Ozone Diurnal Cycles Research Articles

Microwave radiometer observations of the ozone diurnal cycle and its short-term variability over Switzerland

Abstract. In Switzerland, two ground-based ozone microwave radiometers are operated in the vicinity of each other (ca. 40 km): the GROund-based Millimeter-wave Ozone Spectrometer (GROMOS) in Bern (Institute of Applied Physics) and the Stratospheric Ozone MOnitoring RAdiometer (SOMORA) in Payerne (MeteoSwiss). Recently, their calibration and retrieval algorithms have been fully harmonized, and updated time series are now available since 2009. Using these harmonized ozone time series, we investigate and cross-validate the strato–mesospheric ozone diurnal cycle derived from the two instruments and compare it with various model-based datasets: the dedicated GEOS-GMI Diurnal Ozone Climatology (GDOC) based on the Goddard Earth Observing System (GEOS-5) general circulation model, the Belgian Assimilation System for Chemical ObsErvations (BASCOE) – a chemical transport model driven by ERA5 dynamics, and a set of free-running simulations from the Whole Atmosphere Community Climate Model (WACCM). Overall, the two instruments show very similar ozone diurnal cycles at all seasons and pressure levels, and the models compare well with each other. There is a good agreement between the models and the measurements at most seasons and pressure levels, and the largest discrepancies can be explained by the limited vertical resolution of the microwave radiometers. However, as in a similar study over Mauna Loa, some discrepancies remain near the stratopause, at the transition region between ozone daytime accumulation and depletion. We report similar delays in the onset of the modelled ozone diurnal depletion in the lower mesosphere. Using the newly harmonized time series of GROMOS and SOMORA radiometers, we present the first observations of short-term (sub-monthly) ozone diurnal cycle variability at mid-latitudes. The short-term variability is observed in the upper stratosphere during wintertime, when the mean monthly cycle has a small amplitude and when the dynamics are more important. This is shown in the form of strong enhancements of the diurnal cycle, reaching up to 4–5 times the amplitude of the mean monthly cycle. We show that BASCOE is able to capture some of these events, and we present a case study of one such event following the minor sudden stratospheric warming of January 2015. Our analysis of this event supports the conclusions of a previous modelling study, attributing regional variability of the ozone diurnal cycle to regional anomalies in nitrogen oxide (NOx) concentrations. However, we also find periods with an enhanced diurnal cycle that do not show much change in NOx and where other processes might be dominant (e.g. atmospheric tides). Given its importance, we believe that the short-term variability of the ozone diurnal cycle should be further investigated over the globe, for instance using the BASCOE model.

Meteorological and chemical controls on surface ozone diurnal variability in Beijing: A clustering-based perspective

This study aimed to reveal the dominant, recurring patterns of surface ozone diurnal cycles in Beijing, their quantitative characteristics, namely, occurrence frequency and transition probability, and their potential linkages to meteorological and chemical processes based on multi-source observations (surface ozone, satellite-based ozone precursors, ground-based and sonde-based meteorological measurements) and reanalyses (ERA5 meteorological and TCR2 chemical reanalyses). By applying K-means clustering, four distinct ozone diurnal patterns (Cluster 1: all-day low anomaly; Cluster 2: nighttime high-daytime low anomaly; Cluster 3: nighttime low-daytime high anomaly; Cluster 4: extreme pollution pattern with all-day high anomaly) are objectively identified from 2013 to 2020 warm-season (May–September) surface ozone measurements. Despite a relatively low total occurrence, the extreme pollution pattern tended to persist over an extended period (average persistence of 2.3 days). Composite analyses of multi-source meteorology indicate that the elevated nocturnal ozone in Cluster 2 is associated with the compensating downdraft flow of nocturnal convective storms. By contrast, elevated nocturnal ozone in Cluster 4 was associated with the prevailing low-level jet (LLJ) over the whole North China Plain (NCP) region. The LLJ-related dynamical processes trigger regional transport in the residual layer and downward mixing from the residual layer, elevating the background ozone level in downstream Beijing. Moreover, Cluster 4 is under the control of 500-hPa anomalous high pressure, which leads to daytime hot-dry weather favoring the occurrence of extreme afternoon pollution via photochemical ozone production. From Cluster 1 to Cluster 4, satellite-based formaldehyde (CH2O) increased by 71% accompanied by less change in nitrogen dioxide (NO2), causing a shift in ozone production sensitivity. Because extreme pollution pattern typically occurs in the transition regime, synergetic emission control of volatile organic compounds (VOCs) and nitrogen oxides (NOx) would be effective in alleviating severe ozone pollution in Beijing.

Clustering Surface Ozone Diurnal Cycles to Understand the Impact of Circulation Patterns in Houston, TX

AbstractThe diurnal cycle of surface ozone is directly influenced by the chemistry and meteorological processes which affect a region. This study uniquely employs a clustering methodology to examine the complete diurnal pattern of surface ozone for the Houston‐Galveston‐Brazoria region and links the identified patterns to meteorological regimes for June, July, and August of 3 years (2011, 2014, and 2015). Four features were implemented into the clustering algorithm: ozone rate of decrease at night, daily minimum before sunrise, rate of increase after sunrise, and an average of afternoon ozone. Four clusters were chosen, ranging from a mostly flat diurnal pattern with low mixing ratios (~20 ppbv) throughout the day (Cluster 1), to a more variable diurnal cycle with very high mixing ratios (>70 ppbv) in the afternoon (Cluster 4). The clusters are found to associate with distinctive circulation patterns and well‐known regional meteorological processes, such as the low‐level jet and Bermuda High. The uneven distribution of the clusters between the years helps elucidate ozone interannual variability due to meteorology: Cluster 4 had 0 days assigned from 2014 due to the greater influence of circulation patterns bringing clean air from the Gulf of Mexico. We show that the clustering method better characterizes ozone variability than the simplistic method of dividing peak ozone into quantiles. With the clustering analysis, we demonstrate that the ozone diurnal pattern holds more value than just peak ozone hours of the day in providing a clearer understanding of ozone variability and associated meteorological processes.

Evaluation of NASA's high-resolution global composition simulations: Understanding a pollution event in the Chesapeake Bay during the summer 2017 OWLETS campaign

Recirculation of pollutants due to a bay breeze effect is a key meteorological mechanism impacting air quality near urban coastal areas, but regional and global chemical transport models have historically struggled to capture this phenomenon. We present a case study of a high ozone (O3) episode observed over the Chesapeake Bay during the NASA Ozone Water-Land Environmental Transition Study (OWLETS) in summer 2017. OWLETS included a complementary suite of ground-based and airborne observations, with which we characterize the meteorological and chemical context of this event and develop a framework to evaluate model performance. Two publicly-available NASA global high-resolution coupled chemistry-meteorology models (CCMMs) are investigated: GEOS-CF and MERRA2-GMI. The GEOS-CF R2 value for comparisons between the NASA Sherpa C-23 aircraft measurements to the GEOS-CF resulted in good agreement (R2: 0.67) on July 19th and fair agreement (R2: 0.55) for July 20th. Compared to surface observations, we find the GEOS-CF product with a 25 × 25 km2 grid box, at an hourly (R2: 0.62 to 0.87) and 15-min (R2: 0.64 to 0.87) interval for six regional sites outperforms the hourly nominally 50 × 50 km2 gridded MERRA2-GMI (R2: 0.53 to 0.76) for four of the six sites, suggesting it is better capable of simulating complex chemical and meteorological features associated with ozone transport within the Chesapeake Bay airshed. When the GEOS-CF product was compared to the TOLNet LiDAR observations at both NASA Langley Research Center (LaRC) and the Chesapeake Bay Bridge Tunnel (CBBT), the median differences at LaRC were −6 to 8% and at CBBT were ±7% between 400 and 2000 m ASL. This indicates that, for this case study, the GEOS-CF is able to simulate surface level ozone diurnal cycles and vertical ozone profiles at small scales between the surface level and 2000 m ASL. Evaluating global chemical model simulations at sub-regional scales will help air quality scientists understand the complex processes occurring at small spatial and temporal scales within complex surface terrain changes, simulating nighttime chemistry and deposition, and the potential to use global chemical transport simulations in support of regional and sub-regional field campaigns.

The differing impact of air stagnation on summer ozone across Europe

We have examined the joint impact of daily maximum temperature and air stagnation on observed maximum daily 8-h running average (MDA8) and hourly near-surface ozone (O3) over eight regions of Europe for the summer period 1998–2015. The percentage of stagnant area in a region is a better predictor of summer MDA8 O3 in central/southern Europe (correlation coefficient R = 0.50–0.70) than in the north (R = 0.06–0.39). The correlations of MDA8 O3 with temperature are higher than with stagnation for most regions, and the impact of stagnation on those correlations is region-dependent. MDA8 O3 mixing ratios consistently increase over central/southern Europe under stagnant conditions, but this has not been found for some temperature ranges in the north. Under non-stagnant situations and relatively high temperatures (20–25 °C), southerly advection often brings aged air masses from more polluted areas to the receptor regions in the north. Furthermore, the occurrence of stagnation tends to amplify the ozone diurnal cycles in the center/south of the continent, yielding lower nighttime and higher daytime mixing ratios than on non-stagnant days, again with a less clear effect in the north. Changes in planetary boundary layer height, accumulation of ozone and precursors, and subsequent photochemical production during stagnant days are the presumed underlying mechanisms for the amplification of the diurnal cycles. The results from this study prove that the effects of stagnation on summer ozone are regionally dependent across Europe. Consequently, climate model projections of increases in stagnation should not directly be translated into degraded air quality without a proper assessment of the regional impacts.

Influences of planetary boundary layer mixing parameterization on summertime surface ozone concentration and dry deposition over North China

We present a regional modeling study that analyzes how planetary boundary layer (PBL) and surface layer parameterizations influence surface ozone concentrations and dry deposition fluxes over the Beijing-Tianjin-Hebei region in summer (July 2015). We use the Weather Research and Forecasting Model coupled to Chemistry (WRF-Chem) to simulate surface ozone concentration and dry deposition, and examine three PBL schemes: the Yonsei University (YSU), Mellor–Yamada–Janjić (MYJ), and Asymmetric Convective Model version 2 (ACM2) schemes. The model sensitivity to surface layer schemes is also tested by coupling the ACM2 PBL scheme with either the revised MM5-similarity scheme or the Pleim-Xiu surface layer scheme. Key physical and chemical factors for ozone dry deposition parameterization are analyzed to explore the root causes of model discrepancies. We find that all simulations overestimate the summertime daily mean ozone concentrations over North China (42 ppbv in observations vs. 43–50 ppbv in model results for July 2015) that are caused by high biases in daytime ozone and partly compensated by low biases in nighttime ozone. The YSU scheme has the largest overestimate in daily mean ozone concentration, but best reproduces the ozone diurnal cycle. The ACM2 scheme shows the largest underestimates of surface ozone over North China during nighttime, which can be explained by its weakest vertical mixing leading to high NOx concentrations and strong ozone titration near surface. The choices of PBL and surface layer schemes lead to over 20% differences in ozone dry deposition fluxes due to differences in simulated surface ozone concentrations and dry deposition velocities. We find the differences in dry deposition velocity are mainly caused by differences in Monin-Obukhov length during nighttime and surface temperature during daytime. Our study emphasizes the needs to better understand these key PBL and surface factors for reducing the uncertainties in model simulation of surface ozone concentration and dry deposition.

Systematic bias in evaluating chemical transport models with maximum daily 8 h average (MDA8) surface ozone for air quality applications: a case study with GEOS-Chem v9.02

Abstract. Chemical transport models frequently evaluate their simulation of surface ozone with observations of the maximum daily 8 h average (MDA8) concentration, which is the standard air quality policy metric. This requires successful simulation of the surface ozone diurnal cycle including nighttime depletion, but models often have difficulty simulating this diurnal cycle for a number of reasons, including (1) vertical grid structure in the surface layer, (2) timing of changes in mixed layer dynamics and ozone deposition velocity across the day–night transition, (3) poor representation of nighttime stratification, and (4) uncertainties in ozone nighttime deposition. We analyze the problem with the GEOS-Chem model, taking as a representative case study the Southeast US during the NASA SEAC4RS aircraft campaign in August–September 2013. The model is unbiased relative to the daytime mixed layer aircraft observations but has a mean +8 ppb bias at its lowest level (65 m) relative to MDA8 surface ozone observations. The bias can be corrected to +5 ppb by implicit sampling of the model at the 10 m altitude of the surface observations. The model does not capture frequent observed occurrences of <20 ppb MDA8 surface ozone on rainy days, possibly because of enhanced ozone deposition to wet surfaces that is unaccounted for. Restricting the surface ozone evaluation to dry days still shows inconsistencies with MDA8 ozone because of model errors in the ozone diurnal cycle. Restricting the evaluation to afternoon ozone completely removes the bias. We conclude that better representation of diurnal variations in mixed layer dynamics and ozone deposition velocities is needed in models to properly describe the diurnal cycle of ozone.

Impact of typhoon periphery on high ozone and high aerosol pollution in the Pearl River Delta region

This paper analyzes observation data in the Pearl River Delta (PRD) region from 2012 to 2013, and explores the impact of typhoon periphery on high ozone and high aerosol pollution episodes (double high episodes). Observation analysis show that severe tropical storms to severe typhoons are mainly located in the range of 10°N-30°N, 116°E-135°E when double high episodes occur. Meanwhile, obvious high temperature, low humidity, low wind speed, high actinic flux, high aerosol optical depth (AOD), and high single scattering albedo (SSA) can be observed in double high episodes.The diurnal cycle of the PM2.5 is significant in double high episodes, and the average peak concentration in the afternoon can exceed 90 μg/m3. The diurnal cycle of PM2.5 in non–double high episodes is not significant, and the average value is about 34–39 μg/m3. The ozone peak concentration in double high episodes is 81–103 ppbv, which is about 27–40 ppbv higher than that of non–double high episodes. High correlation can be found between the aerosol and ozone diurnal cycles in double high episodes, and r2 reaches 0.76. In double high episodes, black carbon, nitrate, and sea salt decrease while sulfate, ammonium, secondary organic carbon, and total PM2.5 significantly increase in the afternoon. The growth of PM2.5 in double high episodes is mainly contributed by scattered fine particles from photochemical processes and transmission. The mechanisms that control the double high episodes in the PRD are described below. Ozone and aerosol begin to accumulate under unfavorable meteorological conditions. Via local photochemical processes and external transport, the scattered aerosol increases and leads to an increase in multiple scattering and actinic radiation, which is in turn more favorable for photochemical reaction and further increases the ozone concentration. Meanwhile, high oxidizability promotes the formation of scattered aerosol, creating positive feedback. In addition, the scattered aerosol increases backscattering, which increases the photolysis rate and ozone concentration in the middle and upper boundary layer. Meanwhile, downdraft and turbulence transports high-concentration ozone to the ground.

Evaluation of summertime surface ozone in Kanto area of Japan using a semi-regional model and observation

Surface ozone is an air pollutant and harmful to human life. The spatial distribution of the air pollution has been estimated by chemical transport models, but still there are large uncertainties depending on detailed condition of the region. In this study, we extended Goto et al. (2015a) for implementing a chemical transport model to simulate short-lived gases such as ozone over Kanto area (around Tokyo in Japan) for August 2010. Comparison of simulation results with observed data indicated that the model had ability to capture observed ozone diurnal cycles over the target region with high correlation coefficients (0.69–0.81). The simulation result showed a vital role of meteorological conditions in the model performance. The correlation coefficients were much higher (0.78–0.87) and biases were lower (<35%) when the meteorological conditions were stable. In contrast, dominance of local pressure system and an associated complex wind field were main reasons for overestimated surface ozone concentrations in the unstable weather conditions.This study helped achieve a better understanding of the chemistry transport model performance under unstable meteorological conditions in the Kanto area. Maximal association between meteorological factors and surface ozone distribution was revealed. In addition, uncertainty of emission inventories of ozone precursors especially the underestimate NOx level certainly contributed to high level surface ozone during nighttime in this study.

Procuring the regional urbanization and industrialization effect on ozone pollution in Pearl River Delta of Guangdong, China

The decade before 2006 was a period of growing regional urbanization and industrialization. Also, the 5-year plan period (2001–2005) is a half decade of fast regional urban expansion with high population growth within the PRD city clusters, At the same time, the PRD region becomes a leading manufacturing center of the world. Industrial expansion in this region of GD was characterized by the spread of industrial activities from hot spots in GZ city and around Foshan and Dongguan to widely distributed patches of industrial active districts, mostly in the seven newly agglomerate formed and urbanizing prefectural municipalities. To provide transportation communication between GZ city and the municipal cities, a system of highways was built and a region road-network surfaced for the connection of the expanding municipal cities and their sub-urban areas as well as the patches of industrial districts, within their municipalities Also, a network of power plants was built at its periphery. As a result, the ozone precursors, NOx and VOC emission from vehicles, industries and power plants increased region wide, and in particular in the newly formed prefectural municipalities. Hence, the whole region was undergoing a redistribution of O3 precursors and ozone pollution, following the path of redistribution of population and industrial activity in the Pearl River Delta region of Guangdong. Study and report on O3 pollution before 2000 is rare and there were only a few case studies in 2000 and 2004. By 2006, O3 data supplied by the newly established PRD Regional Air Quality Monitoring Network become available for the study of regional O3 pollution. Also relevant information on the new dimension of regional urbanization and industrialization for the PRD region of GD, as well as a highly resolved temporal and spatial PRD region NOx and VOC emission inventory surfaced. Thus, 2006 is a good year to procure the regional urbanization and industrial effect, and to produce a snapshot of the regional and area specific ozone pollution in the PRD region of GD. Patterns of annual ozone variation and seasonal ozone diurnal cycles for urban/sub-urban areas, for metropolitan city, for rural areas and for special area in the PRD region of Guangdong are presented for future reference.

Global model simulation of summertime U.S. ozone diurnal cycle and its sensitivity to PBL mixing, spatial resolution, and emissions

Simulation of summertime U.S. surface ozone diurnal cycle is influenced by the model representation of planetary boundary layer (PBL) mixing, spatial resolution, and precursor emissions. These factors are investigated here for five major regions (Northeast, Midwest, Southeast, California, and Southwest) by using the Model for Ozone And Related chemical Tracers version 2.4 (MOZART-2.4), with important modifications, to conduct sensitivity experiments for summer 1999 with three PBL mixing schemes, two horizontal resolutions and two emissions datasets. Among these factors, the PBL mixing is dominant. The default non-local scheme well reproduces the observed ozone diurnal variation, where the timing for the afternoon maximum and the morning minimum is within 1 h of the observed; biases for the minimum are less than 5 ppb except over the Southeast; and the ozone maximum–minimum contrast (OMMC) is within 10 ppb of observations except for the overprediction by 18.9 ppb over the Northeast. In contrast, the local scheme significantly overestimates the OMMC by 10–34 ppb over all regions as ozone and precursors are trapped too close to the ground. On the other hand, the full-mixing assumption underestimates the OMMC by 0–25 ppb, except over the Northeast, as the nighttime ozone decline is greatly underpredicted. As compared to PBL mixing, the effects of horizontal resolutions and precursor emissions being used are smaller but non-negligible. Overall, with the non-local mixing scheme, relatively high horizontal resolution (∼1.1°) and updated emissions data, the modified MOZART is capable of simulating the main features of the observed ozone diurnal cycle.

Evaluation of long-term ozone simulations from seven regional air quality models and their ensemble

Long-term ozone simulations from seven regional air quality models, the Unified EMEP model, LOTOS-EUROS, CHIMERE, RCG, MATCH, DEHM and TM5, are intercompared and compared to ozone measurements within the framework of the EuroDelta experiment, designed to assess air quality improvement at the European scale in response to emission reduction scenarios for 2020. Modelled ozone concentrations for the year 2001 are evaluated. The models reproduce the main features of the ozone diurnal cycle, but generally overestimate daytime ozone. LOTOS-EUROS and RCG have a more pronounced diurnal cycle variation than observations, while the reverse occurs for TM5. CHIMERE has a large positive bias, which can be explained by a systematic bias in boundary conditions. The other models and the “ensemble model”, whose concentrations are by definition averaged over all models, represent accurately the diurnal cycle. The ability of the models to simulate day-to-day daily ozone average or maxima variability is examined by means of percentiles, root mean square errors and correlations. In general, daily maxima are better simulated than daily averages, and summertime concentrations are better simulated than wintertime concentrations. Summertime correlations range between 0.5 and 0.7 for daily averages and 0.6 and 0.8 for daily maxima. Two health-related indicators are used, the number of days of exceedance of the 120 μ g m - 3 threshold for the daily maximal 8-h ozone concentration and the SOMO35. Both are well reproduced in terms of frequency, but the simultaneity of occurrence of exceedance days between observations and simulations is not well captured. The advantage of using an ensemble of models instead of a single model for the assessment of air quality is demonstrated. The ensemble average concentrations almost always exhibit a closer proximity to observations than any of the models. We also show that the spread of the model ensemble is fairly representative of the uncertainty in the simulations.

On the relationship of HO2 + RO2 with j(O1D) during the Free Tropospheric Experiment (FREETEX '96) at the Jungfraujoch Observatory(3580 m above sea level) in the Swiss Alps

Continuous measurements of peroxy radicals (HO2+RO2), the photolysis rate coefficients j(NO2) and j(O1D), and a number of other trace gases including O3, NOx, and CO, were carried out at the alpine site at Jungfraujoch (3580 m asl), Switzerland, in April/May 1996 during the Free Tropospheric Experiment (FREETEX '96). In spring, the Jungfraujoch Observatory is very often located in the lower free troposphere. A mean midday concentration of 17.1±2.2 parts per trillion by volume (pptv) was calculated from continuous measurements of the sum of inorganic and organic peroxy radicals using the chemical amplification technique. The average diurnal cycles for peroxy radicals and the photolysis rate coefficient j)O1D) during FREETEX '96 showed a statistically more significant square root than first‐order relationship, indicative of a relatively clean atmosphere at the mountain site, where peroxy radical self‐ and cross‐reactions were the dominant peroxy radical loss processes. The average ozone diurnal cycle during FREETEX '96 displayed an increase of about 2 ppbv during the daytime despite the square root relationship. Similar results were derived from individual days during the campaign. A simple photochemical box model indicated that there is a regime in NO concentration from about 20 pptv to 100 pptv where a better square root (than first‐order) correlation between peroxy radicals and j(O1D) is accompanied by net ozone production. Taking into account that median daytime concentrations of NO at Jungfraujoch range from 51 pptv in March to 93 pptv in April, it is suggested that Jungfraujoch, during FREETEX '96, has been in the net ozone production regime in spite of the observed square root dependence of peroxy radicals on j(O1D)

Cloud influence on ozone diurnal cycle in the marine boundary layer at Réunion Island

The influence of clouds on the diurnal cycle of ozone concentration in the Réunion Island marine boundary layer is investigated. Indeed the orographic cloud layer plays an important role in the chemistry of ozone through several ways. The two major ones are the enhancement of photodissociation processes in the interstitial air relative to clear sky conditions and the perturbation of the NO x photostationary state through the scavenging of HO 2 and CH 3O 2 radicals. The relative efficiency of these two ozone sinks depends on the time of day, since the first one dominates the second one in early morning and in late afternoon, while the contrary is observed at midday. The cloud's influence allows us to simulate more than 4 ppbv destruction of ozone, with a range of concentrations of 25–30 ppbv, this 4 ppbv of ozone destruction being the upper limit of gas-phase simulation. Thus we think that the greater destruction of the marine boundary layer ozone observed in Réunion Island relative to the mean 4 ppbv is due to clouds.

Ozone nighttime recovery in the marine boundary layer: Measurement and simulation of the ozone diurnal cycle at Reunion Island

We describe the diurnal cycle of ozone in the marine boundary layer measured at Reunion Island (21°S, 55°E) in the western part of the Indian Ocean in August‐September 1995. Results from a box chemistry model are compared with ozone measurements at Reunion Island. We focus on the peak‐to‐peak amplitude of ozone concentration, since our measurements show a variation of about 4 parts per billion by volume, which is close to the value obtained by Johnson et al. [1990] during the Soviet‐American Gases and Aerosols (SAGA) 1987 Indian Ocean cruise. Different dynamical mechanisms are examined in order to reproduce such a variation. We conclude that the most important one is the exchange between the ozone‐rich free troposphere and the ozone‐poor boundary layer. This exchange is supposed to be more important during the night than during the day, allowing ozone nighttime recovery. This is the key point of the observed diurnal cycle, since daytime ozone photochemistry is well described by the model. Then we assume an entrainment velocity equal to 1 mm s−1 during the day and 14 mm s−1 during the night to closely match our measurements. Topography influences, together with clouds, are presumed to be responsible for this difference between nighttime and daytime entrainment velocities of free tropospheric air into the boundary layer at Reunion Island. Over the open ocean the difference of the turbulent flux of sensible heat between the day and the night explains the strong ozone nighttime recovery observed by us and by Johnson et al. [1990].

Stratospheric effects of 27‐day solar ultraviolet variations: An analysis of UARS MLS ozone and temperature data

The characteristics of upper stratospheric ozone and temperature responses at low latitudes to short‐term solar ultraviolet variations are studied by using 1000 days of UARS microwave limb sounder (MLS) and solar stellar irradiance comparison experiment data. Consistent with previous analyses of Nimbus 7 solar backscattered ultraviolet (SBUV) data, the high‐pass‐filtered solar flux in the 200‐to 205‐nm interval is most strongly correlated with MLS ozone measurements at tropical latitudes near 4 hPa with a sensitivity of about 0.4% for each 1% change in the solar flux. Reproducibility tests, power spectral, and coherency estimates support the reality of the observed ozone response at this level. The MLS solar UV/ozone response is significantly reduced at levels above ∼2 hPa as compared to earlier results based on SBUV data. This reduction appears to be a consequence of the ozone diurnal cycle at high altitudes combined with the necessary inclusion of nighttime records in calculating the MLS ozone zonal averages. Some evidence is obtained for an MLS solar UV/temperature response near the stratopause, but coherency tests are negative. Future analyses of independent data records having similar local time coverage as that of Nimbus 7 SBUV are needed to establish more definitively whether any significant change in the upper stratospheric UV response has occurred.

A study of peroxy radicals and ozone photochemistry at coastal sites in the northern and southern hemispheres

Peroxy radicals and other important species relevant to ozone photochemistry, including ozone, its photolysis rate coefficient jO(1D), NOx (NO + NO2), and peroxides, were measured at the coastal sites of Cape Grim, Tasmania, in January/February 1995 during the Southern Ocean Atmospheric Photochemistry Experiment (SOAPEX 1) and Mace Head, Western Ireland, in May 1995 during the Atlantic Atmospheric Photochemistry Experiment (ATAPEX 1). At both sites it was observed that the relationship between peroxy radical (HO2 + RO2) concentrations and jO(1D) switched from a square root dependence in clean oceanic or “baseline” air to a first‐order dependence in more polluted air. Simple algorithms derived from a photochemical reaction scheme indicate that this switch‐over occurs when atmospheric NO levels are sufficient for peroxy radical reaction with NO to compete with radical recombination reactions. At this crucial point, net tropospheric ozone production is expected to occur and was observed in the ozone diurnal cycles when the peroxy radical/jO(1D) dependencies became first order. The peroxy radical/jO(1D) relationships imply that ozone production exceeds destruction at NO levels of 55±30 parts per trillion by volume (pptv) at Mace Head during late spring and 23±20 pptv at Cape Grim during summer, suggesting that the tropospheric ozone production potential of the southern hemisphere is more responsive to the availability of NO than that of the northern hemisphere.