Towards the Closure of Reactivity and Volatile Organic Compound Budgets in the Troposphere using in situ Observations
A diversity of reactive gasses, known as volatile organic compounds (VOCs) are ubiquitous in the Earth’s atmosphere. Interactions between VOCs, OH + HO2 (HOx), and NO + NO2 (NOx) affect tropospheric oxidation capacity, regional air quality, and climate radiative forcing. Although it is estimated that the over 105 different VOCs have been measured to date, studies suggest that there exist unmeasured VOCs. These studies used direct field observations of hydroxyl (OH) reactivity (inverse OH lifetime) to determine the total reactivity of ambient air. We can assess our understanding of reactive gases, such as VOCs, by comparing measurements of OH reactivity and the OH reactivity calculated from speciated VOC and trace gas observations. The discrepancy between the observed OH reactivity and the OH reactivity calculated from observed reactive gases has been termed the “missing OH reactivity”. Up to 70% missing OH reactivity has been reported at field sites at the interface of urban and rural surroundings. This motivated us to explore the levels of reactive trace gases and OH reactivity in the Seoul Metropolitan Area (SMA) and a nearby forest. During this study we found that about 70 – 80% of the OH reactivity was missing at the forest field site. This motivated us to return to the site with a more comprehensive instrumentation suite, including a PTR-ToF-MS. We improve the discrepancy between the measured and calculated OH reactivity by roughly 20% with an additional 6 s-1 of OH reactivity calculated from typically unmeasured compounds detected by the PTR-ToF-MS. The potential of these typically unmeasured compounds to reconcile large portions of OH reactivity, and their potential to play a significant role in regional photochemistry motivated us to investigate PTR-ToF-MS data sets from three photochemically distinct field sites. I conducted an analysis of PTR-ToF-MS data sets taken across three photochemically distinct field sites raging from pristine to polluted. We cataloged over 200 mass peaks that to our knowledge, have not been reported in previous literature.