macjeff Posted September 23, 2022 Posted September 23, 2022 This question will show my lack of understanding of Air Quality. I was asked for the local AQI for my house. I went to https://www.airnow.gov/?city=Hamilton&state=VA&country=USA and got 32 as my AQI. They said should be 0-50 which is good Where in purple air is this? see screenshot
bpwwer Posted September 23, 2022 Posted September 23, 2022 The EPA AQI is probably the equivalent but that site says it's the nearcastAQI. I don't know if nearcastAQI and the EPA AQI use the same formula or not. The EPA AQI Category in the node server corresponds the ranges that the EPA lists (good, moderate, bad, etc.). It looks to be about the same as what's shown on the AirNow site. The Purple Air bases most of this off the PM2.5 level. The Purple Air is reporting that at .7ppm while on AirNow it's listing it as 8 (over 10x as high). Based on that I would expect the AQI values to be different.
macjeff Posted September 23, 2022 Author Posted September 23, 2022 Then there must be different measurements because the site shows 31 now (Was 32) and that matches what weather apps say. So I am confused.
bpwwer Posted September 23, 2022 Posted September 23, 2022 Why do you expect them to match? I don't know where the Purple Air sensor is located as I couldn't find it on their map, but the sensor reporting for AirNow seems to be located near the freeway and all the Purple Air sensors I could see on the map are located many miles east of that. Air quality is always much worse near freeways. Also, the AirNow reading seems to be based on PM2.5 and Ozone and I believe the Purple Air sensors just look at PM2.5.
dbuss Posted September 23, 2022 Posted September 23, 2022 Currently Air Now shows 31 AQI for my area. My Purple Air AQI is currently 6. The Air Now reporting area for my area is for two counties and neither one is the county I live in.
macjeff Posted September 23, 2022 Author Posted September 23, 2022 Look up 20158 There are 3 within 10 miles of each other. All three are pretty close right now in low 30's. So I guess I dont understand what the 2.5 is as the same 3 in purple are show 1-3 not in the 30's. Thats what I dont understand. They may not match but thats 10+ times difference.
bpwwer Posted September 23, 2022 Posted September 23, 2022 It doesn't look like they're measuring the same things. If I enter 20158 in AirNow and check the box for PM2.5 most of those near that area disappear the ones that are left drop in value to below 10. It looks like the sensors you're looking at in AirNow are measuring Ozone. I don't believe the Purple Air monitors can measure Ozone, they are just PM2.5 and PM1.0 monitors. PM2.5 and PM1.0 refer to the size of the particles that the monitor can detect. Even two monitors next to each other in the same physical location will not match exactly. There are a lot of variables that go into calculating a AQI value and most inexpensive (like Purple Air) sensors are not very accurate. Not compared to sensors that cost 10-100x. The Purple Air monitors can give you a good representation of the air quality within a very small radius of the sensor (probably a couple of hundred yards) for the pollutants it can detect. You might want to read through this: https://www2.purpleair.com/community/faq#hc-how-do-purpleair-sensors-compare-to-regulatory-particulate-matter-sensors
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