Jacquelyn C. Witte

Southern Hemisphere Additional Ozonesondes (SHADOZ) 1998–2000 tropical ozone climatology 2. Tropospheric variability and the zonal wave-one

(1) The first view of stratospheric and tropospheric ozone variability in the Southern Hemisphere tropics is provided by a 3-year record of ozone soundings from the Southern Hemisphere Additional Ozonesondes (SHADOZ) network (http://croc.gsfc.nasa.gov/ shadoz). Observations covering 1998-2000 were made over Ascension Island, Nairobi (Kenya), Irene (South Africa), Reunion Island, Watukosek (Java), Fiji, Tahiti, American Samoa, San Cristobal (Galapagos), and Natal (Brazil). Total, stratospheric, and tropospheric column ozone amounts usually peak between August and November. Other features are a persistent zonal wave-one pattern in total column ozone and signatures of the quasi-biennial oscillation (QBO) in stratospheric ozone. The wave-one is due to a greater concentration of free tropospheric ozone over the tropical Atlantic than the Pacific and appears to be associated with tropical general circulation and seasonal pollution from biomass burning. Tropospheric ozone over the Indian and Pacific Oceans displays influences of the waning 1997-1998 El Nino, seasonal convection, and pollution transport from Africa. The most distinctive feature of SHADOZ tropospheric ozone is variability in the data, e.g., a factor of 3 in column amount at 8 of 10 stations. Seasonal and monthly means may not be robust quantities because statistics are frequently not Gaussian even at sites that are always in tropical air. Models and satellite retrievals should be evaluated on their capability for reproducing tropospheric variability and fine structure. A 1999- 2000 ozone record from Paramaribo, Surinam (6� N, 55� W) (also in SHADOZ) shows a marked contrast to southern tropical ozone because Surinam is often north of the Intertropical Convergence Zone (ITCZ). A more representative tropospheric ozone climatology for models and satellite retrievals requires additional Northern Hemisphere tropical data. INDEXTERMS: 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 1640 Global Change: Remote sensing; 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 9305 Information Related to Geographic Region: Africa; 9325 Information Related to Geographic Region: Atlantic Ocean; KEYWORDS: Free-words-ozone, tropospheric ozone, ozonesondes, satellite ozone, tropical climatology, wave-one, biomass burning, El Nino, satellite retrievals

A tropical Atlantic paradox: Shipboard and satellite views of a tropospheric ozone maximum and wave-one in January-February 1999

During the Aerosols99 trans-Atlantic cruise from Norfolk, VA, to Cape Town, South Africa, daily ozonesondes were launched from the R/V Ronald H Brown between 17 January and 6 February 1999. A composite of tropospheric ozone profiles along the latitudinal transect shows 4 zones, nearly identical to the ozone distribution during a January-February 1993 trans-Atlantic cruise [Weller et al., 1996]. Sondes from the cruise and Ascension Island (8S, 14.5W), as well as the Earth-Probe (EP)/TOMS satellite instrument, show elevated tropospheric ozone (> 35 Dobson Units) throughout the south Atlantic in January 1999. Ozone layers associated with biomass burning north of the ITCZ (Intertropical Convergence Zone) are prominent at 0-5 km from 10-ON, but even higher ozone (100 ppbv, 5-15 km) occurred south of the ITCZ, where it was not burning - an ozone paradox that contributes to a wave-one zonal pattern in tropospheric ozone. Back trajectories, satellite observations and shipboard tracers suggest that the south Atlantic ozone results from a combination of interhemispheric transport, aged stratospheric-upper tropospheric air, and possibly from ozone supplied by lightning nitric oxide.

A trajectory-based estimate of the tropospheric ozone column using the residual method

We estimate the tropospheric column ozone using a forward trajectory model to increase the horizontal resolution of the Aura Microwave Limb Sounder (MLS) derived stratospheric column ozone. Subtracting the MLS stratospheric column from Ozone Monitoring Instrument total column measurements gives the trajectory enhanced tropospheric ozone residual (TTOR). Because of different tropopause definitions, we validate the basic residual technique by computing the 200-hPa-to-surface column and comparing it to the same product from ozonesondes and Tropospheric Emission Spectrometer measurements. Comparisons show good agreement in the tropics and reasonable agreement at middle latitudes, but there is a persistent low bias in the TTOR that may be due to a slight high bias in MLS stratospheric column. With the improved stratospheric column resolution, we note a strong correlation of extratropical tropospheric ozone column anomalies with probable troposphere-stratosphere exchange events or folds. The folds can be identified by their colocation with strong horizontal tropopause gradients. TTOR anomalies due to folds may be mistaken for pollution events since folds often occur in the Atlantic and Pacific pollution corridors. We also compare the 200-hPa-to-surface column with Global Modeling Initiative chemical model estimates of the same quantity. While the tropical comparisons are good, we note that chemical model variations in 200-hPa-to-surface column at middle latitudes are much smaller than seen in the TTOR.

Intercontinental Chemical Transport Experiment Ozonesonde Network Study (IONS) 2004: 1. Summertime upper troposphere/lower stratosphere ozone over northeastern North America

Coordinated ozonesonde launches from the Intercontinental Transport Experiment (INTEX) Ozonesonde Network Study (IONS) (http://croc.gsfc.nasa.gov/intex/ions.html) in July-August 2004 provided nearly 300 O3 profiles from eleven North American sites and the R/V Ronald H. Brown in the Gulf of Maine. With the IONS period dominated by low-pressure conditions over northeastern North America (NENA), the free troposphere in that region was frequently enriched by stratospheric O3. Stratospheric O3 contributions to the NENA tropospheric O3 budget are computed through analyses of O3 laminae (Pierce and Grant, 1998; Teitelbaum et al., 1996), tracers (potential vorticity, water vapor), and trajectories. The lasting influence of stratospheric incursions into the troposphere is demonstrated, and the computed stratospheric contribution to tropospheric column O3 over the R/V Ronald H. Brown and six sites in Michigan, Virginia, Maryland, Rhode Island, and Nova Scotia, 23% ± 3%, is similar to summertime budgets derived from European O3 profiles (Collette and Ancellet, 2005). Analysis of potential vorticity, Wallops ozonesondes (37.9°N, 75.5°W), and Measurements of Ozone by Airbus In-service Aircraft (MOZAIC) O3 profiles for NENA airports in June-July-August 1996–2004 shows that the stratospheric fraction in 2004 may be typical. Boundary layer O3 at Wallops and northeast U.S. sites during IONS also resembled O3 climatology (June-July-August 1996–2003). However, statistical classification of Wallops O3 profiles shows the frequency of profiles with background, nonpolluted boundary layer O3 was greater than normal during IONS.

Regional levels of ozone in the troposphere over eastern Mediterranean

During the last 5 years, information on the regional tropospheric ozone levels over the eastern Mediterranean has become available but was confined to measurements at ground level. Here available information is expanded with measurements at two more rural ground level sites spanning 8° latitude, ozonesonde ascents, lidar observations, ship cruises, and aircraft flights. During wintertime the ozone monthly values are 10-20 ppbv higher than values at other European sites, while during summertime the values are comparable. This results in regional ozone background levels in the area that are above the European Union (EU) 32 ppb/24 hours phytotoxicity limit during the entire year. Late spring lidar observations show that south and southwestern synoptic flows which are associated with Saharan dust events result in lower ozone above the planetary boundary layer (PBL) by 20-35 ppbv as compared to these during northerly flows, which transport air from continental Europe. These lidar observations along with ship measurements during July show that ozone is enhanced 1.5-2.2 times in the continental outflow, when compared to aged maritime air or air from the African continent. These results along with ozonesonde observations suggest that ozone abatement in the area is largely beyond the control of regional emissions and can be controlled only with emission reductions on a European scale. During September, measured background NO and NO 2 levels between 3 and 4.5 km above sea level ranged from 1 to 221 pptv and from 102 to 580 pptv, respectively. The troposphere contains around 40-70 Dobson units of ozone during summer and around 20-30 Dobson units during winter.

Trace gas transport and scavenging in PEM‐Tropics B South Pacific Convergence Zone convection

Analysis of chemical transport on Flight 10 of the 1999 Pacific Exploratory Mission (PEM) Tropics B mission clarifies the role of the South Pacific Convergence Zone (SPCZ) in establishing ozone and other trace gas distributions in the southwestern tropical Pacific. The SPCZ is found to be a barrier to mixing in the lower troposphere but a mechanism for convective mixing of tropical boundary layer air from northeast of the SPCZ with upper tropospheric air arriving from the west. A two-dimensional cloud-resolving model is used to quantify three critical processes in global and regional transport: convective mixing, lightning NOx production, and wet scavenging of soluble species. Very low NO and O3 tropical boundary layer air from the northeastern side of the SPCZ entered the convective updrafts and was transported to the upper troposphere where it mixed with subtropical upper tropospheric air containing much larger NO and O3 mixing ratios that had arrived from Australia. Aircraft observations show that very little NO appears to have been produced by electrical discharges within the SPCZ convection. We estimate that at least 90% of the HNO3 and H2O2 that would have been in upper tropospheric cloud outflow had been removed during transport through the cloud. Lesser percentages are estimated for less soluble species (e.g., <50% for CH3OOH). Net ozone production rates were decreased in the upper troposphere by ∼60% due to the upward transport and outflow of low-NO boundary layer air. However, this outflow mixed with much higher NO air parcels on the southwest edge of the cloud, and the mixture ultimately possessed a net ozone production potential intermediate between those of the air masses on either side of the SPCZ.

Trends and variability in surface ozone over the United States

We investigate the observed trends and interannual variability in surface ozone over the United States using the Global Modeling Initiative chemical transport model. We discuss the roles of meteorology, emissions, and transport from the stratosphere in driving the interannual variability in different regions and seasons. We demonstrate that a hindcast simulation for 1991–2010 can reproduce much of the observed variability and the trends in summertime ozone, with correlation coefficients for seasonally and regionally averaged median ozone ranging from 0.46 to 0.89. Reproducing the interannual variability in winter and spring in the western United States may require higher-resolution models to adequately represent stratosphere-troposphere exchange. Hindcast simulations with fixed versus variable emissions show that changes in anthropogenic emissions drive the observed negative trends in monthly median ozone concentrations in the eastern United States during summer, as well as the observed reduction in the amplitude of the seasonal cycle. The simulation underestimates positive trends in the western United States during spring, but excluding the first 4 years of data removes many of the statistically significant trends in this region. The reduction in the slope of the ozone versus temperature relationship before and after major emission reductions is also well represented by the model. Our results indicate that a global model can reproduce many of the important features of the meteorologically induced ozone variability as well as the emission-driven trends, lending confidence to model projections of future changes in regional surface ozone.

Initial validation of ozone measurements from the High Resolution Dynamics Limb Sounder

Comparisons of the latest High Resolution Dynamics Limb Sounder (HIRDLS) ozone retrievals (v2.04.09) are made with ozonesondes, ground-based lidars, airborne lidar measurements made during the Intercontinental Chemical Transport Experiment–B, and satellite observations. A large visual obstruction blocking over 80% of the HIRDLS field of view presents significant challenges to the data analysis methods and implementation, to the extent that the radiative properties of the obstruction must be accurately characterized in order to adequately correct measured radiances. The radiance correction algorithms updated as of August 2007 are used in the HIRDLS v2.04.09 data presented here. Comparisons indicate that HIRDLS ozone is recoverable between 1 and 100 hPa at middle and high latitudes and between 1 and 50 hPa at low latitudes. Accuracy of better than 10% is indicated between 1 and 30 hPa (HIRDLS generally low) by the majority of the comparisons with coincident measurements, and 5% is indicated between 2 and 10 hPa when compared with some lidars. Between 50 and 100 hPa, at middle and high latitudes, accuracy is 10–20%. The ozone precision is estimated to be generally 5–10% between 1 and 50 hPa. Comparisons with ozonesondes and lidars give strong indication that HIRDLS is capable of resolving fine vertical ozone features (1–2 km) in the region between 1 and 50 hPa. Development is continuing on the radiance correction and the cloud detection and filtering algorithms, and it is hoped that it will be possible to achieve a further reduction in the systematic bias and an increase in the measurement range downward to lower heights (at pressures greater than 50–100 hPa).

The global structure of upper troposphere‐lower stratosphere ozone in GEOS‐5: A multiyear assimilation of EOS Aura data

Eight years of ozone measurements retrieved from the Ozone Monitoring Instrument and the Microwave Limb Sounder, both on the EOS Aura satellite, have been assimilated into the Goddard Earth Observing System Version 5 (GEOS-5) data assimilation system. This study evaluates this assimilated product, highlighting its potential for science. The impact of observations on the GEOS-5 system is explored by examining the spatial distribution of the observation-minus-forecast statistics. Independent data are used for product validation. The correlation of the lower stratospheric (the tropopause to 50 hPa) ozone column with ozonesondes is 0.99 and the (high) bias is 0.5%, indicating the success of the assimilation in reproducing the ozone variability in that layer. The upper tropospheric (500 hPa to the tropopause) assimilated ozone column is about 10% lower than the ozonesonde column, but the correlation is still high (0.87). The assimilation is shown to realistically capture the sharp cross-tropopause gradient in ozone mixing ratio. Occurrence of transport-driven low ozone laminae in the assimilation system is similar to that obtained from the High Resolution Dynamics Limb Sounder (HIRDLS) above the 400 K potential temperature surface, but the assimilation produces fewer laminae than seen by HIRDLS below that surface. Although the assimilation produces about 25% fewer occurrences per day during the 3 years of HIRDLS data, the interannual variability is captured correctly. This data-driven assimilated product is complementary to ozone fields generated from chemistry and transport models. Applications include study of the radiative forcing by ozone and tracer transport near the tropopause.

Assessment and applications of NASA ozone data products derived from Aura OMI/MLS satellite measurements in context of the GMI chemical transport model

Measurements from the Ozone Monitoring Instrument (OMI) and Microwave Limb Sounder (MLS), both on board the Aura spacecraft, have been used to produce daily global maps of column and profile ozone since August 2004. Here we compare and evaluate three strategies to obtain daily maps of tropospheric and stratospheric ozone from OMI and MLS measurements: trajectory mapping, direct profile retrieval, and data assimilation. Evaluation is based on an assessment that includes validation using ozonesondes and comparisons with the Global Modeling Initiative (GMI) chemical transport model. We investigate applications of the three ozone data products from near-decadal and interannual time scales to day-to-day case studies. Interannual changes in zonal mean tropospheric ozone from all of the products in any latitude range are of the order 1–2 Dobson units while changes (increases) over the 8 year Aura record investigated vary by 2–4 Dobson units. It is demonstrated that all of the ozone products can measure and monitor exceptional tropospheric ozone events including major forest fire and pollution transport events. Stratospheric ozone during the Aura record has several anomalous interannual events including split stratospheric warmings in the Northern Hemisphere extratropics that are well captured using the data assimilation ozone profile product. Data assimilation with continuous daily global coverage and vertical ozone profile information is the best of the three strategies at generating a global tropospheric and stratospheric ozone product for science applications.