This doctoral dissertation improvement project will test a method to identify the source areas for nitrogen being deposited as dry deposition in the surrounding landscape. Stable isotopes of nitrogen in dry deposition can be used as a fingerprint to identify possible source areas, but to date it has been unclear whether the isotopic composition of nitrogen that is collected by passive filters is altered significantly during the collection process. This project will test this widely used methodology and determine whether isotopic composition of nitrogen collected on filters can be used to identify the original source areas.
Nitrogen emissions to the atmosphere and subsequent deposition to the landscape has increased significantly over the past several decades, leading to nitrogen leaching off the landscape into waterways and coastal oceans where it causes algal blooms and other environmental problems. Understanding the source areas of nitrogen that is ultimately deposited in specific locations is an important tool in mitigating this environmental problem.
The purpose of this experiment was to determine how effectively atmospheric reactive nitrogen in smog can be measured using passive sampling filters. This experiment will try to determine if human created pollution moves through the atmosphere differently than reactive nitrogen created naturally from the environment by measuring the isotopic composition of reactive nitrogen. Samplers were erected in chambers at the University of California, Riverside (UCR), in which air is filtered of pollutants and then known concentrations of pollutants are pumped back in. The concentrations of nitrogen in the chambers were set to be high, low, and control levels. Atmospheric nitrogen was measured in real time to determine if there were leaks in the system. Filters were installed and collected in sets of three over a four week period to obtain samples of 1, 2, 3, and 4 week exposures to the chambers. Passive samplers were also erected at field sites spanning a 100 mile long section of a previously measured nitrogen deposition gradient. The sites were distributed to include a Source site at UCR, two High deposition sites outside the western boundary of Joshua Tree National Park and two Low deposition sites within Joshua Tree National Park. Filters were installed using the same timing sequence as in the chamber portion of the study. The chamber part of the experiment determined that the filters could consistently measure reactive nitrogen in pollution although there was some variation in measurements in filters that were only exposed for one week. The nitrogen collected by the filters then proved to be consistent in their isotopic composition so that the technique can be used trace human created smog through the atmosphere. The accuracy of the measurements were consistent in both chambers and were most precise after being exposed to the air for three weeks. The field component of the study produced concentrations of nitrogen equivalent to what had previously been measured in previous experiments, with the largest amount of pollution measured at the designated High deposition sites. Comparing the average isotopic signature of the pollution allowed us to determine how quickly human created pollution was deposited to the landscape. Our data indicated that there is a large amount of variability in the amount of pollution that reaches each site over time. During precipitation events, the direction of dominant wind patterns can change bringing air masses in from different directions as well as having the air pollution cleaned from the sky by the rain drops. The variability in air movement is best captured by having multiple filters exposed for different lengths of time. Previous research has developed a correlation between anthropogenic nitrogen deposition and the spread of wildfire based on the increase in invasive grass species. This research provides an extra dimension of analyzing atmospheric nitrogen inputs so that we will have a better understanding of how anthropogenic inputs are moving through the atmosphere. The techniques in the experiment are capable of being used to better our understanding of the nitrogen cycle and how human created nitrogen pollutants are cycled through our atmosphere.