Abstract
Solar eclipses provide unique possibilities to investigate atmospheric processes, such as new particle formation (NPF), important to the global aerosol load and radiative balance. The temporary absence of solar radiation gives particular insight into different oxidation and clustering processes leading to NPF. This is crucial because our mechanistic understanding on how NPF is related to photochemistry is still rather limited. During a partial solar eclipse over Finland in 2015, we found that this phenomenon had prominent effects on atmospheric on-going NPF. During the eclipse, the sources of aerosol precursor gases, such as sulphuric acid and nitrogen- containing highly oxidised organic compounds, decreased considerably, which was followed by a reduced formation of small clusters and nanoparticles and thus termination of NPF. After the eclipse, aerosol precursor molecule concentrations recovered and re-initiated NPF. Our results provide direct evidence on the key role of the photochemical production of sulphuric acid and highly oxidized organic compounds in maintaining atmospheric NPF. Our results also explain the rare occurrence of this phenomenon under dark conditions, as well as its seemingly weak connection with atmospheric ions.
Highlights
Similar boundary conditions are very difficult to find from any other kind of field measurement data
Comparing data from clear-sky and cloudy days to each other would not allow isolating the effect of photochemistry either, as clear-sky and cloudy days tend to have different air mass properties
The ozone concentration usually increases by about 10 ppb from morning to afternoon on springtime NPF event days at our measurement site[24]. The eclipse stalled this ozone increase momentarily, but did not cause any ozone decline as reported in earlier studies conducted in semi-polluted environments[22,25]
Summary
The eclipse day was a sunny, mostly clear-sky NPF event day with light winds (2−3 m/s). The most plausible explanation for a HOM compound not to be affected by a solar eclipse is that its production in the gas phase is initiated, or maintained, by reactions involving ozone rather than photochemistry Another possible explanation, at least for the HOMs that are not extremely low-volatile, is that they do not effectively condense onto pre-existing particles. We conclude that when ozonolysis is the prominent formation route of condensing vapours (like HOMs at night-time, Fig. 3), the resulting ozonolysis products alone cannot grow these sub-3 nm clusters further into new aerosol particles in a boreal forest atmosphere at least in the observed concentration levels. Extremely low-volatile reaction products from the OH-radical oxidation, especially sulphuric acid and a sub-set of (N-)HOMs, are the key compounds for cluster growth and observing NPF. Our results are an important step toward understanding the connections between atmospheric oxidation, NPF and secondary aerosol formation, which is needed for quantifying the complex interplay between future anthropogenic activities, air pollution and changing global climate
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