Previous work suggests that the nitrogen isotopic composition of atmospheric nitrogen oxides might distinguish the influence of different sources in the environment. We report on an automated, mobile, field-based method for the high collection efficiency of atmospheric NOx for N isotopic analysis at an hourly time resolution.
The overall goal of this method is to quantitatively capture gaseous nitrogen oxides in solution in order to measure the nitrogen isotopic composition of this environmentally important gas. This method can help answer key questions in atmospheric chemistry and environmental chemistry fields since nitrogen oxides called NOx contribute to smog formation and acid deposition in the environment. The isotopes of NOx allow us to potentially track the sources and impact of NOx in the environment.
The advantage of our technique is that it can efficiently capture NOx under an variety of conditions. Compared with previously used techniques, our method has been thoroughly validated in both the laboratory and in the field for it's ability to quantitatively capture NOx for isotopic analysis. Visual demonstration of this method is critical to understand this novel system.
It's field deployment requires a compact battery powered package to allow for the portability in a variety of field applications. First, prepare sampling solutions using one molar potassium permanganate stock solution and 10 molar sodium hydroxide. Then, dilute each solution with ultra-pure water to the correct volume.
Remove 25 milliliters of one of the solutions as a laboratory blank and transfer it to a 60 milliliter amber glass bottle. Record from which solution bottle the laboratory blank came. Next, change all filters in the NOx collection system before sampling to ensure that they are working most effectively and efficiently.
Set up the stationary laboratory consisting of the NOx collection system and a chemiluminescence NOx concentration analyzer. Create an inlet to the system with PTFE tubing that at one end points in the direction of the air to be collected and on the other end is fixed to a t-fitting to connect the inlet to both the NOx analyzer and the automated collection system. Turn on the system so that the air is bubbling through the solution, and bubbles are visible.
Once sampling has been performed and the sample is done collecting NOx, collect the solution in a 60 milliliter amber glass bottle and manually remove the bottle from the system. Automatically clean the glass washing bottle using a syringe pump. Take field blanks during the collection for each solution bottle that is used by sending 25 milliliters of solution through the system without turning on the vacuum pump to collect air.
Collect the solution immediately after it is injected into the system. Pour the solution from one sample into a sample beaker and one blank solution into a blank beaker. With vigorous stirring, slowly introduce five milliliters of hydrogen peroxide above the sample beaker so that the tip does not touch the beaker, the stir rod or the solution.
Add the second five milliliter aliquot around the side of the beaker ensuring that all of the sample solution is reduced. Pour the entire contents of each beaker, both the liquid and the brown precipitate into 50 milliliter centrifuge tubes that have been labeled according to the sample or blank number or letter. Following this, centrifuge each batch of samples at 3, 220 times G for 15 minutes.
After centrifugation, pour each supernatant into an amber plastic bottle, and properly dispose of the centrifuge tube. Dip the tip of the syringe being used for injection in a beaker of ultra-pure water and dry it. Rinse the full volume of the syringe with ultra-pure water from a second beaker and discard the water as waste.
Following this, fill the syringe with a small amount of sample to pre-rinse the syringe. After discarding the sample, refill the syringe with sample. And gently knock it to remove any air bubbles so that an accurate volume is measured.
Based on the concentration determined for each sample, inject the appropriate volume into a pre-prepared capped vial with bacteria. After storing the vials overnight in a warm area, inject 0.1 to 0.2 milliliters of 10 molar sodium hydroxide into each sample to lyse the bacteria. A direct comparison of median NOx concentrations from the NOx analyzer and concentrations calculated from the solution and flow measurements indicate that the solution concentrations agree well with NC2 NOx concentrations.
In fact, examination of the percentiles of the distribution of the one minute NOx concentration data show that the solution based NOx concentrations are within the distribution for every collection interval. The NOx concentration of samples collected in urban near road and on road settings spans three orders of magnitude. And the isotope ratios range from minus one to minus 13 per mil.
This shows that the range of isotope ratios in the environment is much larger than the uncertainty of the method which is only 1.5 per mil. Comparison of two collection systems deployed at the same time show excellent agreement for the isotopic data quantified as the absolute deviation between the two data points for each collection period. In urban areas, NOx collection can be done in less than an hour, and samples can be reduced in about five minutes per sample.
This unique method facilitates the characterization of NOx source bloom isotopic composition in a variety of field environments, relatively high time resolution. We have collected emission plumes from on-road vehicles and are comparing their isotopic composition with other major NOx sources. Ultimately the purpose of this method is to accurately collect NOx in the field and use the isotopes of NOx to track it's influence on atmospheric chemistry and acid deposition.
