Reply on RC2

Exposure to ozone (O3) is associated with many human health problems, resulting in tens of thousands of premature deaths annually in Europe. This study quantifies the impact of changes in anthropogenic emissions of O3 precursors on premature deaths from long-term O3 exposure in Europe and the impact of emissions changes during 2005–2015 using the nested-grid chemical transport model Goddard Earth Observing System (GEOS)-Chem and its adjoint. In 2015, it is estimated that a 20% decrease in total anthropogenic emissions in our modeled European domain could prevent 1576 (467–3252) premature deaths from respiratory disease (30 years of age), 70% of which is owing to the decrease in nitrogen oxides (NO x ) emissions. Underlying this aggregate effect is substantial spatial variation. In most of Europe, O3 formation is NO x -limited so that NO x emission reductions help to decrease premature deaths. Yet where O3 formation is NO x -saturated (as in parts of the United Kingdom, Benelux and Germany) emission reductions cause more premature deaths through increased ozone exposure. Despite the overall decreases in anthropogenic emissions, the marginal benefit, expressed as the avoided premature deaths per 1 kg km−2 yr−1 reduction in NO x emissions, is found to generally increase during 2005–2015, with a mean value more than doubling over Europe. This highlights the general trend that O3 formation becomes less sensitive to volatile organic compound emissions and more limited by NO x emissions. An important policy implication of increasing marginal benefits is that more costly regulations of NO x emissions are economically justified even as total anthropogenic emission are declining. NO x contributions from road transport, industry, energy, and residential sectors are most affected by the change in the O3 production regime. Consequently, European regulations of NO x emissions targeted at those sectors will yield the highest health benefits per unit NO x emission of all sources.

We use a global chemical transport model (GEOS‐Chem) driven by a general circulation model (NASA Goddard Institute for Space Studies GCM) to investigate the effects of 2000–2050 global change in climate and emissions (the Intergovernmental Panel on Climate Change A1B scenario) on the global tropospheric ozone budget and on the policy‐relevant background (PRB) ozone in the United States. The PRB ozone, defined as the ozone that would be present in U.S. surface air in the absence of North American anthropogenic emissions, has important implications for setting national air quality standards. We examine separately and then together the effects of changes in climate and anthropogenic emissions of ozone precursors. We find that the 2000–2050 change in global anthropogenic emissions of ozone precursors increases the global tropospheric ozone burden by 17%. The 2000–2050 climate change increases the tropospheric ozone burden by 1.6%, due mostly to lightning in the upper troposphere, and also increases global tropospheric OH by 12%. In the lower troposphere, by contrast, climate change generally decreases the background ozone. The 2000–2050 increase in global anthropogenic emissions of ozone precursors increases PRB ozone by 2–6 ppb in summer; the maximum effect is found in April (3–7 ppb). The summertime PRB ozone decreases by up to 2 ppb with 2000–2050 climate change, except over the Great Plains, where it increases slightly as a result of increasing soil NOx emission. Climate change cancels out the effect of rising global anthropogenic emissions on the summertime PRB ozone in the eastern United States, but there is still a 2–5 ppb increase in the west.

Ozone estimates from observations by the NOAA-16 Solar Backscattered Ultraviolet (SBUV/2) instrument and Television Infrared Observation Satellite (TIROS-N) Operational Vertical Sounder (TOVS) are used to describe the vertical structure of ozone in the anomalous 2002 polar vortex. The SBUV/2 total ozone maps show that the ozone hole was pushed off the Pole and split into two halves due to a split in the midstratospheric polar vortex in late September. The vortex split and the associated transport of high ozone from midlatitudes to the polar region reduced the ozone hole area from 18 × 106 km2 on 20 September to 3 × 106 km2 on 27 September 2002. A 23-yr time series of SBUV/2 daily zonal mean total ozone amounts between 70° and 80°S shows record high values [385 Dobson units (DU)] during the late-September 2002 warming event. The transport and descent of high ozone from low latitudes to high latitudes between 60 and 15 mb contributed to the unusual increase in total column ozone and a small ozone hole estimated using the standard criterion (area with total ozone < 220 DU). In contrast, TOVS observations show an ozone-depleted region between 0 and 24 km, indicating that ozone destruction was present in the elongated but unsplit vortex in the lower stratosphere. During the warming event, the low-ozone regions in the middle and upper stratosphere were not vertically aligned with the low-ozone regions in the upper troposphere and lower stratosphere. This offset in the vertical distribution of ozone resulted in higher total column ozone masking the ozone depletion in the lower stratosphere and resulting in a smaller ozone hole size estimate from satellite total ozone data.

Data from ozonesondes launched at ARIES Nainital (29.40° N, 79.50° E, and 1793 m elevation) are used to evaluate the Atmospheric Infrared Sounder (AIRS) version 6 ozone profiles and total column ozone during the period 2011–2017 over the central Himalaya. The AIRS ozone products are analyzed in terms of retrieval sensitivity, retrieval biases/errors, and ability to retrieve the natural variability of columnar ozone, which has not been done so far from the Himalayan region having complex topography. For a direct comparison, averaging kernels information is used to account for the sensitivity difference between the AIRS and ozonesonde data. We show that AIRS can provide quality data of ozone in the lower and middle troposphere and stratosphere with nominal underestimation (<20 %). However, in the upper troposphere and lower stratosphere (UTLS), we observe a considerable overestimation of the magnitude as high as 102 %. The weighted statistical error analysis of AIRS ozone shows higher positive bias, root mean squared error, and standard deviation in the upper troposphere of about 65 %, 65 %, and 25 %, respectively. Similar to AIRS, Infrared Atmospheric Sounding Interferometer (IASI) and Cross-track Infrared Sounder (CrIS) are also able to produce ozone peaks and gradients successfully. However, the statistical errors are again higher in the UTLS region that are likely related to larger variability of ozone, lower ozone partial pressure and inadequate retrieval information on the surface parameters. The monthly variations of columnar ozone (total, UTLS, and tropospheric) are captured well by AIRS, except the total columnar ozone, which shows a strong bimodal variation, unlike unimodal variation seen in ozonesonde and Ozone Monitoring Instrument (OMI). Increases in ozone of 5–20 % (in 2–6 km altitude) after the biomass burning and during events of downward transport (in 2–16 km altitude) are captured well by AIRS. Ozone radiative forcing (RF) derived from total column ozone matches well between ozonesonde (4.86 mW/m2) and OMI (4.04 mW/m2), while significant RF underestimation is seen in AIRS (2.96 mW/m2). The fragile and complex landscapes of the Himalayas are more sensitive to global climate change, and establishing such biases and error analysis of space-borne sensors will help study the long-term trends and estimate accurate radiative budgets.

The satellite measurements in nadir and limb viewing geometry provide a complementary view of the atmosphere. An effective combination of the limb and nadir measurements can provide a new information about atmospheric composition. In this work, we present tropospheric ozone column datasets that have been created using combination of total ozone column from OMI and TROPOMI with stratospheric ozone column dataset from several available limb-viewing instruments (MLS, OSIRIS, MIPAS, SCIAMACHY, OMPS-LP, GOMOS). We have developed further the methodological aspects of assessment of tropospheric ozone using the residual method using simulations with the chemistry-transport model SILAM. It has been shown that the accurate assessment of ozone in the upper troposphere and the lower stratosphere (UTLS) is of high importance for detecting the ground-level ozone patterns. The stratospheric ozone column is derived from a combination of ozone profiles from several satellite instruments in limb-viewing geometry. We developed a method for the data homogenization, which includes the removal of biases and a-posteriori estimation (validation) of random uncertainties, thus making the data from different instruments compatible with each other. The high horizontal and vertical resolution dataset of ozone profiles is created via interpolation of the limb profiles from each day to 1° × 1° horizonal grid. A new kriging-type interpolation method, which takes into account data uncertainties and the information about natural ozone variations from the SILAM-adjusted ozone field, has been developed. To mitigate the limited accuracy and coverage of the limb profile data in the UTLS, a smooth transition to the model data is applied below the tropopause. This allows estimation of stratospheric ozone column with full coverage of the UTLS. The derived ozone profiles are in very good agreement with collocated ozonesonde measurements. The residual method was successfully applied to OMI and TROPOMI clear-sky total ozone data in combination with the stratospheric ozone column from the high-resolution limb profile dataset. The resulting tropospheric ozone column is in very good agreement with other satellite data. The global distributions of tropospheric ozone exhibit enhancements associated with the regions of high tropospheric ozone production. The main created datasets are (i) monthly 1° × 1° global tropospheric ozone column dataset using OMI and limb instruments, (ii) monthly 1° × 1° global tropospheric ozone column dataset using TROPOMI and limb instruments and (iii) daily 1° × 1° interpolated stratospheric ozone column from limb instruments. Other datasets, which are created as an intermediate step of creating the tropospheric ozone column data, are: (i) daily 1° × 1° clear sky and total ozone column from OMI and TROPOMI (ii) Daily 1° × 1° homogenized and interpolated dataset of ozone profiles and (iii) daily 1° × 1° dataset of ozone profiles from SILAM simulations with adjustment to satellite data. These datasets can be used in various studies related to ozone distributions, variability and trends, both in the troposphere and the stratosphere.

Responses of fine particulate matter (PM2.5) air quality to future climate, land use, and emission changes: Insights from modeling across shared socioeconomic pathways

A series of global actions have been made to address climate change. As a recent developed climate policy, Intended Nationally Determined Contributions (INDC) have renewed attention to the importance of exploring temperature rise levels lower than 2 °C, in particular a long-term limit of 1.5 °C, compared to the preindustrial level. Nonetheless, achieving the 2 °C target under the current INDCs depends on dynamic socioeconomic development pathways. Therefore, this study conducts an integrated assessment of INDCs by taking into account different Shared Socioeconomic Pathways (SSPs). To that end, the CEEP-BIT research community develops the China’s Climate Change Integrated Assessment Model (C3IAM) to assess the climate change under SSPs in the context of with and without INDCs. Three SSPs, including “a green growth strategy” (SSP1), “a more middle-of-the-road development pattern” (SSP2) and “further fragmentation between regions” (SSP3) form the focus of this study. Results show that after considering INDCs, mitigation costs become very low and they have no evident positive changes in three SSPs. In 2100, a temperature rise would occur in SSP1-3, which is 3.20, 3.48 and 3.59 °C, respectively. There are long-term difficulties to keep warming well below 2 °C and pursue efforts toward 1.5 °C target even under INDCs. A drastic reduction in greenhouse gas emissions is needed in order to mitigate potentially catastrophic climate change impacts. This work contributes on realizing the hard link between the earth and socioeconomic systems, as well as extending the economic models by coupling the global CGE model with the economic optimum growth model. In C3IAM, China’s energy consumption and emissions pattern are investigated and refined. This study can provide policy makers and the public a better understanding about pathways through which different scenarios could unfold toward 2100, highlights the real mitigation and adaption challenges faced by climate change and can lead to formulating effective policies.

Abstract. Large-scale changes in ozone precursor emissions affect ozone directly in the short term, and also affect methane, which in turn causes long-term changes in ozone that affect surface ozone air quality. Here we assess the effects of changes in ozone precursor emissions on the long-term change in surface ozone via methane, as a function of the emission region, by modeling 10% reductions in anthropogenic nitrogen oxide (NOx) emissions from each of nine world regions. Reductions in NOx emissions from all world regions increase methane and long-term surface ozone. While this long-term increase is small compared to the intra-regional short-term ozone decrease, it is comparable to or larger than the short-term inter-continental ozone decrease for some source-receptor pairs. The increase in methane and long-term surface ozone per ton of NOx reduced is greatest in tropical and Southern Hemisphere regions, exceeding that from temperate Northern Hemisphere regions by roughly a factor of ten. We also assess changes in premature ozone-related human mortality associated with regional precursor reductions and long-range transport, showing that for 10% regional NOx reductions, the strongest inter-regional influence is for emissions from Europe affecting mortalities in Africa. Reductions of NOx in North America, Europe, the Former Soviet Union, and Australia are shown to reduce more mortalities outside of the source regions than within. Among world regions, NOx reductions in India cause the greatest number of avoided mortalities per ton, mainly in India itself. Finally, by increasing global methane, NOx reductions in one hemisphere tend to cause long-term increases in ozone concentration and mortalities in the opposite hemisphere. Reducing emissions of methane, and to a lesser extent carbon monoxide and non-methane volatile organic compounds, alongside NOx reductions would avoid this disbenefit.

Stratospheric temperature changes are very important for climate systems through atmospheric circulation and stratosphere-troposphere coupling. The relationship between solar modulation potential SMP as a measure of solar activity and variation of temperature in upper, middle and lower stratosphere over Iraq for the period 1948-2014 (for about five solar cycles) has been presented in this study. For upper, middle and lower stratosphere we found that stratospheric temperatures decreased, this descending trend is offset by obviously descending trend in solar activity for the same period. The correlation coefficients between annual solar modulation potential SMP and stratospheric temperature have been examined at pressure levels (10,20,30,50,70 mb) that represent the upper, middle and lower stratosphere over 67 years (1948 to 2014). All levels have positive correlation. The correlation coefficient (R) in upper stratosphere at level 10 mb has been 0.4, in middle stratosphere at level (20, 30 mb) the correlation coefficient (R) has been 5.3 and 0.6 respectively. For lower stratosphere at level (50, 70 mb) R has been 0.57 and 0.68 respectively.

Global distributions of the tropospheric ozone column (TOC) and its error are estimated by assimilating satellite observations of total ozone column and stratospheric ozone profiles in a global chemistry‐transport model. A quantitative comparison of the calculated TOC with collocated ozonesondes observations shows good agreement over a wide range of latitudes for the period studied (July–September 1992), even at the daily timescale. In most cases, the difference with the ozonesonde‐based TOC is within the calculated error estimate. Comparison with the TOC distribution by Thompson and Hudson [1999] does not lead to any quantitative conclusion due to a mix of similarities and differences, even in the geographical distribution of the TOC. A positive TOC bias with respect to ozonesondes is found in this study over Hilo and Irene, probably due to a misrepresentation of the transport of ozone in a region of strong TOC gradients. In September 1992 the LIDAR observations from the TRACE‐A campaign are also assimilated. This is shown to provide a constraint on the TOC that significantly propagates across the Atlantic Ocean, indicating the importance of point measurements of ozone for constraining its large‐scale distribution.

This study aims to investigate the Stratosphere-Troposphere Exchange (STE) events and ozone changes over Irene (25.5° S, 28.1° E). Twelve years of ozonesondes data (2000–2007, 2012–2015) from Irene station operating in the framework of the Southern Hemisphere Additional Ozonesodes (SHADOZ) was used to study the troposphere (0–16 km) and stratosphere (17–28 km) ozone (O3) vertical profiles. Ozone profiles were grouped into three categories (2000–2003, 2004–2007 and 2012–2015) and average composites were calculated for each category. Fifteen O3 enhancement events were identified over the study period. These events were observed in all seasons (one event in summer, four events in autumn, five events in winter and five events in spring); however, they predominantly occur in winter and spring. The STE events presented here are observed to be influenced by the Southern Hemisphere polar vortex. To strengthen the investigation into STE events, advected potential vorticity maps were used, which were assimilated using Modélisation Isentrope du transport Méso–échelle de l’Ozone Stratosphérique par Advection (MIMOSA) model for the 350 K (~12–13 km) isentropic level. These maps indicated transport of high latitude air masses responsible for the reduction of the O3 mole fractions at the lower stratosphere over Irene which coincides with the enhancement of ozone in the upper troposphere. In general, the stratosphere is dominated by higher Modern Retrospective Analysis for Research Application (MERRA-2) potential vorticity (PV) values compared to the troposphere. However, during the STE events, higher PV values from the stratosphere were observed to intrude the troposphere. Ozone decline was observed from 12 km to 24 km with the highest decline occurring from 14 km to 18 km. An average decrease of 6.0% and 9.1% was calculated from 12 to 24 km in 2004–2007 and 2012–2015 respectively, when compared with 2000–2003 average composite. The observed decline occurred in the upper troposphere and lower stratosphere with winter and spring showing more decline compared with summer and autumn.

Domestic versus international contributions on 2050 ozone air quality: How much is convertible by regional control?

Short-term variations in surface UV-B irradiance and total ozone column at Ny-Ålesund during the QAARC campaign

Abstract. An unusually strong and prolonged stratospheric sudden warming (SSW) in January 2006 was the first major SSW for which globally distributed long-lived trace gas data are available covering the upper troposphere through the lower mesosphere. We use Aura Microwave Limb Sounder (MLS), Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS) data, the SLIMCAT Chemistry Transport Model (CTM), and assimilated meteorological analyses to provide a comprehensive picture of transport during this event. The upper tropospheric ridge that triggered the SSW was associated with an elevated tropopause and layering in trace gas profiles in conjunction with stratospheric and tropospheric intrusions. Anomalous poleward transport (with corresponding quasi-isentropic troposphere-to-stratosphere exchange at the lowest levels studied) in the region over the ridge extended well into the lower stratosphere. In the middle and upper stratosphere, the breakdown of the polar vortex transport barrier was seen in a signature of rapid, widespread mixing in trace gases, including CO, H2O, CH4 and N2O. The vortex broke down slightly later and more slowly in the lower than in the middle stratosphere. In the middle and lower stratosphere, small remnants with trace gas values characteristic of the pre-SSW vortex lingered through the weak and slow recovery of the vortex. The upper stratospheric vortex quickly reformed, and, as enhanced diabatic descent set in, CO descended into this strong vortex, echoing the fall vortex development. Trace gas evolution in the SLIMCAT CTM agrees well with that in the satellite trace gas data from the upper troposphere through the middle stratosphere. In the upper stratosphere and lower mesosphere, the SLIMCAT simulation does not capture the strong descent of mesospheric CO and H2O values into the reformed vortex; this poor CTM performance in the upper stratosphere and lower mesosphere results primarily from biases in the diabatic descent in assimilated analyses.

Effect difference of climate change and urbanization on extreme precipitation over the Guangdong-Hong Kong-Macao Greater Bay Area