Atmospheric Trace Constituents Research Articles

Chemistry of riming: the retention of organic and inorganic atmospheric trace constituents

Abstract. During free fall in clouds, ice hydrometeors such as snowflakes and ice particles grow effectively by riming, i.e., the accretion of supercooled droplets. Volatile atmospheric trace constituents dissolved in the supercooled droplets may remain in ice during freezing or may be released back to the gas phase. This process is quantified by retention coefficients. Once in the ice phase the trace constituents may be vertically redistributed by scavenging and subsequent precipitation or by evaporation of these ice hydrometeors at high altitudes. Retention coefficients of the most dominant carboxylic acids and aldehydes found in cloud water were investigated in the Mainz vertical wind tunnel under dry-growth (surface temperature less than 0 °C) riming conditions which are typically prevailing in the mixed-phase zone of convective clouds (i.e., temperatures from −16 to −7 °C and a liquid water content (LWC) of 0. 9 ± 0. 2 g m−3). The mean retention coefficients of formic and acetic acids are found to be 0. 68 ± 0. 09 and 0. 63 ± 0. 19. Oxalic and malonic acids as well as formaldehyde show mean retention coefficients of 0. 97 ± 0. 06, 0. 98 ± 0. 08, and 0. 97 ± 0. 11, respectively. Application of a semi-empirical model on the present and earlier wind tunnel measurements reveals that retention coefficients can be well interpreted by the effective Henry's law constant accounting for solubility and dissociation. A parameterization for the retention coefficients has been derived for substances whose aqueous-phase kinetics are fast compared to mass transport timescales. For other cases, the semi-empirical model in combination with a kinetic approach is suited to determine the retention coefficients. These may be implemented in high-resolution cloud models.

Modeling of gas adsorption by aerosol plumes emitted from industrial sources

Abstract Adsorption of trace atmospheric gases by aerosol particles contributes to the evolution of concentration distribution of the trace constituents and can affect subsequent chemical reactions in the atmosphere. We suggest a two dimensional model of adsorption of trace atmospheric constituents by aerosol particles in air pollution plume emitted from an industrial source. The model is based on an application of theory of turbulent diffusion in the atmospheric boundary layer (ABL) in conjunction with plume dispersion model and model of gas adsorption by porous solid particles. The wind velocity profiles used in the simulations were fitted from our data previously obtained in field measurements conducted in the Northern Negev (Israel) using the experimental wind mast. The developed model allows analyzing spatial and temporal evolution of adsorbate concentration in the gaseous phase as well as in the particulate matter. The adsorbate concentration distributions are calculated for the particulate matter PM2.5–10, which is typical for industrial emissions. Analysis is performed for different meteorological conditions and atmospheric stability classes. It is shown that the concentration of gases adsorbed by aerosol plume strongly depends on the level of atmospheric turbulence. The obtained results are compared with the available experimental data.

An assessment of 10-year NOAA aircraft-based tropospheric ozone profiling in Colorado

The Global Greenhouse Gas Reference Network Aircraft Program at NOAA has sampled ozone and other atmospheric trace constituents in North America for over a decade (2005-present). The method to derive tropospheric ozone climatology from the light aircraft measurements equipped with the 2B Technology instruments is described in this paper. Since ozone instruments at most of aircraft locations are flown once a month, this raises the question of whether the sampling frequency allows for deriving a climatology that can adequately represent ozone seasonal and vertical variability over various locations. Here we interpret the representativeness of the tropospheric ozone climatology derived from these under-sampled observations using hindcast simulations conducted with the Geophysical Fluid Dynamics Laboratory chemistry-climate model (GFDL-AM3). We first focus on ozone measurements from monthly aircraft profiles over the Front Range of Colorado and weekly ozonesondes launched in Boulder, Colorado. The climatology is presented as monthly values separated in 5th, 25th, 50th, 75th, 95th percentiles, and averaged at three vertical layers: lower (1.6–3 km), middle (3–6 km), and upper (6–8 km) troposphere. The aircraft-based climatology is compared to the climatology derived from the nearest located ozonesondes launched from Boulder, Colorado, from GFDL-AM3 co-sampled in time with in-situ observations, and from GFDL-AM3 continuous 3-h samples. Based on these analyses, we recommend the sampling frequency to obtain adequate representation of ozone climatology in the free troposphere. The 3-h sampled AM3 model is used as a benchmark reference for the under-sampled time series. We find that the minimal number of soundings required per month for the all altitude bins (1.6–3, 3–6, and 6–8 km) to sufficiently match the 95% confidence level of the fully sampled monthly ozone means vary between 3 and 5 sounding per month, except in August with a minimum of 6 soundings per month. The middle altitude bin required the least number of samplings per month. We determine the reasonably good agreement between the ozonesondes and aircraft measurements near Boulder suggest that valuable climatologies could be developed from the aircraft sites where no ozonesondes exist even though the aircraft measurements are more limited in number than the ozonesondes. When averaged over a number of years the aircraft data provide valuable information. More frequent sampling could tell us more but the measurements given would indicate that they can provide interesting climatological results.

Daytime formation of nitrous acid at a coastal remote site in Cyprus indicating a common ground source of atmospheric HONO and NO

Abstract. Characterization of daytime sources of nitrous acid (HONO) is crucial to understand atmospheric oxidation and radical cycling in the planetary boundary layer. HONO and numerous other atmospheric trace constituents were measured on the Mediterranean island of Cyprus during the CYPHEX (CYprus PHotochemical EXperiment) campaign in summer 2014. Average volume mixing ratios of HONO were 35 pptv (±25 pptv) with a HONO ∕ NOx ratio of 0.33, which was considerably higher than reported for most other rural and urban regions. Diel profiles of HONO showed peak values in the late morning (60 ± 28 pptv around 09:00 local time) and persistently high mixing ratios during daytime (45 ± 18 pptv), indicating that the photolytic loss of HONO is compensated by a strong daytime source. Budget analyses revealed unidentified sources producing up to 3.4 × 106 molecules cm−3 s−1 of HONO and up to 2.0 × 107 molecules cm−3 s−1 NO. Under humid conditions (relative humidity > 70 %), the source strengths of HONO and NO exhibited a close linear correlation (R2 = 0.72), suggesting a common source that may be attributable to emissions from microbial communities on soil surfaces.

Detection of trends in surface ozone in the presence of climate variability

AbstractTrends in trace atmospheric constituents can be driven not also by trends in their (precursor) emissions but also by trends in meteorology. Here we use ground‐level ozone as an example to highlight the extent to which unforced, low‐frequency climate variability can drive multidecadal trends. Using output from six experiments of the Geophysical Fluid Dynamics Laboratory chemistry‐climate model (CM3), we demonstrate that 20 year trends in surface ozone driven by climate variability alone can be as large as those forced by changes in ozone precursor emissions or by anthropogenic climate change. We highlight regions and seasons where surface ozone is strongly influenced by climate variability and thus where a given forced trend may be more difficult to detect. A corollary is that this approach identifies regions and seasons of low variability, where measurement sites may be most effectively deployed to detect a particular trend driven by changing precursor emissions. We find that the representative concentration pathways 4.5 (RCP4.5) and RCP8.5 forced surface ozone trends in most locations emerge over background variability during the first half of the 21st century. Ozone trends are found to respond mostly to changes in emissions of ozone precursors and unforced climate variability, with a comparatively small impact from anthropogenic climate change. Thus, attempts to attribute observed trends to regional emissions changes require consideration of internal climate variability, particularly for short record lengths and small forced trends.

Airborne measurements and emission estimates of greenhouse gases and other trace constituents from the 2013 California Yosemite Rim wildfire

This paper presents airborne measurements of multiple atmospheric trace constituents including greenhouse gases (such as CO2, CH4, O3) and biomass burning tracers (such as CO, CH3CN) downwind of an exceptionally large wildfire. In summer 2013, the Rim wildfire, ignited just west of the Yosemite National Park, California, and burned over 250,000 acres of the forest during the 2-month period (17 August to 24 October) before it was extinguished. The Rim wildfire plume was intercepted by flights carried out by the NASA Ames Alpha Jet Atmospheric eXperiment (AJAX) on 29 August and the NASA DC-8, as part of SEAC4RS (Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys), on 26 and 27 August during its intense, primary burning period. AJAX revisited the wildfire on 10 September when the conditions were increasingly smoldering, with slower growth. The more extensive payload of the DC-8 helped to bridge key measurements that were not available as part of AJAX (e. g. CO). Data analyses are presented in terms of emission ratios (ER), emission factors (EF) and combustion efficiency and are compared with previous wildfire studies. ERs were 8.0 ppb CH4/(ppm CO2) on 26 August, 6.5 ppb CH4 (ppm CO2)1 on 29 August and 18.3 ppb CH4 (ppm CO2)1 on 10 September 2013. The increase in CH4 ER from 6.5 to 8.0 ppb CH4/(ppm CO2) during the primary burning period to 18.3 ppb CH4/(ppm CO2) during the fire's slower growth period likely indicates enhanced CH4 emissions from increased smoldering combustion relative to flaming combustion. Given the magnitude of the Rim wildfire, the impacts it had on regional air quality and the limited sampling of wildfire emissions in the western United States to date, this study provides a valuable dataset to support forestry and regional air quality management, including observations of ERs of a wide number of species from the Rim wildfire.

High-resolution and Monte Carlo additions to the SASKTRAN radiative transfer model

Abstract. The Optical Spectrograph and InfraRed Imaging System (OSIRIS) instrument on board the Odin spacecraft has been measuring limb-scattered radiance since 2001. The vertical radiance profiles measured as the instrument nods are inverted, with the aid of the SASKTRAN radiative transfer model, to obtain vertical profiles of trace atmospheric constituents. Here we describe two newly developed modes of the SASKTRAN radiative transfer model: a high-spatial-resolution mode and a Monte Carlo mode. The high-spatial-resolution mode is a successive-orders model capable of modelling the multiply scattered radiance when the atmosphere is not spherically symmetric; the Monte Carlo mode is intended for use as a highly accurate reference model. It is shown that the two models agree in a wide variety of solar conditions to within 0.2 %. As an example case for both models, Odin–OSIRIS scans were simulated with the Monte Carlo model and retrieved using the high-resolution model. A systematic bias of up to 4 % in retrieved ozone number density between scans where the instrument is scanning up or scanning down was identified. The bias is largest when the sun is near the horizon and the solar scattering angle is far from 90°. It was found that calculating the multiply scattered diffuse field at five discrete solar zenith angles is sufficient to eliminate the bias for typical Odin–OSIRIS geometries.

The mechanical and thermal setup of the GLORIA spectrometer

Abstract. The novel airborne Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) measures infrared emission of atmospheric trace constituents. GLORIA comprises a cooled imaging Fourier transform spectrometer, which is operated in unpressurized aircraft compartments at ambient temperature. The whole spectrometer is pointed by the gimbal towards the atmospheric target. In order to reach the required sensitivity for atmospheric emission measurements, the spectrometer optics needs to operate at a temperature below 220 K. A lightweight and compact design is mandatory due to limited space and high agility requirements. The cooled optical system needs to withstand high pressure and temperature gradients, humidity, and vibrations. A new cooling system based on carbon dioxide and liquid nitrogen combined with high-performance insulation has been developed to meet the mechanical, thermal, and logistical demands. The challenging mechanical and spatial requirements lead to the development of a novel rigid linear slide design in order to achieve the large optical path difference for high spectral resolution. This paper describes the mechanical and thermal setup of GLORIA and presents the performance results on two different research aircrafts.

Seasonal ambient ammonia and ammonium concentrations in a pilot IMPROVE NHx monitoring network in the western United States

Ammonia and ammonium are important atmospheric trace constituents that affect particulate matter concentrations and contribute to reactive nitrogen deposition. We refer to and measure the sum of ammonia and ammonium as NHx. To better understand concentrations of NHx in remote areas, the Interagency Monitoring of Protected Visual Environments (IMPROVE) fine particulate matter (<2.5 μm) sampler was modified to measure NHx at a subset of locations in the routine IMPROVE network. To sample NHx, an additional IMPROVE PM2.5 sampler was installed. Samples were collected on phosphorous acid impregnated cellulose filters held in polypropylene filter holders. While the standard IMPROVE filter holder is made of Delrin (polyoxymethylene, POM), reactions between collected NHx and formaldehyde released by phosphorous acid degradation of the POM holder produced substantial amounts of artifact methylamine, especially during warm sampling periods. This artifact did not occur with the new polypropylene holder design, and no methylamine was measured above the method detection limit of 0.003 μg/m3. Samples collected using the new IMPROVE NHx sampling system were evaluated against samples collected with a collocated URG annular denuder/filter-pack module for 6 weeks; the observed bias was −7%. The NHx monitors were deployed at a total of nine sites in the U.S. Rocky Mountain and Great Plains regions and to the east, and at Bondville, Illinois, from April 2011 to August 2012. Collocated samplers at Rocky Mountain National Park, Colorado, and Bondville, Illinois, demonstrated excellent measurement precision. The data revealed a pattern of increasing NHx concentrations in late spring/early summer (June) and a decrease in winter, starting in September for most of the sites. This pattern is consistent with expected seasonal patterns in agricultural emissions of ammonia. Sites closer to agricultural sources at Bondville and Cedar Bluff (Kansas), however, still exhibit quite abundant winter NHx, which may reflect continued local agricultural emissions trapped within a shallower winter boundary layer. A probable impact of wildfires on NHx concentrations was observed for Bandelier NM, Chiricahua NM, and Yellowstone NP during summer/fall 2011.

ACE-FTS version 3.0 data set: validation and data processing update

On 12 August 2003, the Canadian-led Atmospheric Chemistry Experiment (ACE) was launched into a 74° inclination orbit at 650 km with the mission objective to measure atmospheric composition using infrared and UV-visible spectroscopy (Bernath et al. 2005). The ACE mission consists of two main instruments, ACE-FTS and MAESTRO (McElroy et al. 2007), which are being used to investigate the chemistry and dynamics of the Earth’s atmosphere. Here, we focus on the high resolution (0.02 cm-1) infrared Fourier Transform Spectrometer, ACE-FTS, that measures in the 750-4400 cm-1 (2.2 to 13.3 µm) spectral region. This instrument has been making regular solar occultation observations for more than nine years. The current ACE-FTS data version (version 3.0) provides profiles of temperature and volume mixing ratios (VMRs) of more than 30 atmospheric trace gas species, as well as 20 subsidiary isotopologues of the most abundant trace atmospheric constituents over a latitude range of ~85°N to ~85°S. This letter describes the current data version and recent validation comparisons and provides a description of our planned updates for the ACE-FTS data set. [...]

Aqueous phase atmospheric oxidation of sulfur dioxide by oxygen : Role of trace atmospheric constituents - Metals, volatile organic compounds and ammonia

Aqueous phase atmospheric oxidation of sulfur dioxide by oxygen : Role of trace atmospheric constituents - Metals, volatile organic compounds and ammonia

Behaviors of Atmospheric particulates and Trace Constituent Elements Classified by Particle Size Range

Behaviors of Atmospheric particulates and Trace Constituent Elements Classified by Particle Size Range

Characterization and source regions of 51 high-CO events observed during Civil Aircraft for the Regular Investigation of the Atmosphere Based on an Instrument Container (CARIBIC) flights between south China and the Philippines, 2005–2008

[1] Carbon monoxide (CO) and other atmospheric trace constituents were measured from onboard an Airbus 340-600 passenger aircraft in the upper troposphere (UT) between south China and the Philippines during Civil Aircraft for the Regular Investigation of the Atmosphere Based on an Instrument Container (CARIBIC) flights from May 2005 until March 2008. A total of 132 events having CO enhancements were observed in the UT over the region during the 81 CARIBIC flights from Frankfurt, Germany, to Manila, Philippines, with a stopover in Guangzhou, China. Among these, 51 high-CO events with enhancements more than 50 ppb above background were observed. For these events enhancements ranged from 52.7 to 221.3 ppb and persisted for 3 to 78 min (∼40 to 1200 km), indicating an influence of strong pollution from biomass/biofuel/fossil fuel burning on the trace gas composition of the UT. Back trajectory analysis shows that south China, the Indochinese Peninsula, and the Philippines/Indonesia are the main source regions of the high-CO events. The composition of air parcels originating from south China was found to be primarily influenced by anthropogenic urban/industrial emissions, while emissions from biomass/biofuel burning contributed substantially to CO enhancements from the Indochinese Peninsula. During the Philippines/Indonesia events, air parcel composition suggests contributions from both biomass/biofuel burning and urban/industrial sources. Long-range transport of air parcels from northeast Asia and India also contributed to CO enhancements in the UT over the region. The general features of regional influence, typical cases, and the contributions of biomass/biofuel burning and anthropogenic emissions are presented and discussed to characterize the air parcels during the observed high-CO events.

Temporal flexibility of permit trading when pollutants are correlated

It is now accepted that climate change is due to the cumulative and joint effect of multiple atmospheric trace constituents. The importance of adopting a comprehensive approach to global warming that would account for multiple interacting pollutants is increasingly recognized. In this paper we propose to extend the analysis of intertemporal permit trading to a framework encompassing multiple correlated pollutants. In doing so our aim is to assess the consequence of providing “when flexibility” to participants in a pollution market scheme with respect to the timing of use of their permits. In the same manner as local pollution hotspots have been identified as potential drawbacks of allowing intra-regional trade, temporal flexibility has the potential to induce a type of “temporal hotspot” of pollution that could undermine the efficiency of intertemporal trading schemes — especially when the stock of a particular pollution induces damage in itself but also impacts on the accumulation of another detrimental pollution stock. In a first step, we assess the impact of various types of correlations (technological and physical) on the socially optimal accumulation of regional and global pollutants. We illustrate that even in the case of a linear damage function the regional stocks may have ambiguous impacts on the global stock. In a second step, we show that it is possible for a global benevolent regulator to have recourse to a set of intertemporal trading schemes to induce individual agents to take socially optimal decisions over time. One requirement is to implement a set of time-dependent intertemporal trading rates. We also analyze the impact on pollutants accumulation of implementing non-optimal intertemporal trading rates.

CHARACTERIZING THE ATMOSPHERES OF TRANSITING ROCKY PLANETS AROUND LATE-TYPE DWARFS

Visible and near-infrared spectra of transiting hot Jupiter planets have recently been observed, revealing some of the atmospheric constituents of their atmospheres. In the near future, it is probable that primary and secondary eclipse observations of Earth-like rocky planets will also be achieved. The characterization of the Earth's transmission spectrum has shown that both major and trace atmospheric constituents may present strong absorption features, including important bio-markers such as water, oxygen and methane. Our simulations using a recently published empirical Earth's transmission spectrum, and the stellar spectra for a variety of stellar types, indicate that the new generation of extremely large telescopes, such as the proposed 42-meter European Extremely Large Telescope(E-ELT), could be capable of retrieving the transmission spectrum of an Earth-like planet around very cool stars and brown dwarfs (Teff < 3100 K). For a twin of Earth around a star with Teff around 3100 K (M4), for example, the spectral features of water vapor, methane, carbon dioxide, and oxygen in the wavelength range between 0.9 and 2.4 micron can simultaneously be detected within a hundred hours of observing time, or even less for a late-M star. Such detection would constitute a proof for the existence of life in that planet. The detection time can be reduced to a few hours for a super-Earth type of planet with twice the Earth's radius.

COMPLEMENTARY ATMOSPHERIC URBAN POLLUTION STUDIES IN THE NORTH-EAST REGION OF ROMANIA, IASI COUNTY

Abstract In order to analyze the dynamics of the air pollutants, both through experiments and new modeling-forecasted environmental software (MAP 3D), preliminary investigations have been initiated. Complementary studies such as meteorological data that influence the physico-chemistry processes of various atmospheric trace constituents at the ground level have been taking into account, too. Key words: map 3D, nitrogen-oxides, ozone, pollution, relative humidity Received: November, 2010; Revised final: January, 2011; Accepted: January, 2011 ∗ Author to whom all correspondence should be addressed: e-mail: sgurlui@uaic.ro; Phone: +40232201197; Fax: +40232201150 1. Introduction The end of the 20 th century and the beginning of the 21 st century have brought to light numerous deficiencies concerning the relationship between people and environment. Only after serious warnings were heard and people have eventually realized that they do have a problem – pollution – did people become aware of the fact that we are not able to efficiently and objectively deal with problems linked with the climate environment. Numerous air pollutants, with variable concentrations, sizes and chemical compositions, depend on the space and time (Castellano et al. 2010; Stefan and Mircea, 1996). Moreover, the study of physical compositions and the chemical transformations of the polluted atmosphere play an important role in reducing the greenhouse emissions and greenhouse gas intensity that affects people, plants, and animals (Ding and Zhu, 2003). The atmospheric pollution of certain areas is influenced by weather (wind, clouds, precipitation, temperature, and relative humidity). In order to characterize the dynamics of the air pollutants both various atmospheric trace constituents (NO, NO

The influence of vegetation, fire spread and fire behaviour on biomass burning and trace gas emissions: results from a process-based model

Abstract. A process-based fire regime model (SPITFIRE) has been developed, coupled with ecosystem dynamics in the LPJ Dynamic Global Vegetation Model, and used to explore fire regimes and the current impact of fire on the terrestrial carbon cycle and associated emissions of trace atmospheric constituents. The model estimates an average release of 2.24 Pg C yr−1 as CO2 from biomass burning during the 1980s and 1990s. Comparison with observed active fire counts shows that the model reproduces where fire occurs and can mimic broad geographic patterns in the peak fire season, although the predicted peak is 1–2 months late in some regions. Modelled fire season length is generally overestimated by about one month, but shows a realistic pattern of differences among biomes. Comparisons with remotely sensed burnt-area products indicate that the model reproduces broad geographic patterns of annual fractional burnt area over most regions, including the boreal forest, although interannual variability in the boreal zone is underestimated.

Daily variations of indoor air-ion and radon concentrations

Air-ions and radon are two atmospheric trace constituents which have two opposite effects on human health: the ions are beneficial, and radon gas is potentially lethal as it increases the risk of lung cancer. In the lower troposphere, radon is the most important generator of the air-ions. Ionization by cosmic rays and radioactive minerals is almost constant in daily cycles, and variation of air-ion concentrations is attributed to changes of the radon activity. Air-ion and radon concentrations in outdoor and indoor space and their vertical gradients in residential buildings were measured. Gerdien type air-ion detector “CDI-06” made in our laboratory and radon monitor “RAD7” were utilized for these measurements. Correlation coefficient between positive air-ion and Rn indoor concentrations was approximately 0.7. Outdoor and indoor peak values were simultaneous while vertical gradient of concentrations in indoor measurements was evident. The indoor experiments showed that positive air-ion concentration could be an alternative method of radon activity concentration evaluation.

Intercomparison of Digital Fast Fourier Transform and Acoustooptical Spectrometers for Microwave Radiometry of the Atmosphere

The Institute of Applied Physics (IAP) of the University of Bern is active in the field of remote sensing of middle atmospheric trace gases such as ozone and water vapor by microwave radiometry. From the measured pressure broadened spectral lines, it is possible to retrieve the vertical distribution of the observed species. The vertical range is dependent on the bandwidth and resolution of the spectrometers. The radiometers from the IAP perform ground-based and airborne measurements. For the spectral analysis, they used acoustooptical spectrometers (AOS) or low-resolution filterbanks until recently. Unfortunately, AOS proved to be deficient under conditions encountered in an aircraft. For this reason, the approach of using novel real-time digital fast Fourier transform (FFT) spectrometers of bandwidth 1 GHz was chosen. In this paper, we present measurements of atmospheric trace constituents using digital FFT spectrometers and compare the results with measurements from the AOS and filterbank. The FFT spectrometer is superior in resolution and system stability as well as in the linearity and stability of the frequency axis. An important point is also the lower costs per bandwidth and resolution. The measured intensities of emitted radiation from the atmosphere from all spectrometer types were in good agreement.

Using quantum cascade lasers with resonant optical cavities as a diagnostic tool

Absorption spectroscopy based on optical resonators is known to be a very sensitive diagnostic technique. For many years the infrared spectral range could not be employed, because of the lack of suitable radiation sources with the required power and tunability. Recent advances in semiconductor laser technology, in particular the advent of quantum cascade lasers (QCL) provides new possibilities for highly sensitive and selective detection of molecular species. Cavity Enhanced Absorption Spectroscopy (CEAS) with a thermoelectrically (TE) cooled cw QCL emitting at 7.66 μm and employing a ∼0.5 m long cavity yielded effective path lengths of 1080 m and a sensitivity of 2 x 10-7 cm-1 Hz-1/2, mainly limited by incomplete averaging over cavity resonances. The molecular concentration detection limit with a 20 s integration time was found to be 6 x 108 molecules/cm3 for N2O and 2 x 109 molecules/cm3 for CH4 which is good enough for the selective measurement of trace atmospheric constituents at 2.2 mbar.