Air pollution recovery still falls short of pre-industrial conditions: sulfur stable isotope analysis of tree rings from two giant trees

Reconstructing long-term records of air pollution and comparing present-day air quality with that of the pre-Industrial Revolution era are crucial for predicting the trajectory of recovery from sulfur-based air pollution. The annual rings of trees contain sulfur compounds that have accumulated over time, and the sulfur stable isotope ratio (δ34S) reflects the past δ34S values in the atmosphere. Although δ34S in tree rings has been analyzed in the past, there are no δ34S data in tree rings from before the Industrial Revolution (1760), when humans began to emit large amounts of sulfur compounds to the atmosphere. Therefore, we analyzed long-term variations in δ34S in the annual rings of two giant Japanese cedars (Cryptomeria japonica) from the Okute-Shinmei Shrine and Ise Jingu in central Japan, spanning over 350 years. The fluctuation trend of δ34S values was statistically divided into periods before and after industrialization. Before industrialization, δ34S values were stable and high, averaging + 6.0 ± 0.6‰ in Okute and + 10.5 ± 0.9‰ in Ise. The values closely match previously reported adjusted δ34S values from Greenland ice cores. After industrialization, values declined sharply owing to fossil fuel use. This decline stabilized around the 1990s, and values increased slightly. Despite these reductions in sulfur emissions, δ34S values remain significantly lower than pre-industrial levels, implying that air quality has not yet returned to pre-industrial conditions.