The other day, a friend brought over an air-quality monitor. It measured a number of different gases and particulates, but the one that caught my eye, since I have a sense of what it should have read, was the CO2 meter. Here’s the setup:
Current global average CO2 concentration is 417.25 ppm.
The CO2 meter in question was reading - bouncing around - between 520 and 560
It was outside
In the evening, about 9:00 pm local time
In a city, but a residential part. Our house is on a ~5000 sq ft lot, and on a non-busy street
It was not particularly near any CO2 sources, other than four humans with open glasses of beer.
My question, in a nutshell, is: is the meter bad, or is something in the 500s actually a reasonable read for this scenario? Curious if anyone knows authoritatively, but also anecdotes about other local outdoors CO2 measurements are welcome.
I don’t live in a dip. The block itself is pretty flat, but if you look over a 30 block range, it does have a bit of a continuous slope. It was probably low 40s (F) and dropping (we ended up calling the evening a bit early because everyone was cold), but I didn’t measure the air pressure.
I don’t think gravity is going to matter unless your CO2 concentration is high enough to meaningfully increase the density of the air parcel in question. Air with 540 ppm of CO2 is going to have almost exactly the same density as air with 417 ppm.
CO2 is a product of carbon and hydrocarbon combustion (not to mention respiration). If you live near an urban/suburban area, then human activities can cause an increase in local CO2 levels. After a brief web search, I found this paper which included this figure, showing CO2 levels at several measuring locations around Salt Lake City. The meter west of the city shows something close to the global average (low 400s), but the ones in the city or east of it were reaching up into the 600s and even 700s.
So…never mind the dip, where were you? You mentioned a block, so I’m guessing suburbs? Anywhere near a major roadway, with prevailing winds coming your way?
As I mentioned in the OP, I’m in a city (Seattle), though a residential part of it. Not on a major road, but I’m not in the suburbs either. I’ll take a look at that paper. That sounds like the kind of data I’m curious about!
Air’s a mixture, and within that mixture, CO2 flows preferentially downhill vs other components. That’s why when you measure CO2 in forest ecology, you also check the advection rates.
The only CO2-meter I have used required calibration to the outside air (and presumed outside air would have CO2 at 400ppm, so that is good design). Was this meter factory calibrated or did it require intermittent re-calibration? What kind of accuracy in the 400-500ppm range was it supposed to have?
I forgot to include a sarcasm tag. It was perfectly fine for showing the relative changes in CO2 in a closed chamber with a plant through a day-night-cycle, but I wouldn’t have accepted students writing a paper on local CO2-levels based on the measurements.
In climate papers, CO2 is specifically characterized as a well mixed greenhouse gas (sometimes denoted as WMGHG). But this refers to its homogeneity on a global scale, implying that the direct CO2 forcing on climate is the same everywhere (not that global warming is the same everywhere, which it absolutely is not). The reason it’s considered to be well mixed is that its residency time in the atmosphere is very long; a rule of thumb is that, on average, it’s more than 100 years.
As for regional variations, I don’t really know. You’re obviously going to get higher readings closer to major CO2 emission sources. But note that although the CO2 relative concentration (PPM) doesn’t change much with altitude, when a CO2 meter measures CO2 as an absolute quantity, it has to be calibrated at a specific altitude.
Nobody, least of all me, is suggesting stratification. Just that, if there were a source of CO2 somewhere (you know, the thing you’re suggesting), that denser gas would flow downhill before complete mixture occurred. That CO2 is denser than air as a whole is not controversial.
Yes, no dip is needed for elevated CO2 in the city. I agree completely. I was just suggesting possible reasons for local higher levels. The “how cold”, for instance, would speak to things like fires and heaters. And topography is definitely something to consider when taking CO2 measurements. That’s what I was taught at uni, anyway.
I’m honestly not sure. It’s not my device, and I don’t have it in my possession now, but it certainly didn’t look like a serious scientific instrument. To my knowledge, it had no mechanism to calibrate it.
Yes, it would. And it would also flow uphill and lefthill and righthill. It flowing in every direction is how the mixing happens. Gases can flow through each other, so the fact that carbon dioxide is denser than air is irrelevant.
As one responsible for calibrating various air monitoring sensors in a very regulated industrial environment, don’t be too concerned with random friends/acquaintances so called “monitor” readings. They can be extremely inaccurate.
Please consult a professional
I’m definitely not going to be consulting any professionals here. At a minimum, I’m just curious about the overall CO2 concentrations in the local area, and how that might differ from global averages. At most, I’d want to measure and track the concentration in my basement. Any recommendations on consumer-grade devices that are accurate enough to be useful? Or are they all just garbage?
We agree that if there’s a release of a large quantity of CO2, it can flow downhill en masse before mixing with ambient air very much. See Lake Nyos, 1986:
Squink’s link (hey, that rhymes!) also shows that when high concentrations of CO2 are slowly emitted into small spaces with little or no circulation (e.g.residential basements, snow caves, etc.), it can linger there.
However, the most common sources for CO2 in a residential area like the OP described are gasoline-fueled cars. Their exhaust is at most 12% CO2 (diesel exhaust has even less), and the first thing that happens to that exhaust after leaving the tailpipe is that it gets mixed with a massive volume of ambient air in the turbulent wake of the car itself. Any ambient wind will then cause this mixture to move across the terrain, resulting in mixing with even larger quantities of air. Whatever mixture is drifting downwind of a roadway, I’d be very surprised if the density were greater than ambient by a sufficient margin to compel said mixture to settle, in a buoyancy-driven manner, into a “dip”.
I am assuming by “dip” you mean a small, very local minimum in elevation. My answer changes if, by “dip” you were referring to a larger terrain feature like a canyon or valley, which may be vulnerable to an atmospheric inversion that can trap pollution emitted near the ground.
However, it must be noted that inversions are a consequence of atmospheric temperature profiles, not differences in composition: an inversion traps a warmer parcel of air near the ground, regardless of whether it’s got an elevated CO2 concentration or not. In other words, high ground-level CO2 concentrations are a consequence of inversions, not a cause of them.
To be clear: I didn’t actually say that CO2 doesn’t pool in low-lying areas. What I said was that its density relative to the surrounding air was irrelevant.
Gases flow in all directions. They flow downhill a bit more than they flow uphill. The amount by which a gas flows down more than up depends on its density. But the density of the surrounding air doesn’t matter. If our atmosphere were composed mostly of sulfur hexafluoride or of hydrogen, carbon dioxide would still pool to the same degree it does here.
It was perfectly fine for showing the relative changes in CO2 in a closed chamber with a plant through a day-night-cycle, but I wouldn’t have accepted students writing a paper on local CO2-levels based on the measurements.