An epidemiological analysis of traffic related air pollution in Denmark implicates NO2 in Parkinson's: a 27% increase in reported cases of PD in the top quartile compared to the bottom quartile[1].

As a sanity check suggesting causality, in vitro analysis shows NO2 radical increases the rate of alpha-synuclein aggregation [2].

References:

[1] "Traffic-Related Air Pollution and Parkinson’s Disease in Denmark: A Case–Control Study"
Beate Ritz, Pei-Chen Lee, Johnni Hansen, Christina Funch Lassen, Matthias Ketzel, Mette Sørensen, and Ole Raaschou-Nielsen
Environmental Health Perspectives, July 2015
http://ehp.niehs.nih.gov/wp-content/...09313.acco.pdf

[2] "Bicarbonate enhances alpha-synuclein oligomerization and nitration: intermediacy of carbonate radical anion and nitrogen dioxide radical."
Christopher Andrekopoulos, Hao Zhang, Joy Joseph, Shasi Kalivendi, and B Kalyanaraman
Biochem J. 2004 Mar 1; 378(Pt 2): 435–447.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1223984/

John