Nonpolar organic compounds as PM_2.5 source tracers: Investigation of their sources and degradation in the Pearl River Delta, China

Abstract

A group of nonpolar organic compounds (NPOCs) in five compound classes including alkanes, polycyclic aromatic hydrocarbons (PAHs), hopanes, steranes, and 1,3,5‐triphenylbenzene were quantified in samples of particulate matter of aerodynamic diameter less than 2.5 μm collected at four sites in the Pearl River Delta (PRD) region, China, over a 2 year period from 2011 to 2012. The four sites include industrial (Nanhai), urban (Guangzhou), urban outskirt (Dongguan), and suburban (Nansha) locations. Some NPOCs are uniquely emitted from particular combustion sources and thereby serving as markers in source apportionment. Based on this multiyear and multisite NPOC data set, spatial and seasonal variations, correlation analysis, and ratio‐ratio plots were used to investigate the source information and degradation of NPOC tracers. In summer, NPOCs showed distinct local emission characteristics, with urban sites having much higher concentrations than suburban sites. In winter, regional transport was an important influence on NPOC levels, driving up concentrations at all sampling sites and diminishing an urban‐suburban spatial gradient. The lighter NPOCs exhibited more prominent seasonal variations. Such spatiotemporal features suggest that their particle‐phase abundance is more influenced by temperature, which is a critical factor in controlling the extent of semivolatile organics partitioned into the aerosol phase. The heavier NPOCs, especially PAHs, showed negligible correlation among the four sites, suggesting more influence from local emissions. Ratio‐ratio plots indicate photodegradation and mixing of various sources for the NPOCs in the PRD. A positive matrix factorization (PMF) analysis of this large NPOC data set suggests that heavier NPOCs are more suitable source indicators than lighter NPOCs. Incorporating particle‐phase light NPOC concentrations in PMF produces a separate factor, which primarily contains those light NPOCs and likely is not a source factor. Total NPOC concentrations predicted using Pankow partitioning theory were explored as PMF inputs; however, the PMF solution is not able to fully explain the input total concentrations or to give reasonable source profiles, suggesting the need for reliable gas‐phase NPOC data before their use in source apportionment studies. In addition, degradation of NPOCs needs to be considered to avoid misinterpretation of PMF source apportionment results.

Non-polar organic compounds in autumn and winter aerosols in a typical city of eastern China: size distribution and impact of gas–particle partitioning on PM<sub>2.5</sub> source apportionment

“…In essence, a haze episode is caused by the distribution of particulate matter with different sizes in atmosphere, leading to a decrease in visibility (Xie et al, 2017). Carbonaceous aerosols contain a large amount of particulate matter, accounting for 30-50 % of PM 2.5 mass concentrations (Yadav et al, 2013;Wang et al, 2016;Ma et al, 2018). Carbonaceous aerosols have a significant influ-ence on environmental and physical processes, including dry and wet deposition, cloud condensation nucleation and heterogeneous reactions (Feng et al, 2006;Li et al, 2017).…”

“…Since carbonaceous aerosols can affect the ambient environment significantly, it is crucial to investigate aerosolassociated organic compounds. Because non-polar organic compounds (NPOCs) can provide specific information on the identification of aerosol sources, they are now of special interest to researchers (Rajput and Sarin, 2014;Wang et al, 2016). n-Alkanes, polycyclic aromatic hydrocarbons (PAHs), hopanes and steranes are four typical NPOC species and are frequently used to apportion sources of ambient particulate matter (Xu et al, 2013;Zhao et al, 2016).…”

“…PAHs are typical semi-volatile organic compounds, which can partition between gas and particle phases in the ambient atmosphere (He and Balasubramanian, 2009;Ma et al, 2011). Recently, research has shown that n-alkanes, hopanes and steranes are also semi-volatile and subject to gas-particle partitioning (Xie et al, 2013(Xie et al, , 2014Wang et al, 2016). Additionally, another crucial factor affecting particle-bound concentrations of NPOCs is their aerodynamic diameter ).…”

“…NPOCs are typically assumed to be stable and nonreactive (Feng et al, 2006;Ma et al, 2011). However, recent research has shown that NPOCs can be oxidized by q OH radicals, RO 2 q radicals and O 3 over atmospherically relevant timescales (Xie et al, 2013;Wang et al, 2016). PAHs, n-alkanes, hopanes and steranes can undergo photochemical oxidation, increasing the production of secondary organic aerosol (Robinson et al, 2006;May et al, 2012).…”

“…Considering the gas-particle partitioning of NPOCs, Xie et al (2013) adopted both gaseous and particulate NPOCs in the PMF model and successfully extracted seven factors. More recently, Wang et al (2016) used data of NPOCs combined with those of organic/elemental compounds (OC/EC), inorganic compounds and elemental compounds as input for the PMF model, and they found that total (gas + particle)-bound concentrations enabled more reasonable source profiles than the single particle phase.…”