Enhancement of a Commercial Cavity Ring-Down Spectrometer for Detection of NO2
Nitrogen dioxide (NO2) plays a key role in atmospheric chemistry and biogeochemical cycles. In polluted atmospheres where photochemical smog is a problem, monitoring of NO2 is necessary to understand the effects of emissions control strategies, to monitor progress in compliance with air quality standards, and to understand the formation and loss of ozone. There is also a need for NO2 vertical profile measurements for satellite validation and to analyze pollution transport in models. Many analytical techniques have been developed to detect NO2 in the atmosphere, but only chemiluminescence is widely commercially available. There remains a need for a fast, light, simple method to directly measure NO2 in the field. In this work we present the modification and characterization of a small commercially available cavity ring-down spectroscopy (CRDS) NO2 detector suitable for surface monitoring and aircraft work (Los Gatos Research, Inc.). A metal oxide scrubber was added to effectively remove NO2, and provide a zero. This enhanced the sensitivity from several parts per billion by volume (ppbv) to 0.2 ppbv integrated over 1 minute. Known interferences by water and particles were removed using Nafion tubing and a 1 μm filter with minimal line losses. A response time of 10 seconds was calculated from a step change in concentration. Ambient measurements were taken alongside a chemiluminescence device with a photolytic converter to evaluate the method with ambient air. The CRDS instrument had a negative 4% bias, and the two methods were highly correlated (R2 = 0.99) over the range of 3 to 50 ppbv.
Characterizing a Quantum Cascade Tunable Infrared Laser Differential Absorption Spectrometer (QC-TILDAS) for Measurements of Atmospheric Ammonia
A compact, fast response Quantum Cascade Tunable Infrared Laser Differential Absorption Spectrometer (QC- TILDAS) for measurements of ammonia has been evaluated under both laboratory and field conditions. Absorption of radiation from a pulsed, thermoelectrically cooled QC laser occurs at reduced pressure in a 76 m path length, 0.5 L volume multiple pass absorption cell. Detection is achieved using a thermoelectrically cooled HgCdTe infrared detector. A novel sampling technique was used, consisting of a short, heated, quartz inlet with a hydrophobic coating to minimize the adsorption of ammonia to surfaces. The inlet contains a critical orifice that reduces the pressure, a virtual impactor for separation of particles and additional ports for delivering ammonia free background air and calibration gas standards. This instrument has been found to have a detection limit of 0.3 ppb with a time resolution of 1 s. The sampling technique has been compared to the results of a conventional lead salt Tunable Diode Laser (TDL) absorption spectrometer during a laboratory intercomparison. Various lengths and types of sample inlet tubing material, heated and unheated, under dry and ambient humidity conditions with ammonia concentrations ranging from 10-1000 ppb were investigated. Preliminary analysis suggests the time response improves with the use of short, PFA tubing sampling lines. No significant improvement was observed when using a heated sampling line and humidity was seen to play an important role on the bi-exponential decay of ammonia. A field intercomparison of the QC-TILDAS with a modified Thermo 42C chemiluminescence based analyzer was also performed at Environment Canada's Centre for Atmospheric Research Experiments (CARE) in the rural town of Egbert, ON between May-July 2008. Background tests and calibrations using two different permeation tube sources and an ammonia gas cylinder were regularly carried out throughout the study. Results indicate a very good correlation (r2>0.9) between the two instruments at the beginning of the study, when regular background subtraction was applied to the QC- TILDAS.
Measurement of Black (BC) and Elemental (EC) Carbon by an Optical and a Thermal- optical Method: An Intercomparison
Elemental or Black carbon (EC or BC) aerosol emitted into the atmosphere from incomplete combustion of fossil fuel, biomass and forest fires absorbs solar radiations and contributes to global warming. EC or BC is defined based on different analytical methods used for measuring the same fraction of carbonaceous aerosol. The different methods give different results which can vary widely. There is no accepted standard available to accurately quantify EC and therefore measurements between different methods need to be compared to reduce the bias. In this study intercomparison of data between two widely used techniques, BC obtained using optical method (non-destructive technique) and EC obtained using thermal-optical methods (destructive technique) are performed on aerosol samples collected on Whatman 41 filter paper from two rural sites, Whiteface Mountain (WFM) and Mayville, NY. Daily aerosol samples from six months at Mayville collected during 1998 and 2002 and fourteen months from WFM collected during 1996 and 2002 were analyzed using Sunset thermal-optical transmittance (TOT) elemental/organic carbon (EC/OC) analyzer and Magee Scientific Transmissiometer (Model OT-21). Total numbers of samples analyzed from the two sites were around 400. Transmissiometer used for BC measurement is based on optical attenuation of light and its working principle is similar to that of widely used Aethalometer. Whatman 41 filters are not suitable for direct EC measurement using EC/OC analyzer, so a pretreatment technique was developed and EC was subsequently transferred on 47 mm quartz filter paper. The total analysis time for individual sample using Transmissiometer is short (∼ 3-5 min) compared with ∼ 20-25 minutes for EC/OC analyzer excluding the time required for chemical pre-treatment (which can be up to 60 min). Reasonably good correlation, r2>0.8 and BC/EC slope close to 1 was obtained for concentrations up to 600 ngm- 3. For concentration >600 ngm-3 the relationship tends to deviate from linearity with BC values biased low. Currently more samples are being analyzed to arrive at firm conclusions. Good correlation obtained between EC and BC measurement could greatly reduced the analysis time for individual measurement from remote to moderate urban environment and the aerosol samples can be saved for measuring other atmospheric species.
Retrieval of Vertical Columns of Sulfur Dioxide From SCIAMACHY and OMI: Air Mass Factor Algorithm Development and Validation
Sulfur dioxide (SO2) is released into the atmosphere as a result of both anthropogenic activities and natural phenomena. SO2 oxidizes rapidly in the atmosphere, leading to aerosol formation and acid deposition. Outstanding questions exist about SO2 emissions and its atmospheric chemistry. Global mapping of atmospheric SO2 concentrations can provide critical information on its emissions and transport and generally improve scientific understanding of its atmospheric chemistry. Here, we present an improved retrieval of sulfur dioxide (SO2) vertical columns from satellite instruments (SCIAMACHY and OMI) that measure solar backscattered UV radiance. Particular attention is devoted to development of a local air mass factor (AMF) algorithm to convert slant columns to vertical columns. For each SCIAMACHY and OMI observation, we calculate an AMF from the relative vertical SO2 distribution (shape factor) determined locally with a 3-D global model of atmospheric chemistry (GEOS-Chem), weighted by altitude-dependent scattering weights computed with a radiative transfer model (LIDORT). Seasonal mean instrument sensitivity to SO2 (AMF) is generally twice as high over ocean than land. Mineral dust can reduce seasonal mean instrument sensitivity by 50%. Mean relative vertical profiles of SO2 simulated with GEOS-Chem and used in the AMF calculation are highly consistent with airborne in situ measurements (INTEX-A and INTEX-B); differences would affect the retrieved SO2 columns by 10%. The retrieved vertical columns are validated (r = 0.9) with coincident airborne in-situ measurements (INTEX-A, INTEX-B, and a campaign over East China). A global uniform AMF would reduce the correlation with aircraft measurements by 0.1 - 0.2. The overall error assessment leads to 45 - 80% errors for yearly averages over the polluted regions. Seasonal mean SO2 columns retrieved from SCIAMACHY and OMI for 2006 are significantly spatially correlated with those from GEOS-Chem, in particular over the United States (r = ∼0.85 for SCIAMACHY and r = ∼0.82 for OMI). Differences in other regions such as South Africa, Nigeria, and the Persian Gulf imply underestimates in emissions.
Trace Gases in the Arctic Atmosphere: Spring Measurements Using the Portable Atmospheric Research Interferometric Spectrometer for the Infrared (PARIS-IR) and the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS)
The first light the High Arctic atmosphere experiences after months of polar night triggers chemical reactions involved in stratospheric ozone depletion. This time of year is also dynamically active, leading to highly variable conditions throughout the middle atmosphere. The differences in the structure of the polar vortex from year to year affect the concentrations of trace gases observed from both the ground and space. The Atmospheric Chemistry Experiment (ACE) is a satellite mission on-board the Canadian satellite SCISAT. ACE is being used to understand the chemical and dynamical processes controlling middle atmosphere ozone distribution in addition to the coupling between chemical processes and climate change, particularly in the Arctic. The primary instrument on SCISAT is the ACE-FTS, a high resolution Fourier Transform Spectrometer. Each spring since 2004, the ground-based version of the ACE-FTS, the Portable Atmospheric Research Interferometric Spectrometer for the Infrared (PARIS-IR) has been deployed at the Polar Environment Atmospheric Research Laboratory (PEARL) in Eureka, Nunavut (80N, 86W) as part of the Canadian Arctic ACE Validation Campaign project. PEARL is ideally located for these campaigns as a large number of ACE overpasses occur between February and March. PARIS-IR is one of eleven ground-based and balloon-borne instruments used in the campaigns each year. It records double-sided interferograms with the same maximum optical path difference (25 cm) as ACE-FTS, resulting in a resolution of 0.02 cm-1. PARIS-IR is designed to measure the full 750 - 4400 cm-1 spectral range with each measurement. This feature allows total column measurements of a range of atmospheric species to be determined from every spectral measurement, creating a data set with high temporal resolution. This provides an opportunity to capture short term variability. PARIS-IR records infrared solar absorption spectra of the atmosphere. Total columns of ozone and related trace gas species (including HCl, HNO3 and HF) are retrieved and retrievals of additional species are under development. Results from 2007 and 2008 and preliminary results from 2009 will be presented and interpreted relative to the varied dynamical conditions observed in each year.
Satellite Retrieval and Ground Based Measurements of NO2
The Ozone Monitoring Instrument aboard the NASA EOS-Aura satellite has been making daily global measurements of a wide range of trace atmospheric chemical species since November 2004. As an important component of anthropogenic air pollution, nitrogen dioxide (NO2) is among the more significant trace gases retrieved from OMI. Its principal sources are fossil fuel and biomass combustion, and so elevated levels are often seen around urban areas, coal-fired power plants, and agricultural burning. The NO2 retrieval algorithm rests on a set of assumptions about the vertical profiles of NO2 concentration in the atmosphere: these were obtained from chemistry and transport models that use emissions inventories as input. The algorithm is known to be rather sensitive in certain parameter regimes. The algorithm is being revised, with a new data release expected to be made later this year. In addition to an updated set of climatological NO2 profiles, the algorithm will use a database of surface reflectivity derived from OMI data. Toronto is an industrial and urban center in a relatively flat area, in which the Ontario Ministry of the Environment operates a network of in situ monitoring instruments, each a few meters above the surface. We present a statistical analysis of the OMI-retrieved NO2 using the agreement between OMI and the in situ measurements as an avenue to characterizing the sensitivity of the OMI product to various parameters in the retrieval, and to help characterize the change in the product due to the algorithm revision.