Large‐scale distributions of tropospheric nitric, formic, and acetic acids over the western Pacific basin during wintertime

Abstract

We report here measurements of the acidic gases nitric (HNO 3 ), formic (HCOOH), and acetic (CH 3 COOH) over the western Pacific basin during the February‐March 1994 Pacific Exploratory Mission‐West (PEM‐West B). These data were obtained aboard the NASA DC‐8 research aircraft as it flew missions in the altitude range of 0.3–12.5 km over equatorial regions near Guam and then further westward encompassing the entire Pacific Rim arc. Aged marine air over the equatorial Pacific generally exhibited mixing ratios of acidic gases <100 parts per trillion by volume (pptv). Near the Asian continent, discrete plumes encountered below 6 km altitude contained up to 8 parts per billion by volume (ppbv) HNO 3 and 10 ppbv HCOOH and CH 3 COOH. Overall there was a general correlation between mixing ratios of acidic gases with those of CO, C 2 H 2 , and C 2 Cl 4 , indicative of emissions from combustion and industrial sources. The latitudinal distributions of HNO 3 and CO showed that the largest mixing ratios were centered around 15°N, while HCOOH, CH 3 COOH, and C 2 Cl 4 peaked at 25°N. The mixing ratios of HCOOH and CH 3 COOH were highly correlated ( r 2 = 0.87) below 6 km altitude, with a slope (0.89) characteristic of the nongrowing season at midlatitudes in the northern hemisphere. Above 6 km altitude, HCOOH and CH 3 COOH were marginally correlated ( r 2 = 0.50), and plumes well defined by CO, C 2 H 2 , and C 2 Cl 4 were depleted in acidic gases, most likely due to scavenging during vertical transport of air masses through convective cloud systems over the Asian continent. In stratospheric air masses, HNO 3 mixing ratios were several parts per billion by volume (ppbv), yielding relationships with O 3 and N 2 O consistent with those previously reported for NO y .