Deriving Global OH Abundance and Atmospheric Lifetimes for Long-Lived Gases: A Search for CH3CCl3 Alternatives Journal of Geophysical Research: Atmospheres

An accurate estimate of global hydroxyl radical (OH) abundance is important for projections of air quality, climate, and stratospheric ozone recovery. As the atmospheric mixing ratios of methyl chloroform (CH3CCl3) (MCF), the commonly used OH reference gas, approaches zero, it is important to find alternative approaches to infer atmospheric OH abundance and variability. The lack of global bottom-up emission inventories is the primary obstacle in choosing a MCF alternative. We illustrate that global emissions of long-lived trace gases can be inferred from their observed mixing ratio differences between the Northern Hemisphere (NH) and Southern Hemisphere (SH), given realistic estimates of their NH-SH exchange time, the emission partitioning between the two hemispheres, and the NH versus SH OH abundance ratio. Using the observed long-term trend and emissions derived from the measured hemispheric gradient, the combination of HFC-32 (CH2F2), HFC-134a (CH2FCF3, HFC-152a (CH3CHF2), and HCFC-22 (CHClF2), instead of a single gas, will be useful as a MCF alternative to infer global and hemispheric OH abundance and trace gas lifetimes. The primary assumption on which this multispecies approach relies is that the OH lifetimes can be estimated by scaling the thermal reaction rates of a reference gas at 272K on global and hemispheric scales. Thus, the derived hemispheric and global OH estimates are forced to reconcile the observed trends and gradient for all four compounds simultaneously. However, currently, observations of these gases from the surface networks do not provide more accurate OH abundance estimate than that from MCF.

准确估算全球羟基自由基（hydroxyl radical，OH）丰度，对于空气质量、气候以及平流层臭氧恢复的预测具有重要意义。作为当前常用的OH参考气体，甲基氯仿（methyl chloroform，CH3CCl3，简称MCF）的大气混合比正逐渐趋近于零，因此亟需探寻替代方案以反演大气OH丰度及其变化规律。全球自下而上排放清单的缺失，是遴选MCF替代气体的首要障碍。本研究表明，若能对南北半球（Northern Hemisphere，NH）与南半球（Southern Hemisphere，SH）间的交换时间、两半球间的排放分配比例以及南北半球OH丰度比值做出合理估算，便可通过观测得到的南北半球混合比差值，反演长寿命痕量气体的全球排放量。结合观测到的长期趋势与由半球梯度反演得到的排放量，采用氢氟烃32（CH2F2，HFC-32）、氢氟烃134a（CH2FCF3，HFC-134a）、氢氟烃152a（CH3CHF2，HFC-152a）与氢氯氟烃22（CHClF2，HCFC-22）的组合而非单一气体作为MCF替代物，将有助于反演全球及半球尺度的OH丰度与痕量气体寿命。该多物种方法的核心假设为：可通过缩放参考气体在272K下的热反应速率，在全球与半球尺度上估算OH的寿命。因此，反演得到的半球及全球OH丰度结果，需同时匹配四种气体的观测趋势与半球梯度特征。但目前，依托地面观测网络获取的上述气体数据，其反演得到的OH丰度精度并未优于基于MCF的反演结果。