1. Introduction
Aerosol, microscopic nanoparticles suspended in the atmosphere, contributes significantly to air quality, human health, regional weather and the global climate [Seinfeld and Pandis, 2016]. For example, aerosol particles can directly absorb and/or scatter solar radiation, and serve as cloud condensation nuclei (CCNs) to influence the cloud formation, the Earth’s hydrological cycle and the radiative forcing [Stocker et al., 2013]. Therefore, it is important to understand the formation and transformation of ambient aerosol, especially in the troposphere of metropolitan centers.
New particle formation (NPF) events significantly increase the ambient aerosol number concentration in a short period of time but our understanding of NPF is limited, especially in urban areas with complex emission inventories of chemicals from various natural and anthropogenic sources [Zhang et al, 2012]. It is generally believed that gaseous sulfuric acid (SA) leads to the NPF by forming freshly nucleated clusters (FNCs), often with the contributions from other chemicals, including ammonia, nitric acid (NA) [Liu et al., 2018; Wang et al., 2020], oxidized organics [Zhang et al., 2004; Wang et al., 2010; Fang et al., 2020], and amines [Almeida et al., 2013; Yao et al 2018]. These small clusters need to grow quickly into a critical diameter (of several nanometers) to avoid being scavenged by coagulation with other particles [Smith et al. 2020]. Therefore, the growth of FNCs are critical in understanding the formation and transformation of aerosol.
Ammonia and NA are generally several orders of magnitude more concentrated than SA in the atmosphere. Wang et al [2020] recently reported rapid ammonia and NA condensation on FNCs at or below 278 K (+5 °C) in laboratory experiments using SA, NA and ammonia concentrations comparable to those commonly found in the urban atmosphere. This significant new result offers new insights into the potential contribution of ammonia and NA condensation to NPF under cold weather conditions.
Amines are emitted into the atmosphere in large amounts from nature sources (e.g., water bodies) and human activities (e.g., agriculture, animal husbandry, sewage and chemical manufacturing) [Cape et al., 2011; Ge et al., 2011a; Qiu and Zhang, 2013]. Some amines are primarily emitted as industrial chemicals: monoethanolamine (MEA) and piperazine (PZ) are widely used in Carbon Capture and Storage (CCS) technology in the post combustion of fossil fuels [Neilsen et al., 2012]. The concentrations of amines in the atmosphere vary significantly; in some extreme cases, amine concentrations at the proximity of their emission sources can be comparable to that of ammonia [SEPA, 2015]. However, ambient amine concentrations are generally at least 1–2 orders of magnitude lower than that of ammonia [Yli-Juuti et al., 2013]. Ammonia and amines are collectively referred as reduced nitrogen compounds (RNCs) in this study (see Table 1 for their structures and abbreviations).
Table 1. The chemical structures and properties of reduced nitrogen compounds (RNCs) involved in this study.