PM_2.5 hourly average measurements from 33 outdoor PurpleAir particle monitors were compared with hourly measurements from 27 nearby US EPA Air Quality System (AQS) stations employing Federal Equivalent Method (FEM) monitors in California over an 18-month (77-week) period. A transparent and reproducible alternative method (ALT) of calculating PM_2.5 from the particle numbers in three size categories was used in place of the estimates provided by Plantower, the manufacturer of the sensors used in PurpleAir monitors. The ALT method was superior in several ways (better precision, lower limit of detection, improved size distribution) compared to Plantower's CF1 or ATM data series. PurpleAir monitors were strongly correlated with the nearby US EPA Air Quality System AQS stations. A calibration factor (CF) ranging between 2.9 and 3.1 was empirically derived for the PurpleAir estimates using the ALT method. This value was based on comparing the average value of 177,329 PurpleAir measurements to the value calculated from the FEM stations. The monitoring period included about 13 weeks showing very high outdoor values due to several major fires covering several hundred thousand acres. The CF during these 13 weeks averaged 2.39, whereas the CF for the remaining 64 weeks averaged about 3.21, suggesting a different response to the smoke from wildfires compared with normal ambient fine particulate matter (PM_2.5). The standard Plantower CF1 data series overestimated the FEM values by about 40%, in agreement with several other studies.

在18个月（77周）的时间内，将33台室外PurpleAir微粒监测仪的PM _2.5每小时平均测量值与加利福尼亚附近27个采用联邦等效方法（FEM）监测仪的美国EPA空气质量系统（AQS）站点的每小时测量值进行了比较。一种透明且可重现的替代方法（ALT），用于计算PM _2.5从三个尺寸类别中的粒子数中提取的粒子被用来代替Plantower提供的估算值，Plantower是PurpleAir监测器中使用的传感器的制造商。与Plantower的CF1或ATM数据系列相比，ALT方法在多种方面（更好的精度，更低的检测下限，改进的尺寸分布）具有优越性。PurpleAir监测仪与附近的美国EPA空气质量系统AQS监测站密切相关。根据经验，使用ALT方法得出的PurpleAir估计值的校准因子（CF）在2.9到3.1之间。该值基于对177,329个PurpleAir测量值的平均值与从FEM站计算得出的值进行比较的结果。监视期包括大约13周，由于几场大火覆盖了数十万英亩，显示出非常高的室外价值。_2.5）。与其他多项研究一致，标准的Plantower CF1数据系列高估了FEM值约40％。