Hornbrooks work in the United States has focused on VOCs and air quality in specific regions. Most recently, Hornbrook was a part of the WE-CAN project which studied the effects of wildfires on air quality and the atmosphere. Wildfires have been a topic of interest to Hornbrook as one of her authored papers, Observations of nonmethane organic compounds during ARCTAS - Part 1: Biomass burning emissions and plume enhancements,' focuses on the effects of wildfires on air quality at distances further away from the fires.
Hornbrook has also contributed to increasing the understanding of anthropogenic emissions in the United States by focusing on VOCs and chemical changes of the atmosphere. In 2018, Hornbrook began to work as a part of the NASA ATom project which was part of the larger goal to prepare for the potential effects of global climate change. Her authored poster, as a result of the project, details the observations made of how the upper troposphere reacts with VOCs emitted by humans. WINTER was a 2015 project based out of northern Virginia that had an emphasis on looking at emissions in the northeastern United States and their effect on pollution over the region as well as the Atlantic Ocean. In 2013, Hornbrook contributed to the NOMADSS project which focused on looking at the effects of anthropogenic emissions in Chicago, Illinois and Gary, Indiana concluding that these emissions led to an increase of mercury in the area.
A significant amount of Hornbrooks research has taken place in the Colorado Front Range. The BEACHON-ROCS study took place at the Manitou Forest Observatory in August, 2010 in order to understand reactive organic gases which can help model the atmospheres oxidation capacity. The 2014 FRAPPE project was another Colorado based project that took a look at air quality in the Colorado Front Range and was partially funded by the state of Colorado. Hornbrook's research helped to understand how some weather patterns are more likely to retain aerosols than other weather patterns. Hornbrook gave a presentation at the Gordon Research Conference where she presented a poster about VOC observations during the FRAPPE project in the Colorado Front Range.
Prior to the FRAPPE project, Hornbrook worked on DC3 in Kansas, an airborne campaign which focused on how convective clouds interact with the upper troposphere at mid latitudes. Hornbrook's group observed that the storms observed possessed a logical relationship on a chemical level.
In 2009, Hornbrook conducted research in Barrow, Alaska as part of the OASIS-2009 campaign (Ocean-Atmosphere-Sea Ice-Snowpack). As part of a larger team, Hornbrook focused on studying VOCs as well as seasonal trends and oxidization events in the arctic. The research concluded that arctic seasonality does have an effect on the chemistry of the atmosphere. OASIS-2009 also provided a large set of data from the arctic that could be used for future analysis.