 |
|
||
|
|
Photochemical Oxidation Products (POP) Group
POP Group Members:
Summary of Activities:The main focus of the POP group during the past year has been field programs. The group participated in three large airborne campaigns and one ground based study. Data is still in the process of being analyzed and final data products will be available later in 2006. These studies have all proven to be fruitful with several interesting results already being found early in the analysis procedure. The studies and some of the results are outlined below.
Antarctic Tropospheric Chemistry Investigation (ANTCI) Nov.-Dec. 2005 Funded by: NSF Polar Programs
This study was conducted in Antarctica during November-December 2005. The broad based goal of this program is to enhance understanding of the processes that control tropospheric levels of HO x , NO x , sulfur, and other trace species over the Antarctic continent. The project included ground based measurements at the South Pole and an airborne component based out of McMurdo Station with a short deployment out of the South Pole. The POP group had a rather large role in the study, in that they were responsible for the design and layout of the experimental package used on the twin otter aircraft, as well as being responsible for measurements of OH, H 2 SO 4 , and MSA; and aircraft state parameters.
Highlights of the study include measurements made on the Antarctic plateau, in glacial valleys, and along the coast. These measurements were aimed at evaluating the detailed dynamical and chemical processes that control spring/summertime levels of NO x /NO y and OH/ HO x , as well as assessing how representative South Pole and previous ground-based coastal measurements are to the larger polar and near shore Antarctica. Based upon observations of elevated concentrations of NO and OH from previous studies at the South Pole, it has been hypothesized that on the polar plateau, NO is produced from NO 3 - photolysis in the surface layer of snow. As air follows the katabatic flow towards the coast, this NO is converted back into HNO 3 and redeposited back to the snow surface where NO 3 - photolysis can occur again. As the airmass descends from the plateau, it travels down glacial valleys. Thus, one could imagine a large flux of NO off the plateau through the glacial valleys. A preliminary look at the data reveals that while larger concentrations of NO (and OH) were observed on the plateau and at the upper entrances to glacial valleys, these concentrations were not making it down the valleys to the exit at the coast. At the exits to the valleys, OH concentrations were elevated as expected with the observed NO, however, due to larger H 2 O concentrations, model runs must be performed to determine the overall contribution of the NO + HO 2 reaction to OH production (secondary production) versus that produced from O 3 photolysis (primary production).
Another highlight of the study is the first airborne measurements of the plume emitting from Mt. Erebus , a volcano located near McMurdo Station. Measurements of H 2 SO 4 and SO 2 inside the plume revealed extremely elevated concentrations compared to upwind of the plume. Following the plume downwind of the volcano, these concentrations fell as expected, giving both chemical and dynamic (dilution) information about the plume. The OH measurements performed will allow information to be gleaned about H 2 SO 4 production rates and lifetimes inside the plume.
Twin Otter aircraft with Mt. Erebus in the background. The plume from the volcano is clearly evident.
Megacity Impacts on Regional and Global Environments (MIRAGE) Mar.-Apr. 2006 Funded by: NCAR/ACD Intercontinental Transport Experiment – B (INTEX-B) Apr.-May 2006 Funded by: NASA
These two studies were conducted back to back using the NCAR C-130 with the same payload. MIRAGE was a large, multi-agency, multi-university study with ground based components in Mexico City, and a multi-aircraft component based out of Veracruz . INTEX-B was a multi-aircraft study with the C-130 based out of Seattle , WA . In these studies, the POP group supplied OH, H 2 SO 4 , MSA, HNO 3, and NH 3 measurements from the NCAR C-130. Of these measurements, NH 3 had never been used before in the field. This technique involved the ionization of the sample flow in which the velocity had been slowed by a factor of 2 from that of the free steam aircraft. This ionization was achieved via a stainless steel corona needle with a 3-4 kV potential applied. Once formed these ions were then directed via electrostatic lenses into a quadrupole mass spectrometer for detection.
Because the goal of the MIRAGE was to study the effects or impacts of Mexico City on the regional and global level, the aircraft sampled outflow from the city in many different environments. One particularly interesting flight was over an oil refinery. Here record levels, larger than any measured before by the POP group, of H 2 SO 4 were seen. These large concentrations (> 10 8 molecule cm -3 ) were accompanied by large concentrations of SO 2 (> 500 ppbv ), and large values of ultra-fine aerosol. By combining these measurements with those of OH, information can be obtained on SO 2 oxidation rates and H 2 SO 4 lifetime. Model simulations of OH will demonstrate strengths and shortcomings of the understanding of gas phase chemistry in this highly complex environment. Another interesting set of observations occurred on a flight through a smoke plume from biomass burning. The environment inside the plume was NO x rich and was accompanied by large concentrations (>10 7 molecule cm -3 ) of OH. Large amounts of NH 3 (> 2 ppbv ) and acetone (a mass monitored, but not calibrated). Both H 2 SO 4 and MSA were suppressed (< 10 6 molecule cm -3 ) inside the plume. This observation is expected however, with the large amount of surface area of particulates present. The goal of INTEX-b was to study the transport and transformation of Asian pollution as it impacts the West coast of North America . Many flights were flown over the North Pacific ocean and were in mainly clean air with the exception of plume encounters. Daytime OH levels were typically low (1-2 x 10 6 molecule cm -3 ) as would be expected in such a clean environment. With plume encounters, both OH and H 2 SO 4 were found to be highly variable with values of OH reaching as high as 1 x 10 7 molecule cm -3 and H 2 SO 4 reaching as high as 1 x 10 8 molecule cm -3 . MSA proved to be interesting during this project with large concentrations seen in high dry layers. As MSA is a product of DMS (a marine emitted compound) oxidation, these measurements can be used as a tracer for air which had significant marine exposure. Chemical Emission, Loss, Transformation and Interactions within Canopies II (CELTIC II ) July -Aug. 2006 Funded by: NCAR/ACD, EPA
CELTIC II was a ground based study conducted in a forested area at Niwot Ridge in the mountains outside Boulder , CO . While the overall goal was to investigate the sources, sinks, and chemistry of organic compounds, one of the minor goals of this project was to try to determine the role of H 2 SO 4 and other types of organic acids in the process of aerosol formation and growth in this type of wooded environment. The role of the POP group was to provide H 2 SO 4 measurements. The results of these measurements were not surprising. H 2 SO 4 concentrations were typically low (< 10 7 molecule cm -3 ), an observation consistent with a clean environment. At times when H 2 SO 4 concentrations did rise above 10 7 molecule cm -3 , they were accompanied by moderate SO 2 concentrations, an indication of upslope air from Denver/Boulder. Though not requested, the POP group did take advantage of the opportunity to perform OH measurements in this wooded mountain environment. Of particular interest were the measurements made at night. The primary mechanism for OH formation is O 3 photolysis via: O 3 + h O( 1 D) + O 2 O( 1 D) + H 2 O 2OH
A secondary mechanism involves the reaction of NO with HO 2 : NO + HO 2 NO 2 + OH At night, in a clean environment, neither photons, h, nor NO should be present in appreciable quantities. Thus OH should not be present at night. Observations of OH made during a previous study conducted in a wooded area of Michigan revealed nighttime OH concentrations as large as 3 x 10 6 molecule cm -3 , values more typical of daytime concentrations. In contrast the POP group's observations from Niwot Ridge indicated that OH concentrations at night were never above 10 6 molecule cm -3 and were typically near the detection limit of 3 x 10 5 molecule cm -3 for a single measurement. It should be pointed out that the Michigan measurements used a Laser Induced Fluorescence (LIF) technique as opposed to the POP groups Chemical Ionization Mass Spectrometry (CIMS) technique. As the two measurements were performed under different conditions in different environments, the same observation would not necessarily be expected. The difference merely points out the need to perform a side by side intercomparison of the two techniques.
Figure of 10 days of OH measurements at Niwot Ridge. These are 30 sec measurements. Note OH is typically < 10 6 molecule cm -3 during the evening-early morning.
|
|
