VARIATION OF FREE TROPOSPHERIC TOTAL NITRATE AT MAUNA-LOA-OBSERVATORY, HAWAII

We measured total nitrate (TN) concentrations (defined as nitric acid vapor plus nitrate aerosol) nightly at the Mauna Loa Observatory (MLO) from September 1988 to September 1991. Using three meteorological cri teria, (condensation nuclei, wind direction, and dew point) we selecte d samples that represented the free troposphere. The 3-year mean and m edian of TN mixing ratios were 113 parts per trillion by volume (pptv) and 93 pptv, respectively. Each year the TN concentrations at MLO dur ing the spring and summer were larger by more than a factor of 2 than those during the fall and winter. The springtime peak TN corresponded to incidents of increased Asian dust over the northern Pacific Ocean. Isentropic back trajectories confirmed that spring air masses with hig h TN originated over the Asian continent. Back trajectories also indic ated that the North American continent was often the source of high TN during the summer. Air mass source regions and flow patterns to MLO w ere classified into five distinct groups of trajectories by cluster an alysis. Despite the different source regions for each of the clusters the distributions of TN were very similar. This suggests that the sour ce region may be less important for determining TN at MLO than such fa ctors as the degree of vertical mixing over source regions, stratosphe ric injection, and wet removal rates.