Limb Sounder Measurements Research Articles

Measurements of Stratospheric Water Vapor at Mauna Loa and the Effect of the Hunga Tonga Eruption

AbstractThe eruption of Hunga Tonga in January 2022 injected an amount of water vapor into the stratosphere that is unprecedented in the satellite era. In the ensuing months Aura Microwave Limb Sounder measurements showed that this plume of water vapor spread from its original injection site at 20.5°S to Mauna Loa, Hawaii at 19.5°N, where an increase was observed in April by the ground‐based Water Vapor Millimeter‐wave Spectrometer instruments. Interannual variations in water vapor occur over Mauna Loa due to both dynamical variations in the tropical stratosphere and variations in the amount of water vapor crossing the tropical tropopause, and we place the observed stratospheric water vapor increase from Hunga Tonga into context of these other variations that have been observed since 2013.

Secondary 12‐Day Planetary Wave in the Mesospheric Water Vapor During the 2016/2017 Unusual Canadian Stratospheric Warming

AbstractThe anomalous strengthening of the stratospheric Aleutian High during the early boreal winter of 2016/2017 led to a rare Canadian stratospheric warming (CSW) event, during which the extent of the wind reversal in the Arctic region is the strongest among CSWs since 2001. Unlike sudden stratospheric warmings, the polar vortex during this CSW is merely distorted, not displaced off the pole or split into two pieces, and the polar mean temperature only has a modest increase of a few kelvins. Using the Aura satellite Microwave Limb Sounder measurements, an exceptionally strong 12‐day wave (12DW) with westward zonal wavenumber 1 is captured in the mesospheric water vapor mixing ratio during the recovery phase of this CSW. Further diagnostic analysis suggests that the baroclinic/barotropic instability related to this CSW triggers the 12DW in the Arctic stratosphere, and meridional advection plays a leading role in the modulation of 12DW on mesospheric water vapor.

Mid-Latitude Mesospheric Zonal Wave 1 and Wave 2 in Recent Boreal Winters

Planetary waves in the mesosphere are studied using observational data and models to establish their origin, as there are indications of their generation independently of waves in the stratosphere. The quantitative relationships between zonal wave 1 and wave 2 were studied with a focus on the mid-latitude mesosphere at 50°N latitude. Aura Microwave Limb Sounder measurements were used to estimate wave amplitudes in geopotential height during sudden stratospheric warmings in recent boreal winters. The moving correlation between the wave amplitudes shows that, in comparison with the anticorrelation in the stratosphere, wave 2 positively correlates with wave 1 and propagates ahead of it in the mesosphere. A positive correlation r = 0.5–0.6, statistically significant at the 95% confidence level, is observed at 1–5-day time lag and in the 75–91 km altitude range, which is the upper mesosphere–mesopause region. Wavelet analysis shows a clear 8-day period in waves 1 and 2 in the mesosphere at 0.01 hPa (80 km), while in the stratosphere–lower mesosphere, the period is twice as long at 16 days; this is statistically significant only in wave 2. Possible sources of mesospheric planetary waves associated with zonal flow instabilities and breaking or dissipation of gravity waves are discussed.

Intriguing aspects of Asian Summer Monsoon Anticyclone Ozone variability from Microwave Limb Sounder measurements

Using the long-term (2005–2019) observations from the Microwave Limb Sounder (MLS) onboard Aqua satellite, the present study delineates the variability of the lower stratospheric ozone (LSO) in the Asian Summer Monsoon Anticyclone (ASMA) region. The climatological features of LSO indicate a peak minimum in the southern flank of ASMA noticed by the geopotential measurements from MLS. Further, the temporal variability of LSO indicates significant interannual variability during the 15 years of MLS observations. Our analysis found that two prominent source regions control the LSO variability at interannual time scales in the ASMA region, associated with the Pacific and Indian Ocean sea surface temperatures. More importantly, the extreme (positive/negative) values in ozone anomalies coincide with the El Niño–Southern Oscillation(El Niño/La Niña) events. Further, the composite analysis reveals a significant change in the mean magnitude of LSO of about 10 ppb between El Niño and La Niña phases. The results indicate that reduced water vapor transport due to subsidence and suppressed convection during the El Niño phase leads to less mixing across the upper troposphere and lower stratosphere, therefore, causing higher concentrations of ozone and vice-versa during the La Niña phase. Hence, this study indicates a strong implication for the climatic perturbations over the ASMA region.

Comparison of Major Sudden Stratospheric Warming Impacts on the Mid-Latitude Mesosphere Based on Local Microwave Radiometer CO Observations in 2018 and 2019

In this paper, a comparison of the impact of major sudden stratospheric warmings (SSWs) in the Arctic in February 2018 (SSW1) and January 2019 (SSW2) on the mid-latitude mesosphere is given. The mesospheric carbon monoxide (CO) and zonal wind in these two major SSW events were observed at altitudes of 70–85 km using a microwave radiometer (MWR) at Kharkiv, Ukraine (50.0°N, 36.3°E). Data from ERA-Interim and MERRA-2 reanalyses and Aura Microwave Limb Sounder measurements were also used. It is shown that: (i) The differences between SSW1 and SSW2, in terms of local variability in zonal wind, temperature, and CO in the stratosphere and mesosphere, were clearly defined by the polar vortex (westerly in cyclonic circulation) and mid-latitude anticyclone (easterly) migrating over the MWR station, therefore; (ii) mesospheric intrusions of CO-rich air into the stratosphere over the Kharkiv region occurred only occasionally, (iii) the larger zonal wave 1–3 amplitudes before SSW1 were followed by weaker polar vortex recovery than that after SSW2, (iv) the strong vortex recovery after SSW2 was supported by earlier event timing (midwinter) favoring vortex cooling due to low solar irradiance and enhanced zonal circulation, and (v) vortex strengthening after SSW2 was accompanied by wave 1–3 amplification in March 2019, which was absent after SSW1. Finally, the influence of the large-scale circulation structures formed in individual major SSW events on the locally recorded characteristics of the atmosphere is discussed.

Recovery and validation of Odin/SMR long-term measurements of mesospheric carbon monoxide

Abstract. The Sub-Millimetre Radiometer (SMR) on board the Odin satellite performs limb sounding measurements of the middle atmosphere to detect molecular emission from different species. Carbon monoxide (CO) is an important tracer of atmospheric dynamics at these altitudes, due to its long photochemical lifetime and high vertical concentration gradient. In this study, we have successfully recovered over 18 years of SMR observations, providing the only dataset to date being so extended in time and stretching out to the polar regions, with regards to satellite-measured mesospheric CO. This new dataset is part of the Odin/SMR version 3.0 level 2 data. Much of the level 1 dataset – except the October 2003 to October 2004 period – was affected by a malfunctioning of the phase-lock loop (PLL) in the front end used for CO observations. Because of this technical issue, the CO line could be shifted away from its normal frequency location, causing the retrieval to fail or leading to an incorrect estimation of the CO concentration. An algorithm was developed to locate the CO line and shift it to its correct location. Nevertheless, another artefact causing an underestimation of the concentration, i.e. a line broadening, stemmed from the PLL malfunctioning. This was accounted for by using a broader response function. The application of these corrections resulted in the recovery of a large amount of data that was previously being flagged as problematic and therefore not processed. A validation study has been carried out, showing how SMR CO volume mixing ratios are in general in good accordance with the other instruments considered in the study. Overall, the agreement is very good between 60 and 80 km altitude, with relative differences close to zero. A positive bias at low altitudes (50–60 km) up to +20 % and a negative bias up to −20 % at high altitudes (80–100 km) were found with respect to the comparison instruments.

Winter 2018 major sudden stratospheric warming impact on midlatitude mesosphere from microwave radiometer measurements

Abstract. The impact of a major sudden stratospheric warming (SSW) in the Arctic in February 2018 on the midlatitude mesosphere is investigated by performing the microwave radiometer measurements of carbon monoxide (CO) and zonal wind above Kharkiv, Ukraine (50.0∘ N, 36.3∘ E). The mesospheric peculiarities of this SSW event were observed using a recently designed and installed microwave radiometer in eastern Europe for the first time. Data from the ERA-Interim and MERRA-2 reanalyses, as well as the Aura microwave limb sounder measurements, are also used. Microwave observations of the daily CO profiles in January–March 2018 allowed for the retrieval of mesospheric zonal wind at 70–85 km (below the winter mesopause) over the Kharkiv site. Reversal of the mesospheric westerly from about 10 m s−1 to an easterly wind of about −10 m s−1 around 10 February was observed. The local microwave observations at our Northern Hemisphere (NH) midlatitude site combined with reanalysis data show wide-ranging daily variability in CO, zonal wind, and temperature in the mesosphere and stratosphere during the SSW of 2018. The observed local CO variability can be explained mainly by horizontal air mass redistribution due to planetary wave activity. Replacement of the CO-rich polar vortex air by CO-poor air of the surrounding area led to a significant mesospheric CO decrease over the station during the SSW and fragmentation of the vortex over the station at the SSW start caused enhanced stratospheric CO at about 30 km. The results of microwave measurements of CO and zonal wind in the midlatitude mesosphere at 70–85 km altitudes, which still are not adequately covered by ground-based observations, are useful for improving our understanding of the SSW impacts in this region.

3-D tomographic limb sounder retrieval techniques: irregular grids and Laplacian regularisation

Abstract. Multiple limb sounder measurements of the same atmospheric region taken from different directions can be combined in a 3-D tomographic retrieval. Mathematically, this is a computationally expensive inverse modelling problem. It typically requires an introduction of some general knowledge of the atmosphere (regularisation) due to its underdetermined nature. This paper introduces a consistent, physically motivated (no ad-hoc parameters) variant of the Tikhonov regularisation scheme based on spatial derivatives of the first-order and Laplacian. As shown by a case study with synthetic data, this scheme, combined with irregular grid retrieval methods employing Delaunay triangulation, improves both upon the quality and the computational cost of 3-D tomography. It also eliminates grid dependence and the need to tune parameters for each use case. The few physical parameters required can be derived from in situ measurements and model data. Tests show that a 82 % reduction in the number of grid points and 50 % reduction in total computation time, compared to previous methods, could be achieved without compromising results. An efficient Monte Carlo technique was also adopted for accuracy estimation of the new retrievals.

11 Years of Rayleigh Lidar Observations of Gravity Wave Activity Above the Southern Tip of South America

AbstractGravity wave (GW) activity is analyzed using temperature (T) data retrieved from a Rayleigh light detection and ranging (lidar) at Río Gallegos, Argentina (51.6°S, 69.3°W). GW characteristics are derived from 302 nights of observations providing more than 1,018 hr of high‐resolution lidar data between 20‐ and 56‐km height from August 2005 to December 2015. T measurements are performed by a Differential Absorption Lidar instrument. This lidar was the southernmost outside Antarctica until the end of 2017. Río Gallegos is an exceptional place to observe large amplitude GW. Every lidar measurement is classified according to its relative position to the polar vortex. The lidar measurements are compared with collocated Sounding of the Atmosphere using Broadband Emission Radiometry and Global Positioning System‐Radio Occultation data. The different instruments show different windows of the GW spectrum, providing complementary observations. In general, the geometric mean of the specific GW potential energy (PE) is larger during winter and spring than during summer and autumn. The largest geometric mean of PE is found inside the vortex and decreases monotonically at its edge, outside it and when there is no vortex. The same behavior is observed with satellite data. On average, it can be seen that lidar observations provide larger PE values than limb sounding measurements. From a Morlet continuous wavelet transform analysis, three distinct modes are captured from Sounding of the Atmosphere using Broadband Emission Radiometry and from Global Positioning System‐Radio Occultation data at the upper and lower stratosphere, respectively. In particular, a systematic 3.5‐ to 4‐year oscillation, possibly related to El Niño–Southern Oscillation is observed.

Response of Mesospheric HO2 and O3 to Large Solar Proton Events

AbstractA lot of solar proton events (SPEs) have happened during the ongoing solar cycle 24 including extremely large events. Seven large SPEs from 2012 to 2017 were chosen to find the response of polar atmosphere to them. Three SPEs happened in 2012, and the other four SPEs happened in 2013, 2014, 2015, and 2017, respectively. The National Oceanic and Atmospheric Administration Geostationary Operational Environmental Satellites 13 proton observations were used to illustrate the proton flux toward the Earth during each SPE. These energetic protons are deposited in the middle and upper atmosphere and ionize the neutral atmosphere constituents and create radicals HOx (H, OH, and HO2). Enhancements in the upper atmospheric hydroperoxyl radical (HO2) of both polar regions of over 1 ppbv are found in the Aura Microwave Limb Sounder measurements immediately after some SPEs happened. The Microwave Limb Sounder observations of ozone (O3) show decreases of >10% in both polar middle and upper atmosphere for a few days with a peak value of >20–60% over 1–3 days. The compositions of HO2 and ozone for all the selected SPEs indicate that the polar ozone depletions during the SPEs in a way are the result of the HO2 enhancements.

Convectively Driven Tropopause‐Level Cooling and Its Influences on Stratospheric Moisture

AbstractCharacteristics of the tropopause‐level cooling associated with tropical deep convection are examined using CloudSat radar and Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) GPS radio occultation measurements. Extreme deep convection is sampled based on the cloud top height (>17 km) from CloudSat, and colocated temperature profiles from COSMIC are composited around the deep convection. Response of moisture to the tropopause‐level cooling is also examined in the upper troposphere and lower stratosphere using microwave limb sounder measurements. The composite temperature shows an anomalous warming in the troposphere and a significant cooling near the tropopause (at 16–19 km) when deep convection occurs over the western Pacific, particularly during periods with active Madden‐Julian Oscillation (MJO). The composite of the tropopause cooling has a large horizontal scale (~6,000 km in longitude) with minimum temperature anomaly of ~ −2 K, and it lasts more than 2 weeks with support of mesoscale convective clusters embedded within the envelope of the MJO. The water vapor anomalies show strong correlation with the temperature anomalies (i.e., dry anomaly in the cold anomaly), showing that the convectively driven tropopause cooling actively dehydrate the lower stratosphere in the western Pacific region. The moisture is also affected by anomalous Matsuno‐Gill‐type circulation associated with the cold anomaly, in which dry air spreads over a wide range in the tropical tropopause layer (TTL). These results suggest that convectively driven tropopause cooling and associated transient circulation play an important role in the large‐scale dehydration process in the TTL.

Decline in Antarctic Ozone Depletion and Lower Stratospheric Chlorine Determined From Aura Microwave Limb Sounder Observations

AbstractAttribution of Antarctic ozone recovery to the Montreal protocol requires evidence that (1) Antarctic chlorine levels are declining and (2) there is a reduction in ozone depletion in response to a chlorine decline. We use Aura Microwave Limb Sounder measurements of O3, HCl, and N2O to demonstrate that inorganic chlorine (Cly) from 2013 to 2016 was 223 ± 93 parts per trillion lower in the Antarctic lower stratosphere than from 2004 to 2007 and that column ozone depletion declined in response. The mean Cly decline rate, ~0.8%/yr, agrees with the expected rate based on chlorofluorocarbon lifetimes. N2O measurements are crucial for identifying changes in stratospheric Cly loading independent of dynamical variability. From 2005 to 2016, the ozone depletion and Cly time series show matching periods of decline, stability, and increase. The observed sensitivity of O3 depletion to changing Cly agrees with the sensitivity simulated by the Global Modeling Initiative chemistry transport model integrated with Modern Era Retrospective Analysis for Research and Applications 2 meteorology.

First Observations of Short‐Period Eastward Propagating Planetary Waves From the Stratosphere to the Lower Thermosphere (110 km) in Winter Antarctica

AbstractUnique Fe lidar observations in May 2014 at McMurdo, combined with Aura‐Microwave Limb Sounder measurements, lead to a new discovery that the amplitudes of 4 day and 2.5 day planetary waves (PWs) grow rapidly from 1–2 K at 100 km to over 10 K at 110 km. This report is also the first observation of short‐period (1–5 days) eastward propagating PWs from 30 km all the way to 110 km. The Specified Dynamics‐Whole Atmosphere Community Climate Model reproduces the observed three dominant peaks of amplitudes in temperature and coherent vertical phase structures. The data‐model comparison indicates a possible mechanism: After PWs originated from the stratosphere dissipate along the critical level, the surviving waves are amplified by in situ instability in the mesosphere and lower thermosphere, resulting in the second (third) peak in geopotential (temperature). This third peak in temperature explains the PW amplitude growth from 100 to 110 km.

A comprehensive overview of the climatological composition of the Asian summer monsoon anticyclone based on 10 years of Aura Microwave Limb Sounder measurements

AbstractIntense deep convection associated with the Asian summer monsoon (ASM) lofts surface pollutants to the upper troposphere/lower stratosphere (UTLS), where strong winds and long chemical lifetimes allow intercontinental transport, affecting atmospheric composition around the globe. The Aura Microwave Limb Sounder (MLS), launched in 2004, makes simultaneous colocated measurements of trace gases and cloud ice water content (a proxy for deep convection) in the UTLS on a daily basis. Here we exploit the dense spatial and temporal coverage, long‐term data record, extensive measurement suite, and insensitivity to aerosol and most clouds of Aura MLS to characterize the climatological (2005–2014) composition of the ASM anticyclone throughout its annual life cycle. We use version 4 MLS data to quantify spatial and temporal variations in both tropospheric (H2O, CO, CH3Cl, CH3CN, CH3OH) and stratospheric (O3, HNO3, HCl) tracers on four potential temperature surfaces (350–410 K). Inside the mature anticyclone, all species exhibit substantial changes, not only from their premonsoon distributions in the ASM region but also from their summertime distributions in the rest of the hemisphere. Different tracers exhibit dissimilar seasonal evolution, and the exact location and timing of their extreme values vary. Although individual aspects of the anticyclone have been described previously, we present a uniquely comprehensive overview of the climatological seasonal evolution of the ASM and its impact on UTLS composition. This work provides valuable context for planned in situ measurements as well as a benchmark for model evaluation and future investigations of interannual variability and long‐term changes in monsoon processes.

Synergy between middle infrared and millimeter-wave limb sounding of atmospheric temperature and minor constituents

Abstract. Synergistic exploitation of redundant and complementary information from independent observations of the same target remains a major issue in atmospheric remote sounding and increasing attention is devoted to investigate optimized or innovative methods for the combination of two or more measured data sets. This paper focuses on the synergy between middle infrared and millimeter-wave limb sounding measurements of atmospheric composition and temperature and reports the results of a study conducted as part of the preparatory activities of the PREMIER (Process Exploration through Measurements of Infrared and millimeter-wave Emitted Radiation) mission candidate to the Core Missions of the European Space Agency (ESA) Earth Explorer 7. The activity was based on data acquired by the MIPAS-STR (Michelson Interferometer for Passive Atmospheric Sounding – STRatospheric aircraft) and MARSCHALS (Millimetre-wave Airborne Receivers for Spectroscopic CHaracterisation in Atmospheric Limb Sounding) instruments on-board the high-altitude research aircraft M-55 Geophysica during the flight of the PremierEx (PREMIER Experiment) campaign on 10 March 2010 from Kiruna, Sweden, for observation of the Arctic upper troposphere and lower stratosphere. The cloud coverage observed along the flight provided representative test cases to evaluate the synergy in three different scenarios: low clouds in the first part, no clouds in the central part and high tropospheric clouds at the end. The calculation of synergistic profiles of four atmospheric targets (i.e., O3, HNO3, H2O and temperature) was performed using a posteriori combination of individual retrieved profiles, i.e., Level 2 (L2) data rather than simultaneous inversion of observed radiances, i.e., Level 1 (L1) data. An innovative method of data fusion, based on the Measurement Space Solution (MSS) was applied along with the standard approach of inversion of MARSCHALS spectral radiances using MIPAS-STR retrieval products as a priori information (L1 + L2 method). A quantitative estimate and cross-check of the results of MSS data fusion and (L1 + L2) method was achieved based on a specific set of quantifiers including the total retrieval error, the number of degrees of freedom, the relative information distribution and the synergy factor.

Multi-instrument gravity-wave measurements over Tierra del Fuego and the Drake Passage – Part 1: Potential energies and vertical wavelengths from AIRS, COSMIC, HIRDLS, MLS-Aura, SAAMER, SABER and radiosondes

Abstract. Gravity waves in the terrestrial atmosphere are a vital geophysical process, acting to transport energy and momentum on a wide range of scales and to couple the various atmospheric layers. Despite the importance of these waves, the many studies to date have often exhibited very dissimilar results, and it remains unclear whether these differences are primarily instrumental or methodological. Here, we address this problem by comparing observations made by a diverse range of the most widely used gravity-wave-resolving instruments in a common geographic region around the southern Andes and Drake Passage, an area known to exhibit strong wave activity. Specifically, we use data from three limb-sounding radiometers (Microwave Limb Sounder, MLS-Aura; HIgh Resolution Dynamics Limb Sounder, HIRDLS; Sounding of the Atmosphere using Broadband Emission Radiometry, SABER), the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) GPS-RO constellation, a ground-based meteor radar, the Advanced Infrared Sounder (AIRS) infrared nadir sounder and radiosondes to examine the gravity wave potential energy (GWPE) and vertical wavelengths (λz) of individual gravity-wave packets from the lower troposphere to the edge of the lower thermosphere ( ∼ 100 km). Our results show important similarities and differences. Limb sounder measurements show high intercorrelation, typically > 0.80 between any instrument pair. Meteor radar observations agree in form with the limb sounders, despite vast technical differences. AIRS and radiosonde observations tend to be uncorrelated or anticorrelated with the other data sets, suggesting very different behaviour of the wave field in the different spectral regimes accessed by each instrument. Evidence of wave dissipation is seen, and varies strongly with season. Observed GWPE for individual wave packets exhibits a log-normal distribution, with short-timescale intermittency dominating over a well-repeated monthly-median seasonal cycle. GWPE and λz exhibit strong correlations with the stratospheric winds, but not with local surface winds. Our results provide guidance for interpretation and intercomparison of such data sets in their full context.

Derived methane in the stratosphere and lower mesosphere from Aura Microwave Limb Sounder measurements of nitrous oxide, water vapor, and carbon monoxide

The global distribution of methane (CH4) in the stratosphere and lower mesosphere has been derived using coincident measurements of water vapor (H2O), carbon monoxide (CO), and nitrous oxide (N2O) from the Microwave Limb Sounder (MLS) instrument on the Aura satellite. The derivation method is based on empirical relationships between these species established using observations from the Atmospheric Chemistry Experiment—Fourier Transform Spectrometer (ACE-FTS) on the SCISAT I satellite. The observed correlation between CH4 and N2O from ACE-FTS is used to derive CH4 from MLS measurements of N2O in the lower stratosphere, extending from a pressure of 100 hPa to a range of 30–10 hPa, depending on atmospheric conditions. In the upper stratosphere and lower mesosphere, between 30–10 hPa and 0.1 hPa, correlations between CH4 and H2O are used to derive CH4 from MLS measurements of H2O. Coincident MLS measurements of CO are utilized to separate two distinct air mass regimes in the CH4 - H2O relationship. This new methane data set covers all seasons and latitudes observed by MLS over the course of the Aura mission from 2004 to 2014. Examples are shown demonstrating the consistency of MLS derived CH4 with other trace gas measurements.

Trace gas evolution in the lowermost stratosphere from Aura Microwave Limb Sounder measurements

[1] Daily global measurements from NASA's Aura Microwave Limb Sounder (MLS) allow comprehensive investigation of interhemispheric and interannual variations in chemical and transport processes throughout the lowermost stratosphere (LMS). We analyze nearly seven years of MLS O3, HNO3, HCl and ClO measurements along with meteorological analyses to place chemical processing in and dispersal of processed air from the winter polar lowermost vortex and subvortex in a global context. The MLS data, the first simultaneous observations of HCl and ClO covering much of the LMS, reveal that chlorine activation is widespread in the Antarctic subvortex and can occur to a significant degree in the Arctic subvortex. Unusually low temperatures and strong, prolonged chlorine activation in the lowermost vortex and subvortex promoted large ozone losses there in the 2006 (and 2008) Antarctic and 2004/2005 Arctic winters, consistent with reported record low column ozone. Processed air dispersing from the decaying vortex in spring induces rapid changes in extravortex trace gas abundances. After vortex breakdown, the subtropical jet/tropopause becomes the major transport barrier in the LMS. Quasi-isentropic transport of tropical tropospheric air into the LMS, associated with the summer monsoon circulations, leads to decreases in extratropical O3, HNO3, and HCl in both hemispheres. Strong mixing in the summertime LMS homogenizes extratropical trace gas fields. MLS measurements in the tropics show signatures of monsoon-related cross-equatorial stratosphere-to-troposphere transport. Observed seasonal and interannual variations in trace gas abundances in the LMS are consistent with variations in the strength of transport barriers diagnosed from meteorological analyses.

Impact of East Asian summer monsoon on the air quality over China: View from space

Tropospheric O3 columns retrieved from Ozone Monitoring Instrument and Microwave Limb Sounder measurements, CO columns retrieved from Measurements of Pollution in the Troposphere, and tropospheric O3 and CO concentrations retrieved from the Tropospheric Emission Spectrometer from May to August in 2006 are analyzed using the Regional Chemical and Transport Model to investigate the impact of the East Asian summer monsoon on the air quality over China. The observed and simulated migrations of O3 and CO are in good agreement, demonstrating that the summer monsoon significantly affects the air quality over southeastern China, and this influence extends to central East China from June to July. Enhancements of CO and O3 over southeastern China disappear after the onset of the summer monsoon and reemerge in August after the monsoon wanes. The premonsoon high O3 concentrations over southern China are due to photochemical production from pollutant emissions and the O3 transport from the stratosphere. In the summer monsoon season, the O3 concentrations are relatively low over monsoon‐affected regions because of the transport of marine air masses and weak photochemical activity. We find that the monsoon system strongly modulates the pollution problem over a large portion of East China in summer, depending on its strength and tempo‐spatial extension. Model results also suggest that transport from the stratosphere and long‐range transport from East China and South/central Asia all make significant contributions to O3 enhancements over West China. Satellite observations provide valuable information for investigating the monsoon impact on air quality, particularly for the regions with limited in situ measurements.

A study of tropospheric ozone column enhancements over North America using satellite data and a global chemical transport model

Tropospheric ozone columns (TCOs) have been calculated from the differences between the Aura Ozone Monitoring Instrument (OMI) Total Ozone Mapping System (TOMS) total ozone (level 2 version 3) and the Aura Microwave Limb Sounding (MLS) measurements of stratospheric ozone (version 2.2). These OMI‐MLS TCOs were compared against ozonesonde measurements from the Intercontinental Chemical Transport Experiment (INTEX) Ozonesonde Network Study (IONS) campaign over North America in spring and summer, 2006. The OMI‐MLS potential vorticity mapped TCOs are smaller than IONS TCOs by 5.9 DU (9.9 ppb when expressed as volume mixing ratio) with a standard deviation of the differences of 8.4 DU (14.4 ppb) and a standard error of the mean differences of approximately 0.5 DU (0.7 ppb). Compared to previously published versions, these OMI‐MLS TCOs are an additional 2 DU smaller relative to ozonesonde measurements. The extra 2 DU arises from changes in OMI (∼−3 to −6 DU) and MLS (−1 to 3 DU), giving a net change of −2 DU. OMI‐MLS TCOs derived using OMI Differential Optical Absorption Spectroscopy (DOAS) show similar differences in summer, but these TCOs are smaller than the sondes by only 2 DU (5 ppb) in spring. OMI‐MLS TCOs derived from TOMS total ozone retrievals lead to better results when validated against IONS data, with less noise and a better seasonal consistency. Tropospheric ozone columns were also compared to those from GEOS‐Chem model simulations in main distribution features. In the spring and summer of 2005 and 2006, the most dominant enhancement features are a tongue of enhancement stretching from around Yellow Sea northeastward into the Pacific and an enhancement band over the North America centered over the eastern United States and the adjacent ocean. The OMI‐MLS TCO enhancements over the western Pacific and over the eastern United States increased from March to June and then decreased. In the GEOS‐Chem model simulations, the monthly variation tendency is similar to that of satellite data over the west Pacific but the decrease tendency from June into August over eastern United States is less dramatic. A springtime TCO enhancement event of a few days duration over coastal California was investigated to demonstrate the ability of OMI‐MLS mapped TCO columns in capturing ozone enhancements associated with stratospheric intrusions and trans‐Pacific transport. Tagged ozone model simulations support the stratospheric contributions to the high TCOs over coastal California and over the Baja peninsula, and meteorological fields indicate that the stratospheric intrusions are associated with Rossby wave breaking events. Furthermore, back trajectory studies and comparisons of GEOS‐Chem standard simulations and sensitivity runs with Asia anthropogenic emissions turned off provide evidence that the high tropospheric ozone columns over coastal California near Santa Barbara, California, has been influenced by cross‐Pacific transport. Two‐day‐average maps of tropospheric ozone columns from Aura OMI‐MLS TCOs also indicate cross‐Pacific propagating features.