Text S8. Estimation of the condensation rate of MEA and NA.
We first estimated the reaction rate constant based on the lab
experiments2, which showed that ammonia and NA can
condense on clusters (with core diameters of 5–10 nm and primarily
consisted of ammonium sulfate) at a rate of ~ 45 nm per
hour. In 45 minutes, the clusters grew into nanoparticles with diameters
of 39–44 nm and a number concentration of ~
105 cm−3. Assuming the net increase
in particle size was solely from the condensation of ammonium nitrate
with a material density of 1.72 g cm−3, we estimated
that the condensation flux of ammonium nitrate on nanoparticles was
1.4–2.1 × 107cm−3 s−1. Since the experiments were
conducted with 1915 pptv ammonia and 24 pptv NA at 1 atm and 278 K, the
concentrations of ammonia and NA were estimated to be 5 ×
1010 and 6 × 108cm−3, respectively. The observed condensation rate of
ammonia and nitrate at 278 K is therefore estimated as 4.5–6.5 ×
10−13cm3 s−1. As discussed in theAtmospheric Implications of the main text, it is reasonable to
assume that MEA will condense with NA at a rate no slower than that of
ammonia.
Typically, at 278 K, MEA will react with hydroxyl radicals ∙ OH (with a
typical ambient concentration of c OH =
106 cm−3) at a rate ofk OH = 8 × 10−11cm3 s−1.11 If MEA
will condense with NA at a rate of k NA = 5 ×
10−13 cm3 s−1 at
278 K and the ambient concentration of NA is c NA= 108 – 1010cm−3, the relative contribution of NA to the
condensation removal of MEA in the atmosphere can be estimated as:k NA ∙c NA/(k OH ∙c OH) = 0.6 – 60, suggesting that the
contribution of NA assisted condensation removal of MEA in the
atmosphere can be as significant as gas-phase oxidations.
Table S1. The chemical structures and properties of reduced nitrogen
compounds (RNCs) involved in this study.