Synergistic HNO3-H2SO4-NH3 upper tropospheric particle formation

New particle formation in the upper free troposphere is a major global source of cloud condensation nuclei (CCN)^1-4. However, the precursor vapours that drive the process are not well understood. With experiments performed under upper tropospheric conditions in the CERN CLOUD chamber, we show that nitric acid, sulfuric acid and ammonia form particles synergistically, at rates that are orders of magnitude faster than those from any two of the three components. The importance of this mechanism depends on the availability of ammonia, which was previously thought to be efficiently scavenged by cloud droplets during convection. However, surprisingly high concentrations of ammonia and ammonium nitrate have recently been observed in the upper troposphere over the Asian monsoon region^5,6. Once particles have formed, co-condensation of ammonia and abundant nitric acid alone is sufficient to drive rapid growth to CCN sizes with only trace sulfate. Moreover, our measurements show that these CCN are also highly efficient ice nucleating particles-comparable to desert dust. Our model simulations confirm that ammonia is efficiently convected aloft during the Asian monsoon, driving rapid, multi-acid HNO₃-H₂SO₄-NH₃ nucleation in the upper troposphere and producing ice nucleating particles that spread across the mid-latitude Northern Hemisphere.

Aircraft measurement campaign on summer cloud microphysical properties over the Tibetan Plateau

We reported the first aircraft campaign on summer cloud microphysical properties conducted in July of 2014 over the Tibetan Plateau during the third Tibetan Plateau Atmospheric Sciences Experiment (TIPEX-III), and demonstrated that the summer clouds over the Tibetan Plateau were primarily characterized as mixed-phase cumulus clouds induced by strong solar radiation heating. Moreover, the characteristic number concentration of cloud droplets (2~50 μm in diameter) in developing cumuli was around 10 cm⁻³, which was about 1~2 orders of magnitudes lower than other continent and ocean regions, and that for large drops (>50 μm in diameter) was around 10⁻³ cm⁻³, which was also lower than other regions. The droplet spectrum distributions (DSDs) of cloud drops were much wider than other regions, indicating that the cumulus clouds over the plateau could form precipitation easier than that in other regions. Ice microphysics was characterized as very active glaciation and riming processes with high supercooled water content, which caused the formation of high concentration of graupel particles in clouds. The findings of this study suggest that these unique cloud microphysical properties formed by the high topography and clean environment of the Tibetan Plateau could induce higher precipitation efficiency when airflow passed over the plateau, so that the plateau could act as a regional “water tower”.

Substantial Seasonal Contribution of Observed Biogenic Sulfate Particles to Cloud Condensation Nuclei

Biogenic sources contribute to cloud condensation nuclei (CCN) in the clean marine atmosphere, but few measurements exist to constrain climate model simulations of their importance. The chemical composition of individual atmospheric aerosol particles showed two types of sulfate-containing particles in clean marine air masses in addition to mass-based Estimated Salt particles. Both types of sulfate particles lack combustion tracers and correlate, for some conditions, to atmospheric or seawater dimethyl sulfide (DMS) concentrations, which means their source was largely biogenic. The first type is identified as New Sulfate because their large sulfate mass fraction (63% sulfate) and association with entrainment conditions means they could have formed by nucleation in the free troposphere. The second type is Added Sulfate particles (38% sulfate), because they are preexisting particles onto which additional sulfate condensed. New Sulfate particles accounted for 31% (7 cm-3) and 33% (36 cm-3) CCN at 0.1% supersaturation in late-autumn and late-spring, respectively, whereas sea spray provided 55% (13 cm-3) in late-autumn but only 4% (4 cm-3) in late-spring. Our results show a clear seasonal difference in the marine CCN budget, which illustrates how important phytoplankton-produced DMS emissions are for CCN in the North Atlantic.

Polarization lidar for atmospheric monitoring

Aerosol plays an important role in global climate and weather changes. Polarization lidar captures parallel and perpendicular signals from atmosphere to research aerosols. The lidar system we used has three emission wavelengths and could obtain the atmospheric aerosol extinction coefficient, backscattering coefficient and depolarization ratio. In this paper, the design of the lidar is described. The methods of data acquisition and inversion are given. Some recent results are presented.

Assessment of the clear-sky bias issue using continuous PM10 data from two AERONET sites in Korea

A bias in clear-sky conditions that will be involved in estimating particulate matter (PM) concentration from aerosol optical depth (AOD) was examined using PM₁₀ from two Aerosol Robotic Network sites in Korea. The study periods were between 2004 and 2007 at Anmyon and between 2003 and 2011 at Gosan, when both PM₁₀ and AOD were available. Mean PM₁₀ when AOD was available (PM_AOD) was higher than that from all PM₁₀ data (PM_all) by 5.1 and 9.9μg/m³ at Anmyon and Gosan, which accounted for 11% and 26% of PM_all, respectively. Because of a difference between mean PM₁₀ under daytime clear-sky conditions (PM_clear) and PM_AOD, the variations in ΔPM₁₀, the difference of PM_all from PM_clear rather than from PM_AOD, were investigated. Although monthly variations in ΔPM₁₀ at the two sites were different, they were positively correlated to those in ΔT, similarly defined as ΔPM₁₀ except for temperature, at both sites. ΔPM₁₀ at Anmyon decreased to a negative value in January due to an influence of the Siberian continental high-pressure system while ΔPM₁₀ at Gosan was high in winter due to an effect of photochemical production at higher temperatures than at Anmyon.

Winter time chemical characteristics of aerosols over the Bay of Bengal: continental influence

As part of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) conducted under the Geosphere Biosphere Programme of Indian Space Research Organisation, ship-based aerosol sampling was carried out over the marine environment of Bay of Bengal (BoB) during the northern winter months of December 2008 to January 2009. About 101 aerosol samples were collected, covering the region from 3.4° to 21° N latitude and 76° to 98° E longitude-the largest area covered-including the south east (SE) BoB for the first time. These samples were subjected to gravimetric and chemical analysis and the total aerosol loading as well the mass concentration of the ionic species namely F(-), Cl(-), Br(-), NO2 (-), NO3 (-), PO4 (2-), SO4 (2-), NH4 (+), etc. and the metallic species, Na, Mg, Ca, K, Al, Fe, Mn, Zn, and Pb were estimated for each sample. Based on the spatial distribution of individual chemical species, the air flow pattern, and airmass back trajectory analysis, the source characteristics of aerosols for different regions of BoB were identified. Significant level of continental pollution was noticed over BoB during winter. While transport of pollution from Indo-Gangetic Plain (IGP) contributed to aerosols over north BoB, those over SE BoB were influenced by SE Asia. A quantitative study on the wind-induced production of sea salt aerosols and a case study on the species dependent effect of rainfall are also presented in this paper.

Ensemble-based simultaneous emission estimates and improved forecast of radioactive pollution from nuclear power plant accidents: application to ETEX tracer experiment

The accidental release of radioactive materials from nuclear power plant leads to radioactive pollution. We apply an augmented ensemble Kalman filter (EnKF) with a chemical transport model to jointly estimate the emissions of Perfluoromethylcyclohexane (PMCH), a tracer substitute for radionuclides, from a point source during the European Tracer Experiment, and to improve the forecast of its dispersion downwind. We perturb wind fields to account for meteorological uncertainties. We expand the state vector of PMCH concentrations through continuously adding an a priori emission rate for each succeeding assimilation cycle. We adopt a time-correlated red noise to simulate the temporal emission fluctuation. The improved EnKF system rapidly updates (and reduces) the excessively large initial first-guess emissions, thereby significantly improves subsequent forecasts (r = 0.83, p < 0.001). It retrieves 94% of the total PMCH released and substantially reduces transport error (>80% average reduction of the normalized mean square error).

Interactions between biomass-burning aerosols and clouds over Southeast Asia: current status, challenges, and perspectives

The interactions between aerosols, clouds, and precipitation remain among the largest sources of uncertainty in the Earth's energy budget. Biomass-burning aerosols are a key feature of the global aerosol system, with significant annually-repeating fires in several parts of the world, including Southeast Asia (SEA). SEA in particular provides a "natural laboratory" for these studies, as smoke travels from source regions downwind in which it is coupled to persistent stratocumulus decks. However, SEA has been under-exploited for these studies. This review summarizes previous related field campaigns in SEA, with a focus on the ongoing Seven South East Asian Studies (7-SEAS) and results from the most recent BASELInE deployment. Progress from remote sensing and modeling studies, along with the challenges faced for these studies, are also discussed. We suggest that improvements to our knowledge of these aerosol/cloud effects require the synergistic use of field measurements with remote sensing and modeling tools.

Temporal variation of aerosol optical depth and associated shortwave radiative forcing over a coastal site along the west coast of India

Optical characterization of aerosol was performed by assessing the columnar aerosol optical depth (AOD) and angstrom wavelength exponent (α) using data from the Microtops II Sunphotometer. The data were collected on cloud free days over Goa, a coastal site along the west coast of India, from January to December 2008. Along with the composite aerosol, the black carbon (BC) mass concentration from the Aethalometer was also analyzed. The AOD0.500 μm and angstrom wavelength exponent (α) were in the range of 0.26 to 0.7 and 0.52 to 1.33, respectively, indicative of a significant seasonal shift in aerosol characteristics during the study period. The monthly mean AOD0.500 μm exhibited a bi-modal distribution, with a primary peak in April (0.7) and a secondary peak in October (0.54), whereas the minimum of 0.26 was observed in May. The monthly mean BC mass concentration varied between 0.31 μg/m(3) and 4.5 μg/m(3), and the single scattering albedo (SSA), estimated using the OPAC model, ranged from 0.87 to 0.97. Modeled aerosol optical properties were used to estimate the direct aerosol shortwave radiative forcing (DASRF) in the wavelength range 0.25 μm4.0 μm. The monthly mean forcing at the surface, at the top of the atmosphere (TOA) and in the atmosphere varied between -14.1 Wm(-2) and -35.6 Wm(-2), -6.7 Wm(-2) and -13.4 Wm(-2) and 5.5 Wm(-2) to 22.5 Wm(-2), respectively. These results indicate that the annual SSA cycle in the atmosphere is regulated by BC (absorbing aerosol), resulting in a positive forcing; however, the surface forcing was governed by the natural aerosol scattering, which yielded a negative forcing. These two conditions neutralized, resulting in a negative forcing at the TOA that remains nearly constant throughout the year.

Long-range transport of biomass burning emissions based on organic molecular markers and carbonaceous thermal distribution

Semi-continuous organic carbon (OC), elemental carbon (EC), and organic molecular markers were analyzed using the thermal optical transmittance method at the Gosan supersite (on Jeju Island, Korea), which has been widely used as a regional background site for East Asia. The Carbonaceous Thermal Distribution (CTD) method, which can provide detailed carbon signature characteristics relative to analytical temperature, was used to improve the carbon fractionation of the analytical method. Ground-based measurements were conducted from October 25 to November 5, 2010. During the sampling period, one high OC concentration event and two characteristic periods were observed. Considering the thermal distribution patterns, the relationship between the EC and black carbon (BC) by optical measurements, the backward trajectories, the aerosol optical thickness, the PM10 concentrations from the 316 PM-network sites that were operated by the Ministry of Environment in Korea, and the organic molecular markers, such as levoglucosan, PAHs, and organic acids, we concluded that the event was influenced by long-range transport from biomass burning emissions. This study discusses the CTD analysis with organic molecular marker concentrations, extracts and interprets additional carbon fractions from a semi-continuous data set, and provides knowledge regarding the origin of carbon sources and their behaviors.

North-south cross sections of the vertical aerosol distribution over the Atlantic Ocean from multiwavelength Raman/polarization lidar during Polarstern cruises

Shipborne aerosol lidar observations were performed aboard the research vessel Polarstern in 2009 and 2010 during three north-south cruises from about 50°N to 50°S. The aerosol data set provides an excellent opportunity to characterize and contrast the vertical aerosol distribution over the Atlantic Ocean in the polluted northern and relatively clean southern hemisphere. Three case studies, an observed pure Saharan dust plume, a Patagonian dust plume east of South America, and a case of a mixed dust/smoke plume west of Central Africa are exemplarily shown and discussed by means of their optical properties. The meridional transatlantic cruises were used to determine the latitudinal cross section of the aerosol optical thickness (AOT). Profiles of particle backscatter and extinction coefficients are presented as mean profiles for latitudinal belts to contrast northern- and southern-hemispheric aerosol loads and optical effects. Results of lidar observations at Punta Arenas (53°S), Chile, and Stellenbosch (34°S), South Africa, are shown and confirm the lower frequency of occurrence of free-tropospheric aerosol in the southern hemisphere than in the northern hemisphere. The maximum latitudinal mean AOT of 0.27 was found in the northern tropics (0- 15°N) in the Saharan outflow region. Marine AOT is typically 0.05 ± 0.03. Particle optical properties are presented separately for the marine boundary layer and the free troposphere. Concerning the contrast between the anthropogenically influenced midlatitudinal aerosol conditions in the 30- 60°N belt and the respective belt in the southern hemisphere over the remote Atlantic, it is found that the AOT and extinction coefficients for the vertical column from 0-5km (total aerosol column) and 1-5km height (lofted aerosol above the marine boundary layer) are a factor of 1.6 and 2 higher at northern midlatitudes than at respective southern midlatitudes, and a factor of 2.5 higher than at the clean marine southern-hemispheric site of Punta Arenas. The strong contrast is confined to the lowermost 3km of the atmosphere.

The effect of smoke, dust, and pollution aerosol on shallow cloud development over the Atlantic Ocean

Clouds developing in a polluted environment tend to have more numerous but smaller droplets. This property may lead to suppression of precipitation and longer cloud lifetime. Absorption of incoming solar radiation by aerosols, however, can reduce the cloud cover. The net aerosol effect on clouds is currently the largest uncertainty in evaluating climate forcing. Using large statistics of 1-km resolution MODIS (Moderate Resolution Imaging Spectroradiometer) satellite data, we study the aerosol effect on shallow water clouds, separately in four regions of the Atlantic Ocean, for June through August 2002: marine aerosol (30 degrees S-20 degrees S), smoke (20 degrees S-5 degrees N), mineral dust (5 degrees N-25 degrees N), and pollution aerosols (30 degrees N- 60 degrees N). All four aerosol types affect the cloud droplet size. We also find that the coverage of shallow clouds increases in all of the cases by 0.2-0.4 from clean to polluted, smoky, or dusty conditions. Covariability analysis with meteorological parameters associates most of this change to aerosol, for each of the four regions and 3 months studied. In our opinion, there is low probability that the net aerosol effect can be explained by coincidental, unresolved, changes in meteorological conditions that also accumulate aerosol, or errors in the data, although further in situ measurements and model developments are needed to fully understand the processes. The radiative effect at the top of the atmosphere incurred by the aerosol effect on the shallow clouds and solar radiation is -11 +/- 3 W/m2 for the 3 months studied; 2/3 of it is due to the aerosol-induced cloud changes, and 1/3 is due to aerosol direct radiative effect.

The oxidative capacity of the troposphere: Coupling of field measurements of OH and a global chemistry transport model

A combination of in situ, ground-based observations of marine boundary layer OH concentrations performed by laser-induced fluorescence at Mace Head, Ireland and Cape Grim, Tasmania, and a global chemistry-transport model (GEOS-CHEM) are used to obtain an estimate of the mean concentration of OH in the global troposphere. The model OH field is constrained to the geographically sparse, observed OH concentration averaged over the duration of the measurement campaigns to remove diurnal and synoptic variability. The mean northern and southern hemispheric OH concentrations obtained are 0.91 x 10(6) cm(-3) and 1.03 x 10(6) cm(-3) respectively, consistent with values determined from methyl chloroform observations. The observational OH dataset is heavily biased towards mid-latitude summer and autumn observations in the northern hemisphere, while the global oxidising capacity is dominated by the tropics which is observed extremely sparsely; the implications of these geographical distributions are discussed.

Ground based radon-222 observations and their application to atmospheric studies

The aim of this paper is to review recent trends in the application of ground based radon observations to atmospheric research. In spite of over four decades of atmospheric radon monitoring, only in the past decade has the potential of this passive tracer been realised through a series of atmospheric model evaluation studies. Firstly, the key operational requirements for baseline radon detectors are briefly discussed, including lower limit of detection and response time. Then, current radon-related benchmarks for the evaluation of regional and global models are reviewed, with particular consideration given to the implications of data availability, resolution, site location and model spatial/temporal resolution. An 8-year subset of radon observations from the Cape Grim Baseline Air Pollution Station is used to suggest new benchmarks that exploit long-term data sets. Lastly an overview is presented of a technique that uses radon to estimate regional fluxes of climatically sensitive gases, with specific examples for CO2, CH4 and N2O.