The missing growth in the boreal forest
Authors: Stolzenburg Dominik , Sarnela Nina , Kulmala Markku , Lehtipalo Katrianne , Kangasluoma Juha 
Published: March 2022

&lt;p&gt;New particle formation is a significant source of aerosols in the boreal forest and the associated northern hemisphere, influencing the cloud condensation nuclei budget and affecting the radiative energy balance. This aerosol-cloud-climate interaction is largely driven by the abundance of oxygenated organics affecting aerosol growth &amp;#160;(Roldin &lt;em&gt;et al.&lt;/em&gt;, 2019). Biogenic volatile organic compounds emitted from trees oxidize in the atmosphere and form low volatility compounds capable of condensing onto freshly formed particles from the smallest sizes onwards (Stolzenburg &lt;em&gt;et al.&lt;/em&gt;, 2018). As the particle growth is in competition with their removal by larger pre-existing aerosols, it is of crucial importance for the climate relevance of new particle formation. However, growth by oxidized organics is still not sufficiently understood to fully predict the role of NPF in the boreal forest system.&lt;/p&gt;&lt;p&gt;During spring time, the abundance of oxidized organics together with their estimated volatilities can explain the observed nanoparticle growth rates (Mohr &lt;em&gt;et al.&lt;/em&gt;, 2019). Here, we show that this situation dramatically changes during the summer months, where a much higher levels of condensable organics is measured but nanoparticle growth rates are only modestly affected. We demonstrate that there is no clear correlation between the measured abundance of highly oxygenated organic molecules and simultaneously measured growth rates similar to estimates of previous studies (Yli-Juuti &lt;em&gt;et al.&lt;/em&gt;, 2016). This is also reflected in the long-term trends in the boreal forest: While biogenic emissions should be increasing due to higher temperatures, observed particle growth rates have been declining.&lt;/p&gt;&lt;p&gt;We hypothesize that multiphase chemistry influencing the activity of different organics in the particle phase is responsible for the current lack of understanding of these processes and provide first model results to support that hypothesis. Moreover, we show that this missing understanding in predicting growth from oxygenated organics also affects growth predictions in other environments such as Chinese megacities. Altogether, our work clearly illustrates the need for better understanding the condensation behaviour of organic molecules.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References:&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Mohr, C. &lt;em&gt;et al.&lt;/em&gt; (2019) &amp;#8216;Molecular identification of organic vapors driving atmospheric nanoparticle growth&amp;#8217;, &lt;em&gt;Nat. Commun.&lt;/em&gt;, 10(1), p. 4442. doi: 10.1038/s41467-019-12473-2.&lt;/p&gt;&lt;p&gt;Roldin, P. &lt;em&gt;et al.&lt;/em&gt; (2019) &amp;#8216;The role of highly oxygenated organic molecules in the Boreal aerosol-cloud-climate system&amp;#8217;, &lt;em&gt;Nat. Commun.&lt;/em&gt;, 10(1), p. 4370. doi: 10.1038/s41467-019-12338-8.&lt;/p&gt;&lt;p&gt;Stolzenburg, D. &lt;em&gt;et al.&lt;/em&gt; (2018) &amp;#8216;Rapid growth of organic aerosol nanoparticles over a wide tropospheric temperature range&amp;#8217;, &lt;em&gt;P. Nat. Acad. Sci. USA&lt;/em&gt;. National Academy of Sciences, 115(37), pp. 9122&amp;#8211;9127. doi: 10.1073/pnas.1807604115.&lt;/p&gt;&lt;p&gt;Yli-Juuti, T. &lt;em&gt;et al.&lt;/em&gt; (2016) &amp;#8216;Analysis of sub-3 nm particle growth in connection with sulfuric acid in a boreal forest&amp;#8217;, &lt;em&gt;Boreal Environ. Res.&lt;/em&gt;, 21, pp. 287&amp;#8211;298.&lt;/p&gt;