The field of environmental chemistry comprises many research questions that are both challenging to scientists and relevant to society. Research in our group applies state-of-the-science analytical techniques to address issues including urban air quality, climate change, acid precipitation, and ecosystem function. Earth systems are difficult to fully characterize because of the range of temporal and spatial scales involved, and the large number of feedbacks, non-linearities, and interactions between systems. Models developed to represent these systems are often poorly constrained due to the limited availability of long term, precise, and accurate observations with adequate geographic coverage and spatial resolution. Our focus is on field measurements, particularly of reactive nitrogen compounds, that can be used to evaluate our understanding of the rates and mechanisms of chemical transformations in the environment.