With smoky skies, it’s essential to factor air quality

CONCORD, CA—In this most surreal year, Bay Area residents have faced an additional crisis by enduring continuing episodes of unhealthy air due to the recent wildfire activity.

A mid-August thunderstorm triggered an outbreak of local fires, some of which are still burning into September. Wildfire-spawning weather conditions returned on Labor Day weekend as triple digit temperatures combined with dry offshore winds to scorch the state.

The healthiness of our air is defined in terms of an Air Quality Index (AQI). It is not measured directly but calculated from measurements for each of the six air pollutants the Environmental Protection Agency (EPA) has deemed potentially unhealthy. EPA regulatory standards serve as a yardstick for the AQI, with the value of 100 serving as a boundary between healthy and unhealthy conditions.

When smoke from wildfires spoils our air, the AQI index is based on measurements of PM2.5. These are airborne particles that measure 2.5 microns in diameter, about 100 times thinner than the diameter of a human hair. PM 2.5 pollutants can be especially hazardous to human health, since these small particles penetrate deeply into the lungs.

The AQI index is divided into sectors that relate to human health standards. According to Bay Area Air Quality Management District (BAAQMD) guidelines, an AQI value between 100 and 150 is considered unhealthy for sensitive groups, between 150 and 300 is defined as unhealthy to very unhealthy and values over 300 are considered hazardous.

Purple Haze

Prior to this fire season, AQI values in the East Bay were calculated at one of three BAAQMD monitoring sites, with the highest value applied to the entire region. This year, a new database has emerged online called PurpleAir. (I like to think of it as Purple Haze.)

PurpleAir is an internet-connected network of measurements that use relatively inexpensive PM2.5 sensors to calculate site-specific AQI values. To date, private citizens and school facilities have installed several hundred monitors in the East Bay.

PurpleAir displays a map view of the real-time data and uses color-coded circles, each color representing one of the six AQI index used by the BAAQMD. The impressive display shows AQI color bands that provide reasonably accurate microclimate analyses of air quality with regard to smoke.

I use the term “reasonably accurate,” because there are some cautions one should take when interpreting the data.

One concerns the existence of bad data. Some (not too many) sensors wildly overestimate the AQI, while some underestimate. On a very smoky day, there might be a few purple circles (unhealthy AQI) amongst a sea of green (healthy AQI) readings, and vice versa. The user should visually disregard the outliers.

Differing measurement techniques

Another concern is that due to differing measurement techniques, the AQI numbers generated by PurpleAir PM2.5 monitors tends to overestimate the BAAQMD AQI values. Purple offers a conversion factor based on a Lane (Oregon) Regional Air Pollution Authority study. To visualize the adjusted data, the user needs to change the conversion choice (in a box on the lower left side of the map) from “none” to “LRAPA.”

Fire season came early this year, and poor air quality has afflicted Bay Area residents for several weeks with no end in sight. Fortunately, we can track real-time AQI index values on a microclimate basis thanks to enterprises like PurpleAir.

COVID and now wildfires. We need a vaccine and rainfall to get out of this mess.

Woody Whitlatch is a meteorologist retired from PG&E. Email your questions or comments to clayton_909@yahoo.com