2.3 Data Analysis

2.3

2.3.1 DATA ANALYSIS

Preliminary Analysis of Concentrations Data

A preliminary analysis was done to utilise concentrations of VOC (volatile organic compound) data measured by the MultiRAE detector from 4 - 14 July 2019 at the 111 schools and concentrations data of the three gases measured by the Gasmet DX4030 at 42 schools from 17 - 18 March 2019. Integrating the data from the Jabatan Alam Sekitar air quality station (station ID CA34J) with the MultiRAE and Gasmet measurements, the possible sources at each school can be estimated using wind speed and direction. The linking parameter between the two datasets is the timestamp of the measurement.

The assumptions made for this analysis are that the VOC is carried directly downwind without lateral or vertical dispersion and that the winds did not meander. It is important to note that 84 out of 111 schools (77%) recorded VOC concentrations below the detection limit. Therefore, this analysis is focused on the spikes in the VOC concentration.

2.3.2 Detailed Analysis of Concentrations Data

In a more detailed analysis, using the merged data, the backward trajectories of the gases were calculated. This is done by first calculating the downwind distance of the maximum concentration using the Gaussian Plume Model. The dispersion parameters, or the sigma’s (σ), were estimated using the solar radiation and wind speed data and the Pasquill’s atmospheric stability classes. The solar radiation is also used to linearly interpolate the boundary layer height for the minimum and maximum range of 300m to 1000m.

Once the distance with the maximum concentration was calculated, it is combined with wind direction to estimate the downwind source using polar coordinates mapped into Universal Transverse Mercator and latitude-longitude coordinates. This generates multiple sources (> 250 sources) in the Pasir Gudang area. Thus, to summarise the sources into a legible plot, the kernel density method is used to combine the sources into “density” contours that highlight the locations with the highest number of occurrences of the multiple sources. The colour range is from red (approximately 250) to yellow (approximately 10).