Relative absence of something is often more difficult to quantify than its presence. This paper uses satellite observations and global model data to show that it is uncertainties in the quantification of clean-sky conditions that contribute most to current uncertainties in estimates of the sensitivity of cloud properties to varying degrees of aerosol loading. This is a novel result that should prove of broad interest to the aerosol-cloud-climate community for reframing how the problem is approached, by focusing on improved quantification of clean conditions.

This paper investigates the influence of internationally-transported pollution on China, with a specific highlight on the formation of secondary PM2.5 in the form of nitrate. While sources from within China have traditionally been of most interest for domestic air quality policy, these have diminished over recent years, so sources from outside China may become more significant. The topic of transboundary exchange of air pollution has long been studied in other parts of the world, in particular among CLRTAP signatory countries in North America and Europe, but East Asian transboundary pollution represents a different challenge, in part owing to differences in geography and emissions, but also compared to Europe in particular, the transportation scales are much larger. This work not only quantifies the impacts of long distance pollution on Chinese air quality, but also highlights the complex chemical interactions between the local and transboundary pollutants. Papers such as this will likely influence the debate regarding international controls of air pollutants.

In the last decade it was discovered that autoxidation of monoterpenes produces highly oxidised organic molecules (HOM) in the atmosphere. These have low volatility and produce secondary organic aerosols that are relevant to climate and human health. Autoxidation involves organic peroxy radicals which undergo one or more intramolecular H-shifts with subsequent O2 addition leading to the formation of HOMs. The experimental and theoretical elucidation of the mechanism is challenging due to the large number and isomerism of possible intermediates and their numerous reaction pathways. In the present study, selective isotope labeling was combined with high-resolution mass spectrometry to greatly enhance the possibilities to identify relevant reaction pathways. Selective isotopic labeling of organic molecules is a powerful method for studying reaction mechanisms, and it is currently underutilized in the community. This is a great example of using this method, hopefully paving the way to more studies of this sort to come.