Ppb Yr Research Articles

Variations of atmospheric CO concentration from 2004 to 2019 at the Mt. Waliguan station in China

Carbon monoxide (CO) is an important indirect greenhouse gas and has been measuring by many sites around the world. In this study, a 14-years (2004–2007 as well as 2010–2019) observed atmospheric CO record at the Mount. Waliguan (WLG), the only global station of the World Meteorological Organization (WMO) and Global Atmosphere Watch (GAW) in Eurasia, was presented. The characteristics of atmospheric CO including daily and seasonal cycles, long-term trends, and spatiotemporal source distributions in different episode were compared and analyzed. Our results show that there was an increasing trend for atmospheric CO with the highest annual average of 129.7 ± 0.2 (σ = 4.6) ppb in 2007 in earlier years but a decreasing trend in later years with an annual average of 110.7 ± 0.2 (σ = 5.3) ppb in 2019. The characteristics of atmospheric CO at the site was dominated by regional economy development strategy such as the China's western development strategy, carbon emission reduction strategy in China's economic development plan. After the infrastructure construction period from 2004 to 2007, the regional emissions of CO at the Qinghai and Gansu provinces stimulated the atmospheric CO at the WLG and changed the patterns of seasonal variation, source regions as well as the long-term trends. A weak decreasing trend with value of −0.47 (σ = 0.01) ppb yr−1 was observed during the observation period, which was less than most of the stations in China. On the whole, we concluded that the CO mole fractions observed at the Mt. Waliguan station might only denote the conditions in regional scale instead of the well mixed condition in Eurasia.

Long-term trend of ozone in southern China reveals future mitigation strategy for air pollution

The 14-year trends of surface ozone were examined using observations from 12 urban sites and 4 regional sites over the Pearl River Delta (PRD) region in China over the 2006–2019 period. Surface ozone exhibits upward trends in the range of 0.28–1.02 ppb yr−1 at urban sites and mixed trends in the range of −0.29 – 0.21 ppb yr−1 at regional sites. Meanwhile, surface Ox (Ox = O3+NO2) mixing ratios exhibit downward trends at 9 urban sites and 3 regional sites in the range of −0.01 to −0.54 ppb yr−1, revealing that the ozone rise at urban sites is predominantly attributed to the reduced titration effect by nitric oxide (NO). The further decline of surface NO2 mixing ratios could drive the decreases in Ox mixing ratios and the increases in ozone mixing ratios at urban sites. In addition, surface Ox concentrations show a good correlation with aerosol concentrations, implying the dominant contributions of secondary production to aerosol and the consistent control strategies of ozone and aerosol pollution in PRD. For the abatement of ozone pollution, our results highlight the synergistic control of key precursors including nitrogen oxides and volatile organic compounds to counteract the reduced NO titration effect by inhibiting the photochemical production of ozone. At last, we also highlight the importance of establishing more regional sites in comprehensively assessing spatiotemporal changes in surface air pollutants in China.

Subseasonal characteristics and meteorological causes of surface O3 in different East Asian summer monsoon periods over the North China Plain during 2014–2019

In recent years, surface ozone (O3) has increased rapidly over the North China Plain (NCP), which is harmful to human health and crop yields. Summertime O3 over the NCP is remarkably influenced by the East Asian summer monsoon (EASM), while the distinctions of their relations in different monsoon periods are still vague. In this study, we divided the whole monsoon season into the pre-flood period, the Meiyu flood period, and the post-flood period. For each monsoon period, we examined the characteristics and variations of O3 from 2014 to 2019 and the associated atmospheric circulations individually. Based on a multiple linear regression model, we further quantified the meteorological contributions to the year-by-year variation and increasing trend in O3 during different monsoon periods. We found that the maximum daily 8 h-average O3 concentration (O3_MDA8) exhibits subseasonal variations on the mean level (72.5, 76.7, and 63.8 ppb) and trends (3.2, 4.0, and 1.7 ppb yr−1) during the three monsoon periods. Meteorological variations account for 58.9% and 58.6% of the O3 uptrends in the Meiyu flood and the pre-flood periods but exert adverse effects on the O3 increase in the post-flood period (−27.4%). The positive meteorology-driven trend in the pre-flood period is attributed to increased radiation and temperature, and that in the Meiyu flood period is contributed by the rise in radiation and decrease in relative humidity. While in the post-flood period, the decreasing trend mainly results from the increased precipitation. The subseasonal discrepancy is related to the distinct dominance of wave activities in the mid-and-high latitudes and the western Pacific subtropical high (WPSH) in different monsoon periods. The anomalously high pressure conduces to the high-level O3 over the NCP in the pre-flood period. Frequent O3 pollution in the Meiyu flood period is caused by the strong wave amplitudes in the mid-and-high latitudes and a westward stretched and intensified WPSH. While in the post-flood period, the east-west shift of WPSH modulates the precipitation and O3 variations over the NCP region. Among the three periods, we highlighted the Meiyu flood period for the highest O3 level and the most drastic increasing trend, during which more strict emission control should be implemented.

Effect of springtime thermal forcing over Tibetan Plateau on summertime ozone in Central China during the period 1950–2019

In recent years, ozone pollution has attracted wide attention from the scientific community, policymakers and the public. Previous studies have already confirmed the effects of meteorological conditions on regional ozone pollution events in the last two decades. However, due to the lack of historical ozone observations, the relationship of climate variability and ozone pollution has been less discussed. In our study, we use the NCEP/NCAR Reanalysis data and modeled historical surface ozone from CMIP6 (Coupled Model Intercomparison Project Phase 6) to explore the impact of climate variability related to Tibetan Plateau on summertime (June, July and August) ozone in Central China during 1950–2019. The results show that in interdecadal timescale, the annual summertime ozone over Central China is significantly correlated (R = −0.52; P-value <0.05) with the springtime (March, April and May) thermal forcing, indicated by total atmospheric energy (TPE), over Tibetan Plateau. In high TPE years, the East Asian monsoon (southerly wind flow) and vertical ascending motion have been enhanced in summer. It is conducive to both the diffusion of ozone and its precursors and occurrence of cloudy and high humidity weather. These processes lead to lower ozone concentrations up to 6.97 ppb (compared with values averaged during 1950–2019) over Central China. On the contrary, in low TPE years, summertime surface ozone concentrations have been increased by 2.96 ppb (the mean value is 49.00 ppb and the standard deviation is 11.75 ppb for 1950–2019) on average of Central China, due to the weakened East Asian monsoon and weakened vertical ascent, thus the stable weather with low humidity and small near surface wind. After the year 2000, the TPE has increased with a rapid growth rate of 3 × 104 hPa J kg−1 yr−1. However, similar to the reported surface ozone increments based on the modeling results and observations, the CMIP6 summertime surface ozone has shown an increasing trend in the last two decades (0.45 ppb yr−1 and 0.25 ppb yr−1 over Central China and entire China during 1950–2019, respectively). While a decline trend (0.18 ppb yr−1) of ozone from 1980 to 2019 over Central China is calculated after the removal of emission-attributed trend. These results indicate that the ozone increasing trend dependent on the changes of precursor emissions has offset the ozone mitigation attributed to the climate.

Interannual Variability of Atmospheric CH4 and Its Driver Over South Korea Captured by Integrated Data in 2019

Understanding the temporal variability of atmospheric methane (CH4) and its potential drivers can advance the progress toward mitigating changes to the climate. To comprehend interannual variability and spatial characteristics of anomalous CH4 mole fractions and its drivers, we used integrated data from different platforms such as in situ measurements and satellites (TROPOspheric Monitoring Instrument (TROPOMI) and Greenhouse Gases Observing SATellite (GOSAT)) retrievals. A pronounced change of annual growth rate was detected at Anmyeondo (AMY), Republic of Korea, ranging from −16.8 to 31.3 ppb yr−1 as captured in situ through 2015–2020 and 3.9 to 16.4 ppb yr−1 detected by GOSAT through 2014–2019, respectively. High growth rates were discerned in 2016 (31.3 ppb yr−1 and 13.4 ppb yr−1 from in situ and GOSAT, respectively) and 2019 (27.4 ppb yr−1 and 16.4 ppb yr−1 from in situ and GOSAT, respectively). The high growth in 2016 was essentially explained by the strong El Niño event in 2015–2016, whereas the large growth rate in 2019 was not related to ENSO. We suggest that the growth rate that appeared in 2019 was related to soil temperature according to the Noah Land Surface Model. The stable isotopic composition of 13C/12C in CH4 (δ13-CH4) collected by flask-air sampling at AMY during 2014–2019 supported the soil methane hypothesis. The intercept of the Keeling plot for summer and autumn were found to be −53.3‰ and −52.9‰, respectively, which suggested isotopic signature of biogenic emissions. The isotopic values in 2019 exhibited the strongest depletion compared to other periods, which suggests even a stronger biogenic signal. Such changes in the biogenic signal were affected by the variations of soil temperature and soil moisture. We looked more closely at the variability of XCH4 and the relationship with soil properties. The result indicated a spatial distribution of interannual variability, as well as the captured elevated anomaly over the southwest of the domain in autumn 2019, up to 70 ppb, which was largely explained by the combined effect of soil temperature and soil moisture changes, indicating a pixel-wise correlation of XCH4 anomaly with those parameters in the range of 0.5–0.8 with a statistical significance (p &lt; 0.05). This implies that the soil-associated drivers are able to exert a large-scale influence on the regional distribution of CH4 in Korea.

Assessment of ozone concentration data from the northern Zagreb area, Croatia, for the period from 2003 to 2016

A measurement station located in an urban area on the southern slope of the Medvednica Mountain (120 m a.s.l.), close to the Croatian capital Zagreb, provided data for an analysis of the photosmog in the city of Zagreb. Data for the period 2003-2016 obtained from this station and analysed in this work can also be compared with the nearby Puntijarka station (980 m a.s.l.) for which a similar analysis has already been carried out. In Puntijarka station analysis, it has been shown that there is most probably no significant change in ozone concentrations during the observed period. In this study the mean value of the annual ozone volume fractions showed a linear trend of 0.23 ppb yr-1, a growth that is in the worst case scenario among the lowest global prediction, while the seasonal (April-to-September) mean values had a trend of 0.32 ppb yr-1, which is a certain clearly observable growth. The 95-percentile values had trends of 0.009 ppb yr-1 (annual data) and -0.072 ppb yr-1 (seasonal data), respectively. Both of these values show very small changes if any at all. By using FT analysis, with the calculation of uncertainties, we have observed three prominent cycles of 169 ± 4 h (weekly cycle), 24 ± 1 h and 12 ± 1 h (diurnal cycles). Uncertainties were low which strongly indicate that the cycles are present. However, since high concentrations of ozone were observed only sporadically, ozone pollution in the northern part of Zagreb is at the present rather low. A Fourier transformation was used to analyse the data for periodic behaviour, which revealed the existence of diurnal and weekly modulations. Nevertheless, constant monitoring is important and will continue in the future as part of continuous monitoring of the ozone levels in the area.

Measurement report: Changing characteristics of atmospheric CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; in the Tibetan Plateau: records from 1994 to 2019 at the Mount Waliguan station

Abstract. A 26-year, long-term record of atmospheric methane (CH4) measured in situ at the Mount Waliguan (WLG) station, the only World Meteorological Organization (WMO) and Global Atmosphere Watch (GAW) global station in inland Eurasia, is presented. Overall, a nearly continuous increase in atmospheric CH4 was observed at WLG, with a yearly growth rate of 5.1±0.1 parts per billion (ppb) per year during 1994–2019, except for some particular periods with near-zero or negative values, e.g., 1999–2000 and 2004–2006. The average CH4 mole fraction was only 1799.0±0.4 ppb in 1994 but increased to about 133 ppb and reached a historic level of 1932.0±0.1 ppb in 2019. The case study in the Tibetan Plateau showed that the atmospheric CH4 increased rapidly. During some special periods, it is even larger than that of city regions (e.g., 6.7±0.2 ppb yr−1 in 2003–2007). Generally, the characteristics of CH4 varied in different observing periods as follows: (i) the diurnal cycle has become apparent and the amplitudes of the diurnal or seasonal cycles increased over time; (ii) the wind sectors with elevated CH4 mole fractions switched from ENE-E-ESE-SE-SSE sectors (wind directions) in early periods to NNE-NE-ENE-E sectors in later years; (iii) the area of source regions increased as the years progressed, and strong sources shifted from northeast (city regions) to southwest (northern India); and (iv) the annual growth rates in recent years (e.g., 2008–2019) were significantly larger than those in the early periods (e.g., 1994–2007).

Measurement report: Long-term variations in carbon monoxide at a background station in China's Yangtze River Delta region

Abstract. This study analyzed the long-term variations in carbon monoxide (CO) mixing ratios from January 2006 to December 2017 at the Lin'an regional atmospheric background station (LAN; 30.3∘ N, 119.73∘ E, 138 m a.s.l.) in China's Yangtze River Delta (YRD) region. The CO mixing ratios were at their highest (0.69 ± 0.08 ppm) and lowest (0.54 ± 0.06 ppm) in winter and summer, respectively. The average daily variation in CO exhibited a double-peaked pattern, with peaks in the morning and evening and a valley in the afternoon. A significant downward trend of −11.3 ppb yr−1 of CO was observed from 2006 to 2017 at the LAN station, which was in accordance with the negative trends of the average CO mixing ratios and total column retrieved from the satellite data (Measurements of Pollution in the Troposphere, MOPITT) over the YRD region during the same period. The average annual CO mixing ratio at the LAN station in 2017 was 0.51 ± 0.04 ppm, which was significantly lower than that (0.71 ± 0.12 ppm) in 2006. The decrease in CO levels was largest in autumn (−15.7 ppb yr−1), followed by summer (−11.1 ppb yr−1), spring (−10.8 ppb yr−1), and winter (−9.7 ppb yr−1). Moreover, the CO levels under relatively polluted conditions (the annual 95th percentiles) declined even more rapidly (−22.4 ppb yr−1, r=-0.68, p&lt;0.05) from 2006 (0.91 ppm) to 2017 (0.58 ppm), and the CO levels under clean conditions (the annual 5th percentiles) showed decreasing evidence but not statistically significant (r=-0.41, p=0.19) throughout the years. The long-term decline and short-term variations in the CO mixing ratios at the LAN station were mainly attributed to the implementation of the anthropogenic pollution control measures in the YRD region and to events like the Shanghai Expo in 2010 and Hangzhou G20 in 2016. The decreased CO level may influence atmospheric chemistry over the region. The average OH reactivity of CO at the LAN station is estimated to significantly drop from 4.1 ± 0.7 s−1 in 2006 to 3.0 ± 0.3 s−1 in 2017.

Decadal O3 variability at the Mt. Cimone WMO/GAW global station (2,165 m a.s.l., Italy) and comparison with two high-mountain “reference” sites in Europe

Tropospheric ozone (O3) is a greenhouse gas as well as a harmful air pollutant with adverse effects on human health and vegetation: The observation and attribution of its long-term variability are key activities to monitor the effectiveness of pollution reduction protocols. In this work, we present the analysis of multi-annual near-surface O3 (1996–2016) at the Mt. Cimone (CMN, Italian northern Apennines) WMO/GAW global station and the comparison with two “reference” high-mountain sites in Europe: Jungfraujoch (JFJ, Swiss Alps) and Mt. Zugspitze (ZUG/ZSF, German Alps). Negative O3 trends were observed at CMN over the period 1996–2016 (from –0.19 to –0.22 ppb yr–1), with the strongest tendencies as being observed for the warm months (May–September: –0.32 ppb yr–1 during daytime). The magnitude of the calculated O3 trends at CMN are 2 times higher than those calculated for ZUG/ZSF and 3–4 times higher than for JFJ. With respect to JFJ and ZUG/ZSF, higher O3 values were observed at CMN during 2004–2008, while good agreement is found for the remaining periods. We used Lagrangian simulations by the FLEXPART particle dispersion model and near-surface O3 data over different European regions, for investigating the possibility that the appearance of the O3 anomalies at CMN could be related to variability in the atmospheric transport or in near-surface O3 over specific source regions. Even if it was not possible to achieve a general robust explanation for the occurrence of the high O3 values at CMN during 2004–2008, the variability of (1) regional and long-range atmospheric transport at CMN and (2) European near-surface O3 could motivate the observed anomalies in specific seasons and years. Interestingly, we found a long-term variability in air mass transport at JFJ with enhanced (decreased) contributions from Western European (intercontinental regions).

Ozone Trends from Two Decades of Ground Level Observation in Malaysia

We examine the change in surface ozone and its precursor behavior over 20 years at four locations in western Peninsular Malaysia which have undergone urban-commercial development. Trend and correlation analyses were carried out on ozone and oxides of nitrogen observation data over the periods of 1997–2016 as well as the decadal intervals of 1997–2006 and 2007–2016. Diurnal variation composites for decadal intervals were also plotted. Significant increasing ozone concentrations were observed at all locations for the 20-year period, with a range between 0.09 and 0.21 ppb yr−1. The most urbanized location (S3) showed the highest ozone trend. Decadal intervals show that not all stations record significant increasing trends of ozone, with S1 recording decreasing ozone at a rate of −0.44 ppb yr−1 during the latter decade. Correlation analysis showed that only oxides of nitrogen ratios (NO/NO2) had significant inverse relationships with ozone at all stations corresponding to control of ozone by photostationary state reactions. The diurnal composites show that decadal difference in NO/NO2 is mostly influenced by change in nitric oxide concentrations.

Comparison of Atmospheric CO2, CH4, and CO at Two Stations in the Tibetan Plateau of China

AbstractThe research of greenhouse gases on the Tibetan Plateau is of great importance since its unique topography as the third pole of our planet and profound response on the climate change. In this study, we compared the concurrent observations of atmospheric carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO) during 2010–2016 from two stations located on the Tibetan Plateau, which are Mt. Waliguan station (WLG), the only World Meteorological Organization/Global Atmosphere Watch global station in the inland of Eurasia, and Shangri‐La station, a Chinese national station (XGLL). Although both stations are located at remote area, the atmospheric CO2, CH4, and CO concentrations are frequently influenced by regional sources, especially for XGLL throughout the year and WLG in summer. Due to the unique topography and regional conditions, the atmospheric CH4 and CO at both stations display different trends with other sites in China, with higher values in summer. The atmospheric CO2, CH4, and CO at the XGLL mainly represent the conditions in regional scale. As the only World Meteorological Organization/Global Atmosphere Watch global station in the inland of Eurasia, the observation results at WLG can be used to represent the conditions on the Tibetan Plateau, but some of them are frequently influenced by the emissions from the cities located on the east or north east, and some even can be affect by emissions from the Ganges basin in autumn and winter, which should be treated with caution. By subtracting the influences of the cities, we updated the growth rate of 2.45 ± 0.02 ppm yr−1 for CO2, 8.2 ± 0.1 ppb yr−1 for CH4, and −0.4 ± 0.1 ppb yr−1 for CO, compared to the prior estimation of 2.31 ± 0.02 ppm yr−1 for CO2, 8.1 ± 0.1 ppb yr−1 for CH4, and −0.6 ± 0.1 ppb yr−1 for CO on the Tibetan Plateau.

Magnitude, trends, and impacts of ambient long-term ozone exposure in the United States from 2000 to 2015

Abstract. Long-term exposure to ambient ozone (O3) is associated with a variety of impacts, including adverse human-health effects and reduced yields in commercial crops. Ground-level O3 concentrations for assessments are typically predicted using chemical transport models; however such methods often feature biases that can influence impact estimates. Here, we develop and apply artificial neural networks to empirically model long-term O3 exposure over the continental United States from 2000 to 2015, and we generate a measurement-based assessment of impacts on human-health and crop yields. Notably, we found that two commonly used human-health averaging metrics, based on separate epidemiological studies, differ in their trends over the study period. The population-weighted, April–September average of the daily 1 h maximum concentration peaked in 2002 at 55.9 ppb and decreased by 0.43 [95 % CI: 0.28, 0.57] ppb yr−1 between 2000 and 2015, yielding an ∼18 % decrease in normalized human-health impacts. In contrast, there was little change in the population-weighted, annual average of the maximum daily 8 h average concentration between 2000 and 2015, which resulted in a ∼5 % increase in normalized human-health impacts. In both cases, an aging population structure played a substantial role in modulating these trends. Trends of all agriculture-weighted crop-loss metrics indicated yield improvements, with reductions in the estimated national relative yield loss ranging from 1.7 % to 1.9 % for maize, 5.1 % to 7.1 % for soybeans, and 2.7 % for wheat. Overall, these results provide a measurement-based estimate of long-term O3 exposure over the United States, quantify the historical trends of such exposure, and illustrate how different conclusions regarding historical impacts can be made through the use of varying metrics.

Continuous atmospheric CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;, CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; and CO measurements at the Observatoire Pérenne de l'Environnement (OPE) station in France from 2011 to 2018

Abstract. Located in north-east France, the Observatoire Pérenne de l'Environnement (OPE) station was built during the Integrated Carbon Observation System (ICOS) Demonstration Experiment to monitor the greenhouse gases mole fraction. Its continental rural background setting fills the gaps between oceanic or mountain stations and urban stations within the ICOS network. Continuous measurements of several greenhouse gases using high-precision spectrometers started in 2011 on a tall tower with three sampling inlets at 10, 50 and 120 m above ground level (a.g.l.). Measurement quality is regularly assessed using several complementary approaches based on reference high-pressure cylinders, audits using travelling instruments and sets of travelling cylinders (“cucumber” intercomparison programme). Thanks to the quality assurance strategy recommended by ICOS, measurement uncertainties are within the World Meteorological Organisation compatibility goals for carbon dioxide (CO2), methane (CH4) and carbon monoxide (CO). The time series of mixing ratios from 2011 to the end of 2018 are used to analyse trends and diurnal and seasonal cycles. The CO2 and CH4 annual growth rates are 2.4 ppm yr−1 and 8.8 ppb yr−1 respectively for measurements at 120 m a.g.l. over the investigated period. However, no significant trend has been recorded for CO mixing ratios. The afternoon mean residuals (defined as the differences between midday observations and a smooth fitted curve) of these three compounds are significantly stronger during the cold period when inter-species correlations are high, compared to the warm period. The variabilities of residuals show a close link with air mass back-trajectories.

Observed dependence of surface ozone on increasing temperature in Shanghai, China

Eight-year measurements at urban (Xujiahui, XJH) and remote (Dongtan, DT) sites during time period 2010–2017 are employed to examine the surface ozone (O3)-temperature relationship in Shanghai, China. O3 pollution was getting worse in Shanghai, with daily maximum O3 concentrations increasing at a rate of 2.47 ppb yr−1 in urban site. The climate penalty (mO3-T), defined as the slope of O3 change with increasing temperature, exhibited largest values in summer. Summertime O3 increased faster as temperature increased, with mean rates of 6.65 and 13.68 ppb °C−1, respectively in XJH and DT above 30 °C. Sensitivity experiments indicate that the temperature dependence of biogenic volatile organic compounds (VOCs) emissions could be the main chemical driver of the high-temperature O3 response in summer, since the simulated mO3-T are most sensitive to changes of biogenic isoprene emissions. NOx emission reductions strengthened the high-temperature O3 response, with summer mean mO3-T values increasing from 1.52 ppb°C−1 during 2010–2012 to 2.97 ppb °C−1 during 2013–2017. As NOx emissions continue to decrease, the O3 production in urban Shanghai tend to become transitional and the dependence of mO3-T on the biogenic VOC emissions might be weakened. Model results suggest that anthropogenic VOC emission reductions would effectively relieve O3 pollution and reduce the sensitivity of O3 to increasing temperatures in urban Shanghai. Tailored emission reductions as well as scientific city planning strategies should be formulated to balance VOC/NOx ratios, so as to wrestle with the challenges for future O3 pollution under a warming climate.

Trends in N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O and SF&amp;lt;sub&amp;gt;6&amp;lt;/sub&amp;gt; mole fractions in archived air samples from Cape Meares, Oregon (USA), 1978–1996

Abstract. Quantifying historical trends in atmospheric greenhouse gases (GHGs) is important to understanding changes in their budgets and for climate modeling, which simulates historic and projects future climate. Archived samples analyzed using updated measurement techniques and calibration scales can reduce uncertainties in historic records of GHG mole fractions and their trends in time. Here, we present historical measurements of two important GHGs, nitrous oxide (N2O) and sulfur hexafluoride (SF6), collected at the midlatitude Northern Hemisphere station Cape Meares, Oregon (USA, 45.5∘ N, 124∘ W), between 1978 and 1996 in archived air samples from the Oregon Health and Science University – Portland State University (OHSU–PSU) air archive. N2O is the third most important anthropogenically forced GHG behind carbon dioxide (CO2) and methane (CH4). SF6 has a low abundance in the atmosphere, but is one of the most powerful GHGs known. Measurements of atmospheric N2O made during this period are available for select locations, but before mid-1990 they have larger uncertainties than more recent periods due to advancements made in gas chromatography (GC) methods. Few atmospheric SF6 measurements exist pre-1990, particularly in the Northern Hemisphere. The GC system used to measure N2O and SF6 mixing ratios in this work is designed to be fully automated, and is capable of running up to 15 samples per batch. Measurement precision (1σ) of N2O and SF6 is 0.16 % and 1.1 %, respectively (evaluated at 328.7 ppb and 8.8 ppt). Samples were corrected for detector response nonlinearity when measured against our reference standard, with the corrections determined to be 0.14 ppb ppb−1 in N2O and 0.03 ppt ppt−1 in SF6. The mixing ratio of N2O in archived samples is found to be 301.5±0.3 ppb in 1980 and rises to 313.5±0.3 ppb in 1996. The average growth rate over this period is 0.78±0.03 ppb yr−1 (95 % CI). The seasonal amplitude is statistically robust, with a maximum anomaly of 0.3 ppb near April and a minimum near November of −0.4 ppb. Measurements of N2O match well with previously reported values for Cape Meares and other comparable locations. The mixing ratio of SF6 in analyzed samples is found to be 0.85±0.03 ppt in 1980 and rises to 3.83±0.03 ppt in 1996. The average growth rate over this period is 0.17±0.01 ppt yr−1 (95 % CI). The seasonality is statistically robust and has an annual peak amplitude of 0.04 ppt near January and a minimum amplitude of −0.03 ppt near July. These are unique SF6 results from this site and represent a significant increase in the SF6 data available during the 1980s and early 1990s. The mixing ratio and growth rate of SF6 measured compares well to other Northern Hemisphere measurements over this period. From these N2O and SF6 measurements, we conclude that sample integrity is generally robust in the OHSU-PSU air archive for N2O and SF6.

Study on CO data filtering approaches based on observations at two background stations in China.

The identification of regional representative carbon monoxide (CO) measurements that are minimally influenced by local sources/sinks is essential to understand the characteristics of atmospheric CO over a certain region. In this study, three commonly used data filtering approaches were applied to atmospheric CO data obtained from 2010/2011 to 2017 at two World Meteorological Administration/Global Atmospheric Programme (WMO/GAW) regional stations (Lin'an, LAN and Shangdianzi, SDZ) in China, to study their applicability for individual stations. The three methods used were the meteorological conditions (MET), statistical approaches (robust extraction of baseline signal, REBS), and the time scale of the CO variations (standard deviations of the running mean, SDM). The results from the three methods displayed almost the same seasonal cycles at LAN but different variations at SDZ. They each extracted similar yearly CO growth rates at LAN, but there was a large difference at SDZ, with values of -10.6 ± 0.5, -2.2 ± 0.1, and - 23.5 ± 0.3 ppb yr-1 for MET, REBS, and SDM, respectively. The slight decrease observed using REBS at SDZ was mainly due to the biased distribution of CO records, which was a purely statistical method that did not consider topography or meteorological conditions. Thus, the REBS method should be applied cautiously to CO observations at stations like SDZ. The SDM method may overestimate multi-year trends. Among the three approaches, MET may be the most suitable for filtering CO observation records, especially at stations like SDZ with special geographical and meteorological conditions in economically-developed regions.

Impact of El Niño–Southern Oscillation on the interannual variability of methane and tropospheric ozone

Abstract. The interannual variability of the greenhouse gases methane (CH4) and tropospheric ozone (O3) is largely driven by natural variations in global emissions and meteorology. The El Niño–Southern Oscillation (ENSO) is known to influence fire occurrence, wetland emission and atmospheric circulation, affecting sources and sinks of CH4 and tropospheric O3, but there are still important uncertainties associated with the exact mechanism and magnitude of this effect. Here we use a modelling approach to investigate how fires and meteorology control the interannual variability of global carbon monoxide (CO), CH4 and O3 concentrations, particularly during large El Niño events. Using a three-dimensional chemical transport model (TOMCAT) coupled to a sophisticated aerosol microphysics scheme (GLOMAP) we simulate changes to CO, hydroxyl radical (OH) and O3 for the period 1997–2014. We then use an offline radiative transfer model to quantify the climate impact of changes to atmospheric composition as a result of specific drivers. During the El Niño event of 1997–1998, there were increased emissions from biomass burning globally, causing global CO concentrations to increase by more than 40 %. This resulted in decreased global mass-weighted tropospheric OH concentrations of up to 9 % and a consequent 4 % increase in the CH4 atmospheric lifetime. The change in CH4 lifetime led to a 7.5 ppb yr−1 increase in the global mean CH4 growth rate in 1998. Therefore, biomass burning emission of CO could account for 72 % of the total effect of fire emissions on CH4 growth rate in 1998. Our simulations indicate that variations in fire emissions and meteorology associated with El Niño have opposing impacts on tropospheric O3 burden. El Niño-related changes in atmospheric transport and humidity decrease global tropospheric O3 concentrations leading to a −0.03 W m−2 change in the O3 radiative effect (RE). However, enhanced fire emission of precursors such as nitrogen oxides (NOx) and CO increase O3 and lead to an O3 RE of 0.03 W m−2. While globally the two mechanisms nearly cancel out, causing only a small change in global mean O3 RE, the regional changes are large – up to −0.33 W m−2 with potentially important consequences for atmospheric heating and dynamics.

An intercomparison of total column-averaged nitrous oxide between ground-based FTIR TCCON and NDACC measurements at seven sites and comparisons with the GEOS-Chem model

Abstract. Nitrous oxide (N2O) is an important greenhouse gas and it can also generate nitric oxide, which depletes ozone in the stratosphere. It is a common target species of ground-based Fourier transform infrared (FTIR) near-infrared (TCCON) and mid-infrared (NDACC) measurements. Both TCCON and NDACC networks provide a long-term global distribution of atmospheric N2O mole fraction. In this study, the dry-air column-averaged mole fractions of N2O (XN2O) from the TCCON and NDACC measurements are compared against each other at seven sites around the world (Ny-Ålesund, Sodankylä, Bremen, Izaña, Réunion, Wollongong, Lauder) in the time period of 2007–2017. The mean differences in XN2O between TCCON and NDACC (NDACC–TCCON) at these sites are between −3.32 and 1.37 ppb (−1.1 %–0.5 %) with standard deviations between 1.69 and 5.01 ppb (0.5 %–1.6 %), which are within the uncertainties of the two datasets. The NDACC N2O retrieval has good sensitivity throughout the troposphere and stratosphere, while the TCCON retrieval underestimates a deviation from the a priori in the troposphere and overestimates it in the stratosphere. As a result, the TCCON XN2O measurement is strongly affected by its a priori profile. Trends and seasonal cycles of XN2O are derived from the TCCON and NDACC measurements and the nearby surface flask sample measurements and compared with the results from GEOS-Chem model a priori and a posteriori simulations. The trends and seasonal cycles from FTIR measurement at Ny-Ålesund and Sodankylä are strongly affected by the polar winter and the polar vortex. The a posteriori N2O fluxes in the model are optimized based on surface N2O measurements with a 4D-Var inversion method. The XN2O trends from the GEOS-Chem a posteriori simulation (0.97±0.02 (1σ) ppb yr−1) are close to those from the NDACC (0.93±0.04 ppb yr−1) and the surface flask sample measurements (0.93±0.02 ppb yr−1). The XN2O trend from the TCCON measurements is slightly lower (0.81±0.04 ppb yr−1) due to the underestimation of the trend in TCCON a priori simulation. The XN2O trends from the GEOS-Chem a priori simulation are about 1.25 ppb yr−1, and our study confirms that the N2O fluxes from the a priori inventories are overestimated. The seasonal cycles of XN2O from the FTIR measurements and the model simulations are close to each other in the Northern Hemisphere with a maximum in August–October and a minimum in February–April. However, in the Southern Hemisphere, the modeled XN2O values show a minimum in February–April while the FTIR XN2O retrievals show different patterns. By comparing the partial column-averaged N2O from the model and NDACC for three vertical ranges (surface–8, 8–17, 17–50 km), we find that the discrepancy in the XN2O seasonal cycle between the model simulations and the FTIR measurements in the Southern Hemisphere is mainly due to their stratospheric differences.

Recent spatial gradients and time trends in Dhaka, Bangladesh, air pollution and their human health implications

ABSTRACTDhaka, the capital of Bangladesh, is among the most polluted cities in the world. This research evaluates seasonal patterns, day-of-week patterns, spatial gradients, and trends in PM2.5 (<2.5 µm in aerodynamic diameter), PM10 (<10 µm in aerodynamic diameter), and gaseous pollutants concentrations (SO2, NO2, CO, and O3) monitored in Dhaka from 2013 to 2017. It expands on past work by considering multiple monitoring sites and air pollutants. Except for ozone, the average concentrations of these pollutants showed strong seasonal variation, with maximum during winter and minimum during monsoon, with the pollution concentration of PM2.5 and PM10 being roughly five- to sixfold higher during winter versus monsoon. Our comparisons of the pollutant concentrations with Bangladesh NAAQS and U.S. NAAQS limits analysis indicate particulate matter (PM2.5 and PM10) as the air pollutants of greatest concern, as they frequently exceeded the Bangladesh NAAQS and U.S. NAAQS, especially during nonmonsoon time. In contrast, gaseous pollutants reported far fewer exceedances throughout the study period. During the study period, the highest number of exceedances of NAAQS limits in Dhaka City (Darus-Salam site) were found for PM2.5 (72% of total study days), followed by PM10 (40% of total study days), O3 (1.7% of total study days), SO2 (0.38% of total study days), and CO (0.25% of total study days). The trend analyses results showed statistically significant positive slopes over time for SO2 (5.6 ppb yr−1, 95% confidence interval [CI]: 0.7, 10.5) and CO (0.32 ppm yr−1, 95% CI: 0.01, 0.56), which suggest increase in brick kilns operation and high-sulfur diesel use. Though statistically nonsignificant annual decreasing slopes for PM2.5 (−4.6 µg/m3 yr−1, 95% CI: −12.7, 3.6) and PM10 (−2.7 µg/m3 yr−1, 95% CI: −7.9, 2.5) were observed during this study period, the PM2.5 concentration is still too high (~ 82.0 µg/m3) and can cause severe impact on human health.Implications: This study revealed key insights into air quality challenges across Dhaka, Bangladesh, indicating particulate matter (PM) as Dhaka’s most serious air pollutant threat to human health. The results of these analyses indicate that there is a need for immediate further investigations, and action based on those investigations, including the conduct local epidemiological PM exposure-human health effects studies for this city, in order to determine the most public health effective interventions.

Temporal patterns and source regions of atmospheric carbon monoxide at two background stations in China

Atmospheric carbon monoxide (CO) was continuously measured from 2010/2011 to 2017 at Lin'an (LAN) and Shangdianzi (SDZ) background station in the economically developed region in China using cavity ring-down spectrometers. Diurnal variations of CO mixing ratio displayed a bimodal in all seasons, with the peak to trough amplitudes larger than 43.6 ± 18.5 (95% CI) ppb at LAN and 80.5 ± 47.0 ppb at SDZ, reflecting the influence of anthropogenic emissions. Seasonal variation at LAN was in accordance with those observed in the northern hemisphere. Due to the multiple influences of regional sources and meteorological transport, CO at SDZ displayed an abnormal pattern, with a maximum in summer and the minimum in autumn. CO levels decreased at a rate of 16.3 ± 0.3 ppb yr−1 at LAN and 1.3 ± 0.4 ppb yr−1 at SDZ during the six-year period. The 3-day back trajectory by the HYSPLIT model showed that high CO at LAN was mainly from the economically developed regions, including the North China Plain, the Middle-Lower Yangtze Plain and the eastern region of the station. The analysis by concentration-weighted trajectory (CWT) approach indicated that large-scale of strong source regions were uniformly distributed around the LAN site in all seasons. At SDZ, atmospheric CO was severely influenced by monsoons and air masses from the south-southeastern site. Major source areas were identified in megacities such as Beijing, Tianjin, and Tangshan, especially in summer. Consequently, most of the atmospheric CO mole fractions at SDZ could be defined into two subsets, with higher values associated with air masses from the south and lower ones from the north.