## 3 Case Study  
In this section, we explain a case study, including the sampling site and the data sets used for evaluating the proposed method. The proposed pollutant proxy is tested using an extensive measurement data obtained in Jeddah, Saudi Arabia. In particular, we choose to develop a proxy of Ozone (O3) concentration, but the proposed method is also generic for other types of air pollutant. The used data set is one of the most comprehensive measurement in the Middle Eastern region [54]. The sampling was performed in an urban background area of Jeddah city, Kingdom of Saudi Arabia (Figure 4). The site is located at 21.4869\({}^{\circ}\) N, 39.2517\({}^{\circ}\) E and the altitude is 38.7 m above sea level. The measurement site is about 105 m and 1.7 km to the nearest road and the major highway, respectively. The measurements were performed at the height of 3.5 m and 6.7 m for gaseous air pollutants and meteorological parameters above the ground level, respectively. The sampling originally took place from 31 July 2011 to 28 February 2013. However, the O3 concentration data is only available for 258 days. A variety of scientific instruments were involved in the measurement campaign. O3 concentration was monitored by an UV Absorption Ozone Analyser (Model 400E, Teledyne Technologies Company, San Diego, CA, USA), whereas NO, NO2 and NOx concentrations were measured using a chemiluminescence NO/NO2/NOx analyser (Model 200E, Teledyne Technologies Company, San Diego). Syntech Spectras BTEX analyser GC 955, type 600 (Syntech Spectras, Groningen, the Netherlands) was used to measure a specific volatile organic compound (VOC) concentrations, that are benzene, toluene, ethylbenzene and xylenes (BTEX). Particulate matter concentrations, such as PM10, PM2.5 and PM1, and the number concentration (TC) of particles in the diameter size range of 0.25 to 32 \(\upmu\)m were measured with one-minute average by an optical scattering spectrometer (EDM-180D, Grimm Aerosol, Germany). A solar radiation sensor (Vantage Pro2 TMAccessories, Davis Instruments, Hayward, CA, USA) was used for continuous measurement of solar radiation. Finally, meteorological variables, such as wind speed, temperature and relative humidity, were measured using Lufft WS600-UMB Compact Weather Station. The detailed explanation of the sampling site, data sets and the instruments used in this research campaign can be found in the previous works by [PERSON] et al. [54; 55], [PERSON] et al. [56]. Figure 3: The difference between standard NN and Dropout NN [53]. Dropout method randomly drops units (along with their connections) from the NN during training, which prevents units from learning too much and thus potentially avoid overfitting. The dropout NN can also be seen as BNN. O\({}_{3}\) concentration is the major gas component of air pollution in cities. Among other photochemical oxidants, O\({}_{3}\) is one of the widely studied subjects worldwide under the category of air pollution [20]. For example, air pollution in Jeddah was investigated by analysing temporal variations of O\({}_{3}\) and NOx [54] and O\({}_{3}\) formation affected by benzene, toluene, ethylbenzene and xylenes [55]. Several reasons motivate this study to establish an O\({}_{3}\) proxy as the case study. First, there are missing values spread across the data, including a large number of O\({}_{3}\) concentration data, due to the instruments maintenance as well as other issues [56]. The proxy is expected to contribute in filling the missing data to allow better air pollution analysis. Second, the cost operation of official air pollution monitoring stations is typically high due to the number of instruments used. To substitute O\({}_{3}\) monitoring, the proxy can be deployed to mimic the real measurement based on \"Ozone Analyzer\". In this way, the number of instruments involved can be minimised. As a result, the operational cost can be reduced and the number of monitoring stations can be scaled up.