Chemical characteristics and source apportionments of volatile organic compounds (VOCs) before and during the heating season at a regional background site in the North China Plain

Abstract

We investigated the characteristics and sources of volatile organic compounds (VOCs) at a regional background station on the North China Plain from 12 October to 31 December 2017. This study is unique in that emission sources of VOCs and their influence on ozone formation were identified for the first time at a regional background site in North China. The average total mixing ratio of VOCs was 16.0 ± 10.0 ppbv before the heating period (NHP, 12 October to 14 November), and this value was 43% higher than that during the heating period (HP, November 15 to 31 December). Alkanes were the dominant contributors to total VOCs and ranged from 59.9% to 67.1% during the HP and NHP. Relative humidity (RH) and temperature (T) dependence were observed before the HP, and high VOC mixing ratios were observed at relatively low T (−4 °C to 5 °C) and high RH (>60%) conditions and medium T (5 °C < T < 14 °C) and low RH (< 60%) conditions. High mixing ratios of VOCs were observed under RH > 55% and T < −1 °C conditions during the HP. Through bivariate polar plots, medium or low mixing ratios of total VOCs (TVOCs) were found at low wind speeds (<2 m s−1) before the HP, whereas a high mixing ratio was found during the HP, highlighting the important role of local source emissions during the HP. In addition, southwest winds at high wind speeds (>4 m s−1) were essential for the regional transport of pollutants. The contributions of alkenes to LOH (OH radical loss rate) and OFP (ozone formation potential) before and during the HP were as high as 48.8%–62.9% and 49.7% -64.4%, respectively. The OFP and LOH of ethene presented higher levels before and during the HP, elucidating the significant role of alkenes in the formation of O3 at SDZ site. Four emission sources of VOCs, including coal burning, liquefied petroleum gas (LPG) and natural gas (NG) usage, vehicular emissions and gasoline evaporation, were resolved using a positive matrix factorization (PMF) model, with contributions of 45.4%, 15.8%, 23.4% and 15.4%, respectively. The concentration weighted trajectory (CWT) analysis indicated that the main potential source regions were located to the south and southwest of Shangdianzi (SDZ) before the HP, and they were characterized by a narrow pollution band along Hengshui-Baoding-Langfang-Beijing. High CWT values were primarily distributed in a small region in southern SDZ during the HP, emphasizing the role of local emissions and transport from surrounding regions, especially urban areas of Beijing.