Revealing Distinct Photochemical Ages within the Vertical Boundary Layer and Seasons by Observed VOC Species Ratios

Previous studies have revealed the degree of VOC oxidation in air through the ratios of observed VOC species. However, quantifying the air photochemical age (PCA) in the boundary layer (BL) remains a challenge due to the limited vertical measurements and the difficulty in determining OH levels. Here, we explore the PCA across different heights and layers of the BL using the observed VOC ratios with varying reactivity and calculated OH levels using observation-based models. Results show that more reactive species ratios of VOCs generally decrease with height across the BL, while less reactive species ratios show the opposite trend. Given the dominance by OH-exposure and OH levels, estimated PCA values exhibit significant seasonal and vertical variations, especially when differentiated by thermal stratification. In the mixing layer (ML), the PCA is younger, with a minimal vertical gradient, whereas it becomes older in the stable BL and residual layer (RL). Specifically, in summer, the average PCA was 2.0 h in the ML and 2.65 h in the SBL and RL, much younger than in autumn, 6.8 h in the ML and 9.1–10.5 h in the SBL and RL. The PCA could serve as a metric of assessing vertical exchanges and holds significant potential for air quality assessment.