A Multivariate Time Series Approach to Study the Interdependence among O3, NO x , and VOCs in Ambient Urban Atmosphere

A multivariate time series approach vector autoregression (VAR) along with impulse response function and variance decomposition technique has been employed to look into the interrelationship among O₃, NO, NO₂, and volatile organic compounds (VOCs, namely, benzene, ethylbenzene, toluene, and xylene in the present study) using 3 months long continuous time series data of 1 h average concentration of these pollutants at one of the traffic sites in Delhi, India. It is found that the VAR of order 2 (i.e., past two lagged values of 1 h interval) is sufficient to represent the observed time series at the station studied. The impulse response function and variance decomposition analysis indicate that O₃ concentration shows an immediate rise and persists for a longer duration (typically 8–10 h) once the impulse of NO₂, benzene, ethylbenzene, or xylene is given in the ambient environment. However, in case of toluene, the reverse effect has been observed. Since O₃ forms in the troposphere due to photolysis of NO₂, it is not surprising that its impulse triggers O₃ formation in the ambient environment. However, in case of VOCs, this has been attributed to their tendency to show higher inclination toward intermediary reactions leading to the formation of O₃ rather than their (VOCs) reaction with O₃. Among VOCs, only toluene has been observed to show higher inclination toward its reaction with O₃. Apart from this, variance decomposition technique also reveals that the relation of NO with NO₂ is more important than the relation of NO with O₃ creating a conducive atmosphere for O₃ formation in the present scenario. Thus, the multivariate time series approach provides significant insight about the role played by the dominant individual VOCs and NO _x in influencing the O₃ concentration in ambient urban atmosphere whereas a photochemical modeling approach gives an overall view of NO _x and VOCs behavior with respect to O₃ by using the O₃ isopleth technique.