Characteristics of PM2.5-bound secondary organic aerosol tracers in a coastal city in Southeastern China: Seasonal patterns and pollution identification

doi:10.1016/j.atmosenv.2020.117710

Atmospheric Environment

Volume 237, 15 September 2020, 117710

https://doi.org/10.1016/j.atmosenv.2020.117710 Get rights and content

Highlights

•
SO₄²⁻ and H⁺_insitu had an increased impact on low-NOx SOA_I products and later-generation SOA_M products.
•
The HO₂ channel was the main chemical reaction of SOA during haze periods.
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Biomass-burning plumes promoted precursor emissions and the formation of SOA during haze events with regional transport.

Abstract

Secondary organic aerosol (SOA) plays an important role in global climate change and air quality, and SOA tracers most directly characterize the sources and formation mechanisms of SOA. Four seasons of observation of SOA tracers was carried out in a coastal city of southeastern China. Fourteen PM_2.5-bound SOA tracers, including isoprene (SOA_I), α/β-pinene (SOA_M), β-caryophyllene (SOA_C), and toluene (SOA_A), were measured using the GC–MS method. The concentrations of SOA tracers in Fuzhou were 25.9 ± 19.9 ng m⁻³ (SOA_M), 7.45 ± 8.53 ng m⁻³ (SOA_I), 3.15 ± 1.99 ng m⁻³ (SOA_C), and 2.63 ± 1.54 ng m⁻³ (SOA_A). The elevated SOA_I concentration in summer was mainly controlled by high biogenic isoprene emission and strong oxidation, and biomass burning contributed strongly to DHOPA (SOA_A) in fall. SO₄²⁻ and H⁺_insitu had an increased impact on later-generation SOA_I products and low-NOx SOA_M products. Based on the ratio of MGA/MTLs and MBTCA/(PA + PNA), atmospheric oxidation capacity (Ox, =NO₂+O₃) had a significant impact on the aerosol aging of SOA tracers. The increased proportions of low-NOx SOA_I products were 3.23–7.21 times higher than those of high-NOx products from non-haze to haze periods, suggesting the influence of high SO₄²⁻ concentration and RH on the reaction channel for SOA_I formation. The percentages of later-generation SOA_M products during RT were 2.46 times higher than those of first-generation products, suggesting the influence of aerosol aging during the regional transport. Both continental Asian outflow and biomass-burning plumes promoted precursor emissions and the formation process of SOA during the haze pollution events. These related findings help to understand the occurrence, sources and formation of SOA in coastal areas.

Introduction

Secondary organic aerosol (SOA) is an important component of atmospheric aerosols, which have important effects on visibility, climate change and human health (Ervens et al., 2011; Ehn et al., 2014; Liu et al., 2019). SOA is believed to be made up of biogenic SOA (BSOA), which is transformed from biogenic volatile organic compounds, with anthropogenic SOA (ASOA) also playing an important role in urban areas (Fu et al., 2016; Stone et al., 2010; Rattanavaraha et al., 2016). Due to the complexity of SOA precursors and formation mechanisms, there remains a lack of comprehensive understanding of the chemical composition, sources, condensation, and distribution of SOA.

The composition, main sources and reaction mechanisms of SOA can be characterized most directly using SOA tracers (Jaoui et al., 2007; Feng et al., 2013). The temporal and spatial distribution and sources of SOA tracers have been investigated based on field observations (Tang et al., 2018; Ding et al., 2011); related results have shown that emissions of NOx, SO₂, Ox (Ox = O₃+NO₂), sulfate, and POA (primary organic aerosol) originating from anthropogenic activities can increase BSOA yields by different amounts (Carlton et al., 2018; Zhang et al., 2019). Moreover, aerosol acidity, controlled by sulfate, relative humidity (RH), and liquid water content (LWC), increases SOA amounts via the salting-in effect and acid-catalyzed reactions (Li et al., 2013; McFiggans et al., 2019). NOx levels impact the proportions of low- and high-NOx products of isoprene (Hong et al., 2019) and enhance secondary monoterpene reactions via nitrate radical oxidation (Xu et al., 2015). Ox and POA increase SOA yields via atmospheric oxidation and emissions of precursors, respectively (Zhang et al., 2019). SO₂ and sulfate can increase aerosol acidity by providing abundant acidic particles to accelerate SOA production, while high RH and LWC reduce aerosol acidity by influencing viscosity and the phase diffusion of the aerosol particulates to inhibit SOA production (Li et al., 2013; Liu et al., 2014b; Slade et al., 2014). However, there are still uncertainties as to the distribution and sources of SOA tracers, due to the complexity of the different precursors and environmental conditions.

Globally, studies on the distribution characteristics of SOA tracers have mainly concentrated on densely populated areas, rural areas neighboring cities, and remote areas (such as plateau areas and mountain background sites) (Shen et al., 2015; Ghirardo et al., 2016; Lyu et al., 2017; Kleindienst et al., 2012). In China, such studies have mainly concentrated on areas with severe air pollution, such as Beijing-Tianjin-Hebei, the Yangtze River Delta, and the Pearl River Delta region (Hu et al., 2008; Feng et al., 2013; Liu et al., 2014a). However, there is a lack of research on southeastern coastal areas with subtropical features, relatively high humidity, dense vegetation, and strong atmospheric oxidation. The typical geographical environment in the southeastern coastal areas provides a good opportunity to study the pollution characteristics of aerosol under the influence of multiple factors. Our previous ground-based observations in a mountainous forest area showed that BSOA tracers were the largest contributor to SOA (Hong et al., 2019). Moreover, in urban regions, the combined effects of port shipping emission, sea salt aerosol, and monsoon on the formation and transformation of atmospheric aerosols have been observed (Fu et al., 2009; Xu et al., 2018). An in-depth study on the characteristics and sources of SOA tracers in urban agglomerations will therefore be of great significance for understanding the formation of SOA in coastal areas.

In this study, we chose the coastal city of Fuzhou to conduct field monitoring of SOA tracers for one year, and focused on (1) the seasonal pattern of SOA tracers and their contribution to secondary organic carbon (SOC), (2) the effects of high and low NOx, aerosol aging degree, acidity, sulfate, and regional transport on SOA tracers, and (3) source and pollution identification by SOA tracers during typical haze events. These findings will help to understand the formation mechanisms of SOA in coastal areas, and provide a scientific basis for more in-depth research on haze pollution.

Section snippets

Field sampling

The coastal city of Fuzhou (26.11°N, 119.29°E), with 54.7% forest coverage, is the core area for rapid urbanization in the southeast of China. Fuzhou is located at the west coast of the Taiwan Strait, and haze events occasionally occur in winter due to local sources and long-range transport from the Yangtze River Delta. Aerosol sampling campaigns were carried out on the rooftop of a 15 m high building in the old town of the city, surrounded by residential areas. Daily PM_2.5 filter samples were

Concentrations of SOA tracers and estimated SOC concentrations

As shown in Table 1, SOA_M (25.9 ± 19.9 ng m⁻³) was the predominant component of total SOA tracers, followed by SOA_I (7.45 ± 8.53 ng m⁻³), SOA_C (3.15 ± 1.99 ng m⁻³) and SOA_A (2.63 ± 1.54 ng m⁻³). Isoprene mostly originates from deciduous plants and broad-leaved trees, while monoterpene, including α/β-pinene, is mainly emitted by citrus and coniferous plants (Carlton et al., 2009; Fu et al., 2009; Ding et al., 2014; Shrivastava et al., 2017). The ratio of coniferous to broad-leaved forest in

Conclusions

PM_2.5-bound SOA tracers were investigated at an urban site in a coastal area of southeastern China, to characterize the formation and sources SOA and haze pollution indicators. The concentrations of BSOA tracers were comparable to those in most cities and much lower than those in suburban, rural, and mountainous forest sites around the world. SOA_I tracers controlled by precursor emission of isoprene formed the main component in summer, and biomass burning contributed strongly to DHOPA (SOA_A) in

CRediT authorship contribution statement

Taotao Liu: Data curation, Writing - original draft, Writing - review & editing, Visualization. Baoye Hu: Software, Visualization. Xinbei Xu: Methodology. Youwei Hong: Conceptualization, Writing - review & editing, Supervision. Yanru Zhang: Methodology. Xin Wu: Software, Validation. Lingling Xu: Validation, Formal analysis. Mengren Li: Validation, Writing - review & editing. Yanting Chen: Data curation. Xiaoqiu Chen: Investigation. Jinsheng Chen: Resources, Project administration, Writing -

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors declare that there is no conflict of financial interest. This study was funded by the Chinese Academy of Sciences Interdisciplinary Innovation Team Project, the National Natural Science Foundation of China (41575146), the National Key Research and Development Program (2016YFC0112200 & 2016YFC02005), the FJIRSM&IUE Joint Research Fund (RHZX-2019-006), and Young Talents Projects of Institute of Urban Environment, Chinese Academy of Sciences (Y8L0221B20).

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Citation Excerpt :
Especially, the YRD, GD, and NCP were emission hotspots in China; Fuzhou was a high-emission region in Southeastern China, with anthropogenic NOx emissions somewhat declining and anthropogenic VOCs emissions fluctuating (Tables S2 and S3). Some studies have highlighted that the observed deterioration of air quality in the coastal cities of FJ was most likely due to the cross-regional transport of pollutants (e.g., fine particulate matter, peroxyacetyl nitrate, and black carbon) from the YRD and Northern China under certain synoptic circulations (Qin et al., 2016; Wang et al., 2017a; Yan et al., 2019; Deng et al., 2020; Liu et al., 2020a, 2020b; Hu et al., 2020). However, quantitative analysis of cross-regional transport and local chemical production of O3 under typical meteorological conditions was still limited in Fuzhou.
The coastal eco-city of Fuzhou in Southeastern China has experienced severe ozone (O₃) episodes at times in recent years. In this study, three typical synoptic circulations types (CTs) that influenced more than 80% of O₃ polluted days in Fuzhou during 2014-2019 were identified using a subjective approach. The characteristics of meteorological conditions linked to photochemical formation and transport of O₃ under the three CTs were summarized. Comprehensive Air Quality Model with extensions was applied to simulate O₃ episodes and to quantify O₃ sources from different regions in Fuzhou. When Fuzhou was located to the west of a high-pressure system (classified as “East-ridge”), more warm southwesterly currents flowed to Fuzhou, and the effects of cross-regional transport from Guangdong province and high local production promoted the occurrence of O₃ episodes. Under a uniform pressure field with a low-pressure system occurring to the east of Fuzhou (defined as “East-low”), stagnant weather conditions caused the strongest local production of O₃ in the atmospheric boundary layer. Controlled by high-pressure systems over the mainland (categorized as “Inland-high”), northerly airflows enhanced the contribution of cross-regional transport to O₃ in Fuzhou. The abnormal increases of the “East-ridge” and “Inland-high” were closely related to O₃ pollution in Fuzhou in April and May 2018, resulting in the annual maximum number of O₃ polluted days during recent years. Furthermore, the rising number of autumn O₃ episodes in 2017-2019 was mainly related to the “Inland-high”, indicating the aggravation of cross-regional transport and highlighting the necessity of enhanced regional collaboration and efforts in combating O₃ pollution.

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Characteristics of PM2.5-bound secondary organic aerosol tracers in a coastal city in Southeastern China: Seasonal patterns and pollution identification

Highlights

Abstract

Introduction

Section snippets

Field sampling

Concentrations of SOA tracers and estimated SOC concentrations

Conclusions

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgement

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Characteristics of PM_2.5-bound secondary organic aerosol tracers in a coastal city in Southeastern China: Seasonal patterns and pollution identification