[Characteristics of C2-C6 Hydrocarbons During the Winter Air Pollution Period in Beijing Urban Area].

A C2-C6 hydrocarbons monitoring campaign was carried out in the Beijing Southeastern Urban Area during December 2015. Twenty-five compounds excluding benzene were detected by an on-line VOCs analyzer; the sum of their concentrations is referred to as C2-C6 HCs in this study. During the monitoring period, C2-C6 HCs ranged from 12.4×10-9 to 297.5×10-9. The mean value of C2-C6 HCs reached 29.4×10-9, 63.2×10-9, 85.5×10-9, 94.9×10-9, and 131.8×10-9, respectively, in AQ Ⅰ (air quality) (hourly PM2.5<35 µg·m-3), AQ Ⅱ (hourly PM2.5:35-75 µg·m-3), AQ Ⅲ (hourly PM2.5:75-150 µg·m-3), AQ Ⅳ (hourly PM2.5:150-250 µg·m-3), and AQ Ⅴ (hourly PM2.5:>250 µg·m-3). Moreover, the mole percentage of alkanes, alkenes, and ethyne significantly varied, 47% vs. 59%, 45% vs. 30%, and 7% vs. 12% (AQ I vs. AQ V). The diurnal variation of C2-C6 HCs presented two peaks at 08:00-09:00 and 17:00-18:00 not only in clean days (when 24-h PM2.5<75 µg·m-3) but also in polluted days (when 24-h PM2.5>75 µg·m-3). This result is consistent with the normal traffic pattern and indicates the significant impact of vehicle emissions on atmospheric hydrocarbon concentrations. Furthermore, we calculated the HCs/CO (×10-9/×10-6) ratio to prevent the impact of meteorological diffusion on C2-C6 HCs and to trace the physical transport process and the chemical degradation process of hydrocarbons. The C2-C6 HCs/CO ratio and the individual hydrocarbon to CO ratio presented a notable decreasing trend with worsening air quality, 90.6 (AQ Ⅰ), 63.8 (AQ Ⅱ), 56.9 (AQ Ⅲ), 37.4 (AQ Ⅳ), and 36.4 (AQ Ⅴ). However, the rate of decrease in the ratio of individual hydrocarbons to CO in the polluted period (AQ Ⅲ-Ⅴ) relative to the clean period (AQ I-Ⅱ) was never effectively related to the kinetic parameters of the reactions with the OH radical. Therefore, the strong chemical degradation of C2-C6 hydrocarbons in the polluted air was denied as the main reason. The HYSPLIT trajectory model showed that the transported air mass from the north and northwest and from the south and southwest prevail in the clean period and in the polluted period, respectively. Compared to the northern region, there were more sources of fossil fuel combustion in the southern region, which led to a lower HCs/CO ratio for the air mass in the southern region. Therefore, the increase in C2-C6 hydrocarbons during the polluted period was not only caused by the accumulation of local emissions but also by the air mass transport from the south.