Hi r/LosAngeles, we are coming up on a period of potentially unhealthy to hazardous air quality due to fireworks.

What is the forecast?

High pressure is slowly building heading into the holiday weekend, which is thinning the marine layer, and leading to a warming trend (temps will be closer to normal for this time of year). The pattern will also keep the area under an inversion that traps pollution by the surface.

Are there any air quality advisories?

There is an air quality alert (officially a Particle Pollution Advisory) issued by the AQMD. It goes into effect at 5PM on Saturday, and will go through at least 3PM Sunday. AQMD is warning of potentially unhealthy to hazardous air quality for much of the region.

The Particle Pollution Advisory issued by AQMD (as of 5:15PM on 7/2)

What is in the smoke?

Fireworks give off large amounts of particulates of varying sizes. Some of the finer particulates (PM2.5) can get deep into your lungs. The particulates will be made of various compounds like paper, plastic, and metals.

The reason we have metals in the air: firework colors. Each color is created by burning metal salts. Some illegal fireworks may even contain lead.

How long is the smoke sticking around?

Smoke and other gases will come out of the atmosphere through deposition (think of ash falling on cars, or rain cleaning the atmosphere), transport/dilution (wind and mixing dissipating the smoke), or through chemical reactions in the atmosphere (which includes reactions with sunlight).

The rate at which this happens depends on the weather and pollutant. Large particles like ash, dust, etc will settle out of the atmosphere faster, and closer to the source. Fine particulates like PM2.5 can remain suspended in the atmosphere for longer periods of time, and can travel farther from the source. Some gases react and can break down quickly, while others may last longer.

How do I protect myself?

Treat this like wildfire smoke.

• Run air purifiers if you have them. If you don’t there’s a guide from the EPA for a DIY air purifier. Link to Instructions
• If you have central air: run the fan or A/C to recirculate air through your home and draw it through the filter. You’ll want a good enough filter (MERV 13 or higher if your system can handle it), that means you may need to run out to a home improvement store to buy a better filter.
• If you have a window A/C or wall mounted A/C: Only run it if you can recirculate the air/close the outside air vent (some have a setting or lever for this). Do not let it pull outside air into the house.
• Stay inside during the worst air quality, if possible.
• If you have to go out, wear an N95 (or higher) mask.
• Check air quality before heading out: AirNow Smoke Map, AQMD Current Air Quality

Hi r/LosAngeles, I wanted to give a bit of info and thoughts from the perspective of a meteorologist. While atmospheric chemistry was part of my coursework, I am not a chemist, so there are some questions I can’t answer and don’t want to speculate on. Speaking of speculation, this post is not intended to say everything is or is not fine.

I apologize in advance for the length, trying to cover a few items I’ve seen brought up by other Redditors. Here’s a quick list of the questions covered below:

1. What site do you use for air quality numbers for smoke?
2. What is an AQI number?
3. What about PurpleAir?
4. Why is it so bad in the morning?
5. How important is wind?
6. Is the smoke stuck forever?
7. How do I protect myself?
8. One more item re: July 4^(th)

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What site do you use for air quality numbers for smoke?

My preferred site (for smoke) is https://fire.airnow.gov. The map has a combination of air quality sensors from air quality agencies (such as South Coast AQMD), as well as some devices from PurpleAir and Clarity. When you look at the map for our region you will see a few big circles with numbers, these are official air quality sites for South Coast AQMD. You may occasionally see circles with a “T” inside, these are temporary sensors from the agency.

Edit: It appears that not all of South Coast AQMD's temporary sites are not listed on AirNow, you can see them (and the "gridded current air quality") on their website here.

The page allows you to click on any site to see the trend in the data over the last day or so. The map also applies a correction equation to PurpleAir stations.

What is an AQI number?

It is important to remember, as with any weather station, that an observation is showing you the conditions at the time listed, at the specific location of that sensor. The location thing is important…unless a sensor is in your yard, it may not perfectly reflect the conditions where you live. In a case like this, especially with wind, it is possible that you could be sitting in smoke while the AQI sensor a mile away is sitting in clear air.

I’ve seen a few posts/comments about how a sensor is or isn’t showing something, and how it is different from what it looks like outside…this likely comes down to sensor position compared to your position, but time of the observation is part of this too.

AQI numbers are based off the highest pollutant. Sometimes it is Ozone, sometimes it is fine particulate matter (PM2.5). Most public-facing air quality monitoring stations only show certain pollutants: particulates, Ozone, NO2, and CO. Fires put up a mix of particulates and other gases, some of those gases may not be detected by these sensors.

What about PurpleAir?

There’s nothing wrong with using PurpleAir’s map, I would recommend applying EPA correction on the map. Click the gear in the upper left corner, and under “Apply Conversion” select “US EPA.” PurpleAir sensors can read high during some smoke events, so this may help bring the numbers in line with other observations.

Why is it so bad in the morning?

First off, blame everything on the inversion. This helps to create/reinforce the marine layer…but also traps things in the lower atmosphere. At night the air can be more stable, and the layer is shallower, so smoke is unable to dissipate. In a no-wind setup, smoke will be worse in areas closer to the fire. We have had light winds each night, with different directions, and that has contributed to the spread of smoke.

During the day, the layer expands as the air warms and becomes more turbulent. This helps to dissipate the smoke a bit, but it is still very much there and trapped in the layer.

How important is the wind?

Very.

The onshore breeze during the day will carry smoke inland, so expect conditions around the region to change throughout the day. The daytime breeze helps to trap pollution by pushing it inland, with the mountains forming a wall preventing the pollution from escaping. At night we can sometimes develop a weak offshore breeze, which carries the smoke into the Basin.

Is the smoke stuck forever?

No.

Smoke and other gases will come out of the atmosphere through deposition (think of ash falling on cars, or rain cleaning the atmosphere), transport/dilution (wind and mixing dissipating the smoke), or through chemical reactions in the atmosphere (which includes reactions with sunlight).

The rate at which this happens depends on the weather and pollutant. Large particles like ash, dust, etc will settle out of the atmosphere faster, and closer to the fire. Fine particulates like PM2.5 can remain suspended in the atmosphere for longer periods of time, and can travel farther from the source. Some gases react and can break down quickly, while others may last longer.

How do I protect myself?

Treat this like any wildfire smoke.

• Run air purifiers if you have them. If you don’t there’s a guide from the EPA for a DIY air purifier.
• If you have central air: run the fan or A/C to recirculate air through your home and draw it through the filter. You’ll want a good enough filter (MERV 13 or higher if your system can handle it), that means you may need to run out to a home improvement store to buy a better filter.
• If you have a window A/C or wall mounted A/C: Only run it if you can recirculate the air/close the outside air vent (some have a setting or lever for this). Do not let it pull outside air into the house.
• Stay inside if possible.
• If you have to go out, wear an N95 (or higher) mask…but keep in mind this may not filter out some gases that are present in the smoke.

One last thing…

We are less than 2 weeks away from the 4th of July, where we typically see some of our worst air quality around the region due to fireworks. Firework smoke contains fine particulates, bits of plastic, and metals (metal salts create the colors in fireworks)…some illegal fireworks may also contain lead. So the advice above applies for that weekend as well.