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Ion – particle interactions during particle formation and growth at a coniferous forest site in central Europe

In this work, we examined the interaction of ions and neutral particles during atmospheric new particle formation (NPF) events. The analysis is based on simultaneous field measurements of atmospheric ions and total particles using a neutral cluster and air ion spectrometer (NAIS) across the diameter range 2–25 nm. The Waldstein research site is located in a spruce forest in NE Bavaria, Southern Germany, known for enhanced radon concentrations, presumably leading to elevated ionization rates. Our observations show that the occurrence of the ion nucleation mode preceded that of the total particle nucleation mode during all analyzed NPF events. The time difference between the appearance of 2 nm ions and 2 nm total particles was typically about 20 to 30 min. A cross correlation analysis showed a rapid decrease of the time difference between the ion and total modes during the growth process. Eventually, this time delay vanished when both ions and total particles did grow to larger diameters. Considering the growth rates of ions and total particles separately, total particles exhibited enhanced growth rates at diameters below 15 nm. This observation cannot be explained by condensation or coagulation, because these processes would act more efficiently on charged particles compared to neutral particles. To explain our observations, we propose a mechanism including recombination and attachment of continuously present cluster ions with the ion nucleation mode and the neutral nucleation mode, respectively.
Air quality in Delhi during the Commonwealth Games

Air quality during the Commonwealth Games (CWG, held in Delhi in October 2010) is analyzed using a new air quality forecasting system established for the games. The CWG stimulated enhanced efforts to monitor and model air quality in the region. The air quality of Delhi during the CWG had high levels of particles with mean values of PM2.5 and PM10 at the venues of 111 and 238 μg m−3, respectively. Black carbon (BC) accounted for ~ 10% of the PM2.5 mass. It is shown that BC, PM2.5 and PM10 concentrations are well predicted, but with positive biases of ~ 25%. The diurnal variations are also well captured, with both the observations and the modeled values showing nighttime maxima and daytime minima. A new emissions inventory, developed as part of this air quality forecasting initiative, is evaluated by comparing the observed and predicted species-species correlations (i.e., BC : CO; BC : PM2.5; PM2.5 : PM10). Assuming that the observations at these sites are representative and that all the model errors are associated with the emissions, then the modeled concentrations and slopes can be made consistent by scaling the emissions by 0.6 for NOx, 2 for CO, and 0.7 for BC, PM2.5, and PM10. The emission estimates for particles are remarkably good considering the uncertainty in the estimates due to the diverse spread of activities and technologies that take place in Delhi and the rapid rates of change. The contribution of various emission sectors including transportation, power, domestic and industry to surface concentrations are also estimated. Transport, domestic and industrial sectors all make significant contributions to PM levels in Delhi, and the sectoral contributions vary spatially within the city. Ozone levels in Delhi are elevated, with hourly values sometimes exceeding 100 ppb. The continued growth of the transport sector is expected to make ozone pollution a more pressing air pollution problem in Delhi. The sector analysis provides useful inputs into the design of strategies to reduce air pollution levels in Delhi. The contribution for sources outside of Delhi on Delhi air quality range from ~ 25% for BC and PM to ~ 60% for day time ozone. The significant contributions from non-Delhi sources indicates that in Delhi (as has been show elsewhere) these strategies will also need a more regional perspective.
Summertime tropospheric-ozone variability over the Mediterranean basin observed with IASI

The Mediterranean basin is one of the most sensitive regions in the world regarding climate change and air quality. This is partly due to the singular dynamical situation of the Mediterranean basin that leads to tropospheric-ozone concentrations that are among the highest over the Northern Hemisphere. Six years of summertime tropospheric ozone observed by the Infrared Atmospheric Sounding Interferometer (IASI) instrument from 2007 to 2012 have been analysed to document the variability of ozone over this region. The satellite observations have been examined together with meteorological analyses (from ECMWF) to understand the processes driving this variability. Our work confirmed the presence of a steep west–east ozone gradient in the lower troposphere with the highest concentrations observed over the eastern part of the Mediterranean basin. This gradient is mainly explained by diabatic convection over the Persian Gulf during the Indian monsoon season, which induces an important subsidence of ozone-rich air masses from the upper to the lower troposphere over the central and the eastern Mediterranean basin. IASI observations of ozone concentrations at a 3 km height show a clear summertime maximum in July that is well correlated to the maximum of downward transport of ozone-rich air masses from the upper troposphere. Even if this feature is robust over the six analysed years, we have also investigated monthly ozone anomalies – one positive (June 2008) and one negative (June and July 2009) – using daily IASI observations. We show that the relative position and the strength of the meteorological systems (Azores anticyclone and Middle Eastern depression) present over the Mediterranean are key factors in explaining both the variability and the anomalies of ozone in the lower troposphere in this region.
First estimates of global free-tropospheric NO2 abundances derived using a cloud-slicing technique applied to satellite observations from the Aura Ozone Monitoring Instrument (OMI)

We derive free-tropospheric NO2 volume mixing ratios (VMRs) by applying a cloud-slicing technique to data from the Ozone Monitoring Instrument (OMI) on the Aura satellite. In the cloud-slicing approach, the slope of the above-cloud NO2 column versus the cloud scene pressure is proportional to the NO2 VMR. In this work, we use a sample of nearby OMI pixel data from a single orbit for the linear fit. The OMI data include cloud scene pressures from the rotational-Raman algorithm and above-cloud NO2 vertical column density (VCD) (defined as the NO2 column from the cloud scene pressure to the top of the atmosphere) from a differential optical absorption spectroscopy (DOAS) algorithm. We compare OMI-derived NO2 VMRs with in situ aircraft profiles measured during the NASA Intercontinental Chemical Transport Experiment Phase B (INTEX-B) campaign in 2006. The agreement is generally within the estimated uncertainties when appropriate data screening is applied. We then derive a global seasonal climatology of free-tropospheric NO2 VMR in cloudy conditions. Enhanced NO2 in the free troposphere commonly appears near polluted urban locations where NO2 produced in the boundary layer may be transported vertically out of the boundary layer and then horizontally away from the source. Signatures of lightning NO2 are also shown throughout low and middle latitude regions in summer months. A profile analysis of our cloud-slicing data indicates signatures of lightning-generated NO2 in the upper troposphere. Comparison of the climatology with simulations from the global modeling initiative (GMI) for cloudy conditions (cloud optical depth > 10) shows similarities in the spatial patterns of continental pollution outflow. However, there are also some differences in the seasonal variation of free-tropospheric NO2 VMRs near highly populated regions and in areas affected by lightning-generated NOx.
Airborne characterization of smoke marker ratios from prescribed burning

A Particle-Into-Liquid Sampler – Total Organic Carbon (PILS-TOC) and fraction collector system was flown aboard a Twin Otter aircraft sampling prescribed burning emissions in South Carolina in November 2011 to obtain smoke marker measurements. The fraction collector provided 2 min time-integrated offline samples for carbohydrate (i.e., smoke markers levoglucosan, mannosan, and galactosan) analysis by high-performance anion-exchange chromatography with pulsed amperometric detection. Each fire location appeared to have a unique Δlevoglucosan/Δwater-soluble organic carbon (WSOC) ratio (RF01/RF02/RF03/RF05 = 0.163 ± 0.007 μg C μg−1 C, RF08 = 0.115 ± 0.011 μg C μg−1 C, RF09A = 0.072 ± 0.028 μg C μg−1 C, and RF09B = 0.042 ± 0.008 μg C μg−1 C, where RF means research flight). These ratios were comparable to those obtained from controlled laboratory burns and suggested that the emissions sampled during RF01/F02/RF03/RF05 were dominated by the burning of grasses, RF08 by leaves, RF09A by needles, and RF09B by marsh grasses. These findings were further supported by the Δgalactosan/Δlevoglucosan ratios (RF01/RF02/RF03/RF05 = 0.067 ± 0.004 μg μg−1, RF08 = 0.085 ± 0.009 μg μg−1, and RF09A = 0.101 ± 0.029 μg μg−1) obtained as well as by the ground-based fuel and filter sample analyses during RF01/RF02/RF03/RF05. Differences between Δpotassium/Δlevoglucosan ratios obtained for these prescribed fires vs. laboratory-scale measurements suggest that some laboratory burns may not accurately represent potassium emissions from prescribed burns. The Δlevoglucosan/ΔWSOC ratio had no clear dependence on smoke age or fire dynamics suggesting that this ratio is more dependent on the type of fuel being burned. Levoglucosan was stable over a timescale of at least 1.5 h and could be useful to help estimate the air quality impacts of biomass burning.
Mapping the physico-chemical properties of mineral dust in western Africa: mineralogical composition

In the last few years, several ground-based and airborne field campaigns have allowed the exploration of the properties and impacts of mineral dust in western Africa, one of the major emission and transport areas worldwide. In this paper, we explore the synthesis of these observations to provide a large-scale quantitative view of the mineralogical composition and its variability according to source region and time after transport. This work reveals that mineral dust in western Africa is a mixture of clays, quartz, iron and titanium oxides, representing at least 92% of the dust mass. Calcite ranged between 0.3 and 8.4% of the dust mass, depending on the origin. Our data do not show a systematic dependence of the dust mineralogical composition on origin; this is to be the case as, in most of the instances, the data represent the composition of the atmospheric burden after 1–2 days after emission, when air masses mix and give rise to a more uniform dust load. This has implications for the representation of the mineral dust composition in regional and global circulation models and in satellite retrievals. Iron oxides account for 58 ± 7% of the mass of elemental Fe and for between 2 and 5% of the dust mass. Most of them are composed of goethite, representing between 52 and 78% of the iron oxide mass. We estimate that titanium oxides account for 1–2% of the dust mass, depending on whether the dust is of Saharan or Sahelian origin. The mineralogical composition is a critical parameter for estimating the radiative and biogeochemical impact of mineral dust. The results regarding dust composition have been used to estimate the optical properties as well as the iron fractional solubility of Saharan and Sahelian dust. Data presented in this paper are provided in numerical form upon email request while they are being turned into a public database, the Dust-Mapped Archived Properties (DUST-MAP), which is an open repository for compositional data from other source regions in Africa and worldwide.
Airborne flux measurements of biogenic isoprene over California

Biogenic isoprene fluxes were measured onboard the CIRPAS Twin Otter aircraft as part of the California Airborne Biogenic volatile organic compound (BVOC) Emission Research in Natural Ecosystem Transects (CABERNET) campaign during June 2011. The airborne virtual disjunct eddy covariance (AvDEC) approach used measurements from a proton transfer reaction mass spectrometer (PTR–MS) and a wind radome probe to directly determine fluxes of isoprene over 7400 km of flight paths focusing on areas of California predicted to have the largest emissions. The fast Fourier transform (FFT) approach was used to calculate fluxes of isoprene over long transects of more than 15 km, most commonly between 50 and 150 km. The continuous wavelet transformation (CWT) approach was used over the same transects to also calculate instantaneous isoprene fluxes with localization of both frequency and time independent of non-stationarities. Fluxes were generally measured by flying consistently at 400 m ± 50 m (a.g.l.) altitude, and extrapolated to the surface according to the determined flux divergence determined in the racetrack-stacked profiles. The wavelet-derived surface fluxes of isoprene averaged to 2 km spatial resolution showed good correspondence to basal emission factor (BEF) land-cover data sets used to drive BVOC emission models. The surface flux of isoprene was close to zero over Central Valley crops and desert shrublands, but was very high (up to 15 mg m−2 h−1) above oak woodlands, with clear dependence of emissions on temperature and oak density. Isoprene concentrations of up to 8 ppb were observed at aircraft height on the hottest days and over the dominant source regions. Even though the isoprene emissions from agricultural crop regions, shrublands, and coniferous forests were extremely low, observations at the Walnut Grove tower south of Sacramento demonstrate that isoprene oxidation products from the high emitting regions in the surrounding oak woodlands accumulate at night in the residual layer above the valley and mix down into the valley in the morning. Thus, the isoprene emissions surrounding the valley have relevance for the regional photochemistry that is not immediately apparent solely from the direct emission flux distribution. This paper reports the first regional observations of fluxes from specific sources by eddy covariance from an aircraft which can finally constrain statewide isoprene emission inventories used for ozone simulations by state agencies. While previously there was no available means to constrain the biogenic models, our results provide a good understanding of what the major sources of isoprene are in California, their magnitude, and how they are distributed. This data set on isoprene fluxes will be particularly useful for evaluating potential model alternatives which will be dealt with in a separate paper to assess isoprene emission models and their driving variable data sets.
Optical, microphysical and compositional properties of the Eyjafjallajökull volcanic ash

Better characterization of the optical properties of aerosol particles are an essential step to improve atmospheric models and satellite remote sensing, reduce uncertainties in predicting particulate transport, and estimate aerosol forcing and climate change. Even natural aerosols such as mineral dust or particles from volcanic eruptions require better characterization in order to define the background conditions from which anthropogenic perturbations emerge. We present a detailed laboratorial study where the spectral optical properties of the ash from the April–May (2010) Eyjafjallajökull volcanic eruption were derived over a broad spectral range, from ultra-violet (UV) to near-infrared (NIR) wavelengths. Samples of the volcanic ash taken on the ground in the vicinity of the volcano were sieved, re-suspended, and collected on filters to separate particle sizes into fine and mixed (coarse and fine) modes. We derived the spectral mass absorption efficiency αabs [m2g−1] for fine and mixed modes particles in the wavelength range from 300 to 2500 nm from measurements of optical reflectance. We retrieved the imaginary part of the complex refractive index Im(m) from αabs, using Mie–Lorenz and T-matrix theories and considering the size distribution of particles obtained by scanning electron microscopy (SEM), and the grain density of the volcanic ash measured as ρ = 2.16 ± 0.13 g cm−3. Im(m) was found to vary from 0.001 to 0.005 in the measured wavelength range. The dependence of the retrieval on the shape considered for the particles were found to be small and within the uncertainties estimated in our calculation. Fine and mixed modes were also analyzed by X-ray fluorescence, exhibiting distinct elemental composition supporting the optical differences we found between the modes. This is a comprehensive and consistent characterization of spectral absorption and imaginary refractive index, density, size, shape and elemental composition of volcanic ash, which will help constrain assumptions of ash particles in models and remote sensing, thereby narrowing uncertainties in representing these particles both for short-term regional forecasts and long-term climate change.
Estimating regional greenhouse gas fluxes: an uncertainty analysis of planetary boundary layer techniques and bottom-up inventories

Quantification of regional greenhouse gas (GHG) fluxes is essential for establishing mitigation strategies and evaluating their effectiveness. Here, we used multiple top-down approaches and multiple trace gas observations at a tall tower to estimate regional-scale GHG fluxes and evaluate the GHG fluxes derived from bottom-up approaches. We first applied the eddy covariance, equilibrium, inverse modeling (CarbonTracker), and flux aggregation methods using 3 years of carbon dioxide (CO2) measurements on a 244 m tall tower in the upper Midwest, USA. We then applied the equilibrium method for estimating CH4 and N2O fluxes with 1-month high-frequency CH4 and N2O gradient measurements on the tall tower and 1-year concentration measurements on a nearby tall tower, and evaluated the uncertainties of this application. The results indicate that (1) the flux aggregation, eddy covariance, the equilibrium method, and the CarbonTracker product all gave similar seasonal patterns of the regional CO2 flux (105−106 km2, but that the equilibrium method underestimated the July CO2 flux by 52–69%. (2) The annual budget varied among these methods from −54 to −131 g C–CO2 m−2 yr−1, indicating a large uncertainty in the annual CO2 flux estimation. (3) The regional CH4 and N2O emissions according to a top-down method were at least 6 and 2 times higher than the emissions from a bottom-up inventory (Emission Database for Global Atmospheric Research), respectively. (4) The global warming potentials of the CH4 and N2O emissions were equal in magnitude to the cooling benefit of the regional CO2 uptake. The regional GHG budget, including both biological and anthropogenic origins, is estimated at 7 ± 160 g CO2 equivalent m−2 yr−1.
Surface gas pollutants in Lhasa, a highland city of Tibet – current levels and pollution implications

Through several years of development, the city of Lhasa has become one of the most populated and urbanized areas on the highest plateau in the world. In the process of urbanization, current and potential air quality issues have been gradually concerned. To investigate the current status of air pollution in Lhasa, various gas pollutants including NOx, CO, SO2, and O3, were continuously measured from June 2012 to May 2013 at an urban site (29.40° N, 91.08° E, 3650 m a.s.l.). The seasonal variations of primary gas pollutants exhibited a peak from November to January with a large variability. High mixing ratios of primary trace gases almost exclusively occurred under low wind speed and showed no distinct dependence on wind direction, implying local urban emissions to be predominant. A comparison of NO2, CO, and SO2 mixing ratios in summer between 1998 and 2012 indicated a significant increase in emissions of these gas pollutants and a change in their intercorrelations, as a result of a substantial growth in the demand of energy consumption using fossil fuels instead of previously widely used biomass. The pronounced diurnal double peaks of primary trace gases in all seasons suggested automobile exhaust to be a major emission source in Lhasa. The secondary gas pollutant O3 displayed an average diurnal cycle of a shallow flat peak for about 4–5 h in the afternoon and a minimum in the early morning. Nighttime O3 was sometimes completely consumed by the high level of NOx. Seasonally, the variations of O3 mixing ratios displayed a low valley in winter and a peak in spring. In autumn and winter, transport largely contributed to the observed O3 mixing ratios, given its dependence on wind speed and wind direction, while in spring and summer photochemistry played an important role. A more efficient buildup of O3 mixing ratios in the morning and a higher peak in the afternoon was found in summer 2012 than in 1998. An enhancement in O3 mixing ratios would be expected in the future and more attention should be given to O3 photochemistry in response to increasing precursor emissions in this area.