Total Nitrate and MSA Variation at Mauna Loa

B. J. HUEBERT AND L. ZHUANG

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 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. We now measure total nitrate every night from the walkup tower in collaboration with the MLO staff. We have also begun measuring methanesulfonate (MSA) aerosol.

We use a Teflon/nylon filter pack method for collecting atmospheric nitrate. Since August 1988, one filter has been exposed each night from 2000 to 0800 LST. Filters are returned to the University of Hawaii laboratory for extraction and analysis by ion chromatography.

The data from August 1991 to July 1992 was, unfortunately, treated differently from the remainder. These samples were all analyzed as a batch during a brief period between the return of our analytical laboratory from a field deployment in the Azores and its shipment to its new home at the University of Hawaii. Once it became apparent that this data looked very different from previous years, it was no longer possible to replicate the analytical conditions or the standards to resolve questions of its validity. Hence, that data is excluded from the figures in this report.

In 1993 we published a description of gradient measurements of nitric acid and aerosol nitrate at MLO [Lee et al., 1993]. This work showed surface-active species, like nitric acid, often have large gradients near the surface at MLO, raising the potential for underestimating free tropospheric concentrations due to depletion of material upstream of samplers. The deposition velocity of nitric acid to the lava surface varied from 0.3 to 4 cm s^-1.

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 1995 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 1995.

Figure 2 shows monthly averages of 2000 to 0800 LST total nitrate concentrations from September 1988 to December 1995. 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 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., 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 1 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 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. As Figure 3 shows, the annual cycle of MSA is both distinct and different from that of either nitrate or non-seasalt sulfate (NSS). The summer MSA minimum could be due to slower DMS transport, a change in the amount of MSA produced from DMS, or more rapid MSA removal. 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, which are clear indicators of boundary-layer air in our samples.


Fig. 3. Monthly average MSA, sulfate, and total nitrate for 1995.

With the help of the MLO staff, we are continuing our nightly sampling from the tower. 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 which control the form and the range of its annual cycle.

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., 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.

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.

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