I hooked up a PM2.5 sensor[1] I ordered during wildfires a few years ago, and the EPA's AirNow has been far more consistent with what I'm measuring than Purple Air or IQAir.
Accuweather and Windy.com have pretty good forecasts, but the ones that have been the most accurate this week for me are the US Forest Service's BlueSky models.
[1] Plantower PMS7003 are less than $20 on AliExpress. Here's a quick plot of the data https://i.imgur.com/FjABgpP.png including a spike where it was above a grill.
I'm surprised though why the data is so different between airnow and purpleair. On AirNow, SF currently shows unhealthy air (150~) whereas purpleair shows 88 (moderate). Which one of these is more reliable and real-time?
Someone posted a Medium article [0] on the /r/bayarea subreddit where they did some digging into this. Apparently the sensors used by Purple Air rely on a constant that represents the average density of the particles it detects. Because wood smoke particles are less dense than typical PM 2.5 particles, the resulting AQI values are too high.
The Lane Regional Air Protection Agency has developed a conversion formula that is built into Purple Air (if you apply the LRAPA conversion factor in the UI), so you can have a better comparison.
AirNow uses official EPA sensor sites, which are professionally maintained but fewer in number. These will have the highest quality data.
That said, the PurpleAir devices use high quality laser particulate sensors, so they're still significantly more accurate than most consumer devices.
Both are real time, but during a wildfire where smoke is shifting between neighborhoods, I'd go with the PurpleAir site to get a score for your particular neighborhood. Just be sure to turn on the LRAPA scoring model, as others have mentioned.
In addition to what other commenters have said, I'd guess that some purpleair sensors are placed in garages or other places with a local concern for air quality. That may explain some of the local variation you see on purpleair.
I think the main difference is that airnow mainly relies on expensive sensors with government-approved calibration. There are only a few such sensors per county.
Airnow's map shows both the government sensors and the Purple Air sensors, but that seems to be a recent "pilot project". I doubt they use the low-cost sensor data anywhere else.
AirNow's data _can_ be less accurate. For example, punching in a Lamorinda ZIP code only returns results for Oakland ("Oakland reporting area"), even if the ZIP correctly shows the right locale.
PurpleAir lets me see my neighbor's air quality stats...two houses away. PurpleAir is much more relevant and accurate for me.
The shortcoming of the AirNow data is the sparse sampling. If you search for your city you can see the caveat underneath with something like "AnyTown Reporting Area". When you look at the sensor coverage it's much more sparse than the corresponding PurpleAir sensor coverage.
There's also the fire map that includes both public and private sensors, the regular interactive map I don't believe uses all the available sensors.
https://fire.airnow.gov/
> Notice: The Sensor Data Pilot adds a new layer of air quality data from low-cost sensors.
I like PurpleAir's air quality map but felt it was a bit too slow to check frequently. So I used their API to make the fastest air quality site around: https://aqi.today
For those who don't live here, a series of dry thunderstorms last weekend sparked a number of wildfires that have combined to burn more than 1500 square miles (more than 4000 square kilometers) in less than a week.
at least, those 1500 square miles can't burn again... maybe CA need to do controlled burns like FL does (if they don't already do it). IE: Burn dry brush in the forests.
I believe California does, but the scale of the problem is completely different compared to a sate like Florida. California has 2.5x as much land as Florida and a significantly higher percentage of the total land is wilderness or otherwise undeveloped.
Not just wilderness or undeveloped, but highly inaccessible to firefighting crews. The Bay Area has peaks over 4000 feet with lots of fairly sheer areas. Often firefighters have to be dropped in by helicopter.
Florida is mostly low rolling hills with the highest point in the state being ~350 feet above sea level. If Florida has a fire in an inaccessible area, it's usually inaccessible due to water, so more of a help to firefighters than a hindrance.
The federal government owns about 45% of California land. Of the forests, 57% are owned by the federal government; 39% are privately owned by individuals, corporations, or land trusts; and the state, county, or local governments control only 2%.
Also, most fires start in grassy areas. The Camp Fire that destroyed Paradise started as a grass fire on federal land before it spread to shrubs and burned through mostly private land. California has almost 8 million acres of shrubs.
The Woosley Fire that burned almost half of Malibu was started on industrial property owned by Boeing and spread through chaparral along the steep Santa Monica Mountains.
As I recall, Bureau of Land Management tried this and there was a lot of backlash from residents. It’s also quite a bit of land to do a controlled burn, so might not be that feasible.
Purple Air sells you a sensor that you can install, and then takes the data, displays it on their website for lots of people to view, and then presumably makes money off the data you send them? Why should you have to pay for the sensor then?
If you set your sensor to private, you may view it by logging in to the PurpleAir map using a Gmail address that matches the "Owner's Email" in the registration form. Alternatively, you can use the link provided in the registration confirmation email. You may share this link with family and friends.
I have a Purple Air sensor; I view the money I spent on it as helping to sponsor a "citizen science" project. Could the nature of the project change in the future, to where I feel like I was taken advantage of? Sure, I guess. But I'm ok taking that risk.
We are a new startup (currently doing Startup School) to provide air quality monitoring solutions especially for schools.
From the beginning it was important for us to give back to the community so we setup AirGradient for Education providing free tutorials and advise.
We just published our open source and open hardware air quality sensor measuring PM2.5, CO2, Temperature and Humdity. So you can build your own sensor with a small display to measure the air quality in and around your home. We call it the AirGradient DIY sensor.
PS: We are also looking for schools that are interested in improving air quality to learn more about their needs and possible solutions. Please PM me for more information.
Not loud at all. Just a background hum. However, I'm sure it's a function of the box fan motor. I have a cheap one and it's perfectly fine to sleep in the same room.
It's useful to see the pollution numbers on the sensors, but it's also just as interesting to see which sensors have been destroyed and for how long. You can track the fire just by seeing when sensors go offline, if you know where to look.
I sent their team an email two years ago with exactly that feedback, and their response was:
"Thank you for the feedback. We are working on an update that will 'save' your preferred map view when logged in on the PurpleAir website. However, it is not published yet."
See my post above how I am able to bookmark the site after I choose the settings I want and it remember these choices when I visit again.
If you bookmark the website after you have zoomed in on your area and toggled settings (unchecked indoor sensors, LRAPA), it will remember your settings.
Seriously. My partners school was saying they will open this week (on top of COVID, of all things) if the AQI is not in the red. The sensor they're using is indoor. This website is actively doing harm with these configurations, granted, so is the school administration.
COVID is bringing out the worst in all parts of our society. It has been such a stressful time, and I am someone who has some financial security. I can't imagine how tough things are on the average American right now.
Stay strong everybody. We will make it out of this.
Airnow's map is slow to load, but shows smoke, air monitors (including the Purple Air ones^[footnote]), and fires all at once: https://fire.airnow.gov/?lat=37.40&lng=-122.077&zoom=10#
A less detailed, fast, static map for the SF Bay Area is available at http://www.baaqmd.gov.
Last, but not least, there is a forecast of surface-level smoke available at https://hwp-viz.gsd.esrl.noaa.gov/smoke/. I found it from https://twitter.com/NWSBayArea, which is a good source of news for the Bay Area.
[footnote]: To make the numbers match, I had to set PurpleAir to show One-Hour Averages with the LRAPA scoring. See https://imgur.com/a/d6yFtow.