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A continuous spectral aerosol-droplet microphysics model

A two-dimensional (2-D) continuous spectral aerosol-droplet microphysics model is presented and implemented into the Weather Research and Forecasting (WRF) model for large-eddy simulations (LES) of warm clouds. Activation and regeneration of aerosols are treated explicitly in the calculation of condensation/evaporation. The model includes a 2-D spectrum that encompasses wet aerosol particles (i.e., haze droplets), cloud droplets, and drizzle droplets in a continuous and consistent manner and allows for the explicit tracking of aerosol size within cloud droplets due to collision-coalescence. The system of differential equations describing condensation/evaporation (i.e., mass conservation and energy conservation) is solved simultaneously within each grid cell. The model is demonstrated by simulating a marine stratocumulus deck for two different aerosol loadings (100 and 500 cm−3), and comparison with the more traditional microphysics modeling approaches (both 1-D bin and bulk schemes) is evaluated. The simulations suggest that in a 1-D bin microphysics scheme, without regeneration, too few particles are produced and hence the mode of the droplet size spectrum occurs at a larger size relative to the 2-D bin model results. Moreover, with regeneration, the 1-D scheme produces too many small droplets and thus shifts the mode toward smaller sizes. These large shifts in the droplet size distribution can potentially have significant effects on the efficiency of the collision-coalescence process, fall speeds, and ultimately precipitation.
A study on the aerosol optical properties over East Asia using a combination of CMAQ-simulated aerosol optical properties and remote-sensing data via a data assimilation technique

To more accurately estimate direct radiative forcing (DRF) by aerosols, and better investigate particulate pollution over East Asia, precise calculations of the optical properties of aerosols, such as aerosol optical depth (AOD), single scattering albedo (SSA) and aerosol extinction coefficient (σext), are of primary importance. The aerosol optical properties over East Asia were investigated in this study, based on US EPA Models-3/CMAQ v4.5.1 model simulations. The CMAQ model simulations in this study were improved in several ways compared to those in a previous study (Song et al., 2008). Although the details of the improvements were described in the manuscript, the following points should be emphasized: (1) two data assimilation techniques were employed for producing more accurate AOD products and meteorological fields over East Asia; (2) updated/upgraded emission inventories were used in the CMAQ model simulations with a fine grid resolution of 30 × 30 km2; and (3) the 4-D particulate composition calculated from the CMAQ model simulations was converted into 3-D or 4-D aerosol optical products, using the Malm and Hand (2007) algorithm with significant further modifications. The results from the CMAQ model simulations (without assimilation) showed great improvements compared to those from a previous study. For example, in terms of the regression coefficients (R), R values were increased from 0.48–0.68 (previous study) to 0.62–0.79 (this study). The monthly-averaged CMAQ-simulated single scattering albedo (SSA) also agreed well with the AERONET SSA, with the exceptions of the Hong Kong and Taipei sites, where the air qualities were strongly influenced by active biomass burning events from January to April. There were also excellent matches between the vertical profiles of the CMAQ-simulated σext and LIDAR-retrieved σext. It was also found that the contributions of (NH4)2SO4 during summer, NH4NO3 during winter, sea-salt particles during winter and dust particles during spring to the total AOD were large over East Asia. In particular, the largest contribution of NH4NO3 to the total AOD was found over East Asia during winter. Therefore, it was suggested that this contribution of NH4NO3 should not be neglected. In order to produce more accurate AOD products, the CMAQ-simulated AODs were further assimilated with the MODIS-retrieved AODs. Both of the assimilated and AERONET AODs were better correlated with each other than the CMAQ-simulated and AERONET AODs. The obvious benefits from this study would be that with these improved aerosol optical properties, the particulate pollution (e.g. AOD can be served as a proxy to PM2.5 or PM10) and DRF by aerosols over East Asia can be more satisfactorily investigated in future.
Continental-scale enrichment of atmospheric 14CO2 from the nuclear power industry: potential impact on the estimation of fossil fuel-derived CO2

The 14C-free fossil carbon added to atmospheric CO2 by combustion dilutes the atmospheric 14C/C ratio (Δ14C), potentially providing a means to verify fossil CO2 emissions calculated using economic inventories. However, sources of 14C from nuclear power generation and spent fuel reprocessing can counteract this dilution and may bias 14C/C-based estimates of fossil fuel-derived CO2 if these nuclear influences are not correctly accounted for. Previous studies have examined nuclear influences on local scales, but the potential for continental-scale influences on Δ14C has not yet been explored. We estimate annual 14C emissions from each nuclear site in the world and conduct an Eulerian transport modeling study to investigate the continental-scale, steady-state gradients of Δ14C caused by nuclear activities and fossil fuel combustion. Over large regions of Europe, North America and East Asia, nuclear enrichment may offset at least 20% of the fossil fuel dilution in Δ14C, corresponding to potential biases of more than −0.25 ppm in the CO2 attributed to fossil fuel emissions, larger than the bias from plant and soil respiration in some areas. Model grid cells including high 14C-release reactors or fuel reprocessing sites showed much larger nuclear enrichment, despite the coarse model resolution of 1.8°×1.8°. The recent growth of nuclear 14C emissions increased the potential nuclear bias over 1985–2005, suggesting that changing nuclear activities may complicate the use of Δ14C observations to identify trends in fossil fuel emissions. The magnitude of the potential nuclear bias is largely independent of the choice of reference station in the context of continental-scale Eulerian transport and inversion studies, but could potentially be reduced by an appropriate choice of reference station in the context of local-scale assessments.
Aerosol hygroscopicity and CCN activation kinetics in a boreal forest environment during the 2007 EUCAARI campaign

Measurements of size-resolved cloud condensation nuclei (CCN) concentrations, subsaturated hygroscopic growth, size distribution, and chemical composition were collected from March through May, 2007, in the remote Boreal forests of Hyytiälä, Finland, as part of the European Integrated project on Aerosol Cloud Climate and Air Quality Interactions (EUCAARI) campaign. Hygroscopicity parameter, κ, distributions were derived independently from Continuous Flow-Streamwise Thermal Gradient CCN Chamber (CFSTGC) and Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) measurements. CFSTGC-derived κ values for 40, 60, and 80 nm particles range mostly between 0.10 and 0.40 with an average characteristic of highly oxidized organics of 0.20 ± 0.10, indicating that organics play a dominant role for this environment. HTDMA-derived κ were generally 30% lower. Diurnal trends of κ show a minimum at sunrise and a maximum in the late afternoon; this trend covaries with inorganic mass fraction and the m/z 44 organic mass fraction given by a quadrupole aerosol mass spectrometer, further illustrating the importance of organics in aerosol hygroscopicity. The chemical dispersion inferred from the observed κ distributions indicates that while 60 and 80 nm dispersion increases around midday, 40 nm dispersion remains constant. Additionally, 80 nm particles show a markedly higher level of chemical dispersion than both 40 and 60 nm particles. An analysis of droplet activation kinetics for the sizes considered indicates that most of the CCN activate as rapidly as (NH4)2SO4 calibration aerosol.
Emission controls versus meteorological conditions in determining aerosol concentrations in Beijing during the 2008 Olympic Games

A series of emission control measures were undertaken in Beijing and the adjacent provinces in China during the 2008 Beijing Olympic Games on 8–24 August 2008. This provides a unique opportunity for investigating the effectiveness of emission controls on air pollution in Beijing. We conducted a series of numerical experiments over East Asia for the period of July to September 2008 using a coupled meteorology-chemistry model (WRF-Chem). Model can generally reproduce the observed variation of aerosol concentrations. Consistent with observations, modeled concentrations of aerosol species (sulfate, nitrate, ammonium, black carbon, organic carbon, total particulate matter) in Beijing were decreased by 30–50% during the Olympic period compared to the other periods in July and August in 2008 and the same period in 2007. Model results indicate that emission controls were effective in reducing the aerosol concentrations by comparing simulations with and without emission controls. In addition to emission controls, our analysis suggests that meteorological conditions (e.g. wind direction and precipitation) were also important in producing the low aerosol concentrations appearing during the Olympic period. Transport from the regions surrounding Beijing determined the daily variation of aerosol concentrations in Beijing. Based on the budget analysis, we suggest that to improve the air quality over Beijing, emission control strategy should focus on the regional scale instead of the local scale.
Long-term impacts of aerosols on precipitation and lightning over the Pearl River Delta megacity area in China

Seven-year measurements of precipitation, lightning flashes, and visibility from 2000 to 2006 have been analyzed in the Pearl River Delta (PRD) region, China, with a focus on the Guangzhou megacity area. Statistical analysis shows that the occurrence of heavy rainfall (>25 mm per day) and frequency of lightning strikes are reversely correlated to visibility during this period. To elucidate the effects of aerosols on cloud processes, precipitation, and lightning activity, a cloud resolving – Weather Research and Forecasting (CR-WRF) model with a two-moment bulk microphysical scheme is employed to simulate a mesoscale convective system occurring on 28 Match 2009 in the Guangzhou megacity area. The model predicted evolutions of composite radar reflectivity and accumulated precipitation are in agreement with measurements from S-band weather radars and automatic gauge stations. The calculated lightning potential index (LPI) exhibits temporal and spatial consistence with lightning flashes recorded by a local lightning detection network. Sensitivity experiments have been performed to reflect aerosol conditions representative of polluted and clean cases. The simulations suggest that precipitation and LPI are enhanced by about 16% and 50%, respectively, under the polluted aerosol condition. Our results suggest that elevated aerosol loading suppresses light and moderate precipitation (less than 25 mm per day), but enhances heavy precipitation. The responses of hydrometeors and latent heat release to different aerosol loadings reveal the physical mechanism for the precipitation and lightning enhancement in the Guangzhou megacity area, showing more efficient mixed phase processes and intensified convection under the polluted aerosol condition.
The 2005 Study of Organic Aerosols at Riverside (SOAR-1): instrumental intercomparisons and fine particle composition

Multiple state-of-the-art instruments sampled ambient aerosol in Riverside, California during the 2005 Study of Organic Aerosols at Riverside (SOAR) to investigate the chemical composition and potential sources of fine particles (PMf) in the inland region of Southern California. In this paper, we briefly summarize the spatial, meteorological and gas-phase conditions during SOAR-1 (15 July–15 August), provide detailed intercomparisons of high-resolution aerosol mass spectrometer (HR-AMS) measurements against complementary measurements, and report the average composition of PMf including the composition of the organic fraction measured by the HR-AMS. Daily meteorology and gas-phase species concentrations were highly consistent, displaying clear diurnal cycles and weekday/weekend contrast. HR-AMS measurements of non-refractory submicron (NR-PM1) mass are consistent and highly correlated with those from a filter dynamics measurement system tapered-element oscillating microbalance (TEOM), while the correlation between HR-AMS and heated TEOM measurements is lower due to loss of high volatility species including ammonium nitrate from the heated TEOM. Speciated HR-AMS measurements are also consistent with complementary measurements as well as with measurements from a collocated compact AMS while HR-AMS OC is similar to standard semi-continuous Sunset measurements within the combined uncertainties of both instruments. A correction intended to account for the loss of semi-volatile OC from the Sunset, however, yields measurements ~30% higher than either HR-AMS or standard Sunset measurements. On average, organic aerosol (OA) was the single largest component of PMf. OA composition was investigated using both elemental analysis and positive matrix factorization (PMF) of HR-AMS OA spectra. Oxygen is the main heteroatom during SOAR-1, with O/C exhibiting a diurnal minimum of 0.28 during the morning rush hour and maximum of 0.42 during the afternoon. O/C is broadly anti-correlated with H/C, while N/C and S/C (excluding organonitrate (ON) and organosulfate (OS) functionalities) are far lower than O/C at about 0.015 and ~0.001, respectively. When ON and OS estimates are included O/C, N/C, and S/C increase by factors of 1.21, 2, and 30, respectively, while H/C changes are insignificant. The increase in these ratios implies that ON accounts for ~1/2 of the organic nitrogen while OS dominate organic sulfur at this location. Accounting for the estimated ON and OS also improves the agreement between anions and cations measured by HR-AMS by ~8%, while amines have only a very small impact (1%) on this balance. Finally, a number of primary and secondary OA components were resolved by PMF. Among these a hydrocarbon-like OA and two minor, local OA components, one of which was associated with amines, were attributed to primary emissions and contributed a minor fraction (~20%) of OA mass. The remaining OA mass was attributed to a number of secondary oxidized OA (OOA) components including the previously-identified low-volatility and semi-volatile OOA components. In addition, we also report for the first time the presence of two additional OOA components.
A Lagrangian view of convective sources for transport of air across the Tropical Tropopause Layer: distribution, times and the radiative influence of clouds

The tropical tropopause layer (TTL) is a key region controlling transport between the troposphere and the stratosphere. The efficiency of transport across the TTL depends on the continuous interaction between the large-scale advection and the small-scale intermittent convection that reaches the Level of Zero radiative Heating (LZH). The wide range of scales involved presents a significant challenge to determine the sources of convection and quantify transport across the TTL. Here, we use a simple Lagrangian model, termed TTL detrainment model, that combines a large ensemble of 200-day back trajectory calculations with high-resolution fields of brightness temperatures (provided by the CLAUS dataset) in order to determine the ensemble of trajectories that are detrained from convective sources. The trajectories are calculated using the ECMWF ERA-Interim winds and radiative heating rates, and in order to establish the radiative influence of clouds, the latter rates are derived both under all-sky and clear-sky conditions. We show that most trajectories are detrained near the mean LZH with the horizontal distributions of convective sources being highly-localized, even within the space defined by deep convection. As well as modifying the degree of source localization, the radiative heating from clouds facilitates the rapid upwelling of air across the TTL. However, large-scale motion near the fluctuating LZH can lead a significant proportion of trajectories to alternating clear-sky and cloudy regions, thus generating a large dispersion in the vertical transport times. The distributions of vertical transport times are wide and skewed and are largely insensitive to a bias of about ±1 km (∓5 K) in the altitude of cloud top heights (the main sensitivity appearing in the times to escape the immediate neighbourhood of the LZH) while some seasonal and regional transport characteristics are apparent for times up to 60 days. The strong horizontal mixing that characterizes the TTL ensures that most air of convective origin is well-mixed within the tropical and eventually within the extra-tropical lower-stratosphere.
Sources and atmospheric processing of organic aerosol in the Mediterranean: insights from aerosol mass spectrometer factor analysis

Atmospheric particles were measured in the late winter (25 February–26 March 2009) at a remote coastal site on the island of Crete, Greece during the Finokalia Aerosol Measurement Experiment-2009. A quadrupole aerosol mass spectrometer (Q-AMS) was employed to quantify the size-resolved chemical composition of non-refractory submicron aerosol, and a thermodenuder was used to analyze the organic aerosol (OA) volatility. Complementary measurements included particle size distributions from a scanning mobility particle sizer, inorganic and organic particle composition from filter analysis, air ion concentrations, O3, NOx and NOy concentrations, and meteorological measurements. Factor analysis was performed on the OA mass spectra, and the variability in OA composition could best be explained with three OA components. The oxygenated organic aerosol (OOA) was similar in composition and volatility to the summertime OA previously measured at this site and may represent an effective endpoint in particle-phase oxidation of organics. The two other OA components, one associated with amines (Amine-OA) and the other probably associated with the burning of olive branches (OB-OA), had very low volatility but were less oxygenated. Hydrocarbon-like organic aerosol (HOA) was not detected. The absence of OB-OA and Amine-OA in the summer data may be due to lower emissions and/or photochemical conversion of these components to OOA.
Sources of carbonaceous aerosols and deposited black carbon in the Arctic in winter-spring: implications for radiative forcing

We use a global chemical transport model (GEOS-Chem CTM) to interpret observations of black carbon (BC) and organic aerosol (OA) from the NASA ARCTAS aircraft campaign over the North American Arctic in April 2008, as well as longer-term records in surface air and in snow (2007–2009). BC emission inventories for North America, Europe, and Asia in the model are tested by comparison with surface air observations over these source regions. Russian open fires were the dominant source of OA in the Arctic troposphere during ARCTAS but we find that BC was of prevailingly anthropogenic (fossil fuel and biofuel) origin, particularly in surface air. This source attribution is confirmed by correlation of BC and OA with acetonitrile and sulfate in the model and in the observations. Asian emissions are the main anthropogenic source of BC in the free troposphere but European, Russian and North American sources are also important in surface air. Russian anthropogenic emissions appear to dominate the source of BC in Arctic surface air in winter. Model simulations for 2007–2009 (to account for interannual variability of fires) show much higher BC snow content in the Eurasian than the North American Arctic, consistent with the limited observations. We find that anthropogenic sources contribute 90% of BC deposited to Arctic snow in January-March and 60% in April–May 2007–2009. The mean decrease in Arctic snow albedo from BC deposition is estimated to be 0.6% in spring, resulting in a regional surface radiative forcing consistent with previous estimates.