LEXICAL FRAMEWORK IN WORD DESCRIPTION

In this work, the low-volume Partisol Plus samplers and the cascade impactors were employed in order to collect airborne particulate matter for bulk chemical analysis. Bulk chemical composition refers to the relative abundance of the major chemical components in aerosol samples (Harrison and Yin, 2000).

2.2.1 Partisol Plus air samplers

The Partisol Plus Model 2025 Sequential Air Sampler combined the unit of automatic filter exchange capabilities with well-established dichotomous splitting methodology developed by the US EPA. The system contained two supply and two storage magazines, each with a capacity of up to 16 filter cassettes, as well as a filter exchange mechanism that replaced two filter cassettes at the same time. The Partisol fitted with PM₁₀ inlet and can separate particles into fine (PM_2.5) and coarse (PM_2.5-10) size fractions at a flow rate of 16.7 l min^-1 (Figure 2.1).

The device maintained the fine and coarse particle stream by the separated flow controllers at 15.0 and 1.7 l min^-1, respectively. It is important to maintain a constant flow rate during the sampling period so precise flow control is essential to its operation. The particle size discrimination characterietics of both the inlet and the virtual impactor depend critically on specified air velocities. A change in velocity will result in a change in the nominal particle size collected. The air stream containing PM10 is forced into the virtual impactor where the air flow is split. Most or the fine particles make a sharp turn to follow the higher velocity

flow stream and pass onto the fine filter whilst the coarse particles are collected onto the coarse filter. Electronic flow controllers maintain their calibrated settings well, but must be occasionally checked for dust build-up on the sensor that would change heat transfer characteristics. Furthermore, accurate air flow is needed because the mass concentration of atmospheric particles is computed as mass of component species divided by the actual volume of air samples. In this study, Partisol is regularly sent to company for the calibration of the system and for maintaining the consistent operation of the hardware. The routine check for air flow in order to verify the calibration of the Partisol is performed using the sampler’s Audit screen, which is accessed via the Service Mode when the sampler is in the Stop Mode.

The current flow should be read within ±5% of set flow. The coarse particle sample was corrected for the collection of fine particles in the carrier flow. Because a small proportion of the fine particles are collected on coarse particle filter. The calculation of coarse PM is achieved by the correction of fine particles in the carrier flow using the formula, C_c = M_c/V_t – V_c/V_t.C_f (where C_c is the mass concentration of the coarse particle fraction, M_c is the mass collection on coarse particle fraction filter, V_c and V_t are the volumes of air samples through the coarse fraction filters and the sum of coarse and fine fraction filters, respectively, and Cf

is the mass concentration of the fine particle fraction). The system collected particulate matter onto 47 mm diameter filters. The quartz fibre filters, Whatman Grade QM-A Circles, were used in this study. All the quartz filters were baked for 4 h at 500 ^oC before sampling to reduce organic residues (Harrison et al., 2003).

Figure 2.1 The Partisol Plus Model 2025 Sequential air sampler and operating diagram

2.2.2 Micro-Orifice Uniform-Deposit Impactors (MOUDI)

In the study of mass size distribution and form of chemical components, 10-stage micro-orifice uniform-deposit impactors (MOUDI) were employed in order to collect the various size fractions of particulate matter. Basically, the MOUDI is designed such that as the aerosol stream flows through each stage, particles having sufficient inertia will deposit on that particular stage collection plate, whilst smaller particles with insufficient inertia will associate with the stream lines and pass to the next collection stage. The stages are assembled in a stack in order of reducing particle size until the smallest particles are collected at an after filter.

Total nozzle area decreases with increasing stage number, so providing that the volumetric flow rate remains constant, the air velocity increases at each stage. Significant variation in the air flow rate leads to incorrect particle size measurement. The cascade impactors give basically well-defined stage cut-off diameters, which are the aerodynamic diameter of particles that accumulate on any given collection surface, at given inlet air flow rates. The impactor plates in this model were able to rotate for spreading out the particle deposit uniformly over the stage. This minimised particle build up under each nozzle and reduced possible particle blow off by the jets. The calibrated cut-points (d50 –values) at 30 l min^-1 for

the inlet and 10 stages of the MOUDI were 18, 9.9, 6.2, 3.1, 1.8, 1, 0.55, 0.325, 0.175, 0.099 and 0.054 µm. Aluminium foil and Teflon filters (1 µm pore size PTFE) with 47 mm diameter were used as the collection substrates in the impaction stages. The MOUDI was equipped with a back-up filter stage, which contained 37 mm diameter filter. The quartz fibre filters, Whatman Grade QM-A Circles, were used in this stage. In this experiment, the back-up filter stage collected the particulate matter smaller than 0.175 µm.

The system was designed for the injection of ammonia gas into aerosol stream during the sampling as shown in Figure 2.2. The ammonia gas cylinder with concentration of 50 ppm (NH₃ in synthetic air) was supplied by Teflon tube and connected to the MOUDI inlet. The gas flow controller was used to adjust and control the flow rate of ammonia gas to the desire value based on the total air flow rate of the MOUDI. Calibrated rotameters were used to measure the air and ammonia gas flow rate before starting and at the end of air sampling. The inline filter which contained Teflon filter (1 µm) was connected between the gas flow controller and the MOUDI inlet to minimise the impurities of ammonia gas. To ensure that the ammonia is reasonably well mixed with the air coming into the MOUDI, the flow Reynolds number, a dimensionless number, that characterised gas flow through a pipe was also calculated as shown in Appendix B.

Even though concentration of ammonia gas nearby the MOUDI system did not monitor, the dispersion of ammonia gas coming from exhaust stream of MOUDI’s pump might not have any influence on each other. This was because the air samplers were set separately around 5 metres. Additionally, this experiment was operated with the relative low ammonia gas concentration (50 ppb in total air flow).

Figure 2.2 Air sampling by the MOUDI with ammonia experiment