Seasonal variation of SO2, HNO3, NH3 and selected aerosol components at Sonnblick (3106 m a.s.l.)

A two-year record of filter pack measurements illustrates the chemical climatology of nitric acid, sulfur dioxide, ammonia and major aerosol components (particulate nitrate, sulfate and ammonium) at a high alpine site (Mt. Sonnblick, 3106 m a.s.l.). Annual averages (standard deviations are given in parenthesis) were 4(6) nmol m-3 for nitric acid, 8(11) nmol m-3 for ammonia, 6(14) nmol m-3 for sulfur dioxide, 4(9) nmol m-3 for particulate nitrate, 15(19) nmol m-3 for particulate sulfate and 23(35) nmol m-3 for particulate ammonium. Marked seasonal cycles with low concentrations during winter and comparable higher concentrations during summer were observed for all components but sulfur dioxide. The variations are more pronounced for aerosol components than for gases. Based on monthly averages summer to winter concentration ratios were 13, 7 and 10 for aerosol nitrate, sulfate and ammonium, and 3 and 5 for nitric acid and ammonia, respectively. For sulfur dioxide a spring maximum was observed, while little seasonal variation occurred in respect to average summer and winter values. In spring, sulfur dioxide was three times higher, on average, than during the rest of the year. Particulate ammonium and particulate sulfate are the predominant forms compared to gaseous ammonia and sulfur dioxide. Particulate sulfate amounted to 64% of total sulfur, particulate ammonium amounted to 66% of total reduced nitrogen. Nitric acid was more abundant than aerosol nitrate and amounted to 61% of both compounds. In respect to ammonium and sulfate aerosol samples are slightly acidic during most of the year. Lowest neutralization ratios were mainly associated with low concentration values. Concentration ranges and meteorological data suggest that the high alpine site lies within the free troposphere during winter, whereas the influence of boundary layer air becomes more important during summer and a modified mixing layer is formed.