Real-time measurements of aromatic hydrocarbons at a regional background station in North China: Seasonal variations, meteorological effects, and source implications

Abstract

We performed long-term, real-time measurements of BTEX species using the on-line Syntech Spectras GC955 analyzers at a regional background site in the North China Plain (Shangdianzi site), from December 2017 to November 2018. This study is unique in that it is the first long-term measurement of BTEX species in a regional background site in north China. The seasonal averaged mixing ratio of total BTEX ranged from 0.47 ppbv in the summer to 1.44 ppbv in the spring, with the contribution of benzene being the highest at 49–61%, followed by toluene at 25–31%. The maximal values of BTEX in spring were strongly associated with the regional transport under the influence of prevailing southwesterly winds and high WS. The effects of RH and T on the BTEX mixing ratio and composition variations were varied in different seasons. High mixing ratios of BTEX were observed under relatively low T (−15 °C < T < 3 °C) and relatively high RH (60% < RH < 85%) conditions in winter and fall. While significantly low mixing ratios of BTEX were observed under high RH (>85%) and T (>12 °C) conditions during summer. Furthermore, the O3 concentration was generally high (~20–40 ppbv) under dry conditions (RH < 40%) and gradually increased to a high level (>50 ppbv) at high RH (>40%) and T (>12 °C). All BTEX species exhibited evident wind gradients, characterized by high concentrations correlated with winds from the southeast, south, and southwest directions. This was coincided with the results obtained from the concentration weighted trajectory (CWT) analysis, which indicated that the areas with a higher contribution to BTEX concentration were mainly distributed in the south and southwest of Shangdianzi (SDZ). It is worth noting that smaller source areas were found in winter compared with that in other three seasons, owing to the transportation of regional emissions. The significantly lower mean T/B ratios observed during the daytime and nighttime were 0.48 and 0.58, respectively, which implicated the impact of biomass burning at SDZ. The ratios of X/E during the daytime and nighttime were 0.82 and 1.07, respectively, which indicated a relatively more aged air mass at daytime.