Abstract
Due to the complex nature of ambient aerosols arising from the presence of myriads of organic compounds, the chemical reactivity of a particular compound with oxidant/s are studied through chamber experiments under controlled laboratory conditions. Several confounders (RH, T, light intensity, in chamber retention time) are controlled in chamber experiments to study their effect on the chemical transformation of a reactant (exposure variable) and the outcome [kinetic rate constant determination, new product/s formation e.g., secondary organic aerosol (SOA), product/s yield, etc.]. However, under ambient atmospheric conditions, it is not possible to control for these confounders which poses a challenge in assessing the outcome/s accurately. The approach of data interpretation must include randomization for an accurate prediction of the real-world scenario. One of the ways to achieve randomization is possible by the instrumental variable analysis (IVA). In this study, the IVA analysis revealed that the average ratio of fSOC/O3 (ppb−1) was 0.0032 (95% CI: 0.0009, 0.0055) and 0.0033 (95% CI: 0.0001, 0.0065) during daytime of Diwali and Post-Diwali period. However, during rest of the study period the relationship between O3 and fSOC was found to be insignificant. Based on IVA in conjunction with the concentration-weighted trajectory (CWT), cluster analysis, and fire count imageries, causal effect of O3 on SOA formation has been inferred for the daytime when emissions from long-range transport of biomass burning influenced the receptor site. To the best of our knowledge, the IVA has been applied for the first time in this study in the field of atmospheric and aerosol chemistry.
Highlights
Instrumental variable analysis (IVA) has been utilized for understanding the causal effect of election day precipitation on electoral outcomes affecting the voters’ decisions differently (Lind, 2020)
The OC and EC concentrations overall ranged from 31–308 μg m−3 and 5.4–30 μg m−3, respectively
This study finds and discusses the application of instrumental variable analysis (IVA) for inferring causality in atmospheric and aerosol chemistry observations
Summary
Instrumental variable analysis (IVA) has been utilized for understanding the causal effect of election day precipitation on electoral outcomes affecting the voters’ decisions differently (Lind, 2020). IVA has been utilized in “remote sensing” e.g., in estimating error of a geophysical product (Dong et al, 2019). IVA has been applied in the field of social science and statistics e.g., utilization of genetic markers as IV for assessing the effect of the risk factor on the outcome (von Hinke et al, 2016). To the best of our knowledge the application of IVA has not been done so far to assess feasibility of chemical transformations occurring in the atmosphere through gas-phase or multi-phase reactions. There are myriads of organic and inorganic compounds in the gas-phase
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