Characteristics and sources of submicron aerosols above the urban canopy (260 m) in Beijing, China, during the 2014 APEC summit

Abstract

Abstract. The megacity of Beijing has experienced frequent severe fine particle pollution during the last decade. Although the sources and formation mechanisms of aerosol particles have been extensively investigated on the basis of ground measurements, real-time characterization of aerosol particle composition and sources above the urban canopy in Beijing is rare. In this study, we conducted real-time measurements of non-refractory submicron aerosol (NR-PM 1 ) composition at 260 m at the Beijing 325 m meteorological tower (BMT) from 10 October to 12 November 2014, by using an aerosol chemical speciation monitor (ACSM) along with synchronous measurements of size-resolved NR-PM 1 composition near ground level using a high-resolution timeof-flight aerosol mass spectrometer (HR-ToF-AMS). The NR-PM 1 composition above the urban canopy was dominated by organics (46 %), followed by nitrate (27 %) and sulfate (13 %). The high contribution of nitrate and high NO − 3 / SO 2− 4 mass ratios illustrates an important role of nitrate in particulate matter (PM) pollution during the study period. The organic aerosol (OA) was mainly composed of secondary OA (SOA), accounting for 61 % on an average. Different from that measured at the ground site, primary OA (POA) correlated moderately with SOA, likely suggesting a high contribution from regional transport above the urban canopy. The Asia-Pacific Economic Cooperation (APEC) summit with strict emission controls provides a unique opportunity to study the impacts of emission controls on aerosol chemistry. All aerosol species were shown to have significant decreases of 40-80 % during APEC from those measured before APEC, suggesting that emission controls over regional scales substantially reduced PM levels. However, the bulk aerosol composition was relatively similar before and during APEC as a result of synergetic controls of aerosol precursors. In addition to emission controls, the routine circulations of mountain-valley breezes were also found to play an important role in alleviating PM levels and achieving the "APEC blue" effect. The evolution of vertical differences between 260 m and the ground level was also investigated. Our results show complex vertical differences during the formation and evolution of severe haze episodes that are closely related to aerosol sources and boundary-layer dynamics.

Simultaneous measurements of particle number size distributions at ground level and 260 m on a meteorological tower in urban Beijing, China

“…The control measures, such as restricting the number of vehicles, shutting down factories and power plants, stopping construction activities, etc., were even stricter than those implemented during the AsiaPacific Economic Cooperation (APEC) summit in 2014 (Sun et al, 2016a). Several studies have addressed the impacts of regional emission controls on aerosol composition and gaseous species Han et al, 2016;Zhao et al, 2017). The results are overall consistent, showing significant reductions in most aerosol and gaseous species during the control period (CP,.…”

Vertically-resolved Characteristics of Air Pollution during Two Severe Winter Haze Episodes in Urban Beijing, China

“…The detailed sampling setup and calibration of the two instruments, and data analysis have been described in Xu et al (2015b) and Chen et al (2015). Positive matrix factorization (PMF) (Paatero and Tapper, 1994) was also performed to the 10 mass spectra of organic aerosols (OA).…”

Fast particulate nitrate formation via N<sub>2</sub>O<sub>5</sub> uptake aloft in winter in Beijing

“…PM pollution reduces visibility (Lei and Wuebbles, 2013) and has harmful effects on public health (Cao et al, 2012). Particulate nitrate (pNO − 3 ) is an important component of secondary inorganic aerosols and contributes 15-40 % of the PM 2.5 mass concentration in China (Sun et al, 2013(Sun et al, , 2015aChen et al, 2015;Zheng et al, 2015;Wen et al, 2015). The main atmospheric pathways of nitrate formation are (1) the reaction of OH with NO 2 and (2) N 2 O 5 heterogeneous hydrolysis (Seinfeld and Pandis, 2006).…”

“…Wagner et al (2013) show the significant pNO − 3 suppression of N 2 O 5 uptake aloft in the wintertime in Denver, CO, USA; the uptake coefficient is 0.005 when the percentage of pNO − 3 in the PM 2.5 mass concentration is 40 %. As the proportion of nitrate in the particle mass concentration is similarly high in North China during wintertime (Sun et al, 2013(Sun et al, , 2015aChen et al, 2015;Zheng et al, 2015;Wen et al, 2015), herein we fixed the uptake coefficient to 0.005 for the base model initial input. Because the model input of ClNO 2 (Wang et al, 2017a;Wang et al, 2018).…”