Roles of ozone depleting substances and solar activity in observed long-term trends in total ozone column over Indian region

An additive time-series decomposition analysis was performed on the Multi Sensor Reanalysis-2 (MSR-2) monthly mean total ozone column (TOC) time-series dataset spanning over 34 years (January 1979–December 2012) for Indian region (0.0–40.0 °N; 67.5–97.5° E). Statistically significant (p-value <0.05) long-term trends in TOC were estimated in the deseasonalized TOC time series. The role of multiple natural and anthropogenic factors: quasi biennial oscillations (QBO), El-Nino Southern Oscillations (ENSO), cyclic variation in solar activity (SA), and ozone depleting substances (ODS) was investigated to explain the long-term trends in TOC over Indian region. Over sub-tropical Indian region (25.0° N– 40.0° N), declining long-term linear trends were estimated, which varied from ?0.30% to ?1.10% per decade. Interestingly a positive long-term linear-trend (0.10–0.30% per decade) was observed over equatorial-tropical part of Indian region. No statistically significant long-term trend was observed for 30mb Equatorial Zonal Winds and Nino 3.4 index – indicators for QBO and ENSO; however, a positive long-term linear trend of magnitude 17.00 ± 1.18% per decade was observed in effective equivalent stratospheric chlorine – a proxy for ODS, and a negative long-term linear trend of magnitude 12.72 ± 2.86% per decade was observed in 10.7 cm Solar Radio Flux – a representative for SF. It is inferred that over the Indian region above tropic of cancer, about 85.00% of the estimated negative long-term linear trend in TOC can be explained by the increase in the stratospheric concentration of ODS; whereas, decrease in the solar activity accounted for 15.00% of the estimated negative long-term linear trend in TOC over sub-tropical Indian region.     

Sulphur dioxide loadings over megacity Lahore (Pakistan) and adjoining region of Indo-Gangetic Basin

This article presents spatial and temporal variations of planetary boundary layer (PBL) sulphur dioxide (SO₂) over megacity Lahore and adjoining region, a typical representative area in the Indo-Gangetic Basin (IGB) largely influenced by transported volcanic SO₂ from Africa, Middle East, and southern Europe, by using data retrieved from satellite-based Ozone Monitoring Instrument (OMI) during October 2004–September 2015. We find a positive trend of 2.4% per year (slope 0.01 ± 0.005 with y-intercept 0.35 ± 0.03 Dobson Unit (DU), correlation coefficient r = 0.55 and 2-tailed p-value at 0.1) of OMI-SO₂ column with the average value of 0.4 ± 0.05 DU. Strong seasonality of OMI-SO₂ column is observed over the region linked with local meteorology, patterns of anthropogenic emissions, crop residue burning, and vegetation cover. There exists a seasonal high value in winter 0.56 ± 0.24 DU with a peak in December 0.67 ± 0.26 DU. The seasonal lowest value is observed to be 0.29 ± 0.11 DU in wet summer with minimum value in July 0.25 ± 0.06 DU. High growth rates of OMI-SO₂ column over the study region have been observed in January, June, October, and December ranging from 5.7% to 11.6% per year. Satellite data show elevated OMI-SO₂ columns in 2007, 2008, 2011, and 2012 largely contributed by trans-boundary volcanic SO₂. A detailed analysis of volcanic SO₂ transported from Africa and Middle East (Jabal Al-Tair, Dalaffilla, and Nabro volcanoes) over the study area is presented. Air mass trajectories suggest the presence of long-range transported volcanic SO₂ at high altitude levels over Lahore and IGB region during the volcanic episodes. The SO₂ enhancements in PBL during winter season are generally due to significant vertical downdraft of high-altitude volcanic SO₂. For the first time, we present significant influence of volcanic SO₂ from southern Europe (Mt. Etna volcano) reaching over the study area. Daily mean OMI-SO₂ levels up to 21.4, 10.0, 5.6, and 2.4 DU have been noticed due to the eruptions from Dalaffilla, Mt. Etna, Nabro, and Jabal Al-Tair volcanoes, respectively.     

Temporal and spatial variabilities of total ozone column over Portugal

This paper focuses on the spatial-temporal structure of total ozone column (TOC) over Portugal. This relevant region of southwestern Europe has not been evaluated yet in detail due to the lack of continuous and well-covered ground-based TOC measurements. The data used in this study are derived from the NASA's Total Ozone Mapping Spectrometer (TOMS) for the period 1978-2005. The TOC spatial behavior shows no significant longitudinal variability (smaller than 3%). In contrast, the variation in latitude changes between 3.5% and 6% depending on the calendar month. The TOC in the northern Portugal is, on average, higher than that recorded in the South. The temporal variability was analyzed for three scales: long-term, seasonal and short-term. The long-term TOC changes are analyzed between 1978 and 1999 by means of linear least squares fits. The results show an annual TOC trend of (−2.65 ± 0.70)%/decade which is statistically significant at the 95% confidence level. This TOC decrease is smaller than the trends obtained in other midlatitudes regions which could be partially explained by the compensation due to the observed increase in the tropospheric ozone over the Iberian Peninsula. A trend analysis performed for each individual month shows a statistically significant TOC decline between March and October, with a maximum linear trend value of (−7.30 ± 1.45)%/decade in May. The amplitude of the seasonal TOC cycle over Portugal shows a slight dependence in latitude, varying from 28.6 DU (37.5° N) to 33.6 DU (41.5° N). Finally, the short-term variability showed a notable seasonal behavior, with maximum day-to-day TOC changes in winter (~ 6%) and minimum in summer (~ 3%). In addition, the persistence (characterized by the autocorrelation coefficients) strongly decreases after a few days (except in summer months).     

A study of spatial and temporal dynamics of total ozone over Southwest China with multi-source remote-sensing data

This article describes a study of the spatial and temporal dynamics of total ozone over Southwest China using satellite-retrieved total ozone products from 1996 to 2008 and a ground-based Dobson spectrophotometer. The findings indicated that the value of total ozone (265.7 Dobson unit (DU)) over Southwest China is lower than the value (273.7 DU) over the adjacent region at the same latitude by about 8 DU, and is about 13.8 DU lower than the global average at the same latitude (279.5 DU), and that there is a distinctly low-value area due to the higher elevation. The relationship of total ozone and the elevation presents a negative correlation, the terrain being the main factor to affect this condition. In the long term, the variation of total ozone exhibits a slightly increasing trend from 1996 over this region. Total ozone presents an obvious seasonal change, with the largest value appearing in springtime and the smallest appearing in wintertime. The difference between the regional seasonal mean value of total ozone in springtime and wintertime is about 28 DU, although the difference between the maximum and minimum monthly total ozone throughout a year is up to 50 DU. There is a positive correlation between the variation of total ozone and relative humidity. Relative humidity may be an important factor impacting on the pattern of seasonal change of total ozone.     

Long-term changes in total ozone column content and its association with stratospheric temperature over two neighbouring tropical Asian stations

This paper focuses on the long-term declining change in total ozone column (TOC) derived from satellite measurements over a 25 year period over two neighbouring tropical Asian sites, Karachi, and Mt Abu. A strong declining trend was observed in TOC at both sites, with a significance level of over 95% and a higher magnitude of 4 to 10 DU per decade in September to December and a weak statistical significance level of below 85% and a lower magnitude of 2 to 4 DU per decade in pre-monsoon months. However, during the monsoon months, a small declining trend of about 1 DU per decade was observed, but this variation is statistically insignificant. Further, the long-term changes in TOC exhibit seasonal dependence with a more negative change of 10 DU per decade in winter over Karachi and 7 DU per decade over Mt Abu. The consequence of such a net long-term declining change in TOC as high as 10 DU per decade is expected to have serious environmental implications due to an overall increase in ground-level solar UV radiation of 18% over the normal values in the tropics. In order to identify some plausible causes of this depletion trend in ozone concentration with stratospheric temperature and solar activity, it is clear that there is a strong relationship between the seasonal dependence of the long-term declining trend of TOC on air temperature at 10 mb or stratospheric cooling. At the same time, there is a less significant long-term variation in TOC due to altered solar activity levels.     

Assessment of total columnar ozone climatological trends over the Indian sub-continent

In the present study, long term satellite and Dobson spectrophotometer Total Column Ozone (TCO) data have been used to study the interannual variability and also to assess climatological trends in TCO over different geographical locations of Indian sub-continent. TCO data were analyzed for the period 1957 to 2015 over New Delhi (28.63° N, 77.18° E), Varanasi (25.30° N, 83.02° E), Pune (18.53° N, 73.84° E) and Kodaikanal (10.0° N, 77.47° E). An extensive validation was performed for Total Ozone Mapping Spectrometer (TOMS) and Ozone Monitoring Instrument (OMI) retrieved TCO data independently with Dobson Spectrophotometer TCO measurements over New Delhi, Varanasi, Pune and Kodaikanal. The results of this exercise showed good correlation coefficient (r) of 0.87 (0.88), 0.84 (0.82), 0.91 (0.80) and 0.84 (Data not available) respectively. Climatological mean TCO over New Delhi, Varanasi, Pune and Kodaikanal are 275.02 ± 6.44 DU, 269.03 ± 7.34 DU, 260.78 ± 5.07 DU and 258.71 ± 6.36 DU respectively for the period 1957 to 2015. An increasing trend over New Delhi (0.20 DU year^–1), Pune (0.18 DU year^–1), Kodaikanal (0.14 DU year^–1) and decreasing trend over Varanasi (0.01 DU year^–1) were observed. High significance of TCO trend was found at New Delhi (p-value < 0.0001), Pune (p-value = 0.002) and Kodaikanal (p-value = 0.003) with negligible trend over Varanasi with p-value of 0.84. The TCO variations at different geographical locations associated with upper atmospheric meteorological parameters such as lower Stratospheric Temperature (ST) at 65 hPa and Tropopause Height (TH) were also addressed. Annual lower stratospheric temperature shows positive relationship with TCO and Stratospheric ozone over the study sites. Further, decadal variability in TCO with respect to solar activity at New Delhi was also analyzed.     

Total ozone observations at Athens,Greece by satellite-borne and ground-based instrumentation

A comparison is made of total ozone (TOZ) content observations conducted by the Dobson spectrophotometer No. 118, the SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY (SCIAMACHY), the Total Ozone Mapping Spectrometer (TOMS) and the Ozone Monitoring Instrument (OMI) over Athens, Greece, during 1991–2008. Spearman's and Wilcoxon's tests were used to determine the measure of the agreement between the ground-based and satellite column ozone data. The correlation coefficient between Dobson and Nimbus-7, ADEOS, Earth Probe, OMI and SCIAMACHY observations was found to be 0.95, 0.96, 0.94, 0.93 and 0.87, respectively, while the correlation coefficient between total ozone observations of SCIAMACHY and Earth Probe-TOMS and OMI is 0.85 and 0.93, respectively. SCIAMACHY overestimates the column ozone with respect to Dobson, Earth Probe-TOMS and OMI by 10, 15 and 3 DU, respectively, while Dobson underestimates the column ozone with respect to Nimbus-7, ADEOS and OMI by 5, 10 and 8 DU. The results obtained confirm that the Athens Dobson station may continue to be considered as a ground-truth total ozone station for the validation of the satellite column ozone observations. In addition, linear regression analysis of the deseasonalized monthly mean column ozone, as derived from Dobson measurements, gives an increase of +0.33 ± 0.07% per year during 1991–2000 and a decrease of –0.33 ± 0.07% per year for the period 2001–2008.     

On the variation of the tropospheric ozone over Indian region in relation to the meteorological parameters

Using monthly mean satellite measurements of TOMS/SBUV tropospheric ozone residual (TOR) data and meteorological parameters (tropopause height (TPH), 200 hPa geopotential height (GPH) and outgoing longwave radiation (OLR)) during 1979–2001, seasonal variability of TOR data and their association with meteorological parameters are outlined over the Indian region. Prominent higher values of TOR (44–48 DU, which is higher than the globally averaged 31.5 DU) are observed over the northern parts of the country during the summer monsoon season (June–September). Similar to the TOR variation, meteorological parameters (tropopause height, 200 hPa geopotential height and outgoing longwave radiation) also show higher values during the summer monsoon season, suggesting an in phase relationship and strong association between them because of deep convection present during summer monsoon time. The monthly trends in TOR values are found to be positive over the region. TOR has significant positive correlations (5% level) with GPH, and negative correlations with OLR and TPH for the month of September. The oxidation chains initiated by CH₄ and CO show the enhanced photochemical production of ozone that would certainly become hazardous to the ecological system. Interestingly, greenhouse gases (GHG) emissions were found to have continuously increased over the Indian region during the period 1990–2000, indicating more anthropogenic production of ozone precursor gases causing higher level of tropospheric ozone during this period.     

Comparative study of the tropospheric ozone derived from satellite data using different interpolation techniques

Tropospheric ozone (TO) has been derived from the Aura/Ozone Monitoring Instrument (OMI) and the Aura/Microwave Limb Sounder (MLS) over the Indian sub-continent region using a tropospheric ozone residual (TOR) technique. The TO was initially retrieved at a horizontal spatial resolution following that of the Aura/MLS (300 km), which has a lower horizontal spatial resolution than that of the Aura/OMI (25 km). To overcome the limitations imposed by data at a lower spatial resolution, we have introduced a 2D rectangular interpolation (RI) algorithm for effective resampling of data to higher horizontal spatial resolutions. The performance of this algorithm has been evaluated by comparison against existing standard techniques such as nearest neighbourhood (NN) and kriging interpolation as well as comparison against in situ ozonesonde observations. Gridded TO estimates were subsequently generated for the region of interest at 25, 50, and 100 km horizontal spatial resolutions for further study.     

Long-term trends and decadal solar variability in ozone near the tropopause over the Indian region

To investigate the long-term trends and effects of decadal solar variability in the upper tropospheric ozone, data obtained from the Stratospheric Aerosol and Gas Experiment II (SAGE II) aboard the Earth Radiation Budget Satellite (ERBS) during the period 1985–2005 were analysed using a multifunctional regression model over the Indian region (8–40° N; 65–100° E). Analysis of time series spanning these years shows statistically insignificant trends (at the two-sigma level (95% confidence level)) at upper tropospheric pressure levels (10?16 km). This period covers two solar cycles, one lasting from 1985 to 1995 and the other from 1996 to 2005; these are referred to as decade I and decade II, respectively. Since temporal variation in ozone number density indicates 11 year periodicity, trends are statistically significant when calculated separately during each solar cycle. Trend analysis indicates statistically significant positive trends (0.7 ± 1.7% to 3.9 ± 2.9% year^?1 during decade I, and 2.2 ± 1.6% to 4.5 ± 3.0% year^?1 during decade II). In general, higher ozone trends are observed during decade II. Seasonal variation in trends during decade II shows increasing trends during the pre-monsoon (0.8?3.8% year^?1), monsoon (0.8?7.1% year^?1), and post-monsoon (2.8?8.0% year^?1) seasons. The annually averaged solar signal in ozone is found to be of the order of around??5 ± 4.3% to??13.8 ± 6.7%/(100 sfu). Results obtained in the present study are also compared with those obtained by other researchers.     

Atmospheric total ozone column evaluation with a smartphone image sensor

A new method for quantifying the total ozone column (TOC) using a smartphone image sensor has been developed and validated. The TOC has been evaluated for relatively cloud free days at high air masses for solar zenith angles between 49.7° and 76.7° at a sub-tropical site. The method is based on the evaluation of the direct solar irradiances at 305 and 312 nm using the red colour pixel values of the solar disc recorded at these wavelengths by a smartphone camera. Narrow bandpass filters of 2 nm full width at half maximum at each of the two wavelengths were used in turn placed over the camera sensor to directly image the solar disc. The calibration of the pixel values of the solar disc to provide the direct solar irradiances at each of these two wavelengths allowed evaluation of the TOC calibrated to a portable sun photometer. The root mean square error (RMSE) for the smartphone-derived ozone values calibrated to corresponding values from a portable sun photometer was 4.3 Dobson Units (DU). The validation measurements for the smartphone-derived ozone values provided an average residual of 3.5% (up to a maximum of 11%) compared to the corresponding portable sun photometer values, with an RMSE of 8.4 DU during days of intermittent inclement weather conditions. The evaluation of the TOC based on a widely available device such as a smartphone has the potential to increase current citizen science initiatives valued by the general public and school-aged learners by enhancing knowledge and awareness of ozone and the resulting influences on the solar ultraviolet environment.     

Satellite-sensed tropospheric NO2 patterns and anomalies over Indus,Ganges, Brahmaputra,and Meghna river basins

This study presents trends, seasonality, hot spots, and anomalies of tropospheric NO₂ pollution over four basins of Indus, Ganges, Brahmaputra, and Meghna rivers in South Asia using observations from Ozone Monitoring Instrument (OMI) on-board Aura satellite during 2004–2015. For the first time this area, a highly populated and industrialized region with significant emissions of air pollutants, has been discussed collectively. OMI data reveal significantly elevated NO₂ column over the region averaged at (1.9 ± 0.1) × 10¹⁵ molecules cm^–2 (average ± standard deviation of observations) with an increase of 21.12% (slope (0.036 ± 0.004) × 10¹⁵ molecules cm^–2, y-intercept (1.705 ± 0.024) × 10¹⁵ molecules cm^–2, R² = 0.92) during the study period. According to MACCity anthropogenic emissions inventory transportation, energy, residential, and industrial sectors are the major contributors of high NO_x emissions. NO₂ pollution hot spots are identified and their tendencies have been discussed. The hot spots of megacities Lahore (Pakistan) and Dhaka (Bangladesh) are found to be strengthening and expanding over the time. Eastern Ganges Basin shows the highest NO₂ concentration at (2.63 ± 0.22) × 10¹⁵ molecules cm^–2 and growth rate of 3.22% per year mainly linked to power generation, fossil fuel extraction, mining activities, and biomass burning. NO₂ over Indus–Ganges–Brahmaputra–Meghna Basin exhibits seasonal maximum in winter and minimum in monsoon. The highest seasonality is found over Meghna Basin due to large variations in meteorological conditions and large-scale crop-residue burning. Some anomalies in NO₂ levels have been detected linked to intense crop-residue burning events. During these anomalies, exceptionally high levels of daily NO₂ reaching up to 76.23 × 10¹⁵ molecules cm^–2 have been observed over some places in Indus and Meghna Basins.     

Total ozone column measurements using an ultraviolet multi-filter radiometer

We have developed and used a method to retrieve total ozone column (TOC), from Ultraviolet Multi-filter Rotating Shadowband Radiometer (UVMFR) measurements in combination with radiative transfer model calculations. Look-up tables of ratios of the direct solar irradiance at (DI) 305 and 325nm in terms of TOC, solar zenith angle, and aerosol optical depth (AOD) have been constructed and compared with TOC retrievals estimated directly from UVMFR irradiance measurements. Sensitivity analysis of the influence of AOD on the calculated TOC has been investigated and found to be 1 Dobson unit per 0.1 change in AOD. We also examined the impact of ozone effective temperature on the TOC retrieval and found that it leads to a 0.9% change in TOC per K. UVMFR direct irradiance measurements in Athens, Greece, during the period July 2009–May 2014 were used to create a time series of high-temporal-frequency measurements (1 min for cloudless conditions) of TOC, which facilitated an analysis of the diurnal variation of TOC. Comparison of the TOC retrievals from the UVMFR with co-located and synchronous daily TOC retrievals from a Brewer MKIV spectrophotometer showed very good agreement (correlation coefficient 0.98). Daily TOC retrievals from the UVMFR were within ±3% compared with the ones measured by the Ozone Monitoring Instrument overpasses on board the Aura satellite.     

SCIAMACHY/Envisat,OMI/Aura,and ground-based total ozone measurements over Kyiv-Goloseyev station

Validation of satellite ozone measurements is important for data improvement due to instrumental long-term drifts and retrieval algorithm limitations. For satellite data quality estimation, we compare the total ozone content (TOC) derived from the satellite Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY)/Envisat and Ozone Monitoring Instrument (OMI)/Aura spectrometer overpass data and the ground-based measurements made with the Dobson spectrophotometer 040 at the GAW station No. 498 Kyiv-Goloseyev. The station was opened for Dobson ozone measurements in 2010. The results for Direct Sun, Zenith Blue, and Zenith Cloud observations are presented separately, in order to assess the influence of weather conditions (clear or cloudy sky) on the difference between satellite and ground-based measurements. Results from the SCIAMACHY–Dobson and OMI–Dobson difference analyses show small relative overestimation of TOC for satellite data. The ground-based Dobson 040 data are of high quality for Direct Sun and Zenith Blue from AD ((305.5 and 325.0 nm) and (317.5 and 339.9 nm)) pair measurements. Seasonal variations of the difference are seen with maximal satellite–Dobson data discrepancy near the winter solstice. Satellite TOC values are systematically higher than Dobson ones at solar zenith angles larger than 70°. This difference could be explained by seasonal non-uniformity in the satellite data.     

Longitudinal position of the quasi‐stationary wave extremes over the Antarctic region from the TOMS total ozone

Zonal asymmetry of total ozone content (TOC) in the Antarctic region is analysed using the TOMS measurements in 1979–2004. The TOC longitudinal distribution at the individual latitude circles in the latitude band 55–75° S with the 5° step is considered. From the positions of the zonal wave extremes it is revealed that the TOC zonal minimum is displaced eastward in the longitudinal sector 60° W–0° E with total shift of about 50°. The zonal maximum position is rather stable in the quadrant 90° E–180° E. Evidence is obtained that averaged geographical location of the extremes at the five latitude circles reproduces the Antarctic continent boundary in the corresponding longitudinal sectors.     

Characterization of aerosols and pre-cursor gases over Maitri during 24th Indian Antarctica Expedition

Within the framework of the 24th Indian Antarctica Expedition (IAE), observations of total column aerosol optical depth (AOD), ozone (TCO) and precipitable water content (TCW) using a multi-channel solar-radiometer (MICROTOPS-II: Microprocessor-controlled Total Ozone Portable Spectrometer-II), and observations of short-wave global radiative flux using a wide-band pyranometer have been carried out over the Indian Antarctica station Maitri (70.76° S, 11.74° E) and the southern Indian Ocean during December 2004–February 2005. These extensive datasets have been utilized to investigate the aerosol optical, physical and radiative properties, and their interface with simultaneously measured gases. Data over the Oceanic region have been collected from the ship front deck. The daily mean AOD at a characteristic wavelength of 500 nm was found to be 0.042 with an average Angstrom coefficient of 0.24, revealing an abundance of coarse-mode particles. Interestingly, the January fluxes were found to be less by about 20% compared with those in February. The average short-wave direct radiative forcing due to aerosols showed cooling at the surface with an average value of??0.47 Wm^?2. The TCO increased from about 252 DU around 38° S to about 312 DU at 70° S, showing a gradual increase in ozone with increasing latitude. The TCO measured by the surface-based ozone monitor matched reasonably well with that observed by the Total Ozone Mapping Spectrometer (TOMS) satellite sensor within 5%. Variability in ozone on a daily scale during the study period was less than 4% over the Antarctica region.     

Observations and model calculations of direct solar UV irradiances in the Schirmacher region of east Antarctica

Measurements of direct UV irradiances (using a MICROTOPS II Sunphotometer) carried out from a high‐latitude site, Antarctica are presented. The instantaneous irradiances at 305±0.9, 312±0.9 and 320±1.0 nm during a no‐ozone‐hole (13 December 2004) and an ozone‐hole (4 October 2004) period have been observed to be about 0.031, 0.150 and 0.299 W m^?2 and 0.010, 0.049 and 0.102 W m^?2, respectively at local noontime. The observations of the direct UV irradiances at 305±0.9, 312±0.9 and 320±1.0 nm are compared with tropospheric ultraviolet visible (TUV) radiation transfer model calculations. The model estimate shows that, during the ozone‐hole period, a loss of ozone of the order of 44% leads to an increase in irradiance of the order of 410%, 90% and 25% at 305±0.9, 312±0.9 and 320±1.0 nm, respectively. The relationship between change in UV irradiance due to a change in column ozone is obtained using a TUV model and irradiances thus estimated from this relationship are found to compare well with the observed irradiances.     

Long-term trends in the total columns of ozone and its precursor gases derived from satellite measurements during 2004–2015 over three different regions in South Asia: Indo-Gangetic Plain,Himalayas and Tibetan Plateau

ABSTRACT

Long-term (2004–2015) satellite data over three adjacent yet contrasting regions: Indo-Gangetic Plain, Himalayas and Tibetan Plateau (TP) were used to study the spatiotemporal distribution of total ozone column (TOC) and its precursor gases (such as nitrogen dioxide (NO₂), methane (CH₄) and carbon monoxide (CO)). The ozone precursor emission data and forest fire points were used to explore the findings. Trace gases showed increasing trend probably due to increasing emission from South Asia as supported by the Emission Database for Global Atmospheric Research emission data. Strong seasonal variation in trace gases was observed with the highest value during the pre-monsoon season, over three regions, possibly due to the biomass burning, pollution build-up and also long-range transport of pollution. TOC exhibited the similar seasonal variation as shown by the earlier ground-based studies over the region. The total column of precursor gases (except methane) exhibited strong seasonality with the highest column during the pre-monsoon season. Patterns in the variations of TOC and related precursors over the Himalayas were similar with that of the TP. Seasonal climatological trends also exhibited increasing pattern except for CO. This work provides an overview on the long-term TOC and its precursor gases which are necessary to understand the regional climate variability especially over the Himalayas and Tibetan Plateau region.     

Correction of ozone influence on TOA radiance

An accurate atmospheric correction (AC) of Earth remote-sensing data in the spectral region 450–800 nm has to account for the ozone gas absorption influence. Usual operational AC codes employ a fixed ozone concentration corresponding to a climatologic average for a certain region and season, e.g. the mid-latitude summer atmosphere of the Moderate Resolution Atmospheric Transmission (MODTRAN) code. The reasons for a fixed ozone column are that ozone does not vary rapidly on a spatial and temporal scale, and additionally, the look-up table (LUT) size for AC is already big. This means that another degree of freedom for the ozone parameter would dramatically increase the size of the LUT database and the time required for LUT interpolation. In order to account for this effect, we use already existing LUTs that were calculated for a certain ozone reference level, e.g. an ozone column of g = 330 Dobson Units (DU) for MODTRAN’s mid-latitude summer atmosphere. Then the deviation of the top-of-atmosphere (TOA) radiance ΔL(g) from the reference level L(g = 330) is calculated as a function of solar and view geometries. The calculation is performed for a set of 36 wavelengths in the ozone-sensitive spectrum (450–800 nm) and five ozone columns. The last step computes the linear regression coefficients for each wavelength and geometry. The results are stored in a small table (11 kB). It is shown that the ozone influence is accurately accounted for by multiplying the modelled radiance L(g = 330) with a factor depending on g and wavelength yielding TOA radiance relative errors smaller than 0.5% for a wide range of ozone concentrations between 180 and 500 DU. Selected examples of a sensitivity study of the ozone effect on the retrieval of water constituents demonstrate the need to account for ozone in the AC step.     

Intercomparison of satellite and ground-based measurements of ozone,NO2, HF,and HCl near Saint Petersburg,Russia

Regular intercomparison of different observing systems is a part of their testing and validation protocol, which gives the estimates of real measurement errors. The main objective of our study is the comparison of satellite and ground-based measurements of atmospheric composition near Saint Petersburg, Russia. Since early 2009, high-resolution Fourier Transform Infrared (FTIR) solar absorption spectra have been recorded at Peterhof station (59.82° N, 29.88° E), located in the suburbs of Saint Petersburg. We derived column amounts of O₃, HCl, HF, and NO₂ from these spectra using the retrieval codes SFIT2 and PROFFIT. We compared the data retrieved from Bruker 125 HR FTIR measurements with coincident satellite observations of the Microwave Limb Sounding (MLS), Ozone Monitoring Instrument (OMI), Fourier Transform Spectrometer from Atmospheric Chemistry Experiment (ACE-FTS), Global Ozone Monitoring Experiment (GOME and GOME-2), and Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) instruments. The relative differences in ozone columns of FTIR from OMI-TOMS amount within (+3.4 ± 2.9)%, from GOME-2 are (+2.2 ± 3.0)%. The comparison of FTIR and MLS measurements of stratospheric ozone columns gives no mean and 5% of the RMS differences. Measurements of NO₂ columns agree with the mean difference of +9% and the RMS differences within 14–16% for FTIR vs. GOME-2, SCIAMACHY, and OMI. FTIR vs. GOME comparison gives (+6 ± 31)%. HCl columns comparison for FTIR vs. MLS shows ?4.5% in the mean and 12% in the RMS differences. FTIR vs. ACE-FTS comparison (nine cases) gives ?8% and 10% for the mean and the RMS relative differences, respectively. Comparison of HF columns shows (?12 ± 6)% and (?12 ± 11)% for FTIR vs. ACE data v.2.2 and v.3.0, respectively. These figures show that the Peterhof ground-based FTIR measuring system can be used to support the validation of satellite data in the monitoring of stratospheric gases.