Hornbrook is active in international projects that work to understand the atmosphere in a variety of locations. Hornbrook was a part of the 2016 ORCAS campaign by assisting with measuring TOGA reactive gases in the airborne study over the Southern Ocean. The study took a look at carbon dioxide and oxygen's behavior interacting with the Southern Ocean which is one of the most remote oceans on the planet. At the 2016 International Global Atmospheric Chemistry meeting, Hornbrook gave a poster presentation regarding the findings of the ORCAS project specific to VOC observations.
In the early part of 2012, Hornbrook began to work on the TORERO campaign in order to study reactive halogen gases and VOCs in the Tropical Eastern Pacific Ocean. The study was an airborne campaign which took data at various altitudes in order to learn more about atmospheric oxidization capacities at differing altitudes in tropical regions. Hornbrook's contributions to the project increased the understanding of VOCs in the studied area off of the South American Coast. Hornbrook continued to work to understand the atmosphere in tropical locations. The 2014 CONTRAST took place in Guam with the goal of learning how convection in a tropical setting can effect the movement of atmospheric gases.