Aerosol Species Research Articles

Large-Scale Modeling of Absorbing Aerosols and Their Semi-Direct Effects

Radiative effects of absorbing black carbon and mineral dust aerosols are estimated from global aerosol climate model simulations with fixed sea surface temperatures as a boundary condition. Semi-direct effects are approximated as the residual between the total direct radiative effect and the instantaneous direct radiative effect of the simulated absorbing aerosol species. No distinction is made for aerosols from natural and anthropogenic sources. Results for global average are highly uncertain due to high model variability, but consistent with previous estimates. The global average results for black carbon aerosol semi-direct effects are small due to cancellation of regionally positive or negative effects, and may be positive or negative overall, depending on the model setup. The presence of mineral dust aerosol above dark surfaces and below a layer containing black carbon aerosol may enhance the reflectivity and act to enhance the positive radiative effect of black carbon aerosol. When mineral dust is absent the semi-direct effect at the top-of-atmosphere of black carbon aerosol from both anthropogenic and natural sources is −0.03 Wm−2, while averaging to +0.09 Wm−2 if dust is included.

SALSA2.0: The sectional aerosol module of the aerosol–chemistry–climate model ECHAM6.3.0-HAM2.3-MOZ1.0

Abstract. In this paper, we present the implementation and evaluation of the aerosol microphysics module SALSA2.0 in the framework of the aerosol–chemistry–climate model ECHAM-HAMMOZ. It is an alternative microphysics module to the default modal microphysics scheme M7 in ECHAM-HAMMOZ. The SALSA2.0 implementation within ECHAM-HAMMOZ is evaluated against observations of aerosol optical properties, aerosol mass, and size distributions, comparing also to the skill of the M7 implementation. The largest differences between the implementation of SALSA2.0 and M7 are in the methods used for calculating microphysical processes, i.e., nucleation, condensation, coagulation, and hydration. These differences in the microphysics are reflected in the results so that the largest differences between SALSA2.0 and M7 are evident over regions where the aerosol size distribution is heavily modified by the microphysical processing of aerosol particles. Such regions are, for example, highly polluted regions and regions strongly affected by biomass burning. In addition, in a simulation of the 1991 Mt. Pinatubo eruption in which a stratospheric sulfate plume was formed, the global burden and the effective radii of the stratospheric aerosol are very different in SALSA2.0 and M7. While SALSA2.0 was able to reproduce the observed time evolution of the global burden of sulfate and the effective radii of stratospheric aerosol, M7 strongly overestimates the removal of coarse stratospheric particles and thus underestimates the effective radius of stratospheric aerosol. As the mode widths of M7 have been optimized for the troposphere and were not designed to represent stratospheric aerosol, the ability of M7 to simulate the volcano plume was improved by modifying the mode widths, decreasing the standard deviations of the accumulation and coarse modes from 1.59 and 2.0, respectively, to 1.2 similar to what was observed after the Mt. Pinatubo eruption. Overall, SALSA2.0 shows promise in improving the aerosol description of ECHAM-HAMMOZ and can be further improved by implementing methods for aerosol processes that are more suitable for the sectional method, e.g., size-dependent emissions for aerosol species and size-resolved wet deposition.

Wintertime study on bulk composition and stable carbon isotope analysis of ambient aerosols from North India

Ambient aerosols (PM2.5; particulate matter with aerodynamic diameter ≤ 2.5 µm) collected during wintertime from Kanpur (26.30 °N, 80.14 °E; in Indo-Gangetic Plain: IGP) in December 2014 were studied for their chemical composition [organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), major ions] and stable carbon isotope composition (δ13C) of total carbon (TC). Several source samples have also been strategically collected and characterized with δ13C value. Air-mass back trajectories arriving at the receptor site during the study period exhibited predominant influence of north-westerly winds. PM2.5 mass in ambient samples averaged at 240 ± 72 μg m−3 and total carbonaceous aerosols (TCA) concentration averaged at 91 ± 25 μg m−3 in these samples. The OC and EC concentrations contributed nearly 22% and 2%, respectively to the ambient PM2.5 mass. A significant variability in OC/EC ratio (8.7‒14.8; Avg. ± SD: 12 ± 2) and WSOC/OC ratio (0.350.51; 0.43 ± 0.05) has been recorded during the winter season in IGP. The ambient concentrations of secondary inorganic aerosol species (SIA) viz. SO42-, NO3- and NH4+ were 23.5 ± 8.7, 10.5 ± 6.0 and 12.8 ± 4.9 μg m−3, respectively. The δ13C of TC in ambient aerosols varied from −24.5 to −26.2‰. Large scatter between δ13C and TC content in aerosols indicated signature from multiple sources of carbonaceous aerosols due to mixing of air-masses. Future studies with large δ13C data population in conjunction with bulk composition would be required to better characterize the carbonaceous aerosols over the Indian region.

The Havemann-Taylor Fast Radiative Transfer Code (HT-FRTC): A multipurpose code based on principal components

The HT-FRTC can be used across the electromagnetic spectrum from the microwave through to the ultraviolet to calculate transmittance, radiance and flux spectra, which are represented by their principal components. The code uses monochromatic calculations at a small number of frequencies, which are selected by k-means clustering, to predict the principal component scores. It has been found that kernel regression yields more robust predictions than a linear regression. The principal components cover the spectrum at a very high spectral resolution, similar to that of conventional line-by-line models such that the individual spectral lines are resolved. This approach allows very fast line-by-line-like, hyperspectral and broadband simulations for satellite-based, airborne and ground-based sensors. The principal components are derived during a code training phase from monochromatic simulations for a diverse set of atmospheres and surfaces. They are sensor independent and no extra training is required for additional sensors. The HT-FRTC has been trained with all of the trace gases in the HIgh-resolution TRANsmission molecular absorption database (HITRAN) and on a large variety of surface emissivity/reflectance spectra. It can be run for any Lambertian or specular surface. The HT-FRTC is a plane-parallel one-dimensional model but some effects of the Earth’s sphericity are accounted for. Solar, lunar and cosmic microwave background sources have been included. Scattering by frozen and liquid cloud, precipitation particles and twenty different aerosol species has been included, as well as Rayleigh scattering which is significant in the short-wave. The scattering phase function can be fully accounted for by an integrated monochromatic version of the Edwards-Slingo spherical harmonics radiation code or approximately by a modification to the extinction (Chou scaling).

Size-resolved characteristics of inorganic ionic species in atmospheric aerosols at a regional background site on the South African Highveld

Aerosols consist of organic and inorganic species, and the composition and concentration of these species depends on their sources, chemical transformation and sinks. In this study an assessment of major inorganic ions determined in three aerosol particle size ranges collected for 1 year at Welgegund in South Africa was conducted. SO42− and ammonium (NH4+) dominated the PM1 size fraction, while SO42− and nitrate (NO3) dominated the PM1–2.5 and PM2.5–10 size fractions. SO42− had the highest contribution in the two smaller size fractions, while NO3− had the highest contribution in the PM2.5–10 size fraction. SO42− and NO3− levels were attributed to the impacts of aged air masses passing over major anthropogenic source regions. Comparison of inorganic ion concentrations to levels thereof within a source region influencing Welgegund, indicated higher levels of most species within the source region. However, the comparative ratio of SO42− was significantly lower due to SO42− being formed distant from SO2 emissions and submicron SO42− having longer atmospheric residencies. The PM at Welgegund was determined to be acidic, mainly due to high concentrations of SO42−. PM1 and PM1–2.5 fractions revealed a seasonal pattern, with higher inorganic ion concentrations measured from May to September. Higher concentrations were attributed to decreased wet removal, more pronounced inversion layers trapping pollutants, and increases in household combustion and wild fires during winter. Back trajectory analysis also revealed higher concentrations of inorganic ionic species corresponding to air mass movements over anthropogenic source regions.

Temporal variation of oxidative potential of water soluble components of ambient PM2.5 measured by dithiothreitol (DTT) assay

The exposure to ambient fine particulate matter (PM2.5) can induce oxidative stress, contributing to global burden of diseases. The evaluation of the oxidative potential (OP) of PM2.5 is thus critical for the health risk assessment. We collected ambient PM2.5 samples in Hangzhou city, China for four consecutive quarters in the year 2017 and investigated the oxidation property of PM2.5 components by the dithiothreitol (DTT) assay. The annual mean of ambient PM2.5 mass concentrations in 2017 was 63.05 μg m−3 (median: 57.34, range: 6.67–214.33 μg m−3) with the significant seasonal variations ranking as winter > spring > summer > autumn. Secondary inorganic aerosol (SIA) species including SO42−, NO3− and NH4+ totally account for >50% of PM2.5 mass. The annual mean volume-normalized DTT activity (DTTv) showed a relatively high value of 0.62 nmol/min/m3 (median: 0.62, range: 0.11–1.66 nmol/min/m3) and DTTv of four seasons was roughly at the same level, indicating a high annual exposure level of ambient PM2.5. SIA species were correlated well with the corresponding DTTv and showed significant diurnal variations with strong or moderate correlations at day and weak correlations at night, suggesting strong secondary formation in daytime with contribution to the particulate OP. The annual mean mass-normalized DTT activity (DTTm) had a relatively low value of 6.39 pmol/min/μg (median: 5.63, range: 1.99–22.70 pmol/min/μg), indicating low intrinsic oxidative toxicity. The DTTm of four seasons ranked as autumn > winter > spring > summer, indicating seasonal variations of the DTT-active components. The PM2.5 mass concentration is more related to exposure levels than intrinsic properties of components, while OP is determined by the components rather than PM2.5 mass concentration. Our results provide an insight into reactive oxygen species-induced health risk of PM2.5 exposure and decision for subsequent emission control.

How Long should the MISR Record Be when Evaluating Aerosol Optical Depth Climatology in Climate Models?

This study used the nearly continuous 17-year observation record from the Multi- angle Imaging SpectroRadiometer (MISR) instrument on the National Aeronautics and Space Administration (NASA) Terra Earth Observing System satellite to determine which temporal subsets are long enough to define statistically stable speciated aerosol optical depth (AOD) climatologies (i.e., AOD by particle types) for purposes of climate model evaluation. A random subsampling of seasonally averaged total and speciated AOD retrievals was performed to quantitatively assess the statistical stability in the climatology, represented by the minimum record length required for the standard deviation of the subsampled mean AODs to be less than a certain threshold. Our results indicate that the multi-year mean speciated AOD from MISR is stable on a global scale; however, there is substantial regional variability in the assessed stability. This implies that in some regions, even 17 years may not provide a long enough sample to define regional mean total and speciated AOD climatologies. We further investigated the agreement between the statistical stability of total AOD retrievals from MISR and the Moderate Resolution Imaging Spectroradiometer (MODIS), also on the NASA Terra satellite. The difference in the minimum record lengths between MISR and MODIS climatologies of total AOD is less than three years for most of the globe, with the exception of certain regions. Finally, we compared the seasonal cycles in the MISR total and speciated AODs with those simulated by three global chemistry transport models in the regions of climatologically stable speciated AODs. We found that only one model reproduced the observed seasonal cycles of the total and non-absorbing AODs over East China, but the seasonal cycles in total and dust AODs in all models are similar to those from MISR in Western Africa. This work provides a new method for considering the statistical stability of satellite-derived climatologies and illustrates the value of MISR’s speciated AOD data record for evaluating aerosols in global models.

Global relevance of marine organic aerosol as ice nucleating particles

Abstract. Ice nucleating particles (INPs) increase the temperature at which supercooled droplets start to freeze. They are therefore of particular interest in mixed-phase cloud temperature regimes, where supercooled liquid droplets can persist for extended periods of time in the absence of INPs. When INPs are introduced to such an environment, the cloud can quickly glaciate following ice multiplication processes and the Wegener–Bergeron–Findeisen (WBF) process. The WBF process can also cause the ice to grow to precipitation size and precipitate out. All of these processes alter the radiative properties. Despite their potential influence on climate, the ice nucleation ability and importance of different aerosol species is still not well understood and is a field of active research. In this study, we use the aerosol–climate model ECHAM6-HAM2 to examine the global relevance of marine organic aerosol (MOA), which has drawn much interest in recent years as a potentially important INPs in remote marine regions. We address the uncertainties in emissions and ice nucleation activity of MOA with a range of reasonable set-ups and find a wide range of resulting MOA burdens. The relative importance of MOA as an INP compared to dust is investigated and found to depend strongly on the type of ice nucleation parameterisation scheme chosen. On the zonal mean, freezing due to MOA leads to relative increases in the cloud ice occurrence and in-cloud number concentration close to the surface in the polar regions during summer. Slight but consistent decreases in the in-cloud ice crystal effective radius can also be observed over the same regions during all seasons. Regardless, MOA was not found to affect the radiative balance significantly on the global scale, due to its relatively weak ice activity and a low sensitivity of cloud ice properties to heterogeneous ice nucleation in our model.

Diurnal and seasonal variability of PM2.5 and AOD in North China plain: Comparison of MERRA-2 products and ground measurements

North China Plain (NCP) is one of heavily polluted regions that is characterized by a mixture of a myriad of anthropogenic and natural aerosols. A substantial spatial and temporal variations of aerosols and their compositions there poses a good testbed for the validation of model simulations. Aerosol optical depth (AOD) and PM2.5 (particulate matter with aerodynamic diameter < 2.5 μm) concentration products from the Modern Era Retrospective-Analysis for Research and Applications, version 2 (MERRA-2) are evaluated using available independent ground-based in situ and remote sensing products in the NCP. The comparison of MERRA-2 aerosol species to the observations is also performed. Although several satellite and ground-based AOD products are assimilated into the MERRA-2, MERRA-2 AOD is systematically smaller than independent sunphotometer measurements. The biases range from 0.09 (13%) in the summer to 0.17 (33%) in the spring and show little spatial dependence. Daytime AOD variations are captured by the MERRA-2, although MERRA-2 has relatively lower AOD. MERRA-2 produces lower PM2.5 concentration relative to surface measurements in all seasons except in summer. The largest bias is found in the winter (44 μgm−3). On the contrary, summer MERRA-2 PM2.5 is close to surface-measured PM2.5 (with bias of 0.4 μgm−3). MERRA-2 was unable to reproduce diurnal PM2.5 variation. Evaluation of MERRA-2 aerosol species in the winter of 2014 suggests that MERRA-2 could not keep track of dramatic day-to-day variation of aerosols and their species. Potential causes for this deficiency may include a lack of nitrate aerosols (accounting for 20% of PM2.5 concentrations during heavily polluted days). This fault cannot be remedied by assimilation of satellite AODs because they are often missing.

Influence of chemical size distribution on optical properties for ambient submicron particles during severe haze events

Despite of extensive efforts on investigation into characteristics of severe haze pollution in megacities of China, the accurate relationships among the aerosol composition, mass-size distribution and optical properties during pollution episodes remain poorly understood. Here, we conducted in situ measurements of the mass size distribution of submicron aerosol (PM1) species by using a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS), particle light scattering by a Cavity Attenuated Phase Shift ALBedo monitor (CAPS-ALB) and a Photoacoustic Extinctionmeter (PAX) during the winter of 2017 in Shanghai, China. The average PM1 concentration was 85.9 ± 14.7 μg/m3 during the haze episodes, of which was ∼7 times higher than that of clean period (12.1 ± 3.1 μg/m3). Organic aerosol (OA) and inorganic species (SO42− + NO3− + NH4+) contributed 39.9% and 51.2% of the total mass of PM1 during the haze episodes, respectively. OA exhibited a single or bimodal distribution during the haze episodes with the peak concentration of 51.8 μg/m3. There were no obvious differences between ammonium nitrate (NH4NO3) and ammonium sulfate ((NH4)2SO4) during the haze episodes, which exhibited single peak distributions at the sizes of 650–700 nm and ∼700 nm, respectively. The peak positions of OA, NH4NO3 and (NH4)2SO4 in clean period were in the range of 450–500 nm, 550–600 nm and 450–500 nm with the peak concentrations of 5.5 μg/m3, 3.1 μg/m3 and 3.8 μg/m3, respectively. The increased scattering coefficients in the haze episodes were positively correlated with higher secondary inorganic aerosols and organic aerosol (OA). The high scattering coefficient contribution fraction peak diameter of NH4NO3 and (NH4)2SO4 were in the range of 600–800 nm and 600–750 nm with the peak scattering coefficient of 352.6 Mm−1 and 165.7 Mm−1, respectively. The size distribution of scattering for OA showed bimodal modes during all episodes. OA and NH4NO3 were the largest contributors to scattering coefficients of PM1 during the haze episodes, accounting for 45.5% and 37.8%, respectively. The contribution of (NH4)2SO4 to the scattering (24%) exceeded that of NH4NO3 during clean period. Our results elucidated that substantial changes in the aerosols optical properties due to the changes in chemical compositions and size distribution during the haze events.

Contributions of residential coal combustion to the air quality in Beijing–Tianjin–Hebei (BTH), China: a case study

Abstract. In the present study, the WRF-Chem model is used to assess contributions of residential coal combustion (RCC) emissions to the air quality in Beijing–Tianjin–Hebei (BTH) during a persistent air pollution episode from 9 to 25 January 2014. In general, the predicted temporal variations and spatial distributions of the mass concentrations of air pollutants are in good agreement with observations at monitoring sites in BTH. The WRF-Chem model also reasonably reproduces the temporal variations in aerosol species when compared with the aerosol mass spectrometer measurements in Beijing. The RCC emissions play an important role in the haze formation in BTH, contributing about 23.1 % of PM2.5 (fine particulate matter) and 42.6 % of SO2 during the simulation period on average. Organic aerosols dominate the PM2.5 from the RCC emissions in BTH, with a contribution of 42.8 %, followed by sulfate (17.1 %). The air quality in Beijing is remarkably improved when the RCC emissions in BTH and the surrounding areas are excluded in model simulations, with a 30 % decrease in PM2.5 mass concentrations. However, if only the RCC emissions in Beijing are excluded, the local PM2.5 mass concentration is decreased by 18.0 % on average. Our results suggest that the implementation of the residential coal replacement by clean energy sources in Beijing is beneficial to the local air quality. Should residential coal replacement be carried out in BTH and its surrounding areas, the air quality in Beijing would be improved remarkably. Further studies would need to consider uncertainties in the emission inventory and meteorological fields.

Constrained simulation of aerosol species and sources during pre-monsoon season over the Indian subcontinent

This study was designed to deliver a better concurrence between model estimates and observations, of atmospheric aerosol species, and predict their spatial distribution as consistently as possible. A free running aerosol simulation (freesimu) in a general circulation model (GCM) was performed, and further the simulated aerosol optical depth (AOD) was constrained with the observed AOD. The present study was carried out during the pre-monsoon season and for the Tigerz experiment which was conducted at stations over the Indo-Gangetic plain (IGP) and the Himalayan foot-hills in northern India. Our formulation of the constrained aerosol simulation (constrsimu) was based upon an identification of the freesimu with the most consistent estimates of aerosol characteristic among the three freesimu. The three freesimu (differing in source of emissions and model horizontal resolution) were carried out with the general circulation model (GCM) of Laboratoire de Météorologie Dynamique (LMD-ZT GCM). Black carbon (BC), organic carbon (OC), and sulfate-other water soluble (Sul-ows) estimated from constrsimu amounted to 70%–100% compared to that from freesimu being 20%–50% of their measured counterparts. Among the aerosol species, the pre-monsoon mean concentration of dust was considerably high over most part of the Indian subcontinent; the anthropogenic aerosol species were, however, specifically predominant over the IGP (mostly 8–12 μg m−3 for Sul-ows, OC). The constrsimu estimated total submicron aerosol mass concentration revealed its alarmingly high value over the northern and north-western India (> 100 μg m−3 and as high as 300 μg m−3). While the high value of observed AOD was found being mainly due to dust (AOD due to dust greater than 0.3) over the northern–northwestern IGP, it was due to Sul-ows (AOD due to Sul-ows as high as 0.4) over the eastern IGP, eastern coastline, and the Bay of Bengal. Temporal trend of fine (FM) and coarse mode (CM) AOD from constrsimu estimates and that derived from Tigerz experiment were in phase with each other for most of the days and exhibited a strong positive correlation coefficient. Source of Tigerz aerosols was mainly due to a predominant influence of dust from Africa/west Asia followed by that from northwest India, and of anthropogenic emissions originating in the IGP. A 200% increase was inferred for potential black carbon emissions (using India emission inventory implemented in a GCM) to obtain a concurrence between observed and freesimu BC concentration.

Determining the link between hygroscopicity and composition for semi-volatile aerosol species

Abstract. Internally and externally mixed aerosols present significant challenges in assessing the hygroscopicity of each aerosol component. This study presents a new sampling technique which uses differences in volatility to separate mixtures and directly examine their respective composition and hygroscopic contribution. A shared thermodenuder and unheated bypass line are continuously cycled between an aerosol mass spectrometer and a volatility and hygroscopicity tandem differential mobility analyser, allowing real-time comparative analysis of heated and unheated aerosol properties. Measurements have been taken of both chamber-generated secondary organic aerosol and coastal marine aerosol at Cape Grim, Australia, to investigate system performance under diverse conditions. Despite rapidly changing aerosol properties and the need to restrict analysis to a narrow size range, the former experiment separated the hygroscopic influences of ammonium sulfate and two distinct organic components with similar oxygen to carbon ratios but different volatilities. Analysis of the marine aerosol revealed an external mixture of non-sea-salt sulfates and sea spray aerosol, which likely shared similar volatile fractions composed of sulfuric acid and a non-hygroscopic organic component.

Impacts of meteorological nudging on the global dust cycle simulated by NICAM coupled with an aerosol model

In this study, we present simulations of the global dust cycle for present day conditions using a new dust-atmosphere model based on the Non-hydrostatic Icosahedral Atmospheric Model (NICAM) coupled with the Spectral Radiation Transport Model for Aerosol Species (SPRINTARS). We focus on evaluations of the dust simulation with respect to emissions, depositions, surface concentrations, aerosol optical depths (AODs), and the dust-aerosol direct radiative effects (DREs). The sensitivities of the dust simulation to the meteorological fields are also investigated through with and without meteorological nudging. NICAM without meteorology nudging tends to systemically overestimate the 10 m wind speeds by approximately 30%–40%, whereas the daily magnitudes and variations in the 10 m wind speeds are both significantly improved with meteorological nudging, especially over the Sahara Desert. The estimated annual global mean dust emission flux, dust AOD, and dust-aerosol shortwave DRE at the top of the atmosphere with meteorological nudging are 1463 Tg yr−1, 0.033, and −1.3 Wm-2, respectively. Due to the approximately 30%–40% overestimations of the 10 m wind speeds over the two major desert regions, the estimated annual global mean dust emission flux, AOD, and DRE without meteorological nudging are significantly greater than those with meteorological nudging. The overestimations of 10 m wind speeds and the associated dust emissions are mainly caused by the positive biases of wind speeds especially from surface to approximately 2 km and slightly affected by the temperature fields. The monthly variations in the dust depositions over the Atlantic and the surface dust concentrations over the Pacific are all better simulated with meteorological nudging. Compared to both the AERONET (Aerosol Robotics Network)- and MODIS (Moderate-Resolution Imaging Spectroradiometer)- retrieved AODs, the simulated daily AOD variations are significantly improved with meteorological nudging, especially over the dust-aerosol dominated regions. The global and annual mean dust lifetime and size distribution, which are two critical factors for estimating dust radiative effects, are quite similar between the dynamic and nudged NICAMs. We therefore can use the dynamic model to understand climate-dust interactions in a global and annual scale. Furthermore, we can improve the model performances for some applications in regional and seasonal scales by meteorological nudging which probably cannot be achieved by just tuning the dust emission.

The implementation of NEMS GFS Aerosol Component (NGAC) Version 2.0 for global multispecies forecasting at NOAA/NCEP – Part 2: Evaluation of aerosol optical thickness

Abstract. An accurate representation of aerosols in global numerical weather prediction (NWP) models is important to predict major air pollution events and to also understand aerosol effects on short-term weather forecasts. Recently the global aerosol forecast model at NOAA, the NOAA Environmental Modeling System (NEMS) GFS Aerosol Component (NGAC), was upgraded from its dust-only version 1 to include five species of aerosols (black carbon, organic carbon, sulfate, sea salt and dust). This latest upgrade, now called NGACv2, is an in-line aerosol forecast system providing three-dimensional aerosol mixing ratios along with aerosol optical properties, including aerosol optical thickness (AOT), every 3 h up to 5 days at global 1∘×1∘ resolution. In this paper, we evaluated nearly 1.5 years of model AOT at 550 nm with available satellite retrievals, multi-model ensembles and surface observations over different aerosol regimes. Evaluation results show that NGACv2 has high correlations and low root mean square errors associated with African dust and also accurately represented the seasonal shift in aerosol plumes from Africa. Also, the model represented southern African and Canadian forest fires, dust from Asia, and AOT within the US with some degree of success. We have identified model underestimation for some of the aerosol regimes (particularly over Asia) and will investigate this further to improve the model forecast. The addition of a data assimilation capability to NGAC in the near future is expected to provide a positive impact in aerosol forecast by the model.

The implementation of NEMS GFS Aerosol Component (NGAC) Version 2.0 for global multispecies forecasting at NOAA/NCEP – Part 1: Model descriptions

Abstract. The NEMS GFS Aerosol Component Version 2.0 (NGACv2) for global multispecies aerosol forecast has been developed at the National Centers of Environment Prediction (NCEP) in collaboration with the NESDIS Center for Satellite Applications and Research (STAR), the NASA Goddard Space Flight Center (GSFC), and the University at Albany, State University of New York (SUNYA). This paper describes the continuous development of the NGAC system at NCEP after the initial global dust-only forecast implementation (NGAC version 1.0, NGACv1). With NGACv2, additional sea salt, sulfate, organic carbon, and black carbon aerosol species were included. The smoke emissions are from the NESDIS STAR's Global Biomass Burning Product (GBBEPx), blended from the global biomass burning emission product from a constellation of geostationary satellites (GBBEP-Geo) and GSFC's Quick Fire Emission Data Version 2 from a polar-orbiting sensor (QFED2). This implementation advanced the global aerosol forecast capability and made a step forward toward developing a global aerosol data assimilation system. The aerosol products from this system have been used by many applications such as for regional air quality model lateral boundary conditions, satellite sea surface temperature (SST) physical retrievals, and the global solar insolation estimation. Positive impacts have been seen in these applications.

Widespread air pollutants of the North China Plain during the Asian summer monsoon season: a case study

Abstract. During the Asian summer monsoon season, prevailing southeasterly–southwesterly winds are subject to delivering air pollutants from the North China Plain (NCP) to northeast and northwest China. In the present study, the WRF-CHEM model is used to evaluate contributions of trans-boundary transport of NCP emissions to the air quality in northeast and northwest China during a persistent air pollution episode from 22 to 28 May 2015. The WRF-CHEM model generally performs well in capturing the observed temporal variation and spatial distribution of fine particulate matter (PM2.5), ozone (O3), and NO2. The simulated temporal variation of aerosol species is also in good agreement with measurements in Beijing during the episode. Model simulations show that NCP emissions contribute substantially to the PM2.5 level in Liaoning and Shanxi provinces, the adjacent downwind areas of the NCP, with an average of 24.2 and 13.9 µg m−3 during the episode, respectively. The PM2.5 contributions in Jilin and Shaanxi provinces are also appreciable, with an average of 9.6 and 6.5 µg m−3, respectively. The average percentage contributions of NCP emissions to the PM2.5 level in Liaoning, Jilin, Shanxi, and Shaanxi provinces are 40.6, 27.5, 32.2, and 20.9 %, respectively. The NCP emissions contribute remarkably to the O3 level in Liaoning province, with an average of 46.5 µg m−3, varying from 23.9 to 69.5 µg m−3. The O3 level in Shanxi province is also influenced considerably by NCP emissions, with an average contribution of 35.1 µg m−3. The O3 level in Shanxi province is also influenced considerably by NCP emissions, with an average contribution of 35.1 µg m−3. The average O3 contributions of NCP emissions to Jilin and Shaanxi provinces are 28.7 and 20.7 µg m−3, respectively. The average percentage contributions of NCP emissions to the afternoon O3 level in Liaoning, Jilin, Shanxi, and Shaanxi provinces are 27.4, 19.5, 21.2, and 15.8 %, respectively. However, the effect of NCP emissions on the air quality in Inner Mongolia is generally insignificant. Therefore, effective mitigation of NCP emissions not only improves the local air quality, but is also beneficial to the air quality in northeast and northwest China during the Asian summer monsoon season.

Insight into global trends in aerosol composition from 2005 to 2015 inferred from the OMI Ultraviolet Aerosol Index

Abstract. Observations of aerosol scattering and absorption offer valuable information about aerosol composition. We apply a simulation of the Ultraviolet Aerosol Index (UVAI), a method of detecting aerosol absorption from satellite observations, to interpret UVAI values observed by the Ozone Monitoring Instrument (OMI) from 2005 to 2015 to understand global trends in aerosol composition. We conduct our simulation using the vector radiative transfer model VLIDORT with aerosol fields from the global chemical transport model GEOS-Chem. We examine the 2005–2015 trends in individual aerosol species from GEOS-Chem and apply these trends to the UVAI simulation to calculate the change in simulated UVAI due to the trends in individual aerosol species. We find that global trends in the UVAI are largely explained by trends in absorption by mineral dust, absorption by brown carbon, and scattering by secondary inorganic aerosol. Trends in absorption by mineral dust dominate the simulated UVAI trends over North Africa, the Middle East, East Asia, and Australia. The UVAI simulation resolves observed negative UVAI trends well over Australia, but underestimates positive UVAI trends over North Africa and Central Asia near the Aral Sea and underestimates negative UVAI trends over East Asia. We find evidence of an increasing dust source from the desiccating Aral Sea that may not be well represented by the current generation of models. Trends in absorption by brown carbon dominate the simulated UVAI trends over biomass burning regions. The UVAI simulation reproduces observed negative trends over central South America and West Africa, but underestimates observed UVAI trends over boreal forests. Trends in scattering by secondary inorganic aerosol dominate the simulated UVAI trends over the eastern United States and eastern India. The UVAI simulation slightly overestimates the observed positive UVAI trends over the eastern United States and underestimates the observed negative UVAI trends over India. Quantitative simulation of the OMI UVAI offers new insight into global trends in aerosol composition.

Application of satellite observations in conjunction with aerosol reanalysis to characterize long-range transport of African and Asian dust on air quality in the contiguous U.S.

A methodology utilizing multiple aerosol measurements and reanalysis was developed to analyze the long-range transport (LRT) of North African and Asian dust events and their effect on contiguous U.S. (CONUS) air quality.Two events in 2015, that originated inland from North Africa and Asia, were detected with aerosol optical depth (AOD) greater than 0.8 over the Gulf of Mexico (June 21) and west coast of Oregon and Washington (April 19). Large amounts of dust were rising over Sahara-Sahel and Taklamakan deserts during these events. For the North African dust event, dust remained the dominant aerosol species transporting from North Africa to CONUS. For Asian dust, before the Taklamakan dust outbreak traveled across the Pacific Ocean, it mixed with sulfate from East China and organic carbons from Siberia. The trans-Atlantic North African dust emerged and compressed below 5 km over the Azores High, whereas the trans-Pacific Asian dust entered the westerlies (5–10 km) along with the mid-latitude frontal system.For the two cases examined, the North African dust event tended to have more influence on CONUS air quality. During the LRT periods, there were 4 of 17 rural and suburban stations (with AOD > 0.5 as the LRT dust approached) in Texas and Louisiana reporting PM2.5 daily maximum observations greater than 36.3 μg m−3. In contrast, the Asian dust stayed above ground level, and all of the 4 rural and suburban stations (with AOD > 0.5 as the LRT dust approached) in Washington and Oregon observed daily maximum PM2.5 below 22.4 μg m−3. The integration of measurements and modeling was able to successfully characterize the sources and transport patterns of the LRT dust events, and differentiate their influence on CONUS air quality. Even though there was high AOD from the Asian LRT dust detected over the western CONUS, significant impacts on surface air quality was not observed as the dust remained aloft.

Spatial Variability of Sources and Mixing State of Atmospheric Particles in a Metropolitan Area.

Characterizing intracity variations of atmospheric particulate matter has mostly relied on fixed-site monitoring and quantifying variability in terms of different bulk aerosol species. In this study, we performed ground-based mobile measurements using a single-particle mass spectrometer to study spatial patterns of source-specific particles and the evolution of particle mixing state in 21 areas in the metropolitan area of Pittsburgh, PA. We selected sampling areas based on traffic density and restaurant density with each area ranging from 0.2 to 2 km2. Organics dominate particle composition in all of the areas we sampled while the sources of organics differ. The contribution of particles from traffic and restaurant cooking varies greatly on the neighborhood scale. We also investigate how primary and aged components in particles mix across the urban scale. Lastly we quantify and map the particle mixing state for all areas we sampled and discuss the overall pattern of mixing state evolution and its implications. We find that in the upwind and downwind of the urban areas, particles are more internally mixed while in the city center, particle mixing state shows large spatial heterogeneity that is mostly driven by emissions. This study is to our knowledge, the first study to perform fine spatial scale mapping of particle mixing state using ground-based mobile measurement and single-particle mass spectrometry.