Is Nighttime the Right Time for Fresh Air?

It’s time for bed after a long, hot day. The night is cool, so you open the window rather than run the air conditioner unnecessarily. The air is fresh—not hot and humid like in the day. And less seemingly less smoggy too since fewer vehicles and industrial facilities operate at night.

But is the air quality actually better?

Summer Nights In NYC

Let’s look at what the data says here in NYC.

As you may know, Temboo has partnered with numerous environmental groups, community organizations, non-profits, and government agencies to empower them to collect their own environmental data directly with wireless sensors. So we can take a look at a few of the sites in NYC where our partners have been collecting air quality data and check their particulate matter over time.

Looking at the most recent three months of air quality data, covering late Spring and Summer, and defining night at 9pm to 6am yields some interesting results.

The Bronx

A photo of the Bronx River House
The Bronx River House in Starlight Park has multiple air quality sensors around it.

At the Bronx River House in Starlight Park, the overall average AQI reading based on particulate matter concentrations has been 28. But during the day the average AQI is 25, whereas the nighttime average is 34. Additionally, the highest reading over this period, 171, also occurred during the night.

Upper Manhattan

A view from the Baylander boat over the Hudson River
The view over the Hudson River from where the sensor is located, facing North.

Out on an old Navy vessel docked on the Hudson River at 130th Street, the overall average AQI has been 33 over the same period. But similar to the Bronx, the nighttime average (35) is higher than the daytime average (32) though the difference is less than in the Bronx. Interestingly, the highest reading over this period (175) came during the day. But at around 9am that reading was taken closer to nighttime than midday.

Lower East Side

A view of the Lower East Side and the Williamsburg Bridge. Source

Now let’s get off the water and back into the city, close to the Williamsburg Bridge in Manhattan. The average AQI reading here has been 25. But again we see a higher nighttime average (27) than in the daytime (24). We also see here that that highest reading during this period (92) also happened at night, around 11:30pm.

What Is Going on at Night?

In all three cases above the air quality at night has been on average worse than in the day. This raises lots of questions.

How can this be possible when so many fewer people drive at night?

Are there factories or other unknown sources of pollution that operate at night but not during the day?

Does this happen all year round or just during the warmer months?

Is this specific to New York City or the particular locations of these sensors?

These questions can be difficult to answer, but the health and environmental implications are significant.

The Atmosphere After Sunset

An image of the sun setting generated by Dall-e

One thing that we definitively know about night is that the sun is no longer out. This greatly affects the atmosphere, wind, and the mixing of particles in the air.

During the day, the heat from the sun causes the air to move and all the particles in the air to mix. It especially promotes more vertical mixing, allowing pollutants and other particulates room to move higher from the ground and away from people.

However, the cooling temperatures at night and lack of energy from sunlight inhibits this vertical mixing. The pollutants have less ability to lift higher from the ground and can tend to be closer to ground level and to people.

Sometimes this change in the atmosphere can be like a lid being closed over all the particulates, preventing them from escaping away from people. Effectively all the pollution that’s in the air can get more concentrated where people are more likely to breathe it in.

It’s counterintuitive since night air often “feels” fresher and there are fewer cars and factories operating.

But it’s also very specific to the geography of the particular location, its weather patterns, and the human activities happening nearby. Not everywhere has worse air quality at night.

The local atmospheric dynamics of airflow are not always well understood or well observed with current methods and models, so predicting air quality at any particular location is very difficult.

How Can We Address the Problem?

4 versions of The Daily Breather color-coded based on AQI
Examples of The Daily Breather email, color-coded according to Air Quality Index.

Since there is so much that isn’t well understood here, it’s hard to know what steps to take.

But one sure fire to actually know what is happening where you live or work is by having access to air quality data that has been measured nearby and locally instead of modeled or guessed based on readings taken far away.

This issue too is just one of many that makes addressing air quality so complicated. Every location has its own peculiarities, and there is often little historical data at the granular level location-wise that is needed.

That’s why giving communities the ability to measure air quality on their own and share it for education, research, and policy is so important. And that’s why we’re working here at Temboo every day to empower more people to collect environmental data directly for a greener world.

If you’re interested to know more about the environment where you live, sign up for our email service The Daily Breather or, even better, have your organization launch their own hyperlocalized version of the service with your own air quality sensors. Just get in touch.