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.