Measurements of Free Tropospheric Nitric Acid and Aerosols
What does this program measure?
We are measuring nitric acid vapor and particles that contain nitrate, sulfate, chloride, ammonium, oxalate, methanesulfonate, calcium, sodium, potassium, and magnesium, which we call “aerosol ions.” We express the results in terms of the mass of each substance per cubic meter of air, µg/m 3. Sometimes we express the concentrations as moles of substance per 1,000,000,000,000 moles of air, “parts per trillion by volume ” (pptv).
How does this program work?
We pull air through a Teflon filter every night. Filters for each day of the week are exposed to an ambient air stream at the 7m level of a 40m MLO tower. We sample from 8 PM to 8 AM every night to provide the maximum probable exposure to the MLO down slope wind regime. Once a week the filters are sent back to our UH laboratory, where the ions are extracted and analyzed using a technique called "ion chromatography." Nitric acid is collected by a nylon filter and analyzed in the same way at UH.
Why is this research important?
These substances have important effects on climate and atmospheric photochemical reactions. We are trying to assess the impact of continental sources on the air of the mid-Pacific, and to define day-to-day and longer term variations (e.g., seasonal) in background atmospheric nitric acid vapor concentrations.
Are there any trends in the data?
We have seen large year-to-year differences, apparently related to rainout between the source regions and MLO regions in wetter vs. drier years. We have not seen a continual upward or downward trend, though.One of the most interesting trends is the annual variability of these ions.
How does this program fit into the big picture?
What is it's role in global climate change?
Models of climate change must be tested against measurements. One of the largest issues in climate forcing is the role of aerosol particles that scatter sunlight back to space. Our measurements provide one of the few tests of these models’ ability to describe free tropospheric aerosol concentrations. Aerosols in the FT are among the many things that impact the energy balance of the Earth. This project is quantifying one of those impacts so that overall climate models can be made more accurate.
Comments and References
Long-term records of aerosol concentrations are very important assets for testing models. MLO is one of the few places in the world where the mid (free) troposphere can be sampled, so our data set is unique.This work complements our organic aerosol measurements, which have been underway for only a few months. Until recently it was believed that sulfuric acid and sulfate salts were almost the only FT aerosols other than occasional dust from Asia. By measuring both OC and sulfates, we can get a more complete picture of the climatic importance of these particles.
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