DST ObservAir Manual de instruções - Página 16

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5. Best Practices

5.1. Filter replacement

As particulate matter deposits on the filter, the ObservAir uses a simple

mathematical relationship to calculate black carbon (BC) concentrations in

the sample flow as a function of the light attenuation rate through the filter

(Section 1.1.1). As the aerosol filter becomes overly saturated with BC

deposits, however, this underlying relationship degrades and the

ObservAir's BC concentration measurements are underreported (lower

than the true value). This measurement degradation is known as the 'filter

loading artifact', and can be largely avoided by changing the aerosol filter

tab when the optical attenuation (ATN) values exceeds 80: a commonly

accepted ATN threshold for aerosol photometry at 880 nm. While some

measurement error persists below this threshold, it is usually small and

grows larger for higher ATN values. Therefore, it is best practice to only

operate the ObservAir while the optical ATN is < 80, and replace the filter

when this threshold value is exceeded.

Given this optical attenuation limit, the filter's operational life before

requiring replacement depends only on the average BC concentration and

flow rate of air sampled through the sensor. For an average BC

concentration of 1 µg/m

Table 2 shows that the effective filter life ranges from 6.3 to 1.6 days as the

sample flow rate settings increases from 50 to 200 ccm. Filter life is

inversely proportional to both average BC and flow rate, and may be

calculated using the equation below. The equation approximates the total

sampling time required for the aerosol filter to reach an ATN of 80 for the

given input conditions.

FL = Filter life (days)

= Average BC concentration (µg/m

avg

FR = Flow rate (ccm)

5.2.

Filter loading correction

While sensor operation at low optical attenuation levels largely eliminates

the filter loading artifact, this may not always be practical and convenient,

and some BC measurement error necessarily remains. As a result, many

filter loading correction algorithms have been developed to compensate

aerosol absorption photometers' BC measurements as a function of optical

attenuation. These empirical corrections are well documented and

ObservAir Operating Manual

313.5

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generally straightforward to implement. However, the filter loading artifact

is not static – it depends on the emissions source (e.g. biomass vs. diesel),

atmospheric conditions, seasonality, and other factors. Therefore, filter

loading correction algorithms should be calibrated and validated for each

particular application of the ObservAir, and periodically updated over long-

term deployments. Standard filter loading correction procedures for the

ObservAir are under development, and will be released in future versions of

this manual. Helpful resources on the filter loading artifact are provided

below.

Filter loading correction resources:

1. Good, N.; Mölter, A.; Peel, J. L.; Volckens, J. An Accurate Filter Loading

Correction Is Essential for Assessing Personal Exposure to Black Carbon

Using an Aethalometer. J. Expo. Sci. Environ. Epidemiol. 2017, 27 (4),

409–416. https://doi.org/10.1038/jes.2016.71.

2. Jimenez, J.; Claiborn, C.; Larson, T.; Gould, T.; Kirchstetter, T. W.;

Gundel, L. Loading Effect Correction for Real-Time Aethalometer

Measurements of Fresh Diesel Soot. J. Air Waste Manag. Assoc. 2007,

57 (7), 868–873. https://doi.org/10.3155/1047-3289.57.7.868.

3. Virkkula, A.; Mäkelä, T.; Hillamo, R.; Yli-Tuomi, T.; Hirsikko, A.; Hämeri,

K.; Koponen, I. K. A Simple Procedure for Correcting Loading Effects of

Aethalometer Data. J. Air Waste Manag. Assoc. 2007, 57 (10), 1214–

1222. https://doi.org/10.3155/1047-3289.57.10.1214.

5.3. Flow rate setting: Filter life vs. BC resolution

The filter's operational life decreases at higher flow rates, so it may be

tempting to set flow rate at the lowest possible option (25 ccm) and

minimize sensor maintenance. However, effective BC measurement

resolution also depends on flow rate: Since the filter loads up with BC more

rapidly at higher flows, the time rate of light attenuation is more readily

detectable, and BC baseline noise decreases. BC baseline noise represents

the sensor's effective measurement resolution and is shown for various

sampling intervals in Table 4 for a flow rate of 100 ccm. BC baseline noise

is inversely proportional to the sample flow rate. For example, 1-minute BC

noise is around 0.025 µg/m

at 200 ccm (half that shown in Table 4 for

100 ccm). In this way, higher flow rates provide measurements with higher

temporal resolution, but at the expense of filter life. The optimum flow rate

setting maintains adequate BC measurement resolution for the monitoring

application while maximizing filter life such that sensor maintenance

remains convenient and practical. The procedure below outlines the

calculation and selection of an appropriate ObservAir flow rate setting.

ObservAir

Operating Manual