Top 10 Approaches to Sampling of Air

This article throws light upon the ten approaches to sampling of air. Some of the approaches are: 1. Passive Samples 2. Paper Tape Samplers 3. Sampling for Sulphur Dioxide Testing 4. Sampling for Oxides of Nitrogen (NOx) Testing 5. Sampling for Suspended Particulate Matter (SPM) Testing 6. Sampling for Respirable Particulate Matters RSPM Testing and Others.

Sampling of Air: Approach # 1. Passive Samples:

The simples to approach to sampling of gaseous air pollutants involves passive collection onto a chemically treated surface or material. The driving force for collection is diffusion through the air and/or movement due to wind. Once in contact with the collector, the pollutant is retained by chemical reaction.

Results generated by these methods are useful in a relative sense, but because of the variability of the factors affecting collection and retention of the pollutant it is difficult to establish any simple relationship to the airborne concentrations.

Sampling of Air: Approach # 2. Paper Tape Samplers:

The usefulness of the passive samplers systems can be extended if air is forcible drawn through or over the treated surface by means of pump. An example of this is the paper tape sampler shown in Figure 1. Numerour applications have been reported for gases such as S₂H, HCN, NH₃, NO₂, SO₂, CI₂, COCI₂, amines and isocyanates.

In most commercial systems the tape is automatically advanced at selected time intervals to produce a series of discrete spots or samples. Some units also incorporate a direct measurement system for use when stain development takes place in situ. In other it is necessary to remove the tape for measurement or analysis of each spot.

A paper tape system has also been developed into a continuous monitoring system for particulate matter, using the attenuation of low energy radiation.

Sampling of Air: Approach # 3.Sampling for Sulphur Dioxide Testing:

Steps for sampling:

i. Prepare absorbing reagent (sodium tetra-chloromercurate) by dissolving 27.2g mercuric chloride and 11.7 g sodium chloride in 1 lit of water.

ii. Prepare a sampling train of as least 2 gas bubblers (for average reading) properly washed with distilled water and air dried.

iii. Place bubblers in the sampling system securely connecting to the manifold. Check the connections of bubblers with the manifold and the inlet and outlet.

iv. Fill the bubblers with an absorbing reagent with an amount sufficient to last for 24 hours (approximately 15 ml for 8 hours sampling to 50 ml for 24 hours sampling).

v. To eliminate interference of trace metals, if any, 1 drop of 0.01% EDTA solution may be added to the reagent prior to sampling; similarly, effect of oxides of nitrogen may also be eliminated by adding 1 ml of 0.06% sulphamic acid to the reagent at site.

vi. Start the sampler and adjust flow rate to 2 lit/min.

vii. Note the flow rate at the end of the desired sampling period and stop the sampler.

Calculations:

i. Average flow rate (if there is a significant difference in initial and final flow rates)

ii. Total volume of air sample (TVA) in m³ = Avg. flow rate (lit)

iii. µgSO₂/TVA

Maximum amount in µg/m³.

Sampling of Air: Approach # 4. Sampling for Oxides of Nitrogen (NOx) Testing:

Steps for sampling:

i. Prepare absorbing reagent (a solution of sodium hydroxide and arsenite) by dissolving 4 g sodium hydroxide and 1 g of sodium arsenite in 1 lit of distilled water.

ii. Prepare a sampling train of at least 2 gas bubblers (for average reading) properly washed with distilled water and air-dried.

iii. Place bullers in the sampling system securely connecting to the manifold. Check the connections of bubblers with the manifold and the inlet and outlet.

iv. Fill the bubblers with an absorbing reagent with an amount sufficient to last for 24 hours (approximately 15 ml for 8 hours sampling to 50 ml for 24 hours sampling).

v. To eliminate interferences of sulphur dioxide, drop of hydrogen peroxide may be added to the reagent to convert sulphur dioxide into sulphate during analysis.

vi. Start the sampler and adjust flow rate to about 0.2 lit/min for 24 hours sampling.

vii. Note the flow rate at the end of the desired sampling period and stop the sampler.

viii. Transit the sampling train to environmental laboratory carefully with scientific precautions and preserve the sample tubes in a controlled environmental condition.

Calculations:

i. Average flow rate (if there is a significant difference in initial and final flow rates).

ii. Total volume of air sampled (TVA) in m³ = Avg. flow rate (lit)*

iii. µ gNOx/TVA.

Maximum limit in µg/m³

Sampling of Air: Approach # 5. Sampling for Suspended Particulate Matter (SPM) Testing:

Steps for sampling:

i. Condition a filter paper in oven.

ii. Prepare a sampling assembly by uncorking screws of the bracket.

iii. Take the tare (initial) weight of filter paper (Wi).

iv. Place the filter in the sampling system securely and tighten the screws of the bracket.

v. Set the timer for the period of sampling.

vi. Start the sampler and adjust flow rate to about 2 lit/min for 24 hours sampling.

vii. Note the flow rate at the end of the desired sampling period and stop. Take the final weight (wt).

viii. Transit the sampling train to environmental laboratory carefully with scientific precautions.

ix. Condition the filter paper again for the same period as was done prior to sampling.

Calculations:

i. Average flow rate (initial and final flow rates) in lit/min = (Initial flow rate + final flow rate)/2

ii. Total volume of air sampled (TVA) in m³ = Avg. flow rate (lit/min)

iii. Concentration of SPM in ug/m³ = (w_f – w_i) (mg) / TVA (m³)

Maximum limit ug/m³

Sampling of Air: Approach # 6. Sampling for Respirable Particulate Matters RSPM (PM_1.0, PM_2.5, and PM10) Testing:

Steps for sampling:

i. Clean thimble filter box of the Dust Trak.

ii. Prepare a sampling assembly with the required inlet size. At one time only one size can be monitored.

iii. Check the flow rate with the help of accessory given with it.

iv. Set the time constant to 10 second or longer depending upon the required averaging period.

v. Instrument can be switch on the survey mode for spot checking. Instrument will display concentration values in mg/m³.

vi. Set the mode, place it in the environmental chamber and make connection with the batteries.

viii. Connect the outdoor probe.

ix. Switch on the instrument and close the chamber.

x. The capacity of storing the data is about 32000 observations at every second.

xi. It therefore requires.

Calculations:

i. Average flow rate (initial and final flow rates) in lit/min.

= (Initial flow rate + final flow rate)/2

ii. Total volume of air sampled (TVA) in m³ = Avg. flow rate (lit/min)*

iii. Concentration of SPM in ug /m³ = (w_f – w_i) (mg)/TVA (n³)*

Maximum limit in ug/m³

Sampling of Air: Approach # 7. Sampling Method for Ozone (O₃) Analysis:

Steps for sampling:

i. Dynamic calibration may be performed by preparing a large mixture of ozone of air.

ii. The output of an ozonizer is fed into the intake of a small blower delivering air. The mixture of gas however emerges through a short length of pipe.

iii. One portion of this mixture is analyzed in the recorder.

iv. Another portion is analyzed through a faintly blue iodine solution containing starch and potassium iodide.

v. Titrate this solution with sodium thiosulphate (0.1N).

vi. The end point established by comparison with the blue starch-iodine solution.

vii. Mixture of ozone and air in polyester film bag may be prepared and admitted to the analyzer.

viii. The mixture is analyzed at the time it is used since the ozone decomposes gradually.

ix. Analysis is then performed by passing a measured volume through an impinge containing buffered KI solution and determined the released iodine by spectrometer method.

x. Make the second mixture about midscale concentration and adjust sample or reagent flow rates until the recorder reading agrees with absorbance scale according to the analytically determined concentration of the ozone.

xi. Linear or logarithmic chart may be prepared followed by a calibration curve.

Sampling of Air: Approach # 8. Sampling Method for Carbon Monoxide (CO) Testing:

Step for sampling:

i. Sampling begins with conditioning a sampling train and then gas analyzer

ii. Pressure system in preferred to condition the sampling train by installing pump before the analyzer. Reducing valve needs to be fitted between the analyzer and pump to eliminate the pulsing effect of pump on the analyzer.

iii. Flow meter is to be installed just before the analyzer.

iv. A fibre filter is used to capture the particulate matter prior to the optical cell to prevent its interference, because particulate matter often accumulates on the optical cell and reduce the efficiency.

v. To eliminate the interference of water vapour, refrigeration or desiccant with magnesium per chlorate may be used.

Maximum limit in mg / m³:

Sampling of Air: Approach # 9. Sampling Method for Non-Methane Hydrocarbon (NMHC) Testing:

Steps for sampling (routine analysis):

i. Some of the hydrocarbons are modified in ambient by sunlight and by reaction with NOx, the sampling is done in the morning between 6 and 9 am

ii. Prepare a sampling train with all the tube connections to columns.

iii. The flow rate of helium be kept at about 80 ml/min which gives approximate retention time of 1.08 min for methane.

iv. This is required for the pre-concentration method, which limits the lower detection value to 0.1 parts per billion (ppb) by volume.

v. To determine an ambient NMHC concentration, it needs to be pre- concentrated at below the detection limit 0.01 parts per million (ppm) by direct injection using a flame ionization detector (FID).

vi. Cryogenic condensation using liquid nitrogen at the boiling point of 196°C. The liquid nitrogen is most used cryogenic fluid in laboratories.

vii. For separating of C₂ – C₅ hydrocarbons, an alternate column, porapak Q, 80-100 mesh filled in a stainless steel column of 1830 mm x 3 mm inner diameter at 50°C in a nitrogen flow of 30 ml/min.

Calculations:

i. Determine the volume of pre-concentrated sample

ii. The volume of the evacuated cantainer is V in lit.

iii. Total volume of air sampled (TVA) in lit.

= (480-400) (mm)/ 760 (mm)

iv. Correct the gas volume at the temperature of t °C of container = T.VA* 273 / (273+t)

v. Similar pre-concentration step is done replacing the sample container with a container containing standard gas mixture. If the volume of gas same mixture is for pre-concentration. The concentration of each hydrocarbon component is

C = -A_sample /A_std

C = concentration (ppm) of the component in the standard gas mixture; A_sample = area of sample in the chromatograph; A_std = area of standard hydrocarbons.

vi. The amount of each solvent is determined by comparing the chromatograph with standards injected gas concentration. Concentration are then expressed in mg/l or ppm)

Sampling of Air: Approach # 10. Sampling for Volatile Organic Compounds (Benzene, Toluene and Xylene) Analysis:

Steps for sampling:

i. Prepare a gas sampling tube filled with an activated charcoal.

ii. Open a tube at two ends and connect it to a sample pump and pulling air through the tube with the pump. Thus airborne chemicals will be trapped onto the surface of the sorbent.

iii. The sampling is carried out using low flow sampler by keeping the tubes in vertical position to prevent the possibility of channeling that can lead to under sampling

iv. The sampling flow rate could be in the range of 20-100 ml/min (+-2%) for ambient air.

v. Discard sample if breakthrough exceeds 10 percent.

vi. Store the tubes properly wrapped in aluminium foil in a silica gel multi tube storage container in a clean environment

vii. Prepare a mix stock standard solution of 50 µg/µl of benzene, toluene and xylene each in the solvent CS2. Prepare further diluted solutions of concentration range 10, 1.0 and 0.1 µg/µl with CS2 from stock standard in a clean vial and make up to 1 ml solution.

viii. Introduce this solution immediately 1 µl standard solution into the injector of GC directly.

ix. Plot the curve between the concentrations an response (only peaks).

Calculations:

i. Total volume of air sampled (TVA) in m³ = S × t/10⁶

where, S = sampling flow rate (ml/min, t = sampling time (min)

ii. Concentration at ambient condition (µg/m³)

(C × V₁ 1000) /(V₂ × V₃)

C = amount of compound found injection sample volume from standard curve (µg/µl);

V₁ = total volume of sample extracted (ml);

V₂ = volume of sample extract injected into GC (µl)

V₃ = volume of air drawn through the tube (m³)

iii. Black value is subtracted from the concentration.

iv. Concentration at STP (µg/m³)

(C × 101.3 (273+T))/(273 × P)

Where, C = concentration at ambient condition (µg/m³), T = temperature of the ambient air (°C), P = atmospheric pressure (kPa).

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