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Atmospheric Radical Studies (ARS) Group
ARS Group Members:
Summary of Activities:
Research in the Atmospheric Radical Studies project involves development and deployment of instrumentation for the measurement of atmospheric free radicals. Currently, the efforts are focused on detection of peroxy radicals (HO 2 and RO 2 ) from aircraft and ground-based platforms. The main instrument is based on chemical conversion with chemical ionization mass spectroscopy, and is called PeRCIMS ( Pe roxy R adical measurement by C hemical I onization M ass S pectroscopy). The fundamentals were reported in Edwards et al. (2003); there have been significant improvements to the method since publication of that paper.
Following is a brief description of the method. Ambient air is sampled into an inlet controlled at a pressure of 200 mbar through a small (0.5 mm diameter) orifice. The air is diluted about 1:1 with either nitrogen or oxygen, and then reagent gases, nitric oxide and sulfur dioxide, are added. Multi-step chemistry converts the peroxy radicals into gas-phase sulfuric acid with small amplification factors (4-5 sulfuric acid molecules per peroxy radical). The background is determined by adding nitric oxide to the front of the inlet while adding sulfur dioxideat the rear of the gas-phase reaction region. This background accounts for ambient sulfuric acid, sulfuric acid that might be present in the reagents, and sulfuric acid made in the inlet not due to peroxy radicals. The air containing the sulfuric acid is then exposed to air containing reagent ions (negatively charged nitrate ions), which leads to production of product ions (negatively charged bisulfate ions). The ratios of the concentrations of product ions to reagent ions are related to the sulfuric acid concentrations by the ion-molecule reaction rate and the exposure time. These ratios are determined by mass analysis of reagent and product ions. The ions are sampled from the inlet into a vacuum chamber which is designed to remove most of the bath gas while directing the ions in a beam to the entrance of a quadrupole filter. It selects ions based on their mass, and those ions thus allowed to pass through are counted by a channel electron multiplier detector. The count rates for the product and reagent ion masses are proportional to the ion concentrations. Ultimately, the ratios of reagent ion to product ion count rates are related to peroxy radical concentrations entering the inlet via calibrations. They are accomplished by quantitatively phototlyzing water vapor in the presence of small amounts of hydrogen or methane. In the former case, two HO 2 radicals are produced per water vapor molecule photolyzed, while in the latter case one HO 2 and one RO 2 are formed. This calibration method requires accurate knowledge of the water vapor concentration, the intensity of the photolyzing light, and the water vapor absorption cross section. Specific improvements to the instrument are discussed below.
PeRCIMS was deployed in two field campaigns this year. It was installed on the NSF/NCAR C-130 aircraft as part of the ACD POP section's 4-channel CIMS system. This complex and lengthy installation was followed by a series of test flights. The aircraft was deployed to Veracruz , Mexico in late February, 2006, and from that base, studies of the evolution of Mexico City atmospheric emissions were conducted as part of the MIRAGE-Mex/MILAGRO project (funded by NSF and NASA). The campaign ended in late March after 14 flights. In mid-April, the aircraft ferried to Paine Field near Seattle , Washington . From there, studies of aged Asian emissions were done as part of INTEX-B (funded primarily by NASA). It concluded in mid-May after 12 flights.
This research contributes to the ACD Theme of Regional and Global Air Quality, although the instrument developments will contribute to future studies of the UT/LS region in studies such as DC3.
Instrument improvements
In past deployments of PeRCIMS, HO 2 and RO 2 were separated by varying the concentration of NO and SO 2 reagents within the inlet. This separation comes about because at high NO to oxygen ratios, the reaction of NO with RO radicals competes with the reaction of O 2 with RO radicals. The former halts further conversion of RO 2 , while the latter leads to formation of HO 2 most of the time. At low NO to oxygen ratios, RO 2 radicals are measured with nearly the same efficiency as HO 2 . To achieve the required NO to oxygen ratios, pure NO and SO 2 reagents were used. Not only were the pure reagents potentially dangerous, they limited the speed at which the instrument could be switched between the two modes of operation.
European colleagues working on a similar approach to measure peroxy radicals, suggested that varying the oxygen concentration might be a better way to modulate the NO to oxygen ratio. It was found that the sensitivity could be reduced by about a factor of three by alternately diluting the air sample with oxygen or nitrogen at the highest dilutions practical (about 3 parts diluent to 1 part air).
The combination of dilution with oxygen or nitrogen and varying the reagent gas concentrations seemed a likely way to eliminate the use of pure gases, avoid high dilutions and thus higher detection limits, and achieve better separation of HO 2 and RO 2 . Through many laboratory experimental tests, it was determined that conditions could be found in which reagent flows were changed by about a factor of five along with dilution by oxygen or nitrogen at about a 1:1 ratio led to adequate separation of HO 2 and RO 2 . We estimate that about 85% of CH 3 O 2 is measured in the HO 2 + RO 2 mode (low NO to oxygen ratio), while less than 15% is measured in the HO 2 mode (high NO to oxygen ratio).
Implementation of this oxygen dilution modulation method of radical separation required modifications to the inlet and careful monitoring of flow controller calibration. In the most recent deployments, the reagent and diluent flows were dynamically varied with altitude to maintain constant mixing ratios within the inlet.
Field campaigns
The PeRCIMS inlet channel was deployed during two field campaigns this year. First, as part of the ACD-organized MIRAGE campaign, peroxy radical observations were conducted aboard the NSF/NCAR C-130 aircraft based out of Veracruz , Mexico . Flights were planned to study the emission plume of Mexico City both close-in and at some distance downwind. Field data were generated immediately after each flight and submitted to the archive. In September, preliminary data were submitted.
Second, the C-130 aircraft payload was deployed to Paine Field near Seattle , Washington . Asian emissions were studied in conjunction with the NASA DC-8 aircraft that was based in Hawaii and Anchorage. Because of the long distances, these emissions were either extremely aged or had been rapidly transported across the Pacific at higher altitudes.
Analysis will be conducted over the next couple of months in preparation for a science workshop in late October. In both field campaigns, we conducted a number of calibrations on the ground, and also performed secondary calibrations while in flight.
Post field-campaign characterizations
After the campaigns, and after moving our operation to the FL0 laboratory, a number of post-campaign tests were conducted to verify the proper operation of the instrument. These included a full range of actinometry experiments to quantify the intensity of the water photolysis lamp, and radical calibrations over the range of conditions encountered during the campaigns including sensitivities as a function of ambient pressure. Data from these experiments were used to finalize calibration factors used in the treatment of the data.
HIAPER instrument development
The new Gulfstream-V NSF/NCAR aircraft HIAPER was delivered a couple of years ago, and is gradually becoming more available for research activities. Over the next couple of years, several of the HAIS instruments will be completed that will make it even more valuable. Missing from the planned suite of instruments is an OH-HO 2 instrument. Significant effort has been spent to try to secure funds to build light-weight, automated versions of PeRCIMS and the OH SICIMS instruments. Great progress has been made on a plan to build such instruments, but although requests from numerous sources have been made, no awards for this project have been made. Efforts will continue since measurement of HO x radicals is central to projects on the horizon for HIAPER.
Other activities
FL0 move. A significant amount of time and some funds were spent completing the move of the POP section into Foothills Laboratory Zero (FL0). This included organizing items so they can be readily found, and completing plumbing, electrical and other installations for proper use.
DC3 planning. ACD and the UTLS Initiative are leading the organization and implementation of a detailed field campaign to study the chemistry and dynamics of convection including the impact of lightning and heterogeneous chemistry on the composition of the upper troposphere. It is envisioned that the campaign will make heavy use of HIAPER as well as at least one other aircraft. A workshop was conducted in which interested parties from the scientific community were invited to Boulder to help formulate the plans for the experiment. Work has continued through working groups, and will culminate in Scientific and Experimental documents submitted to NSF in January 2007.
Analysis of previous airborne campaigns. Under a NASA grant, analysis of data from previous airborne campaigns has continued. The idea is to use a common numerical model to compare against data collected over a series of campaigns to see if our understanding of radical behavior is consistent and to determine if particular situations (location, pollutant level, altitude) result in deviations from the model.
References:
Edwards, G. D., C. A. Cantrell, S. Stephens, B. Hill, O. Goyea, R. E. Shetter, R. L. Mauldin, E. Kosciuch, D. J. Tanner, F. L. Eisele, Chemical ionization mass spectrometer instrument for measurement of Tropospheric HO 2 and RO 2 , Analy. Chem. 75, 5317-5327, doi:10.1021/ac034402b, 2003.
An Analysis of the Fast Photochemistry in Several Previous Airborne Field Campaigns
Over the past ten years, there were a number of aircraft-based measurement campaigns in which odd hydrogen radicals (OH, HO 2 and RO 2 ) were measured. In the routine course of analysis of the results of these studies, photochemical models constrained by the available measured quantities were used to provide estimates of odd hydrogen radical levels. Over time, the suite of measured quantities has expanded and the model representation of the photochemistry has improved through better characterizations of the important processes.
The present study in progress involves the analysis of several of these studies using the same photochemical model mechanism to examine how our ability to quantify tropospheric odd hydrogen radicals has evolved over time. In particular, this work focuses on those instances in which the model cannot reproduce the radical observations and examines the chemical and physical characteristics of air masses in those situations.
Figures 1 and 2 show examples of products of this work. The measured to model ratio for OH (Figure 1) and HO 2 or HO 2 +RO 2 (Figure 2) are shown versus the observed NO x mixing ratio. Most of the time the model and measured values are within 30%, but there are instances (for example, at high NO x values) in which the observations are significantly greater than the model. This phenomenon has been previously reported in the analysis of several campaigns. It has been suggested that this could be due to the patchy nature of NO x at these high levels and the nonlinear connection between NO x and HO x that results in an apparent measure-model difference when using one-minute average data, but is less of a problem with faster data rates. Situations such as these will be examined in detail with the goal to better understand their causes and tp make recommendations for measurements in future campaigns as well as for photochemical model improvement.
Successful Implementation of a New Method of Analyzing Tropospheric Peroxy Radicals
The comprehensive understanding of oxidation processes in the troposphere can be tested through a combination of observations and numerical modeling. In addition to measurements of source gases (such as oxides of nitrogen and hydrocarbons) and products of atmospheric processing (such as acids and partially oxidized hydrocarbons), quantification of short-lived intermediates can be an important way to check our understanding of tropospheric photochemistry. Among these short-lived intermediates, the peroxy radicals play very important roles. When peroxy radicals react with nitric oxide, they lead to the formation of ozone.
Several methods have been used to measure tropospheric peroxy radicals, but most are restricted to either measuring the first member of the family, hydroperoxyl radical (HO 2 ), or the sum of HO 2 and the organic peroxy radicals (RO 2 ). The ACD/NCAR PeRCIMS instrument has successfully quantified HO 2 and HO 2 +RO 2 in previous field measurement campaigns, but the previous method had not allowed rapid change between the two measurement quantities. New research has lead to a method of measuring both values within one minute. The key breakthrough involved modulation of the concentration ratio of nitric oxide to oxygen within the instrument inlet. Oxygen concentrations are changed by diluting the atmospheric sample with either oxygen or nitrogen. At the same time, the nitric oxide concentration is varied by changing the reagent flow into the inlet. These changes alter the inlet chemistry such that either RO 2 can be measured with similar sensitivity to HO 2 (with high oxygen and low nitric oxide concentrations) or RO 2 measurement sensitivity is decreased to less than 15% of HO 2 . Laboratory and atmospheric sample data using this new method are shown in Figures 1 and 2, respectively. The laboratory data show measurement of a 50:50 mixture of HO 2 and CH 3 O 2 (the predominant peroxy radicals in the background troposphere) in the HO 2 mode (orange) and the HO 2 +RO 2 mode (blue). Other points are background signals. The ambient data show measurements of HO 2 +RO 2 (blue) and HO 2 (pink) as well as the ratio of HO 2 to HO 2 +RO 2 . This analytical method has been applied in several field campaigns in 2006.
Figure 1. Figure 2.
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