Long-term measurements of free-tropospheric sulfate at Mauna Loa: Comparison with global model simulations

We report here on a comparison of three models, Model of the Global Univers al Tracer Transport in the Atmosphere (MOGUNTIA, abbreviated here as MOG), European Center/Hamburg (GCM Model) 3 (ECHAM3, here E3), and European Cente r/Hamburg (GCM Model) 4 (ECHAM4, here E4), with a decade-long time series o f sulfate aerosol measurements (Obs) from the Mauna Loa Observatory (MLO), Hawaii. The observations were sorted with four criteria to eliminate any co ntamination from the local volcanoes. E3 was run for 8 years, and E4 was ru n for 5 years to assess interannual variability. In terms of the climatolog ical average concentration, E3 (51 pmol mol(-1)) was very close to Obs (53 pmol mol(-1)), while MOG (33 pmol mol(-1)) and E4 (134 pmol mol) predicted different concentrations. The interannual variability of the annual average was somewhat larger in Obs than in E3 and E4. The large interannual variab ility is noteworthy; it demonstrates that multiyear time series are essenti al for comparison with climatological models. The seasonal cycle of Obs was reproduced surprisingly well by E3. E4 predicted much higher concentration s, although the seasonal amplitude was reasonable. MOG showed much less sea sonal variability, which suggests that stochastic processes (which are not described by climatological models) may be responsible for much of the tran sport from continents to remote regions. Using tagged sources, E4 found tha t urban/industrial sources in SE Asia and volcanoes were responsible for mo st of the sulfate at MLO. The interannual variability of each month was sim ilar for Obs, E3, and E4. When E4 was run in a nudged mode for 1 year (so t hat it tended to reproduce the actual meteorology) and compared day-by-day to Obs, the R-2 was 0.73. Thus, in spite of significant magnitude differenc es, E4 clearly reproduces many of the features that cause free-tropospheric sulfate concentrations to vary from day-to-day. When we moved the continua l 5-8 km volcanic source in E4 to lower altitudes, the large overestimate w as reduced but not eliminated, suggesting that the high-altitude releases a lone cannot explain the high concentrations predicted by E4. Missing proces ses and the parameterizations of vertical exchange and scavenging are among the possible reasons for the differences between the simulations. While th e discrepancy between the observations and E4 suggests that we first look f or shortcomings in E4's sulfur processing, we cannot exclude the possibilit y that compensating errors in E3 cause its better agreement with the observ ations.