Abstract
Abstract. Despite extensive efforts toward the characterization of submicron aerosols at ground level in the megacity of Beijing, our understanding of aerosol sources and processes at high altitudes remains low. Here we conducted a 3-month real-time measurement of non-refractory submicron aerosol (NR-PM1) species at a height of 260 m from 10 October 2014 to 18 January 2015 using an aerosol chemical speciation monitor. Our results showed a significant change in aerosol composition from the non-heating period (NHP) to the heating period (HP). Organics and chloride showed clear increases during HP due to coal combustion emissions, while nitrate showed substantial decreases from 28 to 15–18 %. We also found that NR-PM1 species in the heating season can have average mass differences of 30–44 % under similar emission sources yet different meteorological conditions. Multi-linear engine 2 (ME-2) using three primary organic aerosol (OA) factors as constraints, i.e., fossil-fuel-related OA (FFOA) dominantly from coal combustion emissions, cooking OA (COA), and biomass burning OA (BBOA) resolved from ground high-resolution aerosol mass spectrometer measurements, was applied to OA mass spectra of ACSM. Two types of secondary OA (SOA) that were well correlated with nitrate and chloride–CO, respectively, were identified. SOA played a dominant role in OA during all periods at 260 m although the contributions were decreased from 72 % during NHP to 58–64 % during HP. The SOA composition also changed significantly from NHP to HP. While the contribution of oxygenated OA (OOA) was decreased from 56–63 to 32–40 %, less oxidized OOA (LO-OOA) showed a large increase from 9–16 to 24–26 %. COA contributed a considerable fraction of OA at high altitude, and the contribution was relatively similar across different periods (10–13 %). In contrast, FFOA showed a large increase during HP due to the influences of coal combustion emissions. We also observed very different OA composition between ground level and 260 m. Particularly, the contributions of COA and BBOA at the ground site were nearly twice those at 260 m, while SOA at 260 m was ∼ 15–34 % higher than that at ground level. Bivariate polar plots and back-trajectory analysis further illustrated the different source regions of OA factors in different seasons.
Highlights
Atmospheric aerosol particles reduce visibility by scattering and absorbing solar radiation and exert detrimental effects on human health (Dockery et al, 1993; IPCC, 2013)
The results highlight the fact that stagnant meteorological conditions with shallow planetary boundary layer (PBL) and low surface wind, traffic, and coal combustion emissions, regional transport, and secondary production of SNA are the major factors leading to the formation of severe haze episodes
The characteristics and sources of non-refractory PM1 (NR-PM1) and organic aerosol (OA) before and during Asia-Pacific Economic Cooperation (APEC) were reported in our previous study (Chen et al, 2015); here we mainly focus on the characterization of submicron aerosols during the heating season and the comparisons with those during non-heating period (NHP) and APEC
Summary
Atmospheric aerosol particles reduce visibility by scattering and absorbing solar radiation and exert detrimental effects on human health (Dockery et al, 1993; IPCC, 2013). Beijing released the first air pollution red alert due to a severe haze episode in December 2015 when the daily PM2.5 concentration exceeded 300 μg m−3 for more than 3 days. The sources and formation mechanisms of haze episodes have been extensively investigated for mitigating air pollution in Beijing in recent years The results highlight the fact that stagnant meteorological conditions with shallow planetary boundary layer (PBL) and low surface wind, traffic, and coal combustion emissions, regional transport, and secondary production of SNA (sulfate, nitrate, and ammonium) are the major factors leading to the formation of severe haze episodes. Our knowledge of the characteristics of aerosol particles is far from complete, mainly due to the fact that most previous studies were conducted at ground sites and were subject to strong influences from local emissions. The measurements and analysis at high heights at urban sites are still very limited, long-term analysis covering different seasons
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