Items

Advanced search

Impact of the Asian monsoon on the extratropical lower stratosphere: trace gas observations during TACTS over Europe 2012

The transport of air masses originating from the Asian monsoon anticyclone into the extratropical upper troposphere and lower stratosphere (Ex-UTLS) above potential temperatures Θ =  380 K was identified during the HALO aircraft mission TACTS in August and September 2012. In situ measurements of CO, O3 and N2O during TACTS flight 2 on 30 August 2012 show the irreversible mixing of aged stratospheric air masses with younger (recently transported from the troposphere) ones within the Ex-UTLS. Backward trajectories calculated with the trajectory module of CLaMS indicate that these tropospherically affected air masses originate from the Asian monsoon anticyclone. These air masses are subsequently transported above potential temperatures Θ =  380 K from the monsoon circulation region into the Ex-UTLS, where they subsequently mix with stratospheric air masses. The overall trace gas distribution measured during TACTS shows that this transport pathway had affected the chemical composition of the Ex-UTLS during boreal summer and autumn 2012. This leads to an intensification of the tropospheric influence on the extratropical lower stratosphere with PV  >  8 pvu within 3 weeks during the TACTS mission. During the same time period a weakening of the tropospheric influence on the lowermost stratosphere (LMS) is determined. The study shows that the transport of air masses originating from the Asian summer monsoon region within the lower stratosphere affects the change in the chemical composition of the Ex-UTLS over Europe and thus contributes to the flushing of the LMS during summer 2012.
Continental pollution in the Western Mediterranean basin: large variability of the aerosol single scattering albedo and influence on the direct shortwave radiative effect

Pollution aerosols strongly influence the composition of the Western Mediterranean basin, but at present little is known on their optical properties. We report in this study in situ observations of the single scattering albedo (ω) of pollution aerosol plumes measured over the Western Mediterranean basin during the TRAQA (TRansport and Air QuAlity) airborne campaign in summer 2012. Cases of pollution export from different source regions around the basin and at different altitudes between ∼  160 and 3500 m above sea level were sampled during the flights. Data from this study show a large variability of ω, with values between 0.84–0.98 at 370 nm and 0.70–0.99 at 950 nm. The single scattering albedo generally decreases with the wavelength, with some exception associated to the mixing of pollution with sea spray or dust particles over the sea surface. The lowest values of ω (0.84–0.70 between 370 and 950 nm) are measured in correspondence of a fresh plume possibly linked to ship emissions over the basin. The range of variability of ω observed in this study seems to be independent of the source region around the basin, as well as of the altitude and aging time of the plumes. The observed variability of ω reflects in a large variability for the complex refractive index of pollution aerosols, which is estimated to span in the large range 1.41–1.77 and 0.002–0.097 for the real and the imaginary parts, respectively, between 370 and 950 nm. Radiative calculations in clear-sky conditions were performed with the GAME radiative transfer model to test the sensitivity of the aerosol shortwave Direct Radiative Effect (DRE) to the variability of ω as observed in this study. Results from the calculations suggest up to a 50 and 30 % change of the forcing efficiency (FE), i.e. the DRE per unit of optical depth, at the surface (−160/−235 W m−2 τ−1 at 60° solar zenith angle) and at the Top-Of-Atmosphere (−137/−92 W m−2 τ−1) for ω varying between its maximum and minimum value. This induces a change of up to an order of magnitude (+23/+143 W m−2 τ−1) for the radiative effect within the atmosphere.
Impacts of current and projected oil palm plantation expansion on air quality over Southeast Asia

Over recent decades oil palm plantations have rapidly expanded across Southeast Asia (SEA). According to the United Nations, oil palm production in SEA increased by a factor of 3 from 1995 to 2010. We investigate the impacts of current (2010) and near-term future (2020) projected oil palm expansion in SEA on surface–atmosphere exchange and the resulting air quality in the region. For this purpose, we use satellite data, high-resolution land maps, and the chemical transport model GEOS-Chem. Relative to a no oil palm plantation scenario (∼ 1990), overall simulated isoprene emissions in the region increased by 13 % due to oil palm plantations in 2010 and a further 11 % in the near-term future. In addition, the expansion of palm plantations leads to local increases in ozone deposition velocities of up to 20 %. The net result of these changes is that oil palm expansion in SEA increases surface O3 by up to 3.5 ppbv over dense urban regions, and in the near-term future could rise more than 4.5 ppbv above baseline levels. Biogenic secondary organic aerosol loadings also increase by up to 1 µg m−3 due to oil palm expansion, and could increase by a further 2.5 µg m−3 in the near-term future. Our analysis indicates that while the impact of recent oil palm expansion on air quality in the region has been significant, the retrieval error and sensitivity of the current constellation of satellite measurements limit our ability to observe these impacts from space. Oil palm expansion is likely to continue to degrade air quality in the region in the coming decade and hinder efforts to achieve air quality regulations in major urban areas such as Kuala Lumpur and Singapore.
Evaluating secondary inorganic aerosols in three dimensions

The spatial distribution of aerosols and their chemical composition dictates whether aerosols have a cooling or a warming effect on the climate system. Hence, properly modeling the three-dimensional distribution of aerosols is a crucial step for coherent climate simulations. Since surface measurement networks only give 2-D data, and most satellites supply integrated column information, it is thus important to integrate aircraft measurements in climate model evaluations. In this study, the vertical distribution of secondary inorganic aerosol (i.e., sulfate, ammonium, and nitrate) is evaluated against a collection of 14 AMS flight campaigns and surface measurements from 2000 to 2010 in the USA and Europe. GISS ModelE2 is used with multiple aerosol microphysics (MATRIX, OMA) and thermodynamic (ISORROPIA II, EQSAM) configurations. Our results show that the MATRIX microphysical scheme improves the model performance for sulfate, but that there is a systematic underestimation of ammonium and nitrate over the USA and Europe in all model configurations. In terms of gaseous precursors, nitric acid concentrations are largely underestimated at the surface while overestimated in the higher levels of the model. Heterogeneous reactions on dust surfaces are an important sink for nitric acid, even high in the troposphere. At high altitudes, nitrate formation is calculated to be ammonia limited. The underestimation of ammonium and nitrate in polluted regions is most likely caused by a too simplified treatment of the NH3 ∕ NH4+ partitioning which affects the HNO3 ∕ NO3− partitioning.
Significant increase of summertime ozone at Mount Tai in Central Eastern China

Tropospheric ozone (O3) is a trace gas playing important roles in atmospheric chemistry, air quality and climate change. In contrast to North America and Europe, long-term measurements of surface O3 are very limited in China. We compile available O3 observations at Mt. Tai – the highest mountain over the North China Plain – during 2003–2015 and analyze the decadal change of O3 and its sources. A linear regression analysis shows that summertime O3 measured at Mt. Tai has increased significantly by 1.7 ppbv yr−1 for June and 2.1 ppbv yr−1 for the July–August average. The observed increase is supported by a global chemistry-climate model hindcast (GFDL-AM3) with O3 precursor emissions varying from year to year over 1980–2014. Analysis of satellite data indicates that the O3 increase was mainly due to the increased emissions of O3 precursors, in particular volatile organic compounds (VOCs). An important finding is that the emissions of nitrogen oxides (NOx) have diminished since 2011, but the increase of VOCs appears to have enhanced the ozone production efficiency and contributed to the observed O3 increase in central eastern China. We present evidence that controlling NOx alone, in the absence of VOC controls, is not sufficient to reduce regional O3 levels in North China in a short period.
Measuring ice- and liquid-water properties in mixed-phase cloud layers at the Leipzig Cloudnet station

An analysis of the Cloudnet data set collected at Leipzig, Germany, with special focus on mixed-phase layered clouds is presented. We derive liquid- and ice-water content together with vertical motions of ice particles falling through cloud base. The ice mass flux is calculated by combining measurements of ice-water content and particle Doppler velocity. The efficiency of heterogeneous ice formation and its impact on cloud lifetime is estimated for different cloud-top temperatures by relating the ice mass flux and the liquid-water content at cloud top. Cloud radar measurements of polarization and Doppler velocity indicate that ice crystals formed in mixed-phase cloud layers with a geometrical thickness of less than 350 m are mostly pristine when they fall out of the cloud.
Chemical characteristics and causes of airborne particulate pollution in warm seasons in Wuhan, central China

Continuous measurements of airborne particles and their chemical compositions were conducted in May, June, October, and November 2014 at an urban site in Wuhan, central China. The results indicate that particle concentrations remained at a relatively high level in Wuhan, with averages of 135.1 ± 4.4 (mean ± 95 % confidence interval) and 118.9 ± 3.7 µg m−3 for PM10 and 81.2 ± 2.6 and 85.3 ± 2.6 µg m−3 for PM2.5 in summer and autumn, respectively. Moreover, PM2.5 levels frequently exceeded the National Standard Level II (i.e., daily average of 75 µg m−3), and six PM2.5 episodes (i.e., daily PM2.5 averages above 75 µg m−3 for 3 or more consecutive days) were captured during the sampling campaign. Potassium was the most abundant element in PM2.5, with an average concentration of 2060.7 ± 82.3 ng m−3; this finding indicates intensive biomass burning in and around Wuhan during the study period, because almost no correlation was found between potassium and mineral elements (iron and calcium). The source apportionment results confirm that biomass burning was the main cause of episodes 1, 3, and 4, with contributions to PM2.5 of 46.6 % ± 3.0 %, 50.8 % ± 1.2 %, and 44.8 % ± 2.6%, respectively, whereas fugitive dust was the leading factor in episode 2. Episodes 5 and 6 resulted mainly from increases in vehicular emissions and secondary inorganic aerosols, and the mass and proportion of NO3− both peaked during episode 6. The high levels of NOx and NH3 and the low temperature during episode 6 were responsible for the increase of NO3−. Moreover, the formation of secondary organic carbon was found to be dominated by aromatics and isoprene in autumn, and the contribution of aromatics to secondary organic carbon increased during the episodes.
Chemical cycling and deposition of atmospheric mercury in polar regions: review of recent measurements and comparison with models

Mercury (Hg) is a worldwide contaminant that can cause adverse health effects to wildlife and humans. While atmospheric modeling traces the link from emissions to deposition of Hg onto environmental surfaces, large uncertainties arise from our incomplete understanding of atmospheric processes (oxidation pathways, deposition, and re-emission). Atmospheric Hg reactivity is exacerbated in high latitudes and there is still much to be learned from polar regions in terms of atmospheric processes. This paper provides a synthesis of the atmospheric Hg monitoring data available in recent years (2011–2015) in the Arctic and in Antarctica along with a comparison of these observations with numerical simulations using four cutting-edge global models. The cycle of atmospheric Hg in the Arctic and in Antarctica presents both similarities and differences. Coastal sites in the two regions are both influenced by springtime atmospheric Hg depletion events and by summertime snowpack re-emission and oceanic evasion of Hg. The cycle of atmospheric Hg differs between the two regions primarily because of their different geography. While Arctic sites are significantly influenced by northern hemispheric Hg emissions especially in winter, coastal Antarctic sites are significantly influenced by the reactivity observed on the East Antarctic ice sheet due to katabatic winds. Based on the comparison of multi-model simulations with observations, this paper discusses whether the processes that affect atmospheric Hg seasonality and interannual variability are appropriately represented in the models and identifies research gaps in our understanding of the atmospheric Hg cycling in high latitudes.
Relating atmospheric N2O concentration to N2O emission strength in the U. S. Corn Belt

Nitrous oxide (N2O) has a high global warming potential and depletes stratospheric ozone. The U. S. Corn Belt plays an important role in the global anthropogenic N2O budget. To date, studies on local surface N2O emission and the atmospheric N2O budget have commonly used Lagrangian models. In the present study, we used an Eulerian model – Weather Research and Forecasting Chemistry (WRF-Chem) model to investigate the relationships between N2O emission in the Corn Belt and observed atmospheric N2O mixing ratios. Modeled hourly N2O mixing ratios were combined with continuous atmospheric N2O measurements at the KCMP tall tower in Minnesota to constrain agricultural N2O emissions. The modeled spatial patterns of atmospheric N2O were validated against discrete observations at multiple tall towers in the NOAA flask network. After optimization of the surface flux, the model reproduced reasonably well the hourly N2O mixing ratios monitored at the KCMP tower. Agricultural N2O emissions in the EDGAR42 database needed to be scaled up by 19.0 to 28.1 fold to represent the true emission in the Corn Belt from June 1–20, 2010 – a peak emission period. Optimized total N2O emissions were 3.00–4.38, 1.52–2.08, 0.61–0.81 and 0.56–0.75 nmol m−2 s−1 from June 1–20, August 1–20, October 1–20 and December 1–20, 2010, respectively. The simulated spatial patterns of atmospheric N2O mixing ratios were in good agreement with the NOAA discrete observations during the strong emission peak in June. Such spatial patterns illustrate that the IPCC (Inter-governmental Panel on Climate Change) underestimate of emissions is not dependent on tower measurement location.
20 years of ClO measurements in the Antarctic lower stratosphere

We present 20 years (1996–2015) of austral springtime measurements of chlorine monoxide (ClO) over Antarctica from the Chlorine Oxide Experiment (ChlOE1) ground-based millimeter wave spectrometer at Scott Base, Antarctica, as well 12 years (2004–2015) of ClO measurements from the Aura Microwave Limb Sounder (MLS). From August onwards we observe a strong increase in lower stratospheric ClO, with a peak column amount usually occurring in early September. From mid-September onwards we observe a strong decrease in ClO. In order to study interannual differences, we focus on a 3-week period from 28 August to 17 September for each year and compare the average column ClO anomalies. These column ClO anomalies are shown to be highly correlated with the average ozone mass deficit for September and October of each year. We also show that anomalies in column ClO are strongly anti-correlated with 30 hPa temperature anomalies, both on a daily and an interannual timescale. Making use of this anti-correlation we calculate the linear dependence of the interannual variations in column ClO on interannual variations in temperature. By making use of this relationship, we can better estimate the underlying trend in the total chlorine (Cly  =  HCl + ClONO2 + HOCl + 2  ×  Cl2 + 2  ×  Cl2O2 + ClO + Cl). The resultant trends in Cly, which determine the long-term trend in ClO, are estimated to be −0.5 ± 0.2, −1.4 ± 0.9, and −0.6 ± 0.4 % year−1, for zonal MLS, Scott Base MLS (both 2004–2015), and ChlOE (1996–2015) respectively. These trends are within 1σ of trends in stratospheric Cly previously found at other latitudes. The decrease in ClO is consistent with the trend expected from regulations enacted under the Montreal Protocol.