MEASUREMENT OF THE GAS-PHASE CONCENTRATION OF H2SO4 AND METHANE SULFONIC-ACID AND ESTIMATES OF H2SO4 PRODUCTION AND LOSS IN THE ATMOSPHERE

Measurements of gas phase sulfuric and methane sulfonic acid (MSA) hav e been performed using a relatively new atmospheric pressure selected ion chemical ionization mass spectrometric technique at two field site s. Both gas phase acids are photooxidation products, and their concent rations are seen to qualitatively follow solar flux. While sulfuric ac id concentrations typically decline in conjunction with declining sola r radiation, they sometimes level off in the mid 10(5) - 10(6) molecul es cm-3 range after dark, even in relatively clean air. The reason for this quasi-stable nighttime sulfuric acid concentration is not well u nderstood but may be a result of a steady state exchange of sulfuric a cid between particles and the gas phase. Measurements of OH, H2SO4, an d SO2 concentrations in conjunction with aerosol number and size distr ibution also made possible the independent calculation of gas phase su lfuric acid production and loss rates. Calculated production and loss rates are seen to agree well in relatively clean air during the daylig ht hours. At night, however, the sulfuric acid concentrations and its calculated loss rate often have a nonzero value. In more polluted air masses, calculated gas phase sulfuric acid losses significantly exceed calculated production if H2SO4/aerosol reaction probabilities of 1.0 or 0.5 are assumed.