Validation of MicroAeth® as a Black Carbon Monitor for Fixed-Site Measurement and Optimization for Personal Exposure Characterization

Abstract

This paper reports on validation experiments with the recently developed microAeth®, a pocket-sized device which is able to obtain real-time and personal measurements of black carbon (BC) aerosol. High reproducibility was observed when comparing the results from six new individual units during fixed-site monitoring out of a window (relative standard deviation [RSD] = 8% ± 5%, N = 1442). The results obtained from the microAeth devices agreed with those obtained from a full size rack mounted Aethalometer, based on both the 1-minute data (R = 0.92, slope = 1.01 ± 0.01, N = 1380) and 24-h average data. The 24-h average of real time data obtained from the microAeths was comparable to the BC concentration obtained from 24-h integrated PM_2.5 filter deposits, as determined by multi-wavelength optical absorption (R = 0.98, slope = 0.92 ± 0.07, N = 12). Rapid environmental changes in relative humidity (RH) and temperature (T) can result in false positive and negative peaks in the real time BC concentrations, though averages > 1-2-hour are only minimally affected. An inlet with a diffusion drier based on Nafion® tubing was developed in order to use BC data with a high time resolution. The data shows that the diffusion drier greatly reduce the impacts from rapid changes in RH and T when the monitoring system is worn in close proximity to the body (e.g., in the vest pocket).

Keywords: Black carbon; Fixed-site monitoring; Humidity; MicroAeth; Personal exposure.

Fig. 1

Time series of six-microAeths fixed-site side-by-side sampling over 24 hrs (1-min interval). The left upper corner plot shows the BC levels between 6:00 am to 7:00 am. * Unit 109 data is corrected because its clock shifted 1 minute at 10:48 a.m. on Apr. 24.

Fig. 2

(a) Comparison between 24 hr BC average from quadruplicate microAeth (y-axis) and 24 hr BC average from Aethalometer (x-axis); (b) Comparison between 24 hr BC average from quadruplicate microAeth (y-axis) and 24 hr BC average from triplicate PM_2.5 samples (x-axis).

Fig. 3

Time series of duplicate personal sampling on a rainy day, where one microAeth unit was used with a short diffusion drier and the other without (regular tubing). The numbers in bracket [] represent the duration of in minutes of the questionable data.

Fig. 4

Time series of duplicate personal sampling on a sunny day, where one microAeth unit was used with a short diffusion drier and the other without (regular tubing). The agreement of the data for the two different units indicates that the diffusion drier does not remove BC significantly.

Fig. 5

Lifespan experiments of silica gel bags in the short drying inlet. Comparing the rate of change in the RH, one observes that the diffusion driers keep the rate of change to < 1% per min in pulse mode and < 2% per min in the 100 %RH mode.

Fig. 6

Lifespan experiments of the long diffusion drier inlet. Comparing the rate of change in the RH, one observes that the diffusion driers keep the rate of change to < 1% per min in pulse mode and < 1% per min in the 100 %RH mode.

Fig. 7

Data collected while walking in and out of air-conditioned building with a HOBO and two microAeth units, one with a short diffusion drier and one without (regular tubing). All three instruments were placed on a tray and thus did not have the temperature buffering advantages of being worn on the body (see text).