP report air pollution on up to 90% of days in the year, with brick kilns, vehicles, industry and fires as the main sources.

The good news is that the sources are known. Salamanca needs Pemex to invest in sulphur. The rest of the corridor needs many mid-sized projects — biomass, industrial heat, fleets, kilns — that private capital can develop.

Sources: Periódico AM, Guanajuato environmental authority air-quality data 2023–2024, Feb 2025 · México Evalúa, Salamanca case study, Jun 2025 · Factor Energético, Salamanca combined cycle enters operation, Feb 2025 · Periódico Correo, 73 SO₂ days in 2025, Jul 2025 · Periódico Correo, 87 SO₂ days in 2025, Sep 2025 · La Silla Rota, SO₂ pre-contingency in Salamanca, Nov 2025 · Periódico AM, particulates in Celaya and Salamanca, Nov 2025 · La Silla Rota, air quality in León, Dec 2025 · Energía a Debate, SO₂ emissions from the National Refining System · UASLP, air pollution in San Luis Potosí, May 2026

Bajío Ventures — a Mexican energy-infrastructure investment group based in Guanajuato: capital, a data platform on the region’s industrial base, and a German industrial channel, with its own advisory presence in Berlin.