​Marine cloud brightening (MCB) is a proposed solar climate intervention strategy that sprays sea salt particles into low marine clouds to make them more reflective and help cool the Earth. While most research has focused on its effects on climate, MCB may also alter atmospheric chemistry. Using a global atmospheric chemical transport model, we examined how MCB affects reactive halogens and oxidants, which are highly reactive chemical species, and important greenhouse gases ozone and methane. We found that MCB can increase troposheric oxidants (hydroxyl radical, nitrogen oxides, and ozone) and shorten methane lifetime, which is the opposite of what previous studies have reported. Additional simulations show that this difference is caused by how the model represents particulate nitrate photolysis, a poorly constrained process. These results demonstrate that uncertainties in particulate nitrate photolysis can substantially influence predictions of how MCB impacts atmospheric chemistry, highlighting the need to better understand this chemical process before fully understanding the impacts of MCB.