Time series analysis of wintertime O₃ and NO_x formation using vector autoregressions

Concentrations of 11 species are reported from continuous measurements taken during a wintertime field study in Utah. Time series data for measured species generally displayed strong diurnal patterns. Six species show a diurnal pattern of daytime maximums (NO, NO_y, O₃, H₂O₂, CH₂O₂, and Cl₂), while five species show a diurnal pattern of night time maximums (NO₂, HONO, ClNO₂, HNO₃, and N₂O₅). Vector autoregression analyses were completed to better understand important species influencing the formation of O₃ and NO_x. For the species studied, r² values of predicted versus measured concentrations ranged from 0.82-0.99. Fitting parameters for the autoregressive matrix, Π, indicated the importance of species precursors. In addition, values of fitting parameters for Π were relatively insensitive to data size, with variations generally <10%. Variable causation was quantified using the Granger causation method. Assuming O₃ and NO_x behave as chemical products, reactants (in order of importance) are as follows: H₂O₂, N₂O₅, HONO, and ClNO₂.