A42B-01 INVITED 10:20h
Reexamining HOx measurements from NASA airborne field campaigns: Lessons learned and future needs
The body of airborne HOx (OH and HO2) observations has grown considerably over recent years. Here, we focus on measurements collected during NASA airborne field campaigns. While results from particular campaigns have raised questions, particularly in the upper troposphere, little has been done to examine the current body of data in a single integrated analysis. In trying to establish consistency between data sets, observations are compared with calculations from a single photochemical model with the most recent reaction rates included. Minor corrections have also been applied to some of the earlier HOx measurements to account for the improved understanding of instrument sampling characteristics that has been established over time. Although this work is ongoing, early results have identified airmass heterogeneity as an issue impacting some previous conclusions regarding HOx. Specifically, model calculations based on 1-minute averages for some flight data differ dramatically from results modeled on much shorter time-scales. There is also some indication that bulk statistical approaches to comparing model calculations and measurements may be influenced by the range of solar zenith angle or primary HOx production encountered. Finally, the buffering of HOx abundance against large changes in production and loss indicates the need for improved measurements of other species (e.g., CH2O) that are more sensitive to changes in photochemical conditions. Current progress on these topics will be presented.
A42B-02 10:50h
Evaluation of the Role of Acetone on HOx Chemistry in the Upper Troposphere
Several studies have indicated that photolysys of acetone atconcentrations of order hundreds of ppt could enhance total HOx (OH+HO2) production by up to 30 percent in the upper troposphere (UT). However, recent measurements of the acetone quantum yield at low temperatures (Blitz et al., Geophysical Research Letters, 31, L06111, doi:10.1029/2003GL018793, 2004) obtain significantly smaller values than previous temperature-independent measurements. In this work, we reevaluate the impact of acetone on HOx production in the UT. Calculations are performed with a photochemical box model constrained by aircraft measurements to estimate the sources of HOx for the Pacific Exploratory Mission-Tropics B (PEM-Tropics B). We carry out a total of ~1400 box model calculations, constrained by input from a merged file of aircraft measurements at a 1 minute resolution. Results show that the calculated contribution of acetone to HOx production is smaller with the new acetone quantum yield, as expected. The median percent contribution of acetone photolysis to HOx production is now more than a factor of 2 smaller than the contribution calculated with the old quantum yield. We present details of the role of acetone as a function of altitude and latitude, and extend our analysis to other aircraft campaigns, in order to assess our current understanding of upper tropospheric HOx chemistry in light of the recent kinetic data.
A42B-03 11:05h
Evaluation of Lightning NOx Treatment in the GMI Model and its Effect on Upper Tropospheric NOx and O3
Ideally, model simulations of tropospheric ozone specify the lightning odd nitrogen (NOx) source in a manner that is consistent in time and space with the convective transport of ozone precursors. This consistency can be achieved if the lightning NOx distribution is parameterized in terms of convective fields from the driving GCM and/or data assimilation system (DAS). The Global Modeling Initiative (GMI) off-line tropospheric chemistry model can be driven by meteorological fields from a variety of GCMs and/or DASs. Therefore, it is an ideal tool to explore the sensitivity of trace gas distributions to the driving meteorological model. In this experiment, we explore the sensitivity of NOx and ozone to the the lightning NOx distribution. The method used to parameterize flash rates in the GMI model will be discussed. Lightning NOx production in the GMI model can be specified in terms of climatological convective cloud top heights from ISCCP or in terms of six-hour averaged and normalized upward convective mass fluxes from the driving meteorological model (GEOS DAS, NCAR CCM3, or GISS GCM). Parameterized lightning distributions from simulations driven by each of these models will be compared with the ISCCP-based flash rate distribution and with lightning observations. The sensitivity of upper tropospheric ozone to the distribution of lightning NOx will be evaluated for simulations with ISCCP-based and meteorological model-based lightning distributions.
A42B-04 11:20h
Characterization of a TD-CIMS instrument for aircraft measurements of PeroxyAcetyl Nitrate (PAN) compounds
Results are presented for a detailed characterization of a Thermal Dissociation - Chemical Ionization Mass Spectrometer (TD-CIMS) method for the atmospheric measurement of Peroxyacetyl Nitrate (PAN, CH3C(O)O2NO2) compounds on board the NOAA P3 research aircraft. A detailed laboratory characterization of the TD-CIMS instrument was carried out in preparation for deployment on a research aircraft. The TD-CIMS was cross-calibrated against a dual channel fast GC/ECD and a total nitrogen oxide (NOy) instrument to determine calibration factors and relative sensitivities for the PAN homologues (PAN, PPN, PiBN, APAN, and MPAN). In addition, the PAN homologues were produced in a 50 liter reaction chamber from which the TD-CIMS was cross calibrated against a long path FTIR connected to the reaction chamber. Some novel PAN homologues (1-hydroxyl-peroxyacetyl nitrate and methoxyformyl nitrate) were also produced in the chamber to investigate the TD-CIMS measurement capabilities. Discussion will highlight the capabilities of the new TD-CIMS and its performance during the recent New England Air Quality Study as part of the 2004 ICARTT intensive. Based on the TD-CIMS performance during the ICARTT campaign it will also be demonstrated that the fast response and high sensitivity of this method is ideal for aircraft platforms.
http://tropchem.al.noaa.gov/
A42B-05 11:35h
Atmospheric Analysis by FT-IR: Ground-Based Solar Absorption Spectroscopy and Open-Path Techniques
Knoxville and East Tennessee air-quality is notable disrepute. The high quantity of pollutants in the local air, especially ozone, is of great concern, particularly as regards the nearby Great Smoky Mountains National Park. A Bomem DA8 FT-IR is used to acquire infrared absorption spectra of the atmosphere. Integration of the absorption spectrometer with various optical systems provides a unique opportunity to analyze the local atmospheric chemical composition. This is done by directing solar radiation into the FT-IR with the use of a sun-tracking system, and also by directing light from the lab and back along a path through open air. Many trace atmospheric constituents are open to this analysis. Vertical concentration profiles of nitrous oxide are determined by fitting solar absorbance lines with SFIT2. Improved fitting of solar spectra has been demonstrated by incorporating the tropospheric nitrous oxide concentration as determined by open-path measurements. Since it is of special interest, tropospheric ozone production and analysis will also be discussed. Seasonal and daily trends of troposheric ozone abundance show correlation with other sources such as the EPA, and recent efforts to correlate solar spectra with open-path spectra will be discussed.
A42B-06 11:50h
Smart Balloon Observations of the Distribution of Tropospheric O$_{3}$ Over the North Atlantic
A series of low-altitude balloon flights were conducted over the North Atlantic during the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) field campaign in July/August 2004. We utilized NOAA's Smart Balloon platform, which carried a suite of meteorological sensors and a miniature O$_{3}$ instrument. We launched four balloons from the northern tip of Long Island that traveled distances ranging from 570 to 6800 km. One of the balloons crossed the Atlantic to the African continent after a 12 day journey. Data was collected with a time resolution of 10 seconds using an Iridium satellite telephone connection. The largest O$_{3}$ mixing ratios were measured directly downwind of New England over the Gulf of Maine at 550 m altitude. The peak value of 195 ppbv was observed during flight 02 on the night of July 20. This flight lasted 49 hours, and ended just north of Prince Edward Island with a median O$_{3}$ mixing ratio of 72 ppbv. During the transatlantic flight, O$_{3}$ mixing ratios were measured over altitudes of 500 to 3400 m and ranged from 30 to 155 ppbv. These highly successful flights indicate that autonomous platforms and low-cost sensors can contribute significantly to our understanding of tropospheric chemistry and the impact of polluted continental outflow on air quality over remote locations.
A42B-07 12:05h
Airborne Formaldehyde and Formic Acid Measurements Using a Pulsed QC Laser During NEAQS/ITCT 2004
We present our quantum cascade laser (QCL) measurements of atmospheric formaldehyde (HCHO) and formic acid (HCOOH) performed on the NOAA P3 aircraft as part of the New England Air Quality Study / Intercontinental Transport and Chemical Transformation project in July-August 2004. Formaldehyde is a key species for studies of photochemical oxidation pathways in the troposphere. Formaldehyde is made throughout the troposphere via photo-initiated oxidation of methane and other hydrocarbons. It is also directly emitted by urban combustion sources including jet aircraft engines, cold-start automobiles, compressed natural gas buses, and other gasoline vehicles without a functioning oxidation catalyst. In the remote troposphere, formaldehyde is a photolytic source of HOx. Secondary organic aerosol mass loading can be attributed to the oxidation of precursor VOCs and HCHO is a valuable indicator of photochemical activity in an aging air mass. The capability to perform airborne formaldehyde measurements in the troposphere is a valuable tool and these measurements can help further our understanding of several atmospheric processes. We detect formaldehyde using Tunable Infrared Laser Differential Absorption Spectroscopy (TILDAS). TILDAS provides sensitive, specific detection with a rapid time response (1 s) that makes it especially suitable for airborne measurements. The recent availability of quantum cascade lasers has led to smaller, more compact TILDAS instruments than has been previously possible using lead-salt tunable diode lasers (TDLs). QCLs operate near-room temperature in pulsed mode which greatly simplifies the deployment logistics for aircraft operation. QCLs have superior spectral mode stability which minimizes operator interaction during flight deployments and offers greater potential for automated operation. In July and August of 2004, we deployed a quantum cascade laser system aboard NOAA's P3 aircraft as part of the New England Air Quality Study (NEAQS). This was our initial experience measuring HCHO and HCOOH. Tropospheric concentrations of HCHO were measured in flight with three different lasers at frequencies 1765, 1774 and 1783 cm$^{-1}$. Each of these wavelength regions allowed a simultaneous measurement of formic acid. This campaign represented the first flight-based measurement of any species using a pulsed quantum cascade laser for mid IR absorption spectroscopy. The aircraft instrument used a heated PTFE inlet system and a formaldehyde-free zero air generator based on the design of the NCAR TDL HCHO system to obtain background spectra. Though the detection method itself is based on an absolute Beer's Law absorption technique, a permeation source of HCHO was added near the top of the sample inlet to characterize inlet effects and provide additional confidence in the measured concentration. We will present measurement results from the campaign including formaldehyde and formic acid profiles measured during transects of forest fire and power plant plumes as well as mapping of the New York City urban outflow plume. We will also present the results of inter-comparisons with other airborne formaldehyde instruments and the relationship of our measurements to those of other atmospheric species (including acetic acid and acetaldehyde).