Radiocarbon tracer measurements of atmospheric hydroxyl radical concentrations

Radiochemical techiques have many untapped applications in atmospheric chemistry, especially when great sensitivity is required. We describe the application of these techniques to the measurement of hydroxyl radical concentrations in the troposphere.     

Vertical profiles of hydrogen chloride in the troposphere

Vertical profiles of gaseous hydrogen chloride have been measured in the lower and middle troposphere. For sampling, denuder tubes coated with porous silica were used. Hydrogen chloride was determined by gas chromatography in combination with a derivatization method. The samples were collected over the Atlantic Ocean northwest of Norway in early September 1981 and over the Mediterranean Sea and north-eastern Spain in December 1981 at altitudes between 0.1 and 7 km. Above the 3 km altitude the mixing ratios are generally very low and relatively uniform with values of 50–100 ppt. Below 3 km, the variations of the HCl-mixing ratios are larger with maximum values of up to 500 ppt. The profiles are discussed with respect to the vertical and horizontal transport conditions and the possible sources and sinks of gaseous hydrogen chloride.     

Fourier transform measurement of NO2 absorption cross-section in the visible range at room temperature

New laboratory measurements of NO₂ absorption cross-section were performed using a Fourier transform spectrometer at 2 and 16 cm^-1 (0.03 and 0.26 nm at 400 nm) in the visible range (380–830 nm) and at room temperature. The use of a Fourier transform spectrometer leads to a very accurate wavenumber scale (0.005 cm^-1, 8×10^-5 nm at 400 nm). The uncertainty on the new measurements is better than 4%. Absolute and differential cross-sections are compared with published data, giving an agreement ranging from 2 to 5% for the absolute values. The discrepancies in the differential cross-sections can however reach 18%. The influence of the cross-sections on the ground-based measurement of the stratospheric NO₂ total amount is also investigated.     

Measurement of PAN in the Polluted Boundary Layer and Free Troposphere Using a Luminol-NO2 Detector Combined with a Thermal Converter

Peroxyacetyl nitrate (PAN,CH³C(O)O₂NO₂) has been measured inthe polluted boundary layer and free troposphere by thermal conversion tonitrogen dioxide (NO₂) followed by detection of thedecomposition product with a Scintrex LMA-3 NO₂-luminolinstrument. Following laboratory tests of the efficiency of PAN conversionand investigations of possible interferences, the technique was evaluated atthe West Beckham TOR (Tropospheric Ozone Research) Station near the northNorfolk coast in Eastern England between September 1989 and August 1990. PANmeasured by the new technique was reasonably well correlated with PANrecorded using electron capture gas chromatography (EC/GC). PAN was alsowell correlated with ozone (O³) in the summer months. Springand autumn episodes of simultaneously high concentrations of PAN andO³ were examined in conjunction with air parcelback-trajectories and synoptic- and local-scale meteorology in a study ofthe sources of photooxidants on the east coast of England. Spring-timemeasurements of PAN made in the free troposphere in a light aircraft ataltitudes up to 3.1 km showed the presence of 0.54 and 0.26 ppbv PAN inpolar maritime and mid-latitude oceanic air masses, respectively. Thetechnique is particularly suited to airborne applications because potentialinterferences are minimised and the frequency of measurements is higher thangenerally achieved with EC/GC methods.     

Biological Impact of Elevated Ocean CO<Subscript>2</Subscript> Concentrations: Lessons from Animal Physiology and Earth History

CO₂ currently accumulating in the atmosphere permeates into ocean surface layers, where it may impact on marine animals in addition to effects caused by global warming. At the same time, several countries are developing scenarios for the disposal of anthropogenic CO₂ in the worlds' oceans, especially the deep sea. Elevated CO₂ partial pressures (hypercapnia) will affect the physiology of water breathing animals, a phenomenon also considered in recent discussions of a role for CO₂ in mass extinction events in earth history. Our current knowledge of CO₂ effects ranges from effects of hypercapnia on acid-base regulation, calcification and growth to influences on respiration, energy turnover and mode of metabolism. The present paper attempts to evaluate critical processes and the thresholds beyond which these effects may become detrimental. CO₂ elicits acidosis not only in the water, but also in tissues and body fluids. Despite compensatory accumulation of bicarbonate, acid-base parameters (pH, bicarbonate and CO₂ levels) and ion levels reach new steady-state values, with specific, long-term effects on metabolic functions. Even though such processes may not be detrimental, they are expected to affect long-term growth and reproduction and may thus be harmful at population and species levels. Sensitivity is maximal in ommastrephid squid, which are characterized by a high metabolic rate and extremely pH-sensitive blood oxygen transport. Acute sensitivity is interpreted to be less in fish with intracellular blood pigments and higher capacities to compensate for CO₂ induced acid-base disturbances than invertebrates. Virtually nothing is known about the degree to which deep-sea fishes are affected by short or long term hypercapnia. Sensitivity to CO₂ is hypothesized to be related to the organizational level of an animal, its energy requirements and mode of life. Long-term effects expected at population and species levels are in line with recent considerations of a detrimental role of CO₂ during mass extinctions in the earth's history. Future research is needed in this area to evaluate critical effects of the various CO₂ disposal scenarios. This revised version was published online in July 2006 with corrections to the Cover Date.     

The trend,seasonal cycle,and sources of tropospheric NO<Subscript>2</Subscript> over China during 1997–2006 based on satellite measurement

The characteristics of spatial and temporal distribution of tropospheric NO2 column density concentration over China are presented, on the basis of measurements from the satellite instruments GOME and SCIAMACHY. From these observations, monthly averaged tropospheric NO2 variations are determined for the period of 1997 to 2006. The trend and seasonal cycle are also investigated. The possible source of tropospheric NO2 over megacity area is discussed in this paper. The results show a large growth of tropospheric NO2 over eastern China, especially above the industrial areas with a fast economical growth, such as, Yangtze Rive Delta region and Pearl River Delta region because of the prominent anthropogenic activity. There is a rapid increase of tropospheric NO2 over megacities in China. For instance, Shanghai had a linear significant increase in NO2 columns of ～20% per year (reference year 1997) in the period of 1997-2006, which is the rapidest increase among all the selected cities. The seasonal pattern of the NO2 concentration shows a difference between the east and west in China. In the eastern part of China, an expected winter maximum in seasonal cycle is found because of the prominent anthropogenic activity and meteorological conditions. In the western part this cycle shows a NO2 maximum in summer time, which is attributed to natural emissions, especially soil emissions and lightning. A quickly increasing vehicle population may contribute to the increase of tropospheric NO2 over megacities in China for the remarkable correlation for vehicle population with tropospheric NO2.     

The trend, seasonal cycle, and sources of tropospheric NO_2 over China during 1997―2006 based on satellite measurement

The characteristics of spatial and temporal distribution of tropospheric NO2 column density concen-tration over China are presented,on the basis of measurements from the satellite instruments GOME and SCIAMACHY.From these observations,monthly averaged tropospheric NO2 variations are deter-mined for the period of 1997 to 2006.The trend and seasonal cycle are also investigated.The possible source of tropospheric NO2 over megacity area is discussed in this paper.The results show a large growth of tropospheric NO2 over eastern China,especially above the industrial areas with a fast eco-nomical growth,such as,Yangtze Rive Delta region and Pearl River Delta region because of the prominent anthropogenic activity.There is a rapid increase of tropospheric NO2 over megacities in China.For instance,Shanghai had a linear significant increase in NO2 columns of ～20% per year(ref-erence year 1997) in the period of 1997―2006,which is the rapidest increase among all the selected cities.The seasonal pattern of the NO2 concentration shows a difference between the east and west in China.In the eastern part of China,an expected winter maximum in seasonal cycle is found because of the prominent anthropogenic activity and meteorological conditions.In the western part this cycle shows a NO2 maximum in summer time,which is attributed to natural emissions,especially soil emissions and lightning.A quickly increasing vehicle population may contribute to the increase of tropo-spheric NO2 over megacities in China for the remarkable correlation for vehicle population with tropospheric NO2.     

New limits on the possible variation of physical constants

Recent detections of high-redshift absorption by both atomic hydrogen and molecular gas in the radio spectra of quasars have provided a powerful tool for measuring possible temporal and spatial variations of physical 'constants' in the universe.
We compare the frequency of high-redshift hydrogen 21-cm absorption with that of associated molecular absorption in two quasars to place new (1σ) upper limits on any variation in y≡g_pα² (where α is the fine-structure constant, and g_p is the proton g -factor of
   
at redshifts z = 0.25 and 0.68. These quasars are separated by a comoving distance of 3000 Mpc ( H ₀= 75 km s⁻¹ Mpc⁻¹ and q ₀). We also derive limits on the time rates of change
   
    between the present epoch and z = 0.68. These limits are more than an order of magnitude smaller than previous results derived from high-redshift measurements.     

High-resolution spectroscopy of the Seyfert galaxy Mrk 595 in the direction of the Cohen H I high-velocity stream

We have obtained a spectrum of the Na D lines of the Seyfert galaxy Mrk 595 with a signal-to-noise ratio (S/N) of about 30. The importance of this sightline is that this galaxy lies in a direction close to the highest column densities of the Cohen high-velocity (HV) stream, and hence represents the best opportunity of detecting this stream in interstellar absorption, as previous searches towards stellar targets up to a distance of 600 pc have not been successful. The H  I column density in the direction of this galaxy implies that the HV gas should be detectable in absorption in the Na D lines. No such detections are made (at the 3σ level) and we consider possible explanations. Hence previous non-detections towards stars do not necessarily mean that the HV gas is more distant than the stars, and we emphasize that the distance of this high-velocity cloud (HVC), which is a key parameter to the understanding of the nature and origin of this feature, remains unknown and indeed may be difficult to establish.     

Additional ultra-high-resolution observations of Ca+ ions in the local interstellar medium

We present ultra-high-resolution (0.35 km s⁻¹ FWHM) observations of the interstellar Ca K line towards seven nearby stars. The spectral resolution was sufficient to resolve the line profiles fully, thereby enabling us to detect hitherto unresolved velocity components, and to obtain accurate measurements of the velocity dispersions ( b values). Absorption components with velocities similar to those expected for the Local Interstellar Cloud (LIC) and the closely associated 'G cloud' were identified towards six of the seven stars. However, in most cases the b values deduced for these components were significantly larger than the b  ≈ 2.2 km s⁻¹ (i.e. T _k ≈ 7000 K, v _t ≈ 1 km s⁻¹) expected for the LIC, and it is argued that this results from the presence of additional, spectrally unresolved, components having similar velocities and physical conditions. For two stars (δ Vel and α Pav) we detect interstellar components with much smaller b values (1.1 ± 0.3 and 0.8 ± 0.1 km s⁻¹, respectively) than are expected for low-density clouds within the Local Bubble. In the case of the narrow α Pav component, we also find an anomalously large Na  i /Ca  ii column density ratio, which is indicative of a relatively high density. Thus it is possible that, in addition to LIC-type clouds, the local interstellar medium contains a population of previously undetected cooler and denser interstellar clouds.