Ppb Year Research Articles

Trend analysis of measured surface ozone at the megacity of Tehran for the summertime 2007–2021

High concentrations of surface ozone in urban and industrial regions worldwide have been a major air quality issue. Tehran, as the capital and the most populous city of Iran, has experienced intense ambient air pollution over the last three decades. This study aims to assess the long-term trends of ozone over this city in recent decades. Data series of maximum 8-h daily average (MDA8) ozone were obtained for the period between 2007 and 2021. To determine the MDA8 ozone trend attributed to the long-term changes in ozone precursor emissions, we isolated the effects of meteorology and short-term ozone variability. The results show a downward trend from 2007 to 2014 (P1) and an upward trend from 2014 onward (P2). The long-term precursor emissions changes contribute to a 29.1% negative trend (=−0.7 ppb year−1) over P1 and an 8.95% upward trend (=0.1 ppb year−1) over P2. To find the causes of the trend transition, we investigated the relationship between the adjusted MDA8 and several ozone precursors, namely NOx and CO. It is found that, over P1, the dominant chemistry regime is mixed, in which the decline of CO and NOx (attributed to industrial sources) is associated with the downward trend of ozone. On the other hand, the upward trend of ozone over P2 is associated with an increase in NOx mostly attributed to road transport (passenger cars) and residential sectors.

Investigation of the multi-year trend of surface ozone and ozone-precursor relationship in Hong Kong

As a highly urbanized city in East Asia, Hong Kong has faced severe photochemical ozone (O3) pollution for years. A comprehensive understanding of pollution characteristics is crucial for effective mitigation of the O3 problem. This study systematically investigated the multi-year trends of surface O3 over the last decade using measurements from 11 selected Air Quality Monitoring Stations (AQMS). The analysis reveals upward trends in the annual average and daily maximum 8-h average (DMA8) O3 from 2011 to 2022, with rates of 0.34 ppb year−1 (p < 0.05) and 0.22 ppb year−1 (p = 0.19), respectively. Though upward trends of DMA8 O3 were observed at urban stations (0.10–0.88 ppb year−1), the significant decreases in NO2 levels at all stations (−0.18 to −1.75 ppb year−1) led to a decline of total oxidants (Ox, O3+NO2). In addition to the daytime O3 peak increase, a clear nighttime O3 peak was present with a significant increasing trend, which was associated with the regional transport of O3-rich plumes and weakened NO titration effect. The relative contributions of local production and regional transport were investigated, and results indicated that local control efforts have predictably alleviated the locally produced Ox, while the regional background Ox still contributes significantly but appears to decline. Furthermore, an observation-based model was employed to examine the evolution of O3-precursors relationships. The results indicate that O3 formation was still VOC-limited in Hong Kong, and the suppressive effect of NOx on O3 formation has decreased in recent years. In terms of abatement strategies, apart from aromatics, we emphasize the significance of carbonyl compounds due to their high reactivity to O3 production.

Rapidly Changing Emissions Drove Substantial Surface and Tropospheric Ozone Increases Over Southeast Asia

AbstractWe combined observations and simulations to assess tropospheric ozone trends over Southeast Asia from 2005 to 2016. Multi‐platform observations showed that surface ozone had been increasing at rates of 0.7–1.2 ppb year−1 over the Peninsular Southeast Asia (PSEA) and 0.2–0.4 ppb year−1 over the Maritime Continents (MC); tropospheric ozone columns had been rising throughout Southeast Asia by 0.21–0.35 DU year−1. These observed ozone trends were better reproduced by simulations driven with satellite‐constrained NOx emissions, indicating that NOx emission growths may have been underestimated for the PSEA and overestimated for the MC in the Community Emissions Data System and the Global Fire Emissions Data set. The surface ozone increases over the PSEA were driven by rapidly growing local emissions, wherein fire emission growths may still be underestimated even with satellite constraints. We highlighted the need for better quantifying Southeast Asian emissions to benefit air quality management.

Analysis of Atmospheric CO2 and CO at Akedala Atmospheric Background Observation Station, a Regional Station in Northwestern China.

Air samples were collected by flasks and analyzed via a Picarro G2401 gas analyzer for carbon dioxide (CO2) and carbon monoxide (CO) at the Akedala Atmospheric Background Station in Xinjiang, China, from September 2009 to December 2019, to analyze the changes in the characteristics of atmospheric CO2 and CO and determine the sources. The results show that the annual average CO2 concentration showed an increasing trend (growth rate: 1.90 ppm year−1), ranging from 389.80 to 410.43 ppm, and the annual average CO concentration also showed an increasing trend (growth rate: 1.78 ppb year−1), ranging from 136.30 to 189.82 ppb. The CO2 concentration and growth rate were the highest in winter, followed by autumn, spring, and summer. The CO concentration and growth rate were also the highest in winter due to anthropogenic emissions, ecosystem effects, and diffusion conditions. The main trajectories of CO2 and CO determined by the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model were parallel to the Irtysh River valley and then passed through the Old Wind Pass. Furthermore, the main source regions of CO2 and CO at the Akedala Station were eastern Kazakhstan, southern Russia, western Mongolia, and the Xinjiang Tianshan North Slope Economic Zone of China. This study reflects the characteristics of long-term changes in CO2 and CO concentrations at the Akedala station and provides fundamental data for the studies on environmental changes and climate change in Central Asia.

Assessment of Impact of Meteorology and Precursor in Long-term Trends of PM and Ozone in a Tropical City

Long-term trends of atmospheric pollutants, particularly ozone (O3) and particulate matter (PM) provide a direct evaluation of the response of the atmosphere to the environmental policies and the variability of anthropogenic and biogenic emissions. Here, we report the assessment of the temporal evolution of the air quality in a tropical urban city (Butuan) in the southern Philippines by evaluating the trends of meteorological conditions (i.e., temperature, R.H., boundary layer height), air pollutants (i.e., PM2.5, NO2, O3) and their precursors (Benzene, Toluene, and Xylene) from 2014 to 2020. During the seven-year measurement, the mean PM2.5 and PM10 mass concentrations were 8.7 ± 3.9 and 24.3 ± 12.0 µg m–3, with no single day exceeded the daily PM limit. The max concentrations of aerosol occurred during the dry season when the loss of particles through wet deposition was limited. Speciation of PM2.5 indicated that fine aerosol was dominated by sea salt and organic matter (OM). Analysis of the ratio of OM and sulfate indicated that the main source of pollution in the city was wildfire/biomass burning. The average O3 and NO2 mixing ratios during the same period were 22.3 ± 9.5 ppb and 8.1 ± 5.4 ppb while increasing at the rate of 0.409 ppb year–1 and 0.683 ppb year–1. The highest O3 concentration occurred during the summer months when photochemistry enhanced the formation of tropospheric O3. The increasing O3 trend was attributed to the contribution of anthropogenic VOCs (AVOCs), based on their ozone-forming potentials (OFPs). The seven-year measurement also showcased the variability of the atmospheric pollutants during the COVID-19 pandemic of 2020, when O3 substantially increased due to reduced vehicle transport activities. Overall, our results provide insights to better comprehend the sources of the variability of O3 and PM on a long-term temporal scale, as well as implications on relevant environmental policies in controlling air pollutants in a tropical developing region.

Trends and Variability of Ozone Pollution over the Mountain-Basin Areas in Sichuan Province during 2013–2020: Synoptic Impacts and Formation Regimes

Sichuan Province, the most industrialized and populated region in southwestern China, has been experiencing severe ozone pollution in the boreal warm season (April–September). With a surface ozone monitoring network and reanalysis dataset, we find that nearly all cities in Sichuan Province showed positive increasing trends in the warm-season ozone levels. The warm-season daily maximum 8-h average (MDA8) ozone levels increased by 2.0 ppb (4.8%) year−1 as a whole, with slightly larger trends in some sites such as a site in Zigong (5.2 ppb year−1). Seasonally, the monthly ozone level in Sichuan peaks from May to August (varies with year). The predominant warm-season synoptic patterns were objectively identified based on concurrent hourly meteorological fields from ERA5. High-pressure systems promote ozone production and result in high ozone concentrations, due to strong solar radiation as well as hot and dry atmospheric conditions. The increased occurrence of high-pressure patterns probably drives the ozone increase in Sichuan. When ozone pollution is relatively weak (with MDA8 ozone around 170 μg m−3), the air quality standard could be achieved in the short term by a 25% reduction of NOx and VOCs emissions. Strengthened emission control is needed when ozone pollution is more severe. Our study provides implications for effective emission control of ozone pollution in Sichuan.

Regional and Urban Column CO Trends and Anomalies as Observed by MOPITT Over 16 Years

AbstractAtmospheric carbon monoxide (CO) concentrations have decreased since the beginning of the century, and the rate of decrease depends on the region. Depending on how regions are chosen, their boundaries may not always align with where there are differences in trends. To address this, we calculate trends within 0.4° × 0.4° grid cells independently throughout the globe using satellite CO retrievals from the Measurements Of Pollution In The Troposphere (MOPITT) satellite instrument from 2002 to 2017. These trends are found with the caveat that there are large singular biomass burning events somewhere nearly every year, and we include examples of large column CO anomalies during sporadic but large burning events in the North American and Eurasian boreal forests, the Amazon, Africa, and Indonesia. CO trends behave similarly within regions that span about a few thousand kilometers. Using TransCom region definitions, we find average trends between −0.9 and 0.1 ppb year−1 (about −0.9 to 0.1% year−1) for 2002–2017. Over 5‐year subsets, trends in TransCom regions vary between −3.6 and 1.8 ppb year−1. This substantial spatial and temporal variability in trends is in agreement with other studies. With an understanding of regional trends, we compare with trends from urban areas. Generally, CO trends within urban areas are indistinguishable from regional trends. This may be because of a combination of noise in the data, the large footprint for MOPITT, or because anthropogenic CO reduction measures were implemented before the MOPITT record began. We provide case studies for a few cities, such as Los Angeles, and find long‐term variation in the rate of change of column CO.

Patterns and trends of ozone and carbon monoxide at Ushuaia (Argentina) observatory

The behavior patterns for surface ozone (O3) and carbon monoxide (CO) were investigated with the hourly mixing ratios registered for nine years (2010 to 2018) at the GAW-WMO (Global Atmospheric Watch – World Meteorological Organization) station of Ushuaia (Argentina). Two behavior patterns were identified for these species after applying a criterion. Specific periods were analyzed in detail, in which nitrogen dioxide (NO2) was also studied. The weather conditions were explored using the ERA5 meteorological fields from the ECMWF (European Centre for Medium-Range Weather Forecasts). Outputs of the chemical transport model from the Copernicus Atmosphere Monitoring Service (CAMS) were used to investigate the spatial distribution of O3, CO and NO2. The more frequent scenario is characterized by the unaltered mixing ratios of O3 and CO for days, with values of 10–15 ppb and 60–70 ppb, respectively, and below 200 ppt for NO2. This pattern occurred with intense westerly airflows associated with the low-pressure system located in the south of the Pacific Ocean. A second pattern was observed in 45.8 ± 3.4% of the days, which was characterized by low O3 mixing ratios (~12 ppb), high NO2 concentrations (above ~3000 ppt) and CO peaks exceeding 400 ppb. This scenario was defined by a high-pressure system blocking the westerly winds, which results in atmospheric stagnation and in the accumulation of trace gases in the lower layers. Finally, the annual trend was explored; no O3 trend was obtained, although an unexpected upward CO trend was identified, with an increase of 1.75 ± 0.49 ppb year−1. To investigate its causes, the annual evolution of the temperature at 850 hPa level (T850) and geopotential height at 500 hPa level (Z500) anomalies along with the CO emissions were analyzed. While no trends were found in weather conditions, the CO emissions showed an increasing trend of 4.4% year−1; therefore, the CO increase could be attributed to the emission changes. In the future, warm climate with more frequent anticyclonic situations, together with a growing trend in emissions, could lead to a drastic change in the current levels of these trace gases, in remote regions such as Ushuaia.

Analysis of the temporal variability of CO2, CH4 and CO concentrations at Lamto, West Africa

The 10-year observations of the atmospheric molar fractions of CO2, CH4 and CO in West Africa were analyzed using a high precision measurement of the Lamto (LTO) station (6°31 N and 5°02 W) in Côte d’Ivoire. At daily scale, high concentrations appear at night with significant peaks around 7 a.m. local time and minimum concentrations in the afternoon for CO2 and CH4. The CO concentrations show two peaks around 8 h and 20 h corresponding to the maximum in road traffic of a northern motorway located 14 km from the station. The long-term increase rates of CH4 (∼7 ppb year−1) and CO2 (∼2.24 ppm year−1) at Lamto are very close to global trends. The variations of the concentrations of the three gases show strong seasonality with a peak in January for all gases and minima in September for CO2 and CH4, and in June for CO. The CO variation suggests a significant impact of fires on the CO, CO2 and CH4 anomalies in the Lamto region during the dry season (December to February). CO and CH4 show strong correlations (at synoptic-scale and monthly based) in January (r = 0.84), February (r = 0.90), April (r = 0.74), November (r = 0.79) and December (r = 0.72) reflecting similar sources of emission for both gases. The trajectories of polluted air masses at LTO, also indicate continental sources of emission associated with Harmattan winds.

Ozone weekend effect in cities: Deep insights for urban air pollution control

Studying weekend-weekday variation in ground-level ozone (O3) allows one to better understand O3 formation conditions, with a potential for developing effective strategies for O3 control. Reducing inappropriately the O3 precursors emissions can either produce no reduction or increase surface O3 concentrations. This paper analyzes the weekend-weekday differences of O3 at 300 rural and 808 urban background stations worldwide from 2005 to 2014, in order to investigate the O3 weekend effect over time and assess the effectiveness of the precursors emissions control policies for reducing O3 levels. Data were analyzed with the non-parametric Mann-Kendall test and Theil-Sen estimator. Rural sites typically did not experience a weekend-weekday effect. In all urban stations, the mean O3 concentration on the weekend was 12% higher than on weekdays. Between 2005 and 2014, the annual mean of daily O3 concentrations increased at 74% of urban sites worldwide (+ 0.41 ppb year−1) and decreased in the United Kingdom (- 0.18 ppb year−1). Over this time period, emissions of O3 precursors declined significantly. However, a greater decline in nitrogen oxides (NOx) emissions caused an increase in Volatile Organic Compounds (VOCs) to NOx ratios leading to O3 formation. In France, South Korea and the United Kingdom, most urban stations showed a significant upward trend (+ 1.15% per year) for O3 weekend effect. Conversely, in Canada, Germany, Japan, Italy and the United States, the O3 weekend effect showed a significant downward trend (- 0.26% per year). Further or inappropriate control of anthropogenic emissions in Canada, Southern Europe, Japan, South Korea and the United States might result in increased daily O3 levels in urban areas.

Zonal Similarity of Long‐Term Changes and Seasonal Cycles of Baseline Ozone at Northern Midlatitudes

AbstractThe lifetime of ozone in the troposphere is approximately 3 weeks. Prevailing westerly winds at northern midlatitudes can transport air around the globe in that time. Hence, within these latitudes zonal similarity is expected in long‐term changes and seasonal cycles of concentrations of baseline ozone. We quantify the degree of zonal similarity by examining eight in situ baseline ozone data sets near the west coasts of North America and Europe, that is, upwind of those continents and downwind of the Pacific and Atlantic Oceans, where the impacts of local and regional ozone sources have been largely mixed into the troposphere, giving the best‐defined baseline ozone signature. Zonal similarity is found in both long‐term changes and seasonal cycles. The decades‐long increase in Northern Hemisphere, midlatitude baseline mixing ratios (average ~0.60 ppb year−1 from 1980–2000), has ended, with a maximum reached in the mid‐2000s, followed by slow decrease (average = −0.09 ± 0.08 ppb year−1 from 2000 to the present). The year of the ozone maximum exhibits little if any statistically significant difference with location, altitude, or season. The ozone seasonal cycle differs markedly between sea‐level coastal stations representative of the marine boundary layer and the free troposphere sampled at elevated sites and by sondes and aircraft. However, within each of these broad tropospheric layers, the seasonal cycles are similar at all locations. Vertical profiles of the parameters that define the long‐term trends and the seasonal cycle are also similar between North America and Europe.

Rapid Increases in Warm-Season Surface Ozone and Resulting Health Impact in China Since 2013

China’s nationwide ozone monitoring network initiated in 2013 has observed severe surface ozone pollution. This network, combined with the recent Tropospheric Ozone Assessment Report (TOAR) data set, offers a more comprehensive view on global surface ozone distribution and trends. Here, we report quantitative estimates of the warm-season (April–September) surface ozone trends and resulting health impacts at Chinese cities in 2013–2019. Both the parametric and nonparametric linear trends for 12 ozone metrics relevant to human health and vegetation exposure are derived. We find that all ozone metrics averaged from Chinese urban sites have increased significantly since 2013. The warm-season daily maximum 8-h average (MDA8) ozone levels increased by 2.4 ppb (5.0%) year–¹, with over 90% of the sites showing positive trends and 30% with trends larger than 3.0 ppb year–¹. These rates are among the fastest trends, even faster in some Chinese cities, compared with the urban ozone trends in any other region worldwide reported in TOAR. Ozone metrics reflecting the cumulative exposure effect on human health and vegetation such as SOMO35 and AOT40 have increased at even faster rates (>10% year–¹). We estimate that the total premature respiratory mortalities attributable to ambient MDA8 ozone exposure in 69 Chinese cities are 64,370 in 2019, which has increased by 60% compared to 2013 levels and requires urgent attention.

Local regressions for decomposing CO2 and CH4 time-series in a semi-arid ecosystem

Local regressions have been widely employed for decomposing atmospheric data series. However, the use of local quadratic regressions is less extended. The current paper is grounded on the hypothesis that local linear regressions are able to capture CO2 and CH4 temporal evolution equally as well as quadratic linear regressions. Thus, the current paper pursues the following goals: (1) to quantify the temporal variability of both gases by using the local linear and local quadratic regression method; (2) to analyse the main statistics of the detrended series over time; (3) to compare results between the local linear and local quadratic regression method. Minimum mixing ratios in late summer and maximum in winter were detected for both gases. Atmospheric increases of an average of 1.98 ppm year−1 for CO2 and 11 ppb year−1 for CH4 were found when applying the local linear regression method. Alternatively, an increase of 2.24 ppm year−1 for CO2 and around 10.34 ppb year−1 for CH4 was obtained when the local quadratic regression method was applied. The Pearson correlation coefficients (0.21–0.40) showed acceptable values due to the large amount of available data. Statistically significant differences for the initial and the smoothed trend, as well as statistically significant differences for the seasonal component, were reported when comparing the local linear with the local quadratic method. Overall, both methods proved easy to apply and both provided acceptable data accuracy.

Trend analysis of surface ozone at suburban Guangzhou, China

The long-term variations of ozone are the combined results of climate change and air quality management. As Guangzhou is under the influence of both subtropical monsoon climate and rapid economic development, the ozone trend in recent years is uncertain. This paper presents the trend analysis of maximum daily average 8 h (MDA8) ozone and daily meteorological observations in Guangzhou from 2008 to 2018, using the Kolmogorov–Zurbenko (KZ) filter method. The observations were conducted at two sites in suburban Guangzhou, thus the datasets were processed in two periods. The first period (P1) is from 2008 to 2013, and the second period (P2) is from 2014 to 2018. Results show that the KZ filter method separates the short-term, seasonal, and long-term components efficiently, leaving a covariance term of 7.3% (5.4%) for P1 (P2). Through linear regression of long-term components, the trends were inferred as −0.06 ± 0.04 ppb year−1 (R2 = 0.00, p < 0.05) for P1, and 0.51 ± 0.08 ppb year−1 (R2 = 0.11, p < 0.05) for P2. It is found that the solar radiation has the strongest impact on ozone. With inclusion of temperature, relative humidity, and wind speed, these four meteorological factors held 71% (76%) variability in baseline ozone (sum of seasonal and long-term ozone) for P1 (P2). After applying the KZ filter method, the results reveal that the variance contribution of emission to long-term ozone variation is larger than that of meteorology in P1, while smaller in P2. Furthermore, 59% of the emission-induced ozone change in P2 could be explained by nitrogen dioxide variation, and their inverse correlation suggests that Guangzhou is mainly under volatile organic compounds-limited regime, despite continuous nitrogen oxides reduction.

Anthropogenic CH4 Emissions in the Yangtze River Delta Based on A “Top-Down” Method

There remains significant uncertainty in the estimation of anthropogenic CH4 emissions at local and regional scales. We used atmospheric CH4 and CO2 concentration data to constrain the anthropogenic CH4 emission in the Yangtze River Delta one of the most populated and economically important regions in China. The observation of atmospheric CH4 and CO2 concentration was carried out from May 2012 to April 2017 at a rural site. A tracer correlation method was used to estimate the anthropogenic CH4 emission in this region, and compared this “top-down” estimate with that obtained with the IPCC inventory method. The annual growth rates of the atmospheric CO2 and CH4 mole fractions are 2.5 ± 0.7 ppm year−1 and 9.5 ± 4.7 ppb year−1, respectively, which are 9% and 53% higher than the values obtained at Waliguan (WLG) station. The average annual anthropogenic CH4 emission is 4.37 (± 0.61) × 109 kg in the YRD (excluding rice cultivation). This “top-down” estimate is 20–70% greater than the estimate based on the IPCC method. We suggest that possible sources for the discrepancy include low biases in the IPCC calculation of emission from landfills, ruminants and the transport sector.

Study of the meteorological influence on ozone in urban areas and their use in assessing ozone trends in all seasons from 2009 to 2015 in Tianjin, China

Ozone pollution in urban areas has increasingly become a topic of intense research in China. Assessing the impact of emission control strategies on O3 levels is complicated by the disturbance of meteorological factors. This study employs a statistical methodology named generalized additive model (GAM) to characterize the relationship between meteorological factors and ozone levels as well as to meteorologically adjust the ozone trends in Tianjin from 2009 to 2015. The results indicate that the afternoon temperature and the morning solar radiation are the leading meteorological factors controlling O3 in Tianjin. GAM proves to be an effective tool in predicting ozone levels, because it could capture 40–77% of the variance in the daily ozone maxima for different seasons. Only 3–5 parameters are incorporated into the final model for each season. During summer months, the most important explanatory variables are those influencing the photochemical production. Whereas in winter, ozone destruction by titration with NO is the dominant mechanism affecting the O3 levels. After adjustment for meteorological effect, general upward trends are evident in ozone levels, which is − 0.34, 0.97, 0.70 and 0.12 ppb year−1 in spring, summer, autumn and winter, respectively, indicating that the emission reduction strategies in Tianjin from 2009 to 2015 appear to be more beneficial in reducing the effect of O3 loss using titration rather than mitigating the photochemical pollution. The clear rising tendency of yearly medians of the meteorologically adjusted ozone (0.24 ppb year−1) strongly suggests formulating a specialized mitigation strategy for regional O3 pollution.

Impacts of tropospheric ozone exposure on peatland microbial consumers

Tropospheric ozone pollution is recognised as an important threat to terrestrial ecosystems but impacts on peatlands are little understood despite the importance of peat as a global carbon store. Here we investigate the impacts of three levels of elevated exposure to tropospheric ozone on peatland microbial communities with a particular focus on testate amoebae, the dominant microbial consumers. We found that intermediate (ambient + 25 ppb O3) and high ozone treatments (ambient +35 ppb summer, +10 ppb year round) led to significant changes in testate amoeba communities, typified by an increase in abundance of Phyrganella spp. and loss of diversity. Phyrganella is often suggested to feed on fungi so the observed community change in testate amoeba could be due to ozone-induced changes in the abundance of fungi or other microbial groups. We do not find evidence for changes in numbers of undifferentiated microalgae, nematodes or rotifers but do find weak evidence for an increase in flagellates and ciliates. Our results provide the first direct data to show the impact of ozone on microbial consumers in peatlands.

Significant increase of surface ozone at a rural site, north of eastern China

Abstract. Ozone pollution in eastern China has become one of the top environmental issues. Quantifying the temporal trend of surface ozone helps to assess the impacts of the anthropogenic precursor reductions and the likely effects of emission control strategies implemented. In this paper, ozone data collected at the Shangdianzi (SDZ) regional atmospheric background station from 2003 to 2015 are presented and analyzed to obtain the variation in the trend of surface ozone in the most polluted region of China, north of eastern China or the North China Plain. A modified Kolmogorov–Zurbenko (KZ) filter method was performed on the maximum daily average 8 h (MDA8) concentrations of ozone to separate the contributions of different factors from the variation of surface ozone and remove the influence of meteorological fluctuations on surface ozone. Results reveal that the short-term, seasonal and long-term components of ozone account for 36.4, 57.6 and 2.2 % of the total variance, respectively. The long-term trend indicates that the MDA8 has undergone a significant increase in the period of 2003–2015, with an average rate of 1.13 ± 0.01 ppb year−1 (R2 = 0.92). It is found that meteorological factors did not significantly influence the long-term variation of ozone and the increase may be completely attributed to changes in emissions. Furthermore, there is no significant correlation between the long-term O3 and NO2 trends. This study suggests that emission changes in VOCs might have played a more important role in the observed increase of surface ozone at SDZ.

Decadal trends of atmospheric methane in East Asia from 1991 to 2013

Discrete air sample measurements of atmospheric methane (CH4) were analyzed at the following East Asian monitoring sites: Mt. Waliguan (WLG), China; Ulaan Uul (UUM), Mongolia; Tae-ahn Peninsula (TAP), Korea; and the remote high-altitude site, Mauna Loa (MLO), Hawaii, for 1991∼2013. The changes of CH4 emission from regional sources resulted in a trend in the difference between the East Asian monitoring sites and MLO. The average annual growth rate in the difference between TAP and MLO has a larger 1σ uncertainty of the trend of 0.3 ppb year−1 compared with WLG and UUM. TAP is influenced by changes in regional sources. Therefore, the annual increase varies greatly from year to year. The East Asian monitoring sites show a different seasonal cycle. The average seasonal variation at TAP has higher standard deviation in July and August. During July and August, there is inflow of air depleted in CH4 by OH radical over the North Pacific Ocean as well as very high CH4 from paddy field in eastern China. The average annual growth rate in the difference between TAP and MLO was increasing with 1.4 ± 1.2 ppb year−1 for the regional polluted continental (RPC) air mass originating from China.

Carbon monoxide over Indian region as observed by MOPITT

A comprehensive study has been carried out on tropospheric carbon monoxide (CO) over the Indian land mass and surrounding oceanic region using the CO retrievals from MOPITT (Measurements of Pollution in the Troposphere) for a period of ∼14 years (2000–2014). The lower-tropospheric CO maximises during winter and shows a broad minimum during summer-monsoon over most of the regions, but with regionally varying seasonal amplitudes. Tropospheric column CO also exhibits a seasonal pattern similar to lower-tropospheric CO. But the upper-tropospheric CO shows an opposite seasonal pattern which peaks during summer monsoon. Columnar CO showed strong positive correlation with fire counts over west, east and north-east India, indicating the dominant role of biomass burning in controlling the seasonal variation of CO. The lower-tropospheric and columnar CO showed decreasing trend of 2.0–3.4 ppb year−1 (1.1–2.0% year−1) and 6.0–13.6 × 1015 molecules cm−2 year−1 (0.3–0.6% year−1) respectively over most of the regions. However, over many land regions trend in columnar CO is not significant. Most strikingly, the upper tropospheric CO showed increasing trend of 1.4–2.4 ppb year−1 (1.8–3.2% year−1). The analysis of biases in the estimated trends due to temporal changes in the MOPITT averaging kernels shows that magnitude of the realistic trend may change depending upon the bias but the sign (positive or negative) of trend remains the same. The decreasing trend in lower tropospheric and columnar CO could be attributed partly to increase in lower tropospheric water vapour and/or tropospheric ozone. The strengthening of convective activity, uplifting the CO to higher altitudes, could be a reason for increasing trend in the upper-tropospheric CO.