dataesr/scientific-paragraphs-categorization huggingface dataset:
igacproject. org/activities/TOAR). Initiated in 2014, TOAR's mission is to provide the research community with an up-to-date scientific assessment of tropospheric ozone's global distribution and trends from the surface to the tropopause. TOAR's primary goals are, 1) Produce the first tropospheric ozone assessment report based on the peer-reviewed literature and new analyses, and 2) Generate easily accessible, documented data on ozone exposure metrics at thousands of measurement sites around the world  (Lefohn et al. , 2018). Through the TOAR surface ozone database  (Schultz et al. , 2017;  hereinafter TOAR-Surface Ozone Database) these ozone metrics are freely accessible for research on the global-scale impact of ozone on climate  (Gaudel et al. , 2018) , human health  (Fleming et al. , 2018)  and ecosystem productivity  (Mills et al.
2. 1) are used  (Viallon et al. 2016). Thus GPT supports the proposed decrease in the ozone absorption cross-section at 253. 65 nm. Because GPT is utilized as a standard and has not been used for ambient ozone measurements in either the historical record or the TOAR database, it is not listed in Table  1. Further information on GPT is included in the Supplemental Material (Text S-1). These standards are propagated, via international coordination of the adoption of ozone absorption coefficients for UV and visible light  (Orphal et al. , 2016) , to the communities using remote sensing methods for ozone measurement in the free troposphere. There is a recommendation to extend this co-ordination to infrared methods  (Orphal et al. , 2016). The available record of surface ozone measurements divides into two periods: the modern period covers approximately 1975 to the present and is defined by the widespread availability of sensitive UV photometers for surface ozone measurements; the historical period covers 1877-1975 and is defined by the use of other techniques and the absence of these UV photometers.
There are a few years of overlap between the periods during the uptake of the modern technology. A set of four criteria were developed and applied to select data for the historical reconstruction: (1) the measurement methods used should be related, through intercomparisions, to the current standard UV absorption photometer method;  (2)  the likelihood for significant contamination of the ozone measurements due to interfering pollutants in the sampled atmosphere should be low; (3) for surface ozone, the air sampled should be representative of the well-mixed boundary layer, and (4) recognizing the uncertainties associated with all of the historical data sets, the measurements should be free from major artifacts or inconsistencies. The datasets that pass these four criteria are explicitly documented.
To commence the reconstruction of a historical tropospheric ozone record, the first of the four criteria must be addressed for each of the ozone data sets examined. As many historical measurement methods pre-date the current UV method, traceability may be derived through published side-by-side intercomparisons with an intermediate method. The surface ozone method/instrument intercomparisons found in the literature are presented in Table  2. The ratio of each pair of methods corresponds to either the ratio of the mean values from each method or the slope of a regression whose intercept is assumed to be zero. Often only this number is recorded. In some cases an uncertainty is cited. Where not explicitly stated, we have assumed that this corresponds to one standard deviation. Table  2  is divided into 5 sections, the first four sections commencing with the comparison of a method with the UV method and then proceeding to other comparisons of that or closely related methods.
, 2013). Moreover, it requires little maintenance during operation. Consequently, it is also the recommended technique for continuous ozone observations in ambient air, e. g. in the WMO Global Atmosphere Watch Programme  (Galbally et al. , 2013) , in the United States (US EPA, 2013), Europe (European Union, 2012), and India  (Central Pollution Control Board, 2009). Data quality and consistency of records within and across networks have improved over time, as seen by comparison of time series  (Logan et al. , 2012; Parrish et al. , 2012) , and station audits by the World Calibration Centre for Surface Ozone, Carbon Monoxide, Methane and Carbon Dioxide (WCC-Empa)  (Buchmann et al. , 2009). The great majority of in-situ records from more than 9000 sites in the TOAR database that have operated for 3 years or more between approximately 1975 and 2015 were recorded with UV absorption photometers.
In fact, in Figure  1  less than 1. 5% of the observed calibration results lie outside the estimated uncertainty range, so for this extensive dataset of UV absorption photometer field calibrations, the results fall well within the uncertainty estimate (4). This reflects the robust nature of properly undertaken ozone measurements with the UV photometric method. The foregoing analysis does not cover all sources of uncertainty, omitting those associated with such causes as inlet losses and potential interferences. The uncertainty of the absorption cross-section is not included, as the calibrations are performed with primary standard analysers based on the same UV absorption cross-section. For the purposes of the subsequent analyses, at a mean level of 30 nmol mol -1 , the total expanded uncertainty (95% confidence) in modern ozone measurements is <2 nmol mol -1.
Although not in common use currently, in the TOAR surface ozone database chemiluminescence is listed as the method of ozone measurement at 627 of more than 9000 sites. The rhodamine-B solid chemiluminescent ozone analyser was developed as an ozonesonde  (Regener 1960)  which was calibrated both with a surface based analyser, traceable to a UV measurement, prior to launch and against the total column ozone measured by a UV (Dobson) instrument at the same location and near the time of the ozonesonde flight. This system was also developed into an aircraft and surface based analyser  (Regener, 1964). The analyser was regularly calibrated with an internal ozone generator that in turn had been calibrated against UV based ozone measurements  (Regener, 1964). This chemiluminescent ozone analyser was used in the 1960s in the USA and Antarctica.
The nitric oxide chemiluminescent ozone analyser is a stable instrument with a sensitive ( ~0. 1 nmol mol -1 ) and fast response ( ~1 s) and not subject to SO 2 or NO 2 interference. A current description of the instrument is available  (Campos et al. , 2006). The analyser requires regular calibration with a standard ozone analyser. There are no records in the TOAR surface ozone database that have employed nitric oxide-based chemiluminescence as the method of measurement, but these analysers have been used extensively in aircraft measurements of tropospheric ozone  (Lenschow et al. 1980 ) for more than 20 years (Section 4. 7).
Ozone measurements in surface air 1870s-1970s For the purpose of reconstructing the historical surface ozone record an extensive literature search was conducted, which unearthed several data sets that have not before been coherently analysed in a single study. As discussed in Section 2, the approach used in TOAR-Observations is to reconstruct regional or zonal average ozone values based on all available historical data sets rather than relying on individual data sets as in previous studies. Following is a description of the application of the four data selection critieria for the historical reconstruction. Criterion 1: all measurements via the Schönbein and related filter paper methods are rejected; the early UV measurements  are corrected to the currently accepted values of absorption cross-sections  (Hearn, 1961) ; the observations made by the Mast Ozone Meter and related transmogrifier, and the Cauer method have been adjusted following Table  2.
As  Fabry (1950)  said, the results are absurd. Serious artifacts and inconsistencies associated with individual datasets are discussed in the Supplemental Material (Text S-3). For every site, the method used, the possible pollution sources, the topography, surface cover and prevailing meteorology and credibility of the results have been examined to assess the suitability of the observations for inclusion in the historical record. One excluded site is explicitly included in the following discussion. This is Montsouris, Paris, which is discussed here because it has been used as a central set of data for a number of previous historical reconstructions of tropospheric ozone. For the other historical data discussed below, in all cases the method is valid and with one exception, the sites are presumed free of interfering pollutants and representative of the well-mixed boundary layer.
Dauvillier (1934)  used the KI-arsenite method at Scorsby Sund on the east coast of Greenland, from November 1932 to August 1933 (Table  5 ). His daily data (reported as 24-hour means) can be notionally divided into two sets: 225 days of data show a background value of about 50 µg m -3 , with less than 5% over 100 µg m -3 (approximately 47 nmol mol -1 ), and 45 days of data that show major events where ozone went as high as 570 µg m -3. The latter maximum is approximately 270 nmol mol -1. Ozone during December 1932 was particularly high, averaging approximately 100 nmol mol -1. This monthly mean is more than twice the highest December monthly mean at Arctic sites in the TOAR Surface Ozone Database. These high events were recorded by  Dauvillier (1934)  as interesting and remarkable. He associated them with aurora and suggested enhanced transport from the stratosphere or ionosphere to the surface air.
For the TROZ network, the largest network for this period, a coulometric KI method (Section 2. 1. 3) was used  (Pruchniewicz 1970 (Pruchniewicz , 1973)). Detailed consideration of the TROZ data reveals issues with (a) its traceability to the modern UV standard, (b) interferences or pollution at the sites, and (c) special features in selection of representative surface ozone data (reported ozone values are representative of daytime maximum values rather than daytime means). These issues are discussed in the Supplemental Material (Text S-3). Because this data set has data from 15 stations (one station was omitted for reasons described below), located between 90°N and 30°S, it potentially dominates the 1950s-1970s historical record. To understand the sensitivity of the historic analysis to the TROZ data and to provide robust conclusions, the quantification of the change in ozone from the historical to the modern period is performed both with and without the TROZ data (see Section 3.
The weighted mean of the accepted observations in the Northern Polar region (1950s-1970s) is 24 nmol mol -1 with a range of 19-34 nmol mol -1. If the College and Barrow data are entirely omitted this average becomes 19 nmol mol -1 , but this value relies entirely upon the TROZ network. In the Northern Temperate region there are multiple data sets. Bowen and Regener (1951)  present ozone data from Capillo Peak Observatory in New Mexico (approximately 2800 m asl). Four years of ozone data were obtained at the Arosa Light Observatory (located on the northern edge of the town of Arosa in a Swiss valley, 1810 m  (Staehelin et al. , 2018) ) using the Ehmert technique  (Perl, 1965). In August 1953 and June/July and September 1954 ozone observations were made utilizing an 80 m tower using the Cauer method (Section 2.
1. 4) at Lindenberg (98 m) in northeastern Germany  (Teichert 1955). Measurements were made at Hohenpeissenberg (975 m) from 1971 to 1975 using the HP-KI method. Six other sites, all from the TROZ network, are listed in Table  4. The TROZ station Lindau is omitted due to the influence of high levels of local pollution  (Fabian and Pruchniewicz, 1976). The long-term site at Arkona on the north coast of Germany using the Cauer method commenced in 1956  (Warmbt 1964; Feister and Warmbt 1987). With the exception of Arkona, the methods used are valid, the sites are presumed free of interfering pollutants and representative of the well mixed boundary layer. With a correction for SO 2 interference  (Warmbt 1964; Feister and Warmbt 1987) , the Arkona data are accepted (see note #12 in Text S-7). Measurements from three sites in Japan made with the Ehmert method are included  (Miyake et al.
Changes in surface ozone The sets of observations that pass the 4 data selection criteria imposed here for historical surface ozone observations are presented in Tables 4-6 and Figures  2 3 4 5 6 , grouped by region. To quantify the changes of ozone levels from the historical (pre 1975) to the modern period , the historical data are compared to all available modern ozone observations in the same regions, according to the criteria described below. The modern data are extracted from the TOAR Surface Ozone Database and averaged across 5 × 5 degree grid cells at 5-year intevals  (Schultz et al. , 2017). The historical observations have been selected to be representative of the well-mixed boundary layer, and the comparable metrics from the TOAR Database for rural stations are daytime average values and daily 8 hour maxima (DMA8).
As the "rural" designation in the gridded average product from the TOAR database  (Schultz et al. , 2017)  excludes sites at elevations above 2000 m (because of the typical increase of ozone with altitude) a proper comparison should exclude the measurements at higher mountain sites. In Europe (Figure  2 ) these are Grands-Mulets, Mont Blanc, Jungfraujoch, Zugspitze, Mt. Norikura in Japan and in North America, Capillo Peak. Except for the value of 63 nmol mol -1 in 1896 at Grands-Mulets, disregarding these eight points does not substantially change the picture in Figure  2  (see Figure  S-2 ). In Figure  2  there is no evidence of a change in rural background ozone during the historical record , as noted in Section 3. 4. The averages for the four historical time periods in Table  4  differ by less than 3 nmol mol -1.
The diurnal variation of surface ozone depends on both the underlying surface and the topography. At remote sites, as are considered here, measurements over flat continental surfaces show a distinct daytime maxima, while measurements over water, snow and ice show little diurnal variation and  In the Northern Polar region (Figure  3 ), and in the Tropics (Figure  5 ), there is also some evidence of an increase, although there are fewer historical datasets with   5.
Year A quantification of the change in ozone mole fraction from the historic to the modern measurements has been performed for each latitude band in Tables 4-6 and is presented in Table  7. Table  7a  presents changes of surface ozone from the historical period to the modern period using all available historical observations below 2000 m elevation. The modern period is based on rural ozone observations below 2000 m for the years 1990-2014, and the metric for the modern data is the 12-hr daytime average. The modern data were extracted from the TOAR Surface Ozone Database and reduced to monthly means   6  for details). Error bars represent standard deviations of the measurement averages (atmospheric variability), not measurement uncertainty. Fiveyear averages (daytime mean) of modern UV measurements at sites below 2000 m, classified as "rural", in the TOAR 5 × 5 degree gridded average product are also shown, both daytime averages (day) and daily 8-hour maxima (DMA8)  (Schultz et al.
Early measurements Historical observations of free tropospheric ozone that pass the relevant data selection criteria are presented in Table  8  and Figure 7. The first direct measurement of ozone from a balloon ascent was made with three flights near Stuttgart, Germany in 1934  (Regener and Regener, 1934). A quartz UV spectrograph was used to observe the change with altitude of the total amount of ozone above the balloon. The derived profile increases very smoothly from a value equivalent to 40 nmol mol -1 near the ground to ~180 nmol mol -1 at 10 km; this integrates to about 40 Dobson units (DU) below 10 km. A similar differential method was used on the manned flight of the high-altitude balloon Explorer II on November 11, 1935 in the western USA. This showed very little ozone below the tropopause: less than 10 DU  (O'Brien et al.
The former are quite low, and failed to detect the tropopause, possibly because of losses in the intake  (Brewer, 1955). These data are excluded based on criterion 4. The latter show an average profile increasing from 30 nmol mol -1 at launch to ~95 nmol mol -1 just below the tropopause, very similar to a typical contemporary profile. While some data sets show a large variability (e. g. Paetzold 1955a, b) , the approach here is to include all of the early measurements that meet the relevant selection criteria (1, 2 and 4). The weighted mean of the observations in Table  8  is 23 DU, with a range of 0-50 DU for the period 1934-1955. The corresponding modern average 0-10 km tropospheric column amount of ozone is 36 DU, for both the northern Europe and the eastern US regions in Figure  7. When the historical datasets are treated as separate observations with equal weight (as for the surface data in Section 3.
BSVertOzone  (Hassler et al. , 2018; Bodeker et al. , 2013)  uses several satellite data sources and a sophisticated regression-interpolation scheme to produce a zonally-averaged climatology in 5° latitude bands from 0 to 70 km. The Trajectory-mapped Ozonesonde dataset for the Stratosphere and Troposphere (TOST), used elsewhere in this paper and in TOAR-Climate  (Gaudel et al. , 2018) , uses the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model  (Draxler and Hess, 1998)  and meteorological fields from National Centers for Environmental Prediction (NCEP) reanalyses to fill the gaps between ozonesonde stations, by extending each ozone record along its trajectory path forward and backward for 4 days. Over this 4-day period ozone production and loss is assumed to be negligible.
Ozone values along these trajectory paths are binned into a 3-dimensional grid of 5° × 5° × 1 km (latitude, longitude, and altitude), from sea level or ground level up to 26 km (Figure  17 ). Tropospheric column ozone (TCO) is calculated from ozone mole fractions below the tropopause, found using the WMO 2 K/km lapse-rate definition applied to the NCEP reanalysis data. TOST has been evaluated using individual ozonesondes, excluded from the mapping, by backward and forward trajectory comparisons, and by comparisons with aircraft profiles and surface monitoring data  (Tarasick et al. , 2010; Liu, G. et al. , 2013; Liu, J. et al. , 2013). Differences are typically about 10% or less, but there are larger biases in the UTLS, the boundary layer, and in areas where ozonesonde measurements are sparse.
, 1998). Quality assurance procedures have not changed since the beginning of the record in 1994. No calibration drift has been observed, nor inconsistency between MOZAIC and IAGOS instruments  (Blot et al. , in preparation). Ozone monitors are calibrated annually to a reference analyser at the Bureau Internationale des Poids et Mesures (BIPM), and also compared every 2 hours to an in-flight ozone calibration source. MOZAIC can be considered a reference dataset (e. g. Thouret et al. , 1998 Thouret et al. , , 2006;; Schnadt Poberaj et al. , 2009; Logan et al. , 2012) , due to its known calibration history. Previous comparisons of MOZAIC/IAGOS data with ozonesondes show negative biases of a few per cent (sonde values higher), with larger differences in the earlier part of the MOZAIC record  (Thouret et al.
3). Unlike ozonesonde sites, airports are typically urban, but MOZAIC/IAGOS ozone data do not appear largely affected by local boundary layer chemistry  (Petetin et al. , 2016 (Petetin et al. , , 2018)). This is also indicated by Figure  20. The IAGOS database (http://www. iagos. org) currently contains data from more than 100,000 vertical profiles of tropospheric ozone, measured during takeoff and landing from 148 airports around the world since August 1994. The data sampled from the ascents and descents at these airports are unevenly distributed both spatially and temporally because the frequency of visits to airports by aircraft that take part in MOZAIC/IAGOS varies, depending on commercial airlines' operating constraints. In particular, data are sparser in the southern hemisphere (Figure  21 ).
5 µm spectral range, but with 0. 5 cm -1 spectral resolution (see Table  S -11 for details). Ozone profiles are retrieved with similar vertical resolution in the troposphere to that of TES. Table  11  summarizes the characteristics of these satellite data products. More detailed descriptions are found in the Supplemental Material. For enhancing sensitivity in the lower troposphere, IASI profiles may also be retrieved using a TP altitudedependent regularization  (Eremenko et al. , 2008) , which optimizes the retrieval constraints to maximize the DOFS in the lower troposphere  (Dufour et al. , 2010 (Dufour et al. , , 2012 (Dufour et al. , , 2015)). These IASI (TP) retrievals are able to depict the horizontal distribution of ozone plumes within the lower troposphere (Figure  22 ) with a relative maximum of sensitivity typically between 3 to 4 km, in case of positive thermal contrasts (i.
Representativeness Ozone is a highly reactive secondary pollutant with many processes such as photochemical formation, deposition and titration playing a role in determining atmospheric mole fractions. Although several thousand ground-based stations measure ozone concentrations at high temporal frequencies world-wide (TOAR-Surface Ozone Database), the globe is nevertheless undersampled, since surface ozone over land surfaces may vary locally on scales of a few kilometres or less. (Here the spatial representativeness of observations down to an urban/regional scale is considered. The issue of measurements being representative of the well-mixed boundary layer addressed earlier with Criterion 3, is on a smaller scale and not addressed in this section. ) Sites are unevenly distributed, with relatively few in the tropics and southern mid-latitudes  (Sofen et al.
, 2016 ; TOAR-Surface ozone database). Spatial representativeness is therefore often the largest source of uncertainty in the use of ground-based data. For a well-calibrated analyzer, measurement uncertainty is in the 1-nmol mol -1 range (Section 2. 2. 2), while two locations with different site characteristics (e. g. urban vs rural) may show average differences an order of magnitude larger. Land use may also change with time, complicating trend analysis by inducing changes that do not reflect background ozone trends. Remote baseline sites with minimal influence of local processes are usually representative of a larger area than sites located for catching the plumes of particular sources (e. g. monitoring stations at curbsides) which reflect local conditions. The application of the data may also determine the area of representativeness, as correlation lengths are typically longer for monthly averages than for daily data (e.
g. Sofen et al. , 2016). Standard attempts to assess the representativeness of in-situ air quality monitoring stations classify the sites into categories like urban, suburban, rural, or remote, in terms of their exposure to sources and sinks (e. g. European Union, 2008). These classifications are somewhat qualitative. More quantitative approaches use station metadata. Ozone concentrations at urban stations are strongly controlled by NOx titration, and so population density (which can be seen as a proxy for NOx emissions) is often used to classify sites as urban, suburban, and rural. The intensity and nature of sources (traffic, industrial or background) can be used to refine the classification. TOAR-Surface ozone database has developed a globally consistent classification scheme based on population density, nighttime light intensity, OMI tropospheric column NO 2 , and station altitude.