A corresponding relationship between formaldehyde and missing •OH reactivity extrapolated in chamber studies using a mixture of nine VOCs and NOx

Abstract

•OH reactivity ($k^{\prime }$), which is the inverse of •OH lifetime and can be determined by measuring the decay rate of •OH radicals as total •OH reactivity ($k_{total}^{\prime }$), or summing reactivity from volatile organic compounds (VOCs), NO_x, and other inorganics as calculated •OH reactivity ($k_{calc}^{\prime }$). $k^{\prime }$ is usually underestimated by $k_{calc}^{\prime }$, with the discrepancy named as missing •OH reactivity ($k_{missing}^{\prime }$), mainly representing the reactivity of unknown VOCs. $k_{missing}^{\prime }$ plays an important role in understanding the overall reactivity balance and the production of photochemical oxidants. Given that the Master Chemical Mechanism (MCM) can simulate $k^{\prime }$ and ozone production, uncertainties arise when apply these models to complex ambient atmospheres. We conducted smog-chamber experiments using VOC mixtures under well-controlled conditions. $k_{total}^{\prime }$ and trace species in the smog-chamber were measured and concurrently simulated by MCM model. The MCM reproduced initial time variations of trace species and $k_{total}^{\prime }$. We found a corresponding relationship of $k_{missing}^{\prime }$/[HCHO] = 1.5 s⁻¹ ppbv⁻¹ based on the smog-chamber experiments and several field campaigns’ results in Japan. We further applied this factor in 2017 Tsukuba campaign to predict $k_{missing}^{\prime }$ based on diurnal HCHO concentration and obtained good performance of $k_{missing}^{\prime }$ when compared with $k_{total}^{\prime }-k_{calc}^{\prime }$. Also, ozone production regimes were assessed and cross-compared with conventional sensitivity indicators includes [HCHO]/[NO₂] and Σk_i[VOC]_i/k_N[NO_x] based on MCM results. This study confirmed that MCM model well-reproduced the kinetic chemistry of smog-chamber experiments containing VOC mixtures during the initial hours, and we propose to use HCHO concentration for model validation when $k_{total}^{\prime }$ is not available.