Abstract
Abstract. We show that the limit of the enhancement of coagulation scavenging of charged particles is 2, that is, doubled compared to the neutral case. Because the particle survival probability decreases exponentially as the coagulation sink increases, everything else being equal, the doubling of the coagulation sink can amount to a dramatic drop in survival probability – squaring the survival probability, p2, where p≤1 is the survival probability in the neutral case. Thus, it is imperative to consider this counterbalancing effect when studying ion-induced new-particle formation and ion-enhanced new-particle growth in the atmosphere.
Highlights
There are many situations in atmospheric phenomena where we care about total particle number, and especially total particle number above some critical size
We focus on the effect of charge on the coagulation sink – we investigate the limit of the enhancement in the coagulation sink of charged particles that can take place in the atmosphere or in experiments. We show that this enhancement limit is asymptotically 2; that is, the coagulation sink of charged particles is double that of the otherwise same neutral particles
The other limit is CoagS±/CoagS0 = 1 as α = γ = 1, when the presence of charge is insignificant – that is, charge is “screened”. This shows that in the limit of a coagulation sink comprised of relatively small particles, coagulation can be greatly enhanced for small charged particles compared to neutral particles of the same size
Summary
There are many situations in atmospheric phenomena where we care about total particle number, and especially total particle number above some critical size. Even considering relatively inefficient diffusion neutralization by primary ions (Mahfouz and Donahue, 2020), the steady-state charged fraction for particles smaller than 7 nm in diameter is extremely small (López-Yglesias and Flagan, 2013a); relatedly, this is why standard scanning particle sizers are ineffective below this diameter. The other limit is CoagS±/CoagS0 = 1 as α = γ = 1, when the presence of charge is insignificant – that is, charge is “screened” This shows that in the limit of a coagulation sink comprised of relatively small particles (that is because the first limit, α 2 and γ 1, only happens if the coagulation sink is comprised of smaller particles as shown in Fig. 1), coagulation can be greatly enhanced for small charged particles compared to neutral particles of the same size
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