Total Nitrate, NSS, and MSA Variation at Mauna Loa
B.J. Huebert, C.R. Adams, and L. Zhuang
Department of Oceanography, University of Hawaii, Honolulu 96822
Introduction
Much of the NO and NO2 emitted into the atmosphere is converted to nitric acid vapor or aerosol nitrate before being removed by dry or wet deposition. This conversion to nitrate is largely complete within a few days of the odd-nitrogen's emission, so that in remote areas such as at the Mauna Loa Observatory, Hawaii (MLO) the total nitrate concentration (vapor plus aerosol) represents a fair estimate of the total odd-nitrogen concentration [Atlas et al., 1992].
With support from the National Science Foundation (NSF), we have measured nitrate concentrations at MLO for several years to help identify the important sources of odd-nitrogen compounds in remote parts of the globe. In collaboration with the MLO staff, we now measure total nitrate every night from the walkup tower. We have also begun measuring methanesulfonate (MSA) and calcium aerosol. For the last decade we have also measured non-sea salt sulfate (NSS) at MLO but since the two active volcanoes on Hawaii emit sulfur dioxide that rapidly forms NSS, we have never tried to interpret it. However, this record is the only decade-long record of free tropospheric (FT) NSS in existence. Therefore, we have developed a method for identifying samples that may have been influenced by the volcanoes so that we can sort out a record of clean free tropospheric NSS. That record is now in use by seven modeling groups around the world as a test of the ability of global chemical transport models to predict FT concentrations of NSS for climate modeling.
Materials and Methods
We use a Teflon/nylon filter pack method for collecting atmospheric nitrate. Since August 1988 one filter has been exposed each night from 2000 LST to 0800 LST. Filters are returned to the University of Hawaii for extraction and analysis by ion chromatography.
Results and Discussion
During our intermittent MLO sampling prior to September 1988, we observed a sharp maximum in nitric acid and aerosol nitrate concentrations in the summer. The search for an explanation for this maximum continues to stimulate our science. The daily total nitrate values for 1996-1997 are plotted in Figure 1. The lowest sustained concentrations are still evident in the winter, with a mix of high-concentration events and cleaner periods in the spring and late summer.
Fig. 1. Nightly concentrations of total nitrate in 1996-1997.
Figure 2 shows monthly averages of 2000 LST to 0800 LST total nitrate concentrations from September 1988 to December 1997. The 1993 data represent the lowest (defendable) concentrations we have observed during our sampling at MLO.
Fig. 2. Monthly average total (aerosol plus vapor) nitrate and NSS versus time.
The concentration of total nitrate at MLO is to a large extent controlled by precipitation scavenging of soluble material during transport from the continents [Lee et al., 1993, 1994] so this interannual variability may be an indicator of changes in large-scale precipitation patterns. The apparently-monotonic decrease in summertime total nitrate from 1988 through 1991 suggests that a cyclic process, such as the southern oscillation, may be reflected in this record. It is certainly reasonable that the transport of continental materials like mineral aerosol and fixed nitrogen (which can be limiting nutrients in certain parts of the Pacific) should be sensitive to changes in large-scale atmospheric circulation patterns. Clearly we need to identify the climatological differences that cause this dramatic change in the annual cycle of nitrate from one year to the next since they may have impacts on phenomena as diverse as marine biological productivity and the earth's radiation budget.
In February of 1995 we began to analyze our filter samples for MSA, since MSA is an indicator of dimethylsulfide (DMS) oxidation [Huebert et al., 1996]. We are interested in the potential that DMS oxidation in the free troposphere may be responsible for much of the MSA (and some of the sulfate) found in ice-cores. The MSA data is different from either nitrate or NSS; while it varies considerably from day to day and individual months may be a bit higher or lower, the (multi-year) mean of monthly average MSA shows no annual variability. MSA averages 17.7 ± 2.5 mm (0.7 ± 0.1 inch) every month of the year. It is clear from the data that the MSA we see is not due to boundary-layer contamination of our samples since it is rarely accompanied by Cl or Na that are clear indicators of boundary-layer air.
The FT values of NSS and Ca show the same seasonality as nitrate (Figure 3). The springtime maximum suggests that Asia may be the source of much of this elevated NSS. Calcium also peaks in the spring due to the input of Asian dust.
Fig. 3. Decade-mean of monthly average NSS and 
with 2 years of Ca means.
Ongoing Research
We are continuing our nightly sampling from the tower with the help of the MLO staff. Although equipment failures and analytical problems unavoidably cause lapses in the data, a very interesting record is emerging. We intend to continue this total nitrate data record in the hopes of identifying those factors that control the form and the range of its annual cycle. We hope in the near future to add sensitive SO2 measurements to the NSS and MSA record.
References
Atlas, E. L., B. A. Ridley, G. Hübler, M. A. Carroll, D. D. Montzka, B. Huebert, R. B. Norton, J. Walega, F. Grahek, and S. Schauffler, Partitioning and budget of NOy species during MLOPEX, J. Geophys. Res., 97, 10,449-10,462, 1992.
Huebert, B. J., D. J. Wylie, L. Zhuang, and J. A. Heath, Production and loss of methanesulfonate and non-sea salt sulfate in the equatorial Pacific marine boundary layer, Geophys. Res. Lett., 23, 737-740, 1996.
Lee, G., L. Zhuang, B. J. Huebert, and T. P. Meyers, Concentration Gradients and Dry Deposition of Nitric Acid Vapor at Mauna Loa Observatory, Hawaii, J. Geophys. Res., 98, 12,661-12,671, 1993.
Lee, G., J. T. Merrill, and B. J. Huebert, Variation of Free Tropospheric Total Nitrate at Mauna Loa Observatory, Hawaii, J. Geophys. Res., 99, 12,821-12,831, 1994.