Chemical Loss Rates Research Articles

Ozone loss in the 2002–2003 Arctic vortex deduced from the assimilation of Odin/SMR O3 and N2O measurements: N2O as a dynamical tracer

AbstractIn this paper we investigate the evolution of the northern polar vortex during the winter 2002–2003 in the lower stratosphere by using assimilated fields of ozone (O3) and nitrous oxide (N2O). Both O3 and N2O used in this study are obtained from the Sub‐Millimetre Radiometer (SMR) aboard the Odin satellite and are assimilated into the global three‐dimensional chemistry transport model of Météo‐France, MOCAGE. O3 is assimilated into the ‘full’ model including both advection and chemistry whereas N2O is only assimilated with advection since it is characterized by good chemical stability in the lower stratosphere. We show the ability of the assimilated N2O field to localize the edge of the polar vortex. The results are compared to the use of the maximum gradient of modified potential vorticity as a vortex edge criterion. The O3 assimilated field serves to evaluate the ozone evolution and to deduce the ozone depletion inside the vortex. The chemical ozone loss is estimated using the vortex‐average technique. The N2O assimilated field is also used to substract out the effect of subsidence in order to extract the actual chemical ozone loss. Results show that the chemical ozone loss is 1.1 ± 0.3 ppmv on the 25 ppbv N2O level between mid‐November and mid‐January, and 0.9 ± 0.2 ppmv on the 50 ppbv N2O level between mid‐November and the end of January. A linear fit over the same periods gives a chemical ozone loss rate of ∼18 ppbv day−1 and ∼9.3 ppbv day−1 on the 25 ppbv and 50 ppbv N2O levels, respectively. The vortex‐averaged ozone loss profile from the O3 assimilated field shows a maximum of 0.98 ppmv at 475 K. Comparisons to other results reported by different authors using different techniques and different observations give satisfactory results. Copyright © 2008 Royal Meteorological Society

Alkyl nitrate (C1‐C3) depth profiles in the tropical Pacific Ocean

This paper reports the first depth profile measurements of methyl, ethyl, isopropyl and n‐propyl nitrates in the tropical Pacific Ocean. Depth profile measurements were made at 22 stations during the Project Halocarbon Air Sea Exchange cruise, in warm pool, equatorial, subequatorial, and gyre waters. The highest concentrations, up to several hundred pM of methyl nitrate, were observed in the central Pacific within 8 degrees of the equator. In general, alkyl nitrate levels were highest in the surface mixed layer, and decreased with depth below the mixed layer. The spatial distribution of the alkyl nitrates suggests that there is a strong source associated with biologically productive ocean regions, that is characterized by high ratios of methyl:ethyl nitrate. However, the data do not allow discrimination between direct biological emissions and photochemistry as production mechanisms. Alkyl nitrates were consistently detectable at several hundred meters depth. On the basis of the estimated chemical loss rate of these compounds, we conclude that deep water alkyl nitrates must be produced in situ. Possible sources include free radical processes initiated by radioactive decay or cosmic rays, enzymatically mediated reactions involving bacteria, or unidentified chemical mechanisms involving dissolved organic matter.

Fast Ozone Loss Around the Polar Vortex During 2002/2003 Arctic Winter Deep Minihole Event

A three-dimensional (3D) off-line chemical transport model (CTM) has been used to study an Arctic ozone minihole event in early December 2002, when the meteorological conditon were severely colder than in recent years. The minimum temperatures were below 188 K on 6 December 2002 and remained below 195 K for the entire month. Such temperatures are low enough for the formation of PSCs (polar stratospheric clouds) which were observed by a visible/near infrared solar occultation instrument by the Polar Ozone and Aerosol Measurement (POAM) III experiment. The SLIMCAT model compares well with the depth and evolution of the ozone distribution as observed by the second European Remote Sensing Satellite (ERS-2) Global Ozone Monitoring Experiment (GOME). The combined effect of a very strong and large intrusion of sub-tropical air, and uplifting of air masses of low ozone concentrations from the troposphere plays important role in the fomation of the ozone minihole. The modelled chemical ozone loss rate was relatively fast in the lower stratosphere in the region of frequent PSCs and higher levels of ClO and BrO. The observed PSCs in the vicinity of the minihole supports the model-predicted location of PSCs and chlorine activation. However, ozone miniholes have been attributed almost entirely to dynamical effects due to its short duration.

Three-Dimensional Model Study of the Antarctic Ozone Hole in 2002 and Comparison with 2000

Abstract An offline 3D chemical transport model (CTM) has been used to study the evolution of the Antarctic ozone hole during the sudden warming event of 2002 and to compare it with similar simulations for 2000. The CTM has a detailed stratospheric chemistry scheme and was forced by ECMWF and Met Office analyses. Both sets of meteorological analyses permit the CTM to produce a good simulation of the evolution of the 2002 vortex and its breakup, based on O3 comparisons with Total Ozone Mapping Spectrometer (TOMS) column data, sonde data, and first results from the Environmental Satellite–Michelson Interferometer for Passive Atmospheric Sounding (ENVISAT–MIPAS) instrument. The ozone chemical loss rates in the polar lower stratosphere in September 2002 were generally less than in 2000, because of the smaller average active chlorine, although around the time of the warming, the largest vortex chemical loss rates were similar to those in 2000 (i.e., −2.6 DU day−1 between 12 and 26 km). However, the disturbed vortex of 2002 caused a somewhat larger influence of polar processing on Southern Hemisphere (SH) midlatitudes in September. Overall, the calculations show that the average SH chemical O3 loss (poleward of 30°S) by September was ∼20 DU less in 2002 compared with 2000. A significant contribution to the much larger observed polar O3 column in September 2002 was due to the enhanced descent at the vortex edge and increased horizontal transport, associated with the distorted vortex.

Incoherent scatter radar study of photochemistry in the E-region

The ionospheric E‐region from 100 km to 150 km is typically regarded as being in photo‐chemical equilibrium (PCE) state during daytime. Incoherent scatter radar observations at Arecibo show that PCE is not always valid. During July 31–Aug. 5, 1992, the vertical ion drift was observed to increase by 50 m/s over a ten kilometer altitude range. The large divergence resulting from the steep velocity gradient reduced the electron concentration by 30%, invalidating the generally held PCE. Observations of non‐PCE allow us to estimate the chemical loss rate. By assuming that the loss rate does not have a day‐to‐day variability, the dissociative recombination rate derived from the incoherent scatter radar (ISR) observation is about 1.0 × 10−7 cm3s−1, which is significantly lower than the laboratory rate of NO+, or O2+. The most plausible explanation for the discrepancy is that the relative concentration of O2+ and NO+ changed significantly when non‐PCE occurred.

The impact of liquefied petroleum gas usage on air quality in Mexico City

Liquefied petroleum gas (LPG) is the main fuel used in the residential sector of the Metropolitan Zone in the Valley of Mexico (MZVM). LPG represents 16% of the total fuel consumption in the MZVM and its demand increased 14% from 1986 to 1999. Propane and butanes, the main compounds of LPG, constituted 29% of all non-methane hydrocarbons found in the air of Mexico City. Some researchers have reported that LPG losses are a significant cause of high ozone concentration in MZVM. Three analyses are carried out in this work to estimate LPG's share of responsibility for MZVM pollution problems. First, the correlation between LPG consumption and three ozone pollution indicators was calculated for the period of 1986–1999. The non-significant correlation of these indicators with LPG consumption in a monthly basis suggests that LPG associated emissions are not the foremost cause of ozone formation. Second, a simulation model is applied to three LPG related emission control strategies to estimate the reduction in the maximum ozone concentration. The most noticeable effect was obtained when both hydrocarbon (HC) and oxides of nitrogen (NO x ) emissions associated with LPG use were totally reduced. The other two strategies, that only reduce HC emissions, had a minimum effect on the ozone concentration. Third, organic compounds consumption in air samples captured and irradiated in outdoor smog chambers is used to determine the chemical loss rate of LPG associated species and aromatics in the MZVM. The smog chamber results showed that 70% of propane and n -butane remain at the end of a 1-day irradiation, therefore they remain in the MZVM atmosphere for several days being the reason for the high concentration of these compounds. LPG associated compounds only account for 18% of ozone formed but aromatics contribute 35% to ozone in smog chamber.

Methyl chloride and methyl bromide degradation in the Southern Ocean

This study presents shipboard measurements of the loss rate constants of methyl bromide and methyl chloride in surface seawater in the Southern Ocean, using a 13C stable isotope incubation technique. The measurements were made during October–December, 2001, on a cruise track extending from Hobart, Tasmania to Buchanan Bay (Mertz Glacier) at the coast of Antarctica (46–67°S, 138–145°E). Significant loss rates were measured for both compounds, even in very cold waters where chemical loss rates were negligible. These observations are attributed to biological uptake, and they explain the tendency for high latitude waters to be undersaturated with respect to atmospheric methyl bromide and methyl chloride. These observations are the first open ocean measurements demonstrating the biological degradation of methyl chloride.

Budget of tropospheric ozone during TOPSE from two chemical transport models

The tropospheric ozone budget during the Tropospheric Ozone Production about the Spring Equinox (TOPSE) campaign has been studied using two chemical transport models (CTMs): HANK and the Model of Ozone and Related chemical Tracers, version 2 (MOZART‐2). The two models have similar chemical schemes but use different meteorological fields, with HANK using MM5 (Pennsylvania State University, National Center for Atmospheric Research Mesoscale Modeling System) and MOZART‐2 driven by European Centre for Medium‐Range Weather Forecasts (ECMWF) fields. Both models simulate ozone in good agreement with the observations but underestimate NOx. The models indicate that in the troposphere, averaged over the northern middle and high latitudes, chemical production of ozone drives the increase of ozone seen in the spring. Both ozone gross chemical production and loss increase greatly over the spring months. The in situ production is much larger than the net stratospheric input, and the deposition and horizontal fluxes are relatively small in comparison to chemical destruction. The net production depends sensitively on the concentrations of H2O, HO2 and NO, which differ slightly in the two models. Both models underestimate the chemical production calculated in a steady state model using TOPSE measurements, but the chemical loss rates agree well. Measures of the stratospheric influence on tropospheric ozone in relation to in situ ozone production are discussed. Two different estimates of the stratospheric fraction of O3 in the Northern Hemisphere troposphere indicate it decreases from 30–50% in February to 15–30% in June. A sensitivity study of the effect of a perturbation in the vertical flux on tropospheric ozone indicates the contribution from the stratosphere is approximately 15%.

On the unexplained stratospheric ozone losses during cold Arctic Januaries

Using a combination of data from Match, POAM II, POAM III and MLS we show that the chemical loss rate of Arctic O3 during January of four cold winters (1992, 1995, 1996, and 2000) is consistently faster than can be accounted for by assuming complete activation of reactive chlorine and standard reaction kinetics. However, O3 loss rates measured during late February and early March 1996 are shown to be consistent with observations of ClO. The faster than expected O3 loss rates during January are shown to occur when air parcels are illuminated at high solar zenith angles (SZAs between ∼85 and 94°), and to result in cumulative O3 loss of ∼0.5 ppmv. The cause of the rapid January O3 loss is unclear, but may be related to a photolytic process at high SZA that is poorly represented by current photochemical models.

Simulation of ozone depletion in spring 2000 with the Chemical Lagrangian Model of the Stratosphere (CLaMS)

Simulations of the development of the chemical composition of the Arctic stratosphere for spring 2000 are made with the Chemical Lagrangian Model of the Stratosphere (CLaMS). The simulations are performed for the entire Northern Hemisphere on four isentropic levels (400–475 K). The initialization in early February is based on observations made from satellite, balloon and ER‐2 aircraft platforms. Tracer‐tracer correlations from balloon‐borne cryosampler (Triple) and ER‐2 measurements, as well as tracer‐PV correlations, are used to derive a comprehensive hemispherical initialization of all relevant chemical trace species. Since significant denitrification has been observed on the ER‐2 flights, a parameterization of the denitrification is derived from NOy and N2O observations on board the ER‐2 aircraft and the temperature history of the air masses under consideration. Over the simulation period from 10 February to 20 March, a chemical ozone depletion of up to 60% was derived for 425–450 K potential temperature. Maximum vortex‐averaged chemical ozone loss rates of 50 ppb d−1 or 4 ppb per sunlight hour were simulated in early March 2000 at the 425 and 450 K potential temperature levels. We show comparisons between the measurements and the simulations for the location of the ER‐2 flight paths in late February and March and the location of the Triple balloon flight. The simulated tracer mixing ratios are in good agreement with the measurements. It was not possible to reproduce the exact details of the inorganic chlorine compounds. The simulation agrees with ClOx observations on the Triple balloon flight but overestimates for the ER‐2 flights. The simulated ozone depletion agrees with estimates from other observations in the 425 and 450 K levels, but is underestimated on the 475 K level.

Stratospheric ozone loss in the 1996/1997 Arctic winter: Evaluation based on multiple trajectory analysis for double‐sounded air parcels by ILAS

Quantitative chemical ozone loss rates and amounts in the Arctic polar vortex for the spring of 1997 are analyzed based on ozone profile data obtained by the Improved Limb Atmospheric Spectrometer (ILAS) using an extension of the Match technique. In this study, we calculated additional multiple trajectories and set very strict criteria to overcome the weakness of the satellite sensor data (lower vertical resolution and larger sampling air mass volume) and to identify more accurately a double‐sounded air mass. On the average inside the inner edge of the vortex boundary (north of about 70°N equivalent latitude), the local ozone loss rate was 50–80 ppbv/day at the maximum during late February between the levels of 450 and 500 K potential temperatures. The integrated ozone loss during February to March reached 2.0 ± 0.1 ppmv at 475–529 K levels, and the column ozone loss between 400 and 600 K during the 2 months was 96 ± 0.3 DU. Using a relative potential vorticity (rPV) scale, the vortex was divided into some rPV belts, and it was shown that the magnitude of the ozone loss increased gradually toward the vortex center from the edge. The maximum ozone loss rate of 6.0 ± 0.6 ppbv/sunlit hour near the vortex center was higher than near the vortex edge by a factor of 2–3. When we expanded the area of interest to include all the data obtained inside the vortex edge (north of about 65°N equivalent latitude), the local ozone loss rate was about 50 ppbv/day at the maximum. This value is slightly larger than that estimated by the Match analysis using ozonesondes for the same winter by ∼10 ppbv/day. Temperature histories of double‐sounded air parcels indicated that the extreme ozone loss in the innermost part of the vortex was observed when the air parcel experienced temperatures below TNAT during the two soundings and had experienced temperatures near Tice in the 10 days prior to the first sounding. These facts suggest that the high ozone loss rate deep inside the vortex in the 1997 Arctic early spring correlates with the presence of type Ia polar stratospheric clouds (PSCs).

Methyl bromide loss rate constants in the north Pacific Ocean

The degradation rate constant of CH3Br in the North Pacific Ocean was measured in surface seawater between September and October 1999, using a stable isotope (13CH3Br) incubation technique. Total degradation rate constants ranged from 0.02 to 0.43 d−1, decreasing in colder waters as a result of the temperature‐dependence of chemical losses. Biological rate constants ranged from 0.01 to 0.20 d−1. In subtropical waters (13‐20°N), biological loss rate constants were small compared to chemical loss rate constants. North of Hawaii, biological processes played an increasingly significant role in CH3Br degradation. In subpolar waters (40‐58°N), biological losses dominated the removal of methyl bromide. Comparison of the measured loss rate constants with surface water CH3Br concentrations suggest that the CH3Br production rate is higher in warm, low latitude waters than in cold subpolar waters at this time of year. Diel studies revealed a midday maximum in biological degradation of methyl bromide.

Observational evidence for the role of denitrification in Arctic stratospheric ozone loss

Severe and extensive denitrification, chlorine activation, and photochemical ozone loss were observed throughout the lower stratosphere in the 1999–2000 Arctic vortex. A large number of air parcels sampled between late February and mid‐March, 2000, were photochemically intercomparable for chemical O3 loss rates. In these air parcels, the temporal evolution of the correlations of O3 with the NOy remaining after denitrification provides strong evidence for the role of NOy in moderating O3 destruction. In 71%‐denitrified air parcels, a chemical O3 destruction rate of 63 ppbv/day was calculated, while in 43%‐denitrified air parcels the destruction rate was only 43 ppbv/day. These observational results show that representative denitrification models will be required for accurate prediction of future Arctic O3 changes.

Severe chemical ozone loss inside the Arctic Polar Vortex during winter 1999–2000 Inferred from in situ airborne measurements

Lower stratospheric in situ observations are used to quantify both the accumulated ozone loss and the ozone chemical loss rates in the Arctic polar vortex during the 1999–2000 winter. Multiple long‐lived trace gas correlations are used to identify parcels in the inner Arctic vortex whose chemical loss rates are unaffected by extra‐vortex intrusions. Ozone‐tracer correlations are then used to calculate ozone chemical loss rates. During the late winter the ozone chemical loss rate is found to be −46±6 (1σ) ppbv/day. By mid‐March 2000, the accumulated ozone chemical loss is 58±4% in the lower stratosphere near 450 K potential temperature (∼19 km altitude).

Methyl bromide loss rates in surface waters of the North Atlantic Ocean, Caribbean Sea, and eastern Pacific Ocean (8°–45°N)

The first‐order loss rate constant of methyl bromide was measured in surface seawater samples from the North Atlantic Ocean, Caribbean Sea, and coastal eastern Pacific Ocean during May‐July 1998. A stable isotope incubation technique was used, which consisted of spiking seawater samples with 13CH3Br and following the loss rate of the isotopically labeled spike with gas chromatography and isotope dilution mass spectrometry. The analysis of both filtered and unfiltered aliquots of seawater provides insight into the relative importance of chemical and particle‐related (presumably biological) degradation pathways. Over the entire cruise, first‐order total degradation rate constants ranged from 0.03–0.40 day−1. On average, higher rate constants were observed during the last part of the cruise in the Caribbean Sea (mean 0.34 day−1) and coastal Pacific waters (0.31 day−1), than in the earlier passage through the North Atlantic Ocean (0.18 day−1, legs 1 and 3). In the warm waters of the Caribbean Sea and the Pacific, total degradation rate constants were controlled primarily by chemical losses (0.30 and 0.25 day−1, respectively). In the colder Atlantic waters the average chemical loss rate constant was lower (0.11 day−1), and biological losses constituted a significant fraction of the total loss rate constant (35%). Chemical loss rate constants varied strongly with water temperature, in good agreement with previous determinations of the rate constant for methyl bromide removal from seawater due to chloride substitution and hydrolysis. Biological losses were detected in most of the water samples analyzed, suggesting that the biological capability to degrade methyl bromide is ubiquitous in the oceans. Rate constants for biological removal exhibited no apparent relationship to seawater temperature, salinity, chlorophyll, or bacterial counts.

Arctic ozone loss in threshold conditions: Match observations in 1997/1998 and 1998/1999

Chemical ozone loss rates inside the Arctic polar vortex were determined in early 1998 and early 1999 by using the Match technique based on coordinated ozonesonde measurements. These two winters provide the only opportunities in recent years to investigate chemical ozone loss in a warm Arctic vortex under threshold conditions, i.e., where the preconditions for chlorine activation, and hence ozone destruction, only occurred occasionally. In 1998, results were obtained in January and February between 410 and 520 K. The overall ozone loss was observed to be largely insignificant, with the exception of late February, when those air parcels exposed to temperatures below 195 K were affected by chemical ozone loss. In 1999, results are confined to the 475 K isentropic level, where no significant ozone loss was observed. Average temperatures were some 8°–10° higher than those in 1995, 1996, and 1997, when substantial chemical ozone loss occurred. The results underline the strong dependence of the chemical ozone loss on the stratospheric temperatures. This study shows that enhanced chlorine alone does not provide a sufficient condition for ozone loss. The evolution of stratospheric temperatures over the next decade will be the determining factor for the amount of wintertime chemical ozone loss in the Arctic stratosphere.

Arctic and Antarctic ozone layer observations: chemical and dynamical aspects of variability and long-term changes in the polar stratosphere

The altitude dependent variability of ozone in the polar stratosphere is regularly observed by balloon-borne ozonesonde observations at Neumayer Station (70°S) in the Antarctic and at Koldewey Station (79°N)in the Arctic. The reasons for observed seasonal and interannual variability and long-term changes are discussed. Differences between the hemispheres are identified and discussed in light of differing dynamical and chemical conditions. Since the mid- 1980s, rapid chemical ozone loss has been recorded in the lower Antarctic stratosphere during the spring season. Using coordinated ozone soundings in some Arctic winters, similar chemical ozone loss rates have been detected related to periods of low temperatures. The currently observed cooling trend of the stratosphere, potentially caused by the increase of anthropogenic greenhouse gases, may further strengthen chemical ozone removal in the Arctic. However, the role of internal climate oscillations in observed temperature trends is still uncertain. First results of a 10000 year integration of a low order climate model indicate significant internal climate variability. on decadal time scales, that may alter the effect of increasing levels of greenhouse gases in the polar stratosphere.

UARS Microwave Limb Sounder observations of denitrification and ozone loss in the 2000 Arctic late winter

The UARS Microwave Limb Sounder obtained measurements of ClO, HNO3, and O3 inside the Arctic lower stratospheric vortex during two intervals in February and March 2000. The data show evidence of significant chemical processing in February, consistent with the exceptionally cold conditions that prevailed earlier in the winter. Ozone at 465 K decreased by 0.04±0.01 ppmv/day in early February, implying chemical loss rates greater than those observed during the 1995–1996 winter, which was also unusually cold in the lower stratosphere. A persistent depression in the HNO3 abundances in late March, well after polar stratospheric cloud activity had ceased, suggests a moderate degree (∼20%) of denitrification around the 465 K level at 70°N and higher equivalent latitudes. This is the strongest evidence yet seen for the occurrence of denitrification in the Arctic over spatial scales large enough to be detected in satellite measurements.

Chemical NOx budget in the upper troposphere over the tropical South Pacific

The chemical NOx budget in the upper troposphere over the tropical South Pacific is analyzed using aircraft measurements made at 6–12 km altitude in September 1996 during the Global Tropospheric Experiment (GTE) Pacific Exploratory Mission (PEM) Tropics A campaign. Chemical loss and production rates of NOx along the aircraft flight tracks are calculated with a photochemical model constrained by observations. Calculations using a standard chemical mechanism show a large missing source for NOx; chemical loss exceeds chemical production by a factor of 2.4 on average. Similar or greater NOx budget imbalances have been reported in analyses of data from previous field studies. Ammonium aerosol concentrations in PEM‐Tropics A generally exceeded sulfate on a charge equivalent basis, and relative humidities were low (median 25% relative to ice). This implies that the aerosol could be dry in which case N2O5 hydrolysis would be suppressed as a sink for NOx. Suppression of NO2O5 hydrolysis and adoption of new measurements of the reaction rate constants for NO2 + OH + M and HNO3 + OH reduces the median chemical imbalance in the NOx budget for PEM‐Tropics A from 2.4 to 1.9. The remaining imbalance cannot be easily explained from known chemistry or long‐range transport of primary NOx and may imply a major gap in our understanding of the chemical cycling of NOx in the free troposphere.

Match observations in the Arctic winter 1996/97: High stratospheric ozone loss rates correlate with low temperatures deep inside the polar vortex

With the Match technique, which is based on the coordinated release of ozonesondes, chemical ozone loss rates in the Arctic stratospheric vortex in early 1997 have been quantified in a vertical region between 400 K and 550 K. Ozone destruction was observed from mid February to mid March in most of these levels, with maximum loss rates between 25 and 45ppbv/day. The vortex averaged loss rates and the accumulated vertically integrated ozone loss have been smaller than in the previous two winters, indicating that the record low ozone columns observed in spring 1997 were partly caused by dynamical effects. The observed ozone loss is inhomogeneous through the vortex with the highest loss rates located in the vortex centre, coinciding with the lowest temperatures. Here the loss rates per sunlit hour reached 6 ppbv/h, while the corresponding vortex averaged rates did not exceed 3.9 ppbv/h.