## 3 Results and Discussion
### BTEX Source Apportionment  
Several studies have reported that the ratio of toluene to benzene (T/B) can be used to find pollutants sources [36; 37]. The T/B ratio can be applied to determine the emission sources of BTEX in ambient air. T/B \(<\) 1 indicates the non-traffic-related sources (biomass/bifuel/coal burning), while T/B \(>\) 1 shows the dominant contribution of traffic-related sources (vehicle emissions) [38]. Figure 5 shows that sources of air pollution in Almaty have a complex nature. During the sampling in January, April, and July, the prevailing sources of BTEX were the combustion of biomass/bifuel/coal. While, in May and October sampling days, BTEX mainly originated from vehicle emissions. In the July sampling period, the lower contribution of vehicle-related sources can be explained by partial lockdown measures (5 July-16 August) due to the second wave of COVID-19 in Kazakhstan [35]. However, CHPs had been operating for a whole year in Almaty what can explain the contribution of coal-burning sources to air pollution. Figure 4: Spatial distribution of total VOCs concentrations during sampling periods in 15, 17, 19, January (a), 3, 5, 7 April (b), 28, 30 April, 3 May (c), 22, 24, 26 July (d), and 21, 23, 25 October (e). The \(m\)-, \(p\)-xylenes to ethylbenzene (X/E) ratio indicates the photochemical age of pollution. \(m\)- and \(p\)-Xylenes are more reactive than ethylbenzene and quickly react with \(\bullet\)OH radicals [39]. The X/E ratio higher than 3 indicates the fresh and local emissions, while the ratio lower than 3 shows aged air masses and consequently emissions from remote sources [40]. The majority of X/E ratios were lower than 3 in all sampling periods, assuming the presence of aged air masses from remote sources (Figure 5). Several X/E ratios (6% of total) were higher or close to 3, which indicate the local, fresh emissions, and a mixture of aged air masses (Figure 5). The strong correlation of ethylbenzene, \(m\)-, \(p\)-xylenes, and \(o\)-xylene (r \(\geq\) 0.9, \(p\)\(\leq\) 0.01) suggests the constant and familiar sources of these VOCs (Table S3 in Supplementary Materials). Based on these results, it can be concluded that the contribution of the remote sources is dominant in Almaty at all sites and periods. The benzene, toluene, and ethylbenzene (B:T:E) ratio is applied to evaluate aromatic sources in ambient air [36; 40; 41]. Biomass/bioruel/coal burning is characterized by the mean relative proportions of B:T:E--0.69:0.27:0.04, traffic emissions--0.31:0.59:0.10, industrial emissions--0.06:0.59:0.35 [36]. B:T:E ratio of each air sample was calculated and plotted in ternary diagrams to understand the emissions sources better. The obtained B:T:E ratio showed that in Almaty, there were two main sources of BTEX during the studied period: traffic emissions and biomass/bioruel/coal burning (Figure 6). For all sampling sites, biomass/bioruel/coal burning was a prevalent source of VOCs on 15, 17, 19 January, with an average B:T:E ratio of 0.87:0.12:0.01. During the heating season, the use of coal increases by more than two times [4], which leads to a dramatic increase in SO\({}_{2}\) emissions from CHP-2 [42] and PM\({}_{2.5}\) concentrations [4]. The moderate correlation of benzene with concentrations of a coal combustion marker SO\({}_{2}\)[43] and PM\({}_{2.5}\) (r \(\geq\) 0.5, \(p\)\(\leq\) 0.01) can also indicate that the burning of solid fuel is one of the primary sources of pollution during the heating season in Almaty (Table S3 in Supplementary Materials). Similar findings were obtained in the Dushanzi district, Northwest China [44], and in a rural area of North China [38; 45], where coal combustion was the main source of VOCs emissions during cold seasons. Figure 5: The ratios of BTEX at different sampling periods. On 3, 5, 7 April, sampling was carried out during the first COVID-19 lockdown (19 March-13 April) under non-traffic conditions, and the B:T:E ratio was 0.61:0.39:0 indicating the coal combustion as the main source of BTEX.