Showing papers by "Paul J. Crutzen published in 2001"

Aerosols, climate, and the hydrological cycle

Abstract: Human activities are releasing tiny particles (aerosols) into the atmosphere. These human-made aerosols enhance scattering and absorption of solar radiation. They also produce brighter clouds that are less efficient at releasing precipitation. These in turn lead to large reductions in the amount of solar irradiance reaching Earth's surface, a corresponding increase in solar heating of the atmosphere, changes in the atmospheric temperature structure, suppression of rainfall, and less efficient removal of pollutants. These aerosol effects can lead to a weaker hydrological cycle, which connects directly to availability and quality of fresh water, a major environmental issue of the 21st century.

Indian Ocean Experiment: An integrated analysis of the climate forcing and effects of the great Indo-Asian haze

Abstract: Every year, from December to April, anthropogenic haze spreads over most of the North Indian Ocean, and South and Southeast Asia. The Indian Ocean Experiment (INDOEX) documented this Indo-Asian haze at scales ranging from individual particles to its contribution to the regional climate forcing. This study integrates the multiplatform observations (satellites, aircraft, ships, surface stations, and balloons) with one- and four-dimensional models to derive the regional aerosol forcing resulting from the direct, the semidirect and the two indirect effects. The haze particles consisted of several inorganic and carbonaceous species, including absorbing black carbon clusters, fly ash, and mineral dust. The most striking result was the large loading of aerosols over most of the South Asian region and the North Indian Ocean. The January to March 1999 visible optical depths were about 0.5 over most of the continent and reached values as large as 0.2 over the equatorial Indian ocean due to long-range transport. The aerosol layer extended as high as 3 km. Black carbon contributed about 14% to the fine particle mass and 11% to the visible optical depth. The single-scattering albedo estimated by several independent methods was consistently around 0.9 both inland and over the open ocean. Anthropogenic sources contributed as much as 80% (±10%) to the aerosol loading and the optical depth. The in situ data, which clearly support the existence of the first indirect effect (increased aerosol concentration producing more cloud drops with smaller effective radii), are used to develop a composite indirect effect scheme. The Indo-Asian aerosols impact the radiative forcing through a complex set of heating (positive forcing) and cooling (negative forcing) processes. Clouds and black carbon emerge as the major players. The dominant factor, however, is the large negative forcing (-20±4 W m^(−2)) at the surface and the comparably large atmospheric heating. Regionally, the absorbing haze decreased the surface solar radiation by an amount comparable to 50% of the total ocean heat flux and nearly doubled the lower tropospheric solar heating. We demonstrate with a general circulation model how this additional heating significantly perturbs the tropical rainfall patterns and the hydrological cycle with implications to global climate.

The Indian Ocean Experiment: Widespread Air Pollution from South and Southeast Asia

Abstract: The Indian Ocean Experiment (INDOEX) was an international, multiplatform field campaign to measure long-range transport of air pollution from South and Southeast Asia toward the Indian Ocean during the dry monsoon season in January to March 1999. Surprisingly high pollution levels were observed over the entire northern Indian Ocean toward the Intertropical Convergence Zone at about 6°S. We show that agricultural burning and especially biofuel use enhance carbon monoxide concentrations. Fossil fuel combustion and biomass burning cause a high aerosol loading. The growing pollution in this region gives rise to extensive air quality degradation with local, regional, and global implications, including a reduction of the oxidizing power of the atmosphere.

An atmospheric chemistry interpretation of mass scans obtained from a proton transfer mass spectrometer flown over the tropical rainforest of Surinam.

Abstract: Data on a variety of organic gases are presented, obtained with a protontransfer mass spectrometer (PTR-MS) operated during the March 1998 LBA/CLAIREairborne measurement campaign, between 60 and 12500 m over the rainforest inSurinam (2° N–5° N, 54° W–57° W). The instrumentcan detect molecules with a proton affinity greater than water, includingalkenes, dialkenes, carbonyls, alcohols, and nitriles. Many such molecules areemitted from the rainforest (e.g., isoprene) or formed from the oxidation ofprimary emissions (e.g., methylvinylketone (MVK) and methacrolein (MACR)).From a comparison with modelled data; the variation with altitude; previouslyreported biogenic emissions and the time and location of the measurement,possible and probable identities for the significant masses encountered in therange 33–140 amu have been deduced.The main observed protonated masses, postulated identities and observedaverage boundary layer mixing ratios over the rainforest were: 33 methanol(1.1 nmol/mol); 42 acetonitrile (190 pmol/mol); 43 multiple possibilities (5.9nmol/mol), 45 acetaldehyde (1.7 nmol/mol), 47 formic acid (not quantified);59 acetone (2.9 nmol/mol), 61 acetic acid (not quantified), 63 dimethylsulphide (DMS) (289 pmol/mol), 69 isoprene (1.7 nmol/mol), 71 MVK + MACR (1.3nmol/mol), 73 methyl ethyl ketone (1.8 nmol/mol), 75 hydroxyacetone (606pmol/mol), 83 C5 isoprene hydroxy carbonylsC5H8O2, methyl furan, and cis 3-hexen-1-ol(732 pmol/mol), 87 C5 carbonyls and methacrylic acid, 95 possibly2-vinyl furan (656 pmol/mol), 97 unknown (305 pmol/mol), 99 cis hexenal (512pmol/mol) and 101 isoprene C5 hydroperoxides (575 pmol/mol). Somespecies agreed well with those derived from an isoprene only photochemicalmodel (e.g., mass 71 MVK + MACR) while others did not and were observed athigher than previously reported mixing ratios (e.g., mass 59 acetone, mass 63DMS). Monoterpenes were not detected above the detection limit of 300pmol/mol. Several species postulated are potentially important sources ofHOx in the free troposphere, e.g., methanol, acetone, methyl ethylketone, methyl vinyl ketone and methacrolein.

Isoprene and its oxidation products methyl vinyl ketone, methacrolein, and isoprene related peroxides measured online over the tropical rain forest of Surinam in March 1998

Abstract: Airborne measurements of volatile organic compounds (VOC) were performed overthe tropical rainforest in Surinam (0–12 km altitude,2°–7° N, 54°–58° W) using the proton transferreaction mass spectrometry (PTR-MS) technique, which allows online monitoringof compounds like isoprene, its oxidation products methyl vinyl ketone,methacrolein, tentatively identified hydroxy-isoprene-hydroperoxides, andseveral other organic compounds. Isoprene volume mixing ratios (VMR) variedfrom below the detection limit at the highest altitudes to about 7 nmol/molin the planetary boundary layer shortly before sunset. Correlations betweenisoprene and its product compounds were made for different times of day andaltitudes, with the isoprene-hydroperoxides showing the highest correlation.Model calculated mixing ratios of the isoprene oxidation products using adetailed hydrocarbon oxidation mechanism, as well as the intercomparisonmeasurement with air samples collected during the flights in canisters andlater analysed with a GC-FID, showed good agreement with the PTR-MSmeasurements, in particular at the higher mixing ratios.Low OH concentrations in the range of 1–3 × 105molecules cm-3 averaged over 24 hours were calculated due to lossof OH and HO2 in the isoprene oxidation chain, thereby stronglyenhancing the lifetime of gases in the forest boundary layer.

Human Impacts on Atmospheric Chemistry

Abstract: An overview is given of the main anthropogenic influences on the chemistry of the atmosphere. Industrial and agricultural activities have altered the chemical composition of the atmosphere in many important ways, which is reflected especially in the distribution and concentrations of ozone in the troposphere and stratosphere. On one hand, as a result of industrial chlorofluorocarbon emissions, ozone has been depleted in unexpected major ways in the polar stratosphere. On the other hand, especially as a result of NO emissions, tropospheric ozone has increased both in the industrial mid-latitude regions and at low latitudes, in the latter mostly because of tropical biomass burning. In the future, growing anthropogenic emissions by developing nations will have an additional effect on the climate and the self-cleaning (oxidation) power of the atmosphere.

On a fundamental problem in implementing flux‐form advection schemes for tracer transport in 3‐dimensional general circulation and chemistry transport models

Abstract: A fundamental difficulty in 3-D models is addressed which can arise due to inconsistencies between advection schemes and winds. It is shown that a model will fail to meet certain basic criteria, desired of 3-D transport models, when the density field computed by the advection scheme from the winds differs from the implied density field based on the surface pressure and the sigma (or hybrid) coordinates. To allow a rigorous mathematical formulation, the focus is on the example of a mass flux advection scheme in a model where the winds and surface pressure are derived from different advection schemes (e.g. a spectral scheme in a climate model or a weather centre model); however, in principle the discussion applies to nearly any situation in which the pressure levels change in a model. To illustrate the potential severity of such problems, a mass conserving grid-to-grid transformation scheme is constructed which only uses the current tracer mass mixing-ratio distribution. It is shown that only one solution exists that is comprehensively valid for any arbitrary tracer distribution, and that this type of correction introduces an additional undesired artificial vertical diffusion component into the model transport that increases with increasing tracer mass mixing-ratio gradients and may exceed the physical vertical transport itself. It is demonstrated that the results of any supplementary fix, either mass fixer or grid-to-grid transformation, are generally unacceptable for global modelling applications. From this, it is concluded that the only alternative which can produce reliable results for any arbitrary tracer is to maintain a consistent grid throughout the entire model time step, where all changes in pressure levels due to modelled advection exactly match the changes implied by the surface pressure at the next time step. Although this is already done in some models, this would require significant changes in the structure of the advection scheme or its input wind fields in several other contemporary general circulation and chemistry transport models.

Isotopic enrichment of nitrous oxide (15N14NO, 14N15NO, 14N14N18O) in the stratosphere and in the laboratory

Abstract: Nitrous oxide (N2O) extracted from stratospheric whole air samples has been analyzed for its 15N and 18O isotopic composition, and strong enrichments in the heavy isotopes are observed concomitant with decreasing N2O mixing ratio. Notably, the N enrichment is strongly different at the two nonequivalent positions in the molecule. Laboratory broadband photolysis experiments at wavelengths representative for the stratosphere confirm that photolysis is the prime cause for the observed fractionation in the stratosphere. However, the in situ stratospheric fractionation constants are significantly reduced compared to the laboratory data, reflecting the importance of dynamic processes. In addition, small but significant variations in the ratio of the two 15N fractionation constants indicate the influence of additional chemical processes like the oxidation of N2O by O(1 D).

The origin of the anomalous or “mass‐independent” oxygen isotope fractionation in tropospheric N2O

Abstract: Analysis of the complete oxygen isotopic composition (16O, 17O, 18O) of tropospheric N2O from various northern hemispheric locations reveals a mass independent anomaly with a 17O excess of ?17O = 1.0±0.2‰ at d18O = 20.7±0.3‰. So far, the origin of this intriguing isotope signature has remained elusive. New laboratory experiments demonstrate that the fractionation during UV photolysis of N2O, which causes 15N and 18O enrichments in the stratosphere, is strictly mass dependent (?17O = 0). To explain the isotope anomaly in atmospheric N2O, we propose a chemical mechanism for heavy oxygen transfer from O3 to N2O. In a first step, the NOx-O3 photochemical interaction leads to the formation of NO2 with significant excess 17O. In a second step, the heavy oxygen anomaly is transferred to N2O via the reaction NO 2 + NH 2 ? N 2 O + H 2 O , as part of the gas phase degradation of ammonia. This small but significant N2O source is of the right magnitude to explain the tropospheric observations.

High concentrations of reactive biogenic VOCs at a high altitude site in late autumn

Abstract: Volatile organic compounds (VOCs) at the Sonnblick Observatory, Austria (3106 m a.s.l.) were measured in autumn 1999 using proton-transfer-reaction mass spectrometry (PTR-MS). Anthropogenic VOCs at this remote site, such as benzene and toluene (50 pptv to 2 ppbv), were primarily traced to air from urban areas. In mid-November, following a hard freeze in the central Alps, a different pattern of VOCs developed similar to that seen with C5 and C6 leaf wound compounds (0.5 to 5 ppbv range). The common origins of these compounds are proposed to be freeze-damaged leaves, inferred from laboratory investigations that identified hexenals, hexanal, methylbutanals, pentenol and pentenone formation in such leaves. We suggest that deciduous forests can release large amounts of reactive VOCs during the autumn, and that these releases may be important for regional tropospheric chemistry.

High acetone concentrations throughout the 0-12 km altitude range over the tropical rainforest in Surinam

Abstract: Airborne measurements of acetone were performed overthe tropical rainforest in Surinam(2°–7° N, 54°–58° W, 0–12 kmaltitude) during the LBA-CLAIRE campaign in March1998, using a novel proton transfer reaction massspectrometer (PTR-MS) that enables the on-linemonitoring of volatile organic compounds (VOC) with ahigher proton affinity than water. The measuredacetone volume mixing ratios ranged from ∼0.1 nmol/molup to ∼8 nmol/mol with an overall average of 2.6nmol/mol and a standard deviation of 1.0 nmol/mol. Theobserved altitude profile and correlations with CO,acetonitrile, propane and wind direction are discussedwith respect to potential acetone sources. No linearcorrelation between acetone and CO mixing ratios wasobserved, at variance with results of previousmeasurement campaigns. The mean acetone/CO ratio(0.022) was substantially higher than typical valuesfound before. The abundance of acetone appears to beinfluenced, but not dominated, by biomass burning,thus suggesting large emissions of acetone and/oracetone precursors, such as possibly 2-propanol, fromliving plants or decaying litter in the rainforest.

Catalysis by NOx as the Main Cause of the Spring to Fall Stratospheric Ozone Decline in the Northern Hemisphere

Abstract: We address the cause of the largely natural total ozone decline in the stratosphere from its spring maximum to fall minimum in the northern hemisphere and show that this is mainly due to NOx-catalyzed ozone destruction. While in an earlier paper, using HALOE satellite observations, an analysis was made only for the year 1993, here we also analyze later years, yielding very similar results. We emphasize the role of the subtropical meridional transport barrier in isolating the chemistry at middle to high latitudes from that over the tropics. We find that for all years net ozone production takes place between the “subtropical barrier”, at about 30° N, and 50° N. Nevertheless, also in this latitude region the ozone content declines due to transport to higher latitudes where very strong chemical ozone loss takes place due to summer time NOx activation. An interesting result of the analysis is that, rather irrespective of the starting values of total ozone in early spring, chemical ozone loss yields about the s...

Ground based gas phase measurements in Surinam during the LBA-Claire 98 Experiment

Abstract: As part of the LBA-CLAIRE-98 experiment, ground level atmosphericconcentrations of O3, CO, hydroperoxides and organic acids weremeasured in the rainforest region in Surinam. Measurements of CO andO3 were also made at a coastal site.

The Position Dependent 15N Enrichment of Nitrous Oxide in the Stratosphere

Abstract: The position dependent 15N fractionation of nitrous oxide (N2O), which cannot be obtained from mass spectrometric analysis on molecular N2O itself, can be determined with high precision using isotope ratio mass spectrometry on the NO+ fragment that is formed on electron impact in the source of an isotope ratio mass spectrometer. Laboratory UV photolysis experiments show that strong position dependent 15N fractionations occur in the photolysis of N2O in the stratosphere, its major atmospheric sink. Measurements on the isotopic composition of stratospheric N2O indeed confirm the presence of strong isotope enrichments, in particular the difference in the fractionation constants for 15N14NO and 14N15NO. The absolute magnitudes of the fractionation constants found in the stratosphere are much smaller, however, than those found in the lab experiments, demonstrating the importance of dynamical and also additional chemical processes like the reaction of N2O with O(1D).

CARIBIC: Results and Status of the Aircraft Research Project CARIBIC

Abstract: The deployment of passenger aircraft for carrying out atmospheric chemistry measurements has an enormous potential that neatly fills the gap between capabilities of research aircraft and of satellites. Research aircraft give detailed information, in particular for short to ultra-short lived species, whereas satellite based remote sensing equipment provide a true global picture. Both methods are known to be expensive. The regular flight schedules of long range aircraft allow a systematic collection of a large range of data of considerable detail at lower cost.