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 NAc NA/(k OHc 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.