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Stop Blaming Drivers for Mexico City’s Smog

This Spring, Mexico City has been choking under some of its worst smog conditions in years. The problem is ozone. Pollution alerts for ozone have been issued repeatedly, triggering “double” driving restrictions that have pulled hundreds of thousands of cars off the road, twice as many as usual. But what if cars are not the problem? I’ve been combing through recent data from Mexico City and the relationship between cars and ozone is tenuous at best.

Big Data

If you want to look at a pollutant that is tightly related to driving, take carbon monoxide. As the figure below shows, carbon monoxide levels in Mexico City tend to peak at 8am or 9am, when the roads are jammed with commuters trying to get to work. Emissions inventories show that 99% of carbon monoxide in Mexico City comes from cars, and you can see this in the daily pattern.

The pattern for carbon monoxide also differs across days of the week. The figure shows only Friday through Monday, but this is enough to be able to compare weekdays to weekends. Friday, Monday, and other weekdays have the biggest peaks. Saturday, and in particular, Sunday, have lower peaks. Again, this reflects driving. After battling traffic all week, people in Mexico City are happy to drive less on weekends.


Ozone has a very different pattern, peaking in the middle of the day when the sun is highest in the sky. There is no peak during the morning commute like you see with carbon monoxide. But even more revealing, notice that the peak for ozone is similar across all days of the week. Weekend ozone levels are just as high as weekday levels, even though many fewer cars are on the road. When people drive less, carbon monoxide levels go down.  Ozone levels? Not so much.

What’s Going On?

I’m an economist, not an atmospheric scientist, but I think the data give a clear picture of what is going on here. Ozone is the classic “secondary” pollutant, meaning that it is not emitted directly but instead is formed in the atmosphere as a product of other pollutants.  The basic recipe for ozone is simple. Take volatile organic compounds (VOCs) and nitrogen oxides (NOx).  Add sun. Chemical reactions happen. Voila!! You get ozone, a pollutant that not only causes smog but also is very dangerous for human health.

But here’s the deal: You need both VOCs and NOx. If you are short one of these two ingredients, you can’t substitute more of the other. In econ-speak, we’d describe this as a “Leontief” production function. In areas where there are lots of VOCs, ozone formation is “NOx-limited”. A reduction in VOCs will have little impact, because the process already has more VOCs than it can use. Similarly, in areas where there is lots of NOx, ozone formation is “VOC-limited”, and a reduction in NOx will have little impact.


Note: Figure 1 from Sillman (1999).  The solid lines represent ozone production rates of 1, 2.5, 5, 10, 15, 20, and 30 parts per billion per hour.  Thus there are different combinations of NOx and VOC that yield the same ozone production rates.

Surprisingly, there is no consensus in the scientific literature on whether Mexico City is NOx- or VOC-limited. See here and here.  But the recent data provide pretty clear evidence that Mexico City is, in fact, VOC-limited. NOx levels are much lower on weekends, but ozone levels are not.


In fact, Sunday ozone levels in Mexico City are actually somewhat higher than other days of the week, consistent with the backward bending part of the curves above. For places that are severely VOC-limited, ozone production can actually increase when NOx concentrations drop. In other words, when VOCs are the constraint to ozone production, more cars on the road can actually reduce ozone levels!

Given this evidence, it seems crazy to try to reduce ozone levels by restricting driving.  Sunday is, in some sense, an extreme version of what could be achieved through driving restrictions. And while many pollutants are indeed lower on Sundays, ozone is not. Driving is not the problem.

Policy Implications

What are the policy implications? First, drop the double driving restrictions on high ozone days. There is no evidence that this has any impact on ozone levels. And, more generally, driving restrictions have been widely shown to be an expensive and ineffective approach to addressing air quality.

If you want to reduce ozone in Mexico City, you have to reduce VOCs. VOCs come from all kinds of things. Paints. Solvents. Adhesives. Cleaning Products. Cosmetics. Even dog poop.  Yes, dog poop. Carlos Álvarez, a chemical engineer at Mexico’s National Polytechnic Institute has calculated that 250,000 tons of dog poop are “deposited” annually on Mexico City’s sidewalks, significantly contributing to VOC emissions.


Note:  Watch your step.  A dog walker in Mexico City’s Hipódromo neighborhood.  Source: Airbnb.

In addition to targeting these sources, it would be worth looking again at transportation.  But rather than restricting driving, it’s time to look at gasoline regulations.

California provides a particularly useful point of comparison. Los Angeles is similar to Mexico City in that both suffer from high ozone levels and both are VOC-limited. Since 1996, California Air Resources Board (CARB) gasoline has been required throughout the state. Considerably more stringent than U.S. national fuel standards, CARB gasoline must meet strict content requirements for olefins and other highly reactive VOCs.

CARB gasoline has been shown to be very effective at reducing ozone. When CARB gasoline was introduced, it reduced Los Angeles ozone levels by 16%, according to research from the Energy Institute’s Max Auffhammer and Ryan Kellogg. California has also achieved additional ozone reductions by enforcing strict requirements for vapor recovery systems which reduce VOC emissions at gas stations when drivers are filling up their tanks.

Similar requirements could work in Mexico City too. Whatever approach is taken, let’s then evaluate the policy using data. Too much is at stake to continue rolling out the same tired policies. Let’s use modern data techniques to quickly and credibly figure out what works and what doesn’t work.



Lucas Davis View All

Lucas Davis is an Associate Professor of Economic Analysis and Policy at the Haas School of Business at the University of California, Berkeley. His research focuses on energy and environmental markets, and in particular, on electricity and natural gas regulation, pricing in competitive and non-competitive markets, and the economic and business impacts of environmental policy.

13 thoughts on “Stop Blaming Drivers for Mexico City’s Smog Leave a comment

  1. Mexicans get offended when a foreigner points out their problems. The problem with pollution in Mexico City is a problem of corruption, ineptitude, and lack of will to resolve it from the source.
    Verification centers have been in the hands of fraudulent, corrupt friends of those in power, who have sold the franchuse to run the Test emmision Centers.

  2. This is a good analysis, however it only covers one of several contaminants that can be tackled by reducing the number of cars on the streets. The author does not mention that air quality in Mexico City has improved over the years largely by policy that gives incentives to newer less-polluting cars. Cars that meet certain emissions standards are exempt from non-driving during pollution alerts up to a certain level; hybrids and electrics are exempted altogether. He is also wrong in mentioning that these are the “worst smog conditions in years” , and fails to understand that the reason behind more days held under a pollution alert is that pollution alert standards have become more demanding over the years, with a big hike last year, which in turn has prompted sales of less polluting cars. TGB

  3. El artículo es muy bueno, excepto por su comentario inicial de que tenemos el peor aire en muchos años. Tenemos el aire más limpio de los últimos 25 años, bajaron los límites para molestar impunemente

  4. As an economist I have been discussing ozone formation with atmospheric scientists. The main conclusion is that ozone is a complicated pollutant. My colleague and I at Uniandes have been studying the case of the car-free days in Bogotá (restricting driving for 1.5 million cars and 0.4 million motorbikes) and our preliminary results indicate that during car-free days ozone levels are higher than in classical working days. It seems that a stringent driving restriction policy instead of reducing ozone it may cause the opposite effect. A possible explanation of this result is Lucas’s interpretation or titration. Several factors intervene in ozone formation, the emission rates per hour, the abundance of pollutants, the speed of chemical reactions and weather. Some of these factors we may not control. Cars are not the only ones that emit VOCs and other pollutants. Involving industries is important because they also contribute to pollution.

  5. A couple of decades ago I was working in Santiago, Chile, which sits in a narrower, deeper valley re the mountains than does Sacramento. Each day we would see the dense smog rise as the day went on and obscure the beautiful Andes. The academics there told us it was the diesel buses and trucks producing the smog, and the fact that auto emissions were much looser than ours! Maybe now there, and in Mexico City, they have reached the stage where cleaner gasoline and diesel would be important!

  6. In previous communications we have asked to the economist Lucas W. Davis to stop making recommendations on the air quality management of Mexico City. His “scientific” work is based on simplistic statistical analysis of the publicly available air pollution data published by the local environmental agency (Davis, Journal of Political Economy 116, 2008; Scientific Reports 7:41652, 2017).

    Economists are not trained to understand the chemical and phisycal processes driving the formation of secondary pollutants in a complex environment as the Metropolitan Area of Mexico City. The air pollution problem in Mexico City has been extensively studied (, Comprehensive studies have found that the olefins emitted by vehicular traffic are the volatile organic compounds (VOCs) triggering the formation of ozone.

    Indeed, driving restrictions have shown to not be the solution to stop the ozone’s threat. They are emergency measures that work merely as temporally reliefs (Velasco & Retama, Sustainable Cities and Society 31, 2017). Without those unpopular measures the environmental crisis experienced this and the previous year would had been even worse.

    Suggesting that pets’ excrements is an important source of ozone precursors demonstrates a complete lack of knowledge of the problem.

    On behalf of many Mexicans trying to stop the air pollution threat, I asked again to L. W. Davis to avoid publishing articles/notes that could bring adverse consequences to millions of people far from his office in Berkeley.

    By the way, L. W. Davis has not responded to the comments raised by his recent publication about Mexico City driving restrictions published in Scientific Reports.

    • Unfortunately, Mexico City has been relying on “emergency” driving restrictions for two decades. Economists conducted studies in the 1990s that showed that such restrictions would be ineffective, and CARB rejected a proposed similar restriction in its 1994 SIP based on our review of those studies. Our reviews of other air quality studies have found substantial errors and misattributions of pollutant links in other cases as well. I’m not familiar enough with this specific situation to say whether Professor Davis is misguided or not, but I have seen enough problems in the past to say that I am more inclined to accept his skepticism than to take your statements at face value.

      • As someone working on the development and understanding of novel catalytic converters, then the question Erik Velasco, is why on earth the Mexican authorities have not implemented more restrictive measures in the last two decades, and “only” rely on the “Hoy no circula”? It is ridiculous!! What about enforcing the use of state-of-the-are catalytic converters for diesel-based transport, which are well-known to emit large NOx quantities which contribute to O3 formation? Do they actually know about the existence of SCR catalytic converters for diesel engines??? In theory, they should follow the equivalent to Euro 4 regulations (from 2005), instead of more stringent Euro 6 (from 2014)?? Ah no, they instead love they hocus-pocus “hoy no circula” solution!!

  7. Ozone formation is a VERY complex issue. It’s not just the gasoline, although that might have an effect, even in CA with reformulated gasoline we saw interesting impacts. Weather drives it, as do emissions, and not just the magnitude, but the timing of emissions are key. The “weekend effect” was just such and effect and was extensively studied in the South Coast Air Basin years ago. The key was on weekends there were fewer diesel truck on the road, which meant less NO2 which titrated out the ozone as it was forming. See:
    Fujita, E.M., W.R. Stockwell, D.E. Campbell, R.E. Keisler, and D.R. Lawson.
    2003a. Evolution of the magnitude and spatial extent of the weekend ozone
    effect in California’s South Coast Air Basin. J. Air Waste Manage. Assoc. 53:802–815. doi:10.1080/10473289.2003.10466225

    Fujita, E.M., D.E. Campbell, D.E., B. Zielinska, J.C. Sagebiel, J.L. Bowen, W.S.
    Goliff, and W.R. Stockwell. 2003b. Diurnal and weekday variations in the
    source contributions of ozone precursors in California’s South Coast Air
    Basin. J. Air Waste Manage. Assoc. 53:844–863. doi:10.1080/

    Recent research suggests this may be lessening, see:
    George T. Wolff , Dennis F. Kahlbaum & Jon M. Heuss (2013) The vanishing
    ozone weekday/weekend effect, Journal of the Air & Waste Management Association, 63:3,
    292-299, DOI: 10.1080/10962247.2012.749312

  8. Good work. Nice to see real data analysis. A big problem in Mexico is that The gasoline company is owned by the government. This the same reason that pollution in communist and former communist countries is far higher than in capitalist countries. The other likely problem is that the smog controls on the cars are non funtional an dthe drivers just bribe their way through the test, It was shown in LA street tests of cars that the really high polluters had found a way around the smog testing. The same testing showed that the Pareto rule fit the smog emissions. so 80% of the smog comes from 20% of the cars. Typically this means a taxi that has high mileage and is not maintained and so bribes its way through any required testing. Unfortunately for Mexico city La Mordida is engrained inthe culture. A real solution would be a private enterprise solution of remote testing and payments for every car found out of compliance. This would need some kind of supervisionn to prevent bribery. Possible sending the data to a remote site so the motorist oculd not bribe the local operator. Since yoir a not a scientist. Remorte sensing work by analysing the spectra of the gas emissions and compares the Co to Co2 ratio which is a good indication.
    lots of other references if you Google remote sensing of automobile emissions

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