Erik Crosman, an atmospheric sciences researcher, to help us better understand the weather factor to inversions (also known as cold air pools.) Crosman answers common questions about what causes the wintertime haze, how it forms, if it’s worse in some places than others and what researchers like him hope to learn about it and why it matters.">

Feb 29, 2016 — There are two components to Utah's inversions. We can see one of them: the pollution that causes the nasty-looking air. The other is the meteorological conditions that trap the pollution. We asked Erik Crosman, an atmospheric sciences researcher, to help us better understand the weather factor to inversions (also known as cold air pools.) Crosman answers common questions about what causes the wintertime haze, how it forms, if it’s worse in some places than others and what researchers like him hope to learn about it and why it matters.


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Interviewer: You know, when we have a nasty inversion it's a combination of a lot of different factors, and of course the one everybody thinks about is the pollution. But there are meteorological conditions as well. That's a big part of that equation, and we want to better understand that. So in order to understand the health factors involved, I thought we should understand the meteorological factors involved.

We're talking to Erik Crosman. He's a professor and researcher in the department of atmospheric sciences at University of Utah. The first thing I want to do is we all can see the inversion when it starts happening, that dirty nasty air, and I live east of State Street. So I'm under the impression that it tends to build up on the Oquirrhs and then move across the valley up to the Wasatch Mountains. Is that how it builds or is does it build a different way?

Dr. Crosman: Yeah it's basically an optical trick on your eyes that it appears like it's always on the other side of the valley. Basically, the way of the atmospheric optics work, it's pretty uniform across the whole valley but you see that gunk cloud farther away.

Interviewer: So the 'me' on the other side of the valley is thinking the same thing, "Oh, it looks like it builds up on the Wasatch and comes back to us.

Dr. Crosman: Exactly, exactly.

Interviewer: But in reality we're all breathing the same nasty air at that point.

Dr. Crosman: To some extent. I will give the caveat, though, that they're very well . . . you know, if you're hiking in the foothills on a day where the inversion is very shallow, it could be totally clean in the foothills and really polluted down along the I-15 corridor. So there are spatial variations that we see. But on those days where it's deep and mucky, it's often an optical effect where it looks bad on the other side of the valley.

Interviewer: And then when an inversion clears out, how does that happen? Does the air just push it up and over the Wasatch primarily, or is it a different route that it goes? See, these are all my perceptions that I don't know.

Dr. Crosman: So the main things we need to get rid of inversion are lots of heating or warming at the surface, or for the most part cooling aloft, and so usually a cold front or a storm system comes in and brings cold air, and that cold air will then be able to come in and mix with and remove our cold air at the surface.

What our typical inversion situations are is we have a high pressure ridge with really cold air in the valley for that entire period, and then really warm air aloft. So it was like 50 degrees at Park City. People were skiing in T-shirts in the mountains. That cold air, around 30 degrees in the Salt Lake Valley was just trapped there. Cold air sinks, warm air rises. It had nowhere to go for those two weeks, and our pollutants just accumulated for that period in the valley. And so we need a storm system with winds and cold air aloft to make it so that you can overturn and mix that air at the surface.

Interviewer: So that's where inversion comes from. Normally, in the valley it would be hotter and as you get higher in altitude it gets cooler, but the inversion is it's colder low and it gets warmer when you get high. Is that . . .

Dr. Crosman: Exactly, it's the opposite of the typical atmospheric situation where temperatures cool at about 10 degrees Celsius per kilometer with height. That's why in our intuitive sense it's always cooler in the mountains in the summer when we go hiking. In the inversion situation, it's warmer in the mountains. That's where we're up there skiing in the warm air and it's much colder in the valley.

Interviewer: Yeah, and cold air is lazy. It doesn't want to go anywhere.

Dr. Crosman: Right, it won't go anywhere . . .

Interviewer: It's not like there's a lid holding it down. It just doesn't want to go.

Dr. Crosman: Exactly. It's just gravitational force. Cold air is heavier and more dense. It sinks to the lowest spot possible, and when we're surrounded by the Wasatch and Oquirrh Mountains, that cold air can't go anywhere laterally, either.

Interviewer: When we have one of these inversion events, it's not that we're producing more pollution, it's just the pollution we're producing can't escape anywhere.

Dr. Crosman: Exactly, and we've gotten cleaner. Our cars are cleaner. We've enacted lots of regulations to pollute less over the years, the last few decades. Even though the population is growing, our total emissions in the Salt Lake Valley have been decreasing, but at the same time, the research from health scientists has shown that we need more and more stringent air quality standards. There are even arguments that those need to be lower, and as those lower, we meet the criteria where we say, "That's bad for your health," more easily, and there we issue more red days. And so that's part of the perception of why it seems like things are getting worse. It's because our standards of what we deem acceptable have gone down.

Interviewer: So along the Wasatch front, are there places where the inversion pollution tends to be worse because of geography?

Dr. Crosman: It depends on the pollution, so for the particulate pollution the PM2.5 that matters in the wintertime episodes. We have instruments on the tracks train that runs from each of the benches, at daybreak up to the bench at the University of Utah, and then all along the I-15 corridor. We do see if we average that during some episodes, that it's higher along the lower parts of the valley, and lower in the benches.

But at any particular snapshot of time, there are times when it's higher on the northeast quadrant of the city and lower in the south west, depending on where the flows are, where the meteorology is. So at any given time it could vary. If you average it all out, in these winter episodes usually the lower you go, the worse it gets.

Interviewer: So you do a lot of work and a lot of research on the meteorology here in the valley and how it affects the inversion and how the pollution is affected by it. To what end does your work help make a difference?

Dr. Crosman: What is the end goal of my research and how do we help society in general? My meteorological studies are to help improve the understanding so that we can improve the numerical models and we can improve the forecasts. And so for instance, this last episode in February was hard to forecast. We didn't think it was going to end this weekend, and thankfully it did on Sunday. If we had better understanding of the meteorology and then better models, we could have predicted that better.

We still have no way of saying three days out, is it going to be a red air day or a yellow. There's a wide range and when you're saying bad versus really bad, that has to do with how deep that structure, how concentrated the cold air pool is in the basin. Part of our research is to better understand that and predict that ahead of time, and then also to give more lead time so that people can prepare and know and they can put the signs up that say, "Don't drive. Carpool" several days in advance instead of the day before or not at all, so more to connect society with our understanding and prediction of it.

From the meteorology side, that's one aspect. Then I'm also involved in some air quality instrumentation with the tracks train and others throughout the valley, and that's getting a better understanding of how these things vary spatially, and how the meteorology changes that. And we can couple that with health science researchers down the road, and then they could say there's a reason why people on the benches, or wherever we find where the pollution is lower, had lower health risks or got sick less. We don't have that data set yet, but our research that we've developed in the last few years is starting to produce that data set. So I think that's an exciting thing down the road.

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