自動車および焼却炉排ガス中の有機酸濃度の測定と発生量の見積り

大気中有機酸の発生源である自動車および焼却炉排ガスについて，有機酸（ギ酸，酢酸，プロピオン酸，シュウ酸）濃度を測定した。ガソリン車の排ガス中有機酸濃度の相加平均値は，ギ酸59.7ppbv，酢酸327ppbv，プロピオン酸12.1ppbv，シュウ酸0.77ppbv（parts per billion by volume）であった。ディーゼル車および二輪車は，ガソリン車よりも有機酸濃度がやや高く，特に二輪車の酢酸は10ppmv（parts per million by volume）以上と高かった。広島大学東広島キャンパス設置の焼却炉排ガス中有機酸の組成および濃度は，ガソリン車と同様だった。自動車有機酸の組成及び濃度は，排気量，走行距離とは特に相関が見られず，これらは主に燃料組成，エンジン機構および排ガス浄化装置により決定すると考えられた。排ガス中有機酸の濃度比を，東広島における降水，露，大気中有機酸のそれと比較すると，降水よりも露および大気中有機酸の濃度比に近いことが示された。測定結果から，日本国内の自動車起源の有機酸年間発生量は，ギ酸3.0×109g yr-1，酢酸5.3×109g yr-1，プロピオン酸4.6×108g yr-1，シュウ酸7.9×107g yr-1と見積もられた。さらに一酸化炭素を指標として大気中有機酸への自動車起源有機酸の寄与を計算すると，ギ酸41％，酢酸18％となった。

The concentrations of organic acids (formic, acetic, propionic and oxalic acids) were measured in automobile and incinerator exhaust gases, which are one of major sources of atmospheric organic acids. The mean concentrations of formic, acetic, propionic and oxalic acids in the exhaust gases of gasoline-powered vehicles (seven automobile models) were 59.7, 327, 12.1 and 0.77ppbv (parts per billion by volume), respectively. The organic acid concentrations in exhaust gases from one diesel-powered automobile model and two motorcycle models were higher than those from the gasoline-powered vehicles. The acetic acid concentrations in the exhaust gases from the motorcycles were particularly high at >10ppmv (parts per million by volume). The type of fuel, engine, and purification system of the exhaust gas probably determine the concentrations of organic acids in the exhaust gas. The concentrations of organic acids in exhaust gas from an incinerator at the Higashi-Hiroshima campus of Hiroshima University (Japan) were similar to those in the exhaust gas from gasoline-powered vehicles. The concentration ratios of formic, acetic and oxalic acids in automobile exhaust gas were compared with those of atmospheric organic acids, precipitation, and dew. The organic acid concentration ratios in automobile exhaust gas were more similar to those in dew and the atmosphere than those in precipitation. Based on the organic acid concentrations in the automobile exhaust gases, the annual emission rates from all automobiles in Japan were estimated to 3.0×109, 5.3×109, 4.6×108 and 7.9×107g yr-1 for formic, acetic, propionic and oxalic acids, respectively, The percentage contributions of formic and acetic acids from automobile exhaust gas to the acid concentrations in the atmosphere were estimated to 41% and 18%, respectively, using carbon monoxide as an indicator of automobile exhaust gas.