Cloud impacts on photochemistry: a new climatology of photolysis rates from the Atmospheric Tomography mission
Samuel R. HallKirk UllmannMichael J. PratherClare M. FlynnLee T. MurrayArlene M. FioreGustavo CorreaSarah A. StrodeStephen D. SteenrodJean‐François LamarqueJ. GuthBéatrice JosseJohannes FlemmingVincent HuijnenN. Luke AbrahamA. T. Archibald
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Abstract. Measurements from actinic flux spectroradiometers on board the NASA DC-8 during the Atmospheric Tomography (ATom) mission provide an extensive set of statistics on how clouds alter photolysis rates (J-values) throughout the remote Pacific and Atlantic Ocean basins. ATom made profiling circumnavigations of the troposphere over four seasons during 2016–2018. J-values are a primary chemical control over tropospheric ozone and methane abundances and their greenhouse effects. Clouds have been recognized for more than three decades as being an important factor in tropospheric chemistry. The ATom climatology of J-values is a unique test of how the chemistry models treat clouds. This work focuses on measurements over the Pacific during the first deployment (ATom-1) in August 2016. Nine global chemistry–climate or –transport models provide J-values for the domains measured in ATom-1. We compare mean profiles over a range of cloudy and clear conditions; but, more importantly, we build a statistical picture of the impact of clouds on J-values through the distribution of the ratio of J-cloudy to J-clear. In detail, the models show largely disparate patterns. When compared with measurements, there is some limited, broad agreement. Models here have resolutions of 50–200 km and thus reduce the occurrence of clear sky when averaging over grid cells. In situ measurements also average the scattered sunlight, but only out to scales of 10 s of km. A primary uncertainty remains in the role of clouds in chemistry, in particular, how models average over cloud fields, and how such averages can simulate measurements.Keywords:
Atmospheric chemistry
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Abstract. In this analysis, Tropospheric Emission Spectrometer (TES) V004 nadir ozone profiles are validated with more than 4400 coinciding ozonesonde measurements taken across the world from the World Ozone and Ultraviolet Radiation Data Centre (WOUDC) during the period 2005–2010. The TES observation operator was applied to the sonde data to ensure a consistent comparison between TES and ozonesonde data, i.e. without the influence of the a priori O3 profile needed to regulate the retrieval. Generally, TES V004 ozone retrievals are biased high by 2–7 ppbv in the troposphere, consistent with validation results from earlier studies. Because of two degrees of freedom for signal in the troposphere, we can distinguish between upper and lower troposphere mean biases, respectively ranging from −0.4 to +13.3 ppbv for the upper troposphere and +3.9 to +6.0 ppbv for the lower troposphere. Focusing on the 464 hPa retrieval level, broadly representative for free tropospheric ozone, we find differences in the TES biases for the Tropics (+3 ppbv), sub-tropics (+5 ppbv), and northern (+7 ppbv) and southern mid-latitudes (+4 ppbv). The relatively long-term record (6 yr) of TES-ozonesonde comparisons, allowed us to quantify temporal variations in TES biases in free tropospheric ozone, at 464 hPa. We find that there are no discernable biases in each of these latitudinal bands; temporal variations in the bias are typically within the uncertainty of the difference between TES and ozone-sondes. Establishing these bias patterns is important in order to make meaningful use of TES O3 data in applications such as model evaluation, trend analysis, or data assimilation.
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Abstract. Ozone in the troposphere is a greenhouse gas and a pollutant, hence, additional understanding of the drivers of tropospheric ozone evolution is essential. The El Niño–Southern Oscillation (ENSO) is a main climate mode and may contribute to the variations of tropospheric ozone. Nevertheless, there is uncertainty regarding the causal influences of ENSO on tropospheric ozone under warming environment. Here, we investigated the links between ENSO and tropospheric ozone using Coupled Modeling Intercomparison Project Phase 6 (CMIP6) data over the period 1850–2014. Our results show that ENSO impacts on tropospheric ozone are primarily found over oceans, while the signature of ENSO over continents is largely nonsignificant. The response of ozone to ENSO may vary depending on specific air pressure levels in the troposphere. These responses are weak in the middle troposphere and are stronger in the upper and lower troposphere. Although there are biases in simulating the signature of ENSO on surface ozone, these signatures in the middle and upper troposphere appear to be more consistent across CMIP6 models. While the response of tropical tropospheric ozone to ENSO is in agreement with previous works, our results suggest that ENSO impacts on tropospheric ozone of the mid-latitude regions over the southern Pacific, Atlantic, and Indian oceans might be more significant than previously understood.
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The small‐scale vertical variability of tropospheric O 3 and CO is examined from a set of simultaneous measurements obtained in July and August 1974, between 55°S and 67°N. From this set of vertical profiles, it is noted that many of the fluctuations are coincident in both species, and a method is presented that quantifies the correlation between the observed O 3 and CO variability. A two‐dimensional depiction of the distribution of these O 3 ‐CO correlations shows that there are regions in the troposphere where these trace gases are positively correlated, while at the same time there are preferred locations where these two species are primarily anticorrelated. The regions of anticorrelation are consistent with the traditional picture of the tropospheric ozone cycle, which suggests that this gas is chemically unreactive in the troposphere. On the other hand, the location and magnitude of the region in which these two species are positively correlated suggest that there is considerable in situ production of tropospheric ozone, which is likewise consistent with the more recent interpretation of the tropospheric ozone cycle that shows that this gas is photochemically active in the troposphere. In addition, the existence of a significant in situ source of tropospheric O 3 in the northern hemisphere is supported by a modeling study which likewise suggests that the observed hemispheric asymmetry in the distribution of tropospheric O 3 is quite likely a result of surface emissions of precursors.
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The vertical distribution of the tropospheric ozone column concentration (OCC) in China from 2005 to 2020 was analysed based on the ozone profile product of the ozone monitoring instrument (OMI). The annual average OCC in the lower troposphere (OCCLT) showed an increasing trend, with an average annual increase of 0.143 DU. The OCC in the middle troposphere showed a downward trend, with an average annual decrease of 0.091 DU. There was a significant negative correlation between the ozone changes in the two layers. The monthly average results show that the peak values of OCCLT occur in May or June, the middle troposphere is significantly influenced by topographic conditions, and the upper troposphere is mainly affected by latitude. Analysis based on multi-source data shows that the reduction in nitrogen oxides (NOx) and the increase in volatile organic compounds (VOCs) weakened the titration of ozone generation, resulting in the increase in OCCLT. The increase in vegetation is closely related to the increase in OCCLT, with a correlation coefficient of up to 0.875. The near-surface temperature increased significantly, which strengthened the photochemical reaction of ozone. In addition, the increase in boundary layer height also plays a positive role in the increase in OCCLT.
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Abstract. Ozone in the troposphere is a greenhouse gas and a pollutant, hence, additional understanding of the drivers of tropospheric ozone evolution is essential. The El Niño–Southern Oscillation (ENSO) is a main climate mode and may contribute to the variations of tropospheric ozone. Nevertheless, there is uncertainty regarding the causal influences of ENSO on tropospheric ozone under warming environment. Here, we investigated the links between ENSO and tropospheric ozone using Coupled Modeling Intercomparison Project Phase 6 (CMIP6) data over the period 1850–2014. Our results show that ENSO impacts on tropospheric ozone are primarily found over oceans, while the signature of ENSO over continents is largely nonsignificant. The response of ozone to ENSO may vary depending on specific air pressure levels in the troposphere. These responses are weak in the middle troposphere and are stronger in the upper and lower troposphere. Although there are biases in simulating the signature of ENSO on surface ozone, these signatures in the middle and upper troposphere appear to be more consistent across CMIP6 models. While the response of tropical tropospheric ozone to ENSO is in agreement with previous works, our results suggest that ENSO impacts on tropospheric ozone of the mid-latitude regions over the southern Pacific, Atlantic, and Indian oceans might be more significant than previously understood.
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In this study, we have performed a set of simulations to detail the evolution of tropospheric ozone from 1890 to 1990. The simulations are compared with available measurements for present‐day conditions and earlier. Using our best estimates of ozone precursors emissions (based on the work by van Aardenne et al. (2001)), we have found a tropospheric ozone burden increase of 71 Tg between 1890 and 1990, an increase of ∼30%. When no anthropogenic emissions in 1890 are considered, this burden increase reaches 88 Tg. The ozone lifetime is shown to have decreased by ∼30%, especially after 1930. It is also shown that the net chemical production in the lower troposphere exceeded that in the free troposphere for the first time in the 1950–1970 period. In addition, the ozone production in this study increased rapidly between 1890 and 1930 and from 1970 to 1990. However, the ozone production efficiency in the troposphere is shown to have decreased during the 20th century, making the troposphere less NO x limited. Finally, a decrease in the OH burden is estimated to be on the order of 8%, matched by a similar increase in the CO lifetime.
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Abstract. A flexible and explicit on-line parameterization for the calculation of tropospheric photodissociation rate constants (J-values) has been integrated into the global Chemistry Transport Model TM5. Here we provide a comprehensive description of this Modified Band Approach (MBA) including details of the optimization procedure employed, the methodology applied for calculating actinic fluxes, the photochemical reaction data used for each chemical species and the parameterizations adopted for improving the description of scattering and absorption by clouds and aerosols. The resulting J-values change markedly throughout the troposphere when compared to the offline approach used to date, with significant increases in the boundary layer and upper troposphere. Conversely, for the middle troposphere a reduction in the actinic flux results in a decrease in J-values. Integrating effects shows that application of the MBA introduces seasonal dependent differences in important trace gas oxidants. Tropospheric ozone changes by ±5% in the seasonal mean mixing ratios throughout the troposphere, which induces changes of ±15% in tropospheric OH. In part this is due to an increase in the re-cycling efficiency of nitrogen oxides. The overall increase in northern hemispheric tropospheric ozone strengthens the oxidizing capacity of the troposphere significantly and reduces the lifetime of CO and CH4 by ~5% and ~4%, respectively. Changes in the tropospheric CO burden, however, are limited to a few percent due to competing effects. Comparing the distribution of tropospheric ozone in the boundary layer and middle troposphere against observations in Europe shows there are improvements in the model performance during boreal winter in the Northern Hemisphere near regions affected by high nitrogen oxide emissions. Monthly mean total columns of nitrogen dioxide and formaldehyde also compare more favorably against OMI and SCIAMACHY total column observations.
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Results from a global three‐dimensional model for tropospheric O 3 ‐NO x ‐hydrocarbon chemistry are presented and evaluated with surface, ozonesonde, and aircraft measurements. Seasonal variations and regional distributions of ozone, NO, peroxyacetylnitrate (PAN), CO, ethane, acetone, and H 2 O 2 are examined. The model reproduces observed NO and PAN concentrations to within a factor of 2 for a wide range of tropospheric regions including the upper troposphere but tends to overestimate HNO 3 concentrations in the remote troposphere (sometimes several fold). This discrepancy implies a missing sink for HNO 3 that does not lead to rapid recycling of NO x ; only in the upper troposphere over the tropical South Atlantic would a fast conversion of HNO 3 to NO x improve the model simulation for NO x . Observed concentrations of acetone are reproduced in the model by including a large biogenic source (15 Tg C yr −1 ), which accounts for 40% of the estimated global source of acetone (37 Tg C yr −1 ). Concentrations of H 2 O 2 in various regions of the troposphere are simulated usually to within a factor of 2, providing a test for HO x chemistry in the model. The model reproduces well the observed concentrations and seasonal variations of ozone in the troposphere, with some exceptions including an underestimate of the vertical gradient across the tropical trade wind inversion. A global budget analysis in the model indicates that the supply and loss of tropospheric ozone are dominated by photochemistry within the troposphere and that NO x . emitted in the southern hemisphere is twice as efficient at producing ozone as NO x emitted in the northern hemisphere.
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Sink (geography)
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