Impact of nucleation on global CCN

Here is what makes cloud science quietly unsettling for policymakers: you can spend decades cutting sulfur from diesel fuel, scrubbing smokestacks, reducing the particles that pour directly from combustion, and the atmosphere will partly make its own replacement. Merikanto and colleagues put a number on that self-assembly in 2009. Forty-five percent of the particles that seed low-level clouds worldwide aren't emitted by anything. They're assembled in the air itself, from gas-phase chemistry, at scales ranging from individual boundary layers to the full depth of the troposphere. That single number reshapes how you think about regulating air quality and predicting climate. To understand why it matters, start with what cloud condensation nuclei, or CCN, actually do. They are the seeds on which cloud droplets form. More CCN means more, smaller droplets, which means brighter, more reflective clouds, which means more sunlight bounced back to space. Fewer CCN tips the balance the other way. CCN are the central variable in what climate scientists call the indirect aerosol effect, which is the largest single source of uncertainty in climate projections. And yet, until this study, nobody had done a rigorous global accounting of where they come from. There are two fundamentally different pathways. Primary emissions dump particles directly into the atmosphere, including sea spray, sulfate, and carbonaceous particles from combustion. These are what most people picture when they think about air pollution. Nucleation works differently: it begins with gas-phase molecules, typically sulfuric acid vapor, that cluster together at the nanometer scale and then grow, over hours to days, until they're large enough to activate as cloud droplets. Merikanto and colleagues argue that distinguishing these two pathways is essential because they respond differently to emission controls and environmental change. Cut smokestacks, and primary particles drop. But nucleation responds to gas-phase precursors, atmospheric oxidants, temperature, and existing particle concentrations. You cannot regulate your way out of nucleation the same way you can regulate your way out of soot. To do the accounting, Merikanto, Spracklen, Mann, Pickering, and Carslaw used GLOMAP, the Global Model of Aerosol Processes, an extension of the three-dimensional chemical transport model TOMCAT. GLOMAP tracks hydrophilic and hydrophobic particle populations across twenty size sections, from three nanometres to ten micrometres, and includes two distinct nucleation treatments for different parts of the atmosphere. Upper-tropospheric nucleation, or UTN, is driven by binary homogeneous nucleation of sulfuric acid and water at altitude. Boundary-layer nucleation, or BLN, operates closer to the surface through a cluster activation mechanism. The experimental logic was elegant: run the model with different combinations of sources switched on and off, and take differences. First, consider primary particles alone, then add UTN, then add BLN, and also combinations that reverse the order. Multiple nucleation parameterizations and three BLN rate constants spanning two orders of magnitude allowed them to bracket uncertainty rather than just report a single number. The central result holds across nearly all of those uncertainty scenarios. Forty-five percent of global low-level cloud CCN at 0.2 percent supersaturation, the degree of water vapor oversaturation at which most stratiform cloud droplets activate, come from nucleation, with a range of 31 to 49 percent when you push the model across its uncertainty envelope. The remaining 55 percent come from primary emissions. Of that nucleated 45 percent, roughly 35 percentage points originated in the free and upper troposphere and were transported downward. Only 10 percentage points came from nucleation occurring directly within the boundary layer. So, three-quarters of the nucleation contribution to low-level clouds was born aloft and rained down into them. Nowhere is that transport signal clearer than over the ocean. In the marine boundary layer, 55 percent of CCN at 0.2 percent supersaturation are from nucleation, which is higher than the global mean. But almost none of it is local: 45 percentage points come from particles entrained from the free troposphere, and only 10 percentage points from nucleation within the boundary layer itself. The ocean, far from continental pollution sources, is largely seeded by particles formed kilometers above and carried laterally over weeks. Free-tropospheric residence times are long enough, several weeks to over a month, that UTN particles spread nearly globally before descending. Even in a heavily polluted region like Southeast Asia, nucleation still accounts for about 19 percent of CCN at 0.2 percent supersaturation. It is everywhere. Supersaturation matters for how you read the numbers. At higher supersaturation, one percent instead of 0.2 percent, the smaller particles produced by nucleation are more likely to activate into droplets, so the nucleation contribution rises. BLN, for instance, goes from contributing 10 percent of CCN at 0.2 percent to 20 percent at one percent. The absolute percentages shift, but the qualitative picture stays the same: nucleation, especially from above, is a dominant global source. Now comes the part that should make emission policymakers pause. The two source categories, primary and nucleated, are not independent. They suppress each other. Existing particles, whether from smokestacks or from entrained upper-tropospheric nucleation, act as a sink for the low-volatility vapors that would otherwise drive boundary-layer nucleation. Take away the existing particles, and BLN expands to partially fill the gap. The atmosphere buffers itself. The numbers that demonstrate this are striking. When Merikanto and colleagues switch off all primary emissions in the model, a radical intervention far beyond any realistic policy, global CCN at 0.2 percent drop by only 20 percent, not the 55 percent you might expect from the source attribution. Meanwhile, total ground-level particle number actually increases by 31 percent because BLN is no longer suppressed. The source mix restructures entirely: in the no-primary simulation, 50 percent of global CCN come from BLN and 50 percent from UTN, whereas in the atmosphere with primaries, UTN dominates the nucleated fraction at roughly 73 percent. The effect of eliminating upper-tropospheric nucleation is equally muted. Switch off UTN, and global CCN at 0.2 percent fall by only 12 percent because BLN compensates. Its contribution to global mean low-level cloud CCN jumps to 48 percent in that scenario and to 60 percent in the marine mean. The suppression runs in both directions. Primary emissions suppress BLN, and UTN-entrained particles suppress BLN, so when either is removed, BLN grows. This is what Merikanto and colleagues mean when they call primary and nucleated CCN "non-linearly coupled." The system resists simple subtraction. Regionally, the responses differ in revealing ways. Over continents, where primary particles account for 59 percent of CCN at 0.2 percent, eliminating primaries reduces continental CCN by 33 percent, a larger response than the global mean. Over the ocean, where primaries account for 44 percent of marine CCN, the reduction is only 16 percent, cushioned by the strong upper-tropospheric nucleation entrainment signal. What this paper established, at its core, is a global accounting of CCN origins that revealed the atmosphere as a partially self-regulating system. Roughly half of the particles that seed low-level clouds are born from chemistry rather than direct emission, and the largest share of those are formed kilometers above the clouds they ultimately influence. That finding directly constrains how climate models should represent CCN, and it complicates any assumption that cutting primary pollution will proportionally reduce cloud-seeding particles. Merikanto and colleagues are careful about what remains uncertain. Boundary-layer nucleation mechanisms are still not fully characterized. Observed BLN rates vary enormously in space and time, and the range of rate constants tested in the model, spanning two orders of magnitude, already captures a wide spread in possible outcomes. The fundamental question of exactly which molecular mechanism drives BLN in different environments remains open. What is not uncertain is the central message. The air is making its own aerosol. It does so on a scale that rivals everything we emit directly. And when we try to change one input, the system adjusts the others. That is not a reason for pessimism — it is a reason for more precise science. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. Read when you can. Listen when you want to.