ALBERT

Description: This paper evaluates the current status of global modeling of the organic aerosol (OA) in the troposphere and analyzes the differences between models as well as between models and observations. Thirty-one global chemistry transport models (CTMs) and general circulation models (GCMs) have participated in this intercomparison, in the framework of AeroCom phase II. The simulation of OA varies greatly between models in terms of the magnitude of primary emissions, secondary OA (SOA) formation, the number of OA species used (2 to 62), the complexity of OA parameterizations (gas-particle partitioning, chemical aging, multiphase chemistry, aerosol microphysics), and the OA physical, chemical and optical properties. The diversity of the global OA simulation results has increased since earlier AeroCom experiments, mainly due to the increasing complexity of the SOA parameterization in models, and the implementation of new, highly uncertain, OA sources. Diversity of over one order of magnitude exists in the modeled vertical distribution of OA concentrations that deserves a dedicated future study. Furthermore, although the OA / OC ratio depends on OA sources and atmospheric processing, and is important for model evaluation against OA and OC observations, it is resolved only by a few global models. The median global primary OA (POA) source strength is 56 Tg a−1 (range 34–144 Tg a−1) and the median SOA source strength (natural and anthropogenic) is 19 Tg a−1 (range 13–121 Tg a−1). Among the models that take into account the semi-volatile SOA nature, the median source is calculated to be 51 Tg a−1 (range 16–121 Tg a−1), much larger than the median value of the models that calculate SOA in a more simplistic way (19 Tg a−1; range 13–20 Tg a−1, with one model at 37 Tg a−1). The median atmospheric burden of OA is 1.4 Tg (24 models in the range of 0.6–2.0 Tg and 4 between 2.0 and 3.8 Tg), with a median OA lifetime of 5.4 days (range 3.8–9.6 days). In models that reported both OA and sulfate burdens, the median value of the OA/sulfate burden ratio is calculated to be 0.77; 13 models calculate a ratio lower than 1, and 9 models higher than 1. For 26 models that reported OA deposition fluxes, the median wet removal is 70 Tg a−1 (range 28–209 Tg a−1), which is on average 85% of the total OA deposition. Fine aerosol organic carbon (OC) and OA observations from continuous monitoring networks and individual field campaigns have been used for model evaluation. At urban locations, the model–observation comparison indicates missing knowledge on anthropogenic OA sources, both strength and seasonality. The combined model–measurements analysis suggests the existence of increased OA levels during summer due to biogenic SOA formation over large areas of the USA that can be of the same order of magnitude as the POA, even at urban locations, and contribute to the measured urban seasonal pattern. Global models are able to simulate the high secondary character of OA observed in the atmosphere as a result of SOA formation and POA aging, although the amount of OA present in the atmosphere remains largely underestimated, with a mean normalized bias (MNB) equal to −0.62 (−0.51) based on the comparison against OC (OA) urban data of all models at the surface, −0.15 (+0.51) when compared with remote measurements, and −0.30 for marine locations with OC data. The mean temporal correlations across all stations are low when compared with OC (OA) measurements: 0.47 (0.52) for urban stations, 0.39 (0.37) for remote stations, and 0.25 for marine stations with OC data. The combination of high (negative) MNB and higher correlation at urban stations when compared with the low MNB and lower correlation at remote sites suggests that knowledge about the processes that govern aerosol processing, transport and removal, on top of their sources, is important at the remote stations. There is no clear change in model skill with increasing model complexity with regard to OC or OA mass concentration. However, the complexity is needed in models in order to distinguish between anthropogenic and natural OA as needed for climate mitigation, and to calculate the impact of OA on climate accurately.

Description: Strong diurnal variation of aerosol has been observed in many heavy polluted regions in China. This variation could alter the distribution of the direct aerosol radiative forcing (DARF) during the whole daytime that could increase the uncertainty of the normally used averaged values. To quantify the effect of using the daily averaged DARF, 196 days of high temporal resolution ground-based data collected in SKYNET Hefei site during the period from 2007 to 2013 is used to perform an assessment. We demonstrate that strong diurnal changes of heavy aerosol loading have an impact on the 24 h averaged DARF when daily averaged optical properties are used to retrieve this quantity. Though there is a high temporal sampling of aerosol properties to be used for calculation of daily averaged ones, statistical errors (up to 5.3 Wm−2 absolutely and 14.6% relatively) in the computed DARF still occur. These errors increase with increasing daily aerosol optical depth (AOD) is also found which indicated the high temporal resolution DARF dataset should be used in the model instead of the normal daily-averaged one, especially under heavy aerosol loading condition.

Description: The usefulness of mercury (Hg) isotopes for tracing the sources and pathways of Hg (and its vectors) in atmospheric fine particles (PM2.5) is uncertain. Here, we measured Hg isotopic compositions in 30 potential source materials and 23 PM2.5 samples collected in four seasons from the megacity Beijing (China), and combined the seasonal variation of both mass-dependent fractionation (represented by the ratio 202Hg/198Hg, δ202Hg) and mass-independent fractionation of isotopes with odd and even mass numbers (represented by Δ199Hg and Δ200Hg, respectively) with geochemical parameters and meteorological data to identify the sources of PM2.5-Hg and possible atmospheric particulate Hg transformation. All PM2.5 samples were strongly enriched in Hg and other heavy metals, and displayed large ranges of both δ202Hg (−2.18 to 0.51 ‰) and Δ199Hg (−0.53 to 0.57 ‰), and small positive Δ200Hg (0.02 to 0.17 ‰). The results indicated that the seasonal variation of Hg isotopic composition (and elemental concentrations) was likely derived from variable contributions from the anthropogenic sources, with continuous input due to industrial activities (e.g. smelting, cement production and coal combustion) in all seasons, while coal combustion dominated in Winter and biomass burning mainly in Autumn. The significant positive Δ199Hg of PM2.5-Hg in Spring and early Summer was likely derived from long-range transported Hg that had undergone extensive photochemical reduction. The study demonstrated that Hg isotopes are a powerful tool for tracing the sources of particulate Hg and its vectors in the atmosphere.

Description: Ground-based observations of the polarization properties of aerosol particles using a polarization optical particle counter (POPC) were performed from October 2013 to January 2015 at a suburban site in the Kyushu area of Japan. By conducting an analysis of online measurements of aerosol composition, we investigated size-dependent polarization characteristics for three typical aerosol types (anthrop ogenic pollutants, dust, and sea salt). We found that, for supermicron particles, its depolarization ratio (DR, the fraction of s-polarized signal in the total back ward light scattering signal) generally increased with the particle size, and a threshold value of 0.1 could be used to identify the irsphericity. In summer, air pollution was less serious, and the DR of aerosol particles was relatively small due to the influence of spherical sea salt particles in high humidity conditions. This study indicated that air masses were transported across the Asian continent and contained not only anthropogenic pollutants, but also large amounts of non-spherical particles (i.e., dust), which could impact on the air quality in western Japan, especially in winter and spring. The variation of number fraction of spherical particles did not correlate with relative humidity averaged along the trajectories of air parcels, indicating the coexistence of hydrophobic substances (e.g., mineral dust and organics), although the sulfate and nitrate mass concentrations were high.

Description: This paper evaluates the current status of global modeling of the organic aerosol (OA) in the troposphere and analyzes the differences between models as well as between models and observations. Thirty-one global chemistry/transport and general circulation models have participated in this intercomparison, in the framework of AeroCom phase II. The simulation of OA varies greatly between models in terms of the magnitude of primary emissions, secondary OA (SOA) formation, the number of OA species used (2 to 62), the complexity of OA parameterizations (gas-particle partitioning, chemical aging, multiphase chemistry, aerosol microphysics), and the OA physical, chemical and optical properties. The diversity of the global OA simulation results has increased since earlier AeroCom experiments, mainly due to the increasing complexity of the SOA parameterization in models, and the implementation of new, highly uncertain, OA sources. Diversity of over an order of magnitude exists in the modeled vertical distribution of OA that deserves a dedicated future study. Furthermore, although the OA / OC ratio depends on OA sources and atmospheric processing and is important for model evaluation against OA and OC observations, it is resolved only by few global models. The median global primary OA (POA) source strength is 56 Tg a−1 (range 34–144 Tg a−1) and the median secondary OA source strength (natural and anthropogenic) is 19 Tg a−1 (range 13–121 Tg a−1). Among the models that take into account the semi-volatile SOA nature, the median source is calculated to be 51 Tg a−1 (range 16–121 Tg a−1), much larger than the median value of the models that calculate SOA in a more simplistic way (19 Tg a−1; range 13–20 Tg a−1, with one model at 37 Tg a−1). The median atmospheric burden of OA is 1.4 Tg (24 models in the range of 0.6–2.0 Tg and 4 between 2.4–3.8 Tg) with a median OA lifetime of 5.4 days (range 3.8–9.6 days). In models that reported both OA and sulfate burdens, the median value of the OA / sulfate burden ratio of is calculated to be 0.77; 13 models calculate a ratio lower than 1, and 9 models higher than 1. For 26 models that reported OA deposition fluxes, the median wet removal is 70 Tg a−1 (range 28–209 Tg a−1), which is on average 85% of the total OA deposition. Fine aerosol organic carbon (OC) and OA observations from continuous monitoring networks and individual field campaigns have been used for model evaluation. At urban locations the model-observation comparison indicates missing knowledge on anthropogenic OA sources, both strength and seasonality. The combined model/measurements analysis suggests the existence of increased OA levels during summer due to biogenic SOA formation over large areas of the USA that can be of the same order of magnitude as the POA, even at urban locations, and contribute to the measured urban seasonal pattern. Global models are able to simulate the high secondary character of OA observed in the atmosphere as a result of SOA formation and of POA aging, although, the amount of OA present in the atmosphere remains largely underestimated, with a mean normalized bias (MNB) equal to −0.62 (−0.51) based on the comparison against OC (OA) urban data of all models at surface, −0.15 (+0.51) when compared with remote measurements, and −0.30 for marine locations with OC data. The correlations overall are low when comparing with OC (OA) measurements: 0.47 (0.52) for urban stations, 0.39 (0.37) for remote, and 0.25 for marine stations with OC data. The combination of high (negative) MNB and higher correlation at urban stations when compared with the low MNB and lower correlation at remote sites suggests that the knowledge about the processes, on top of the sources, are important at the remote stations. There is no clear change in model skill with increasing model complexity with regard to OC or OA mass concentration. However, the complexity is needed in models in order to separate between anthropogenic and natural OA and accurately calculate the impact of OA on climate.

Description: A strong diurnal variation of aerosol has been observed in many heavily polluted regions in China. This variation could affect the direct aerosol radiative forcing (DARF) evaluation if the daily averaged value is used as normal rather than the time-resolved values. To quantify the effect of using the daily averaged DARF, 196 days of high temporal resolution ground-based data collected in SKYNET Hefei site during the period from 2007 to 2013 is used to perform an assessment. We demonstrate that strong diurnal changes of heavy aerosol loading have an impact on the 24-h averaged DARF when daily averaged optical properties are used to retrieve this quantity. The DARF errors varying from −7.6 to 15.6 W m−2 absolutely and from 0.1 to 28.5 % relatively were found between the calculations using daily average aerosol properties, and those using time-resolved aerosol observations. These errors increase with increasing daily aerosol optical depth (AOD) and decreasing daily single-scattering albedo (SSA), indicating that the high temporal resolution DARF data set should be used in the model instead of the normal daily-averaged one, especially under heavy aerosol loading conditions for regional campaign studies. We also found that statistical errors (0.3 W m−2 absolutely and 11.8 % relatively) will be less, which means that the effect of using the daily averaged DARF can be weakened by using a long-term observational data set.

Description: Mass concentrations of black carbon (BC) were determined in June 2006 at the top of Mount Tai (36.26° N, 117.11° E, 1534 m a.s.l.), located in the middle of Central East China, using four different instruments: a multi-angle absorption photometer (5012 MAAP, Thermo), a particle soot absorption photometer (PSAP, Radiance Research), an ECOC semi-continuous analyzer (Sunset Laboratory) and an Aethalometer (AE-21, Magee Scientific). High correlation coefficients (R2〉0.88) were obtained between the measurements of the BC mass concentrations by the different instruments. From the range of the slopes of the linear least-square fittings, we concluded that the BC concentrations regionally-representative of the area were measured in a range with a maximum-to-minimum ratio of 1.5 (an exception was that the BC (PM2.5) concentrations derived from MAAP were ~2 times higher than the optical measurements (PM2.5) derived from the ECOC analyzer). This range is significant, but is still sufficiently narrow to better constrain the large and highly uncertain emission rate of BC from China. In detail, two optical instruments (the MAAP instrument and the PSAP instrument equipped with a heated inlet (400°C)) tended to give higher concentrations than the thermal EC concentrations observed by the ECOC analyzer. The ratios of optical BC to thermal EC showed a positive correlation with the OC/EC ratio reported by the ECOC analyzer, suggesting two possibilities. One is that the optical instruments overestimated BC concentrations in spite of careful cancellation of the scattering effect in the MAAP instrument and the expected evaporation of volatile species by heating the inlet of the PSAP instrument. The other is that the determined split points between OC and EC were too late when a large amount of OC underwent charring during the analysis, resulting in an underestimation of EC by the ECOC analyzer. High ratios of optical BC to thermal EC were recorded when the NOx/NOy ratio was low, implying the coating of the particles became thicker in the aged air mass and resulted in the optical instruments overestimating BC concentrations owing to the lensing effect.

Description: The impact of the East Asia monsoon on the seasonal behavior of O3 in the boundary layer of Eastern China and the west Pacific region was analyzed for 2004–2006 by means of full-year nested chemical transport model simulations and continuous observational data obtained from three inland mountain sites in central and eastern China and three oceanic sites in the west Pacific region. The basic common features of O3 seasonal behaviors over all the monitoring sites are the pre- and post-monsoon peaks with a summer trough. Such bimodal seasonal patterns of O3 are predominant over the region with strong summer monsoon penetration, and become weaker or even disappear outside the monsoon region. The seasonal/geographical distribution of the pre-defined Monsoon Index indicated that the East Asia summer monsoon is responsible for the bimodal seasonal O3 pattern, and also partly account for the differences in the O3 seasonal variations between the inland mountain and oceanic sites. Over the inland mountain sites, the O3 concentration increased gradually from the beginning of the year, reached a maximum in June, decreased rapidly to a minimum in July or August, and then peaked in September or October, thereafter decreased gradually again. Over the oceanic sites, O3 abundance showed a similar increasing trend beginning in January, but then decreased gradually from the end of March, followed by a wide trough with the minimum in July and August and a small peak in October or November. A sensitivity analysis performed by setting China-emission to zero revealed that the chemically produced O3 from China-emission contributed more than 40% of total boundary layer O3 during summertime (60–70% in June) and accounted for about 40 ppb of each peak value over the inland region. In contrast, over the oceanic region in the high monsoon index zone, the contribution of China-emission to total O3 was always less than 20% (

Description: A 3-D regional chemical transport model, the Nested Air Quality Prediction Model System (NAQPMS), with an on-line tracer tagging module was applied to study the source of the near-ground (

Description: The development of tropical depressions within tropical waves over the Atlantic and eastern Pacific is usually preceded by a "surface low along the wave" as if to suggest a hybrid wave-vortex structure in which flow streamlines not only undulate with the waves, but form a closed circulation in the lower troposphere surrounding the low. This structure, equatorward of the easterly jet axis, resembles the familiar critical layer of waves in shear flow, a flow configuration which arguably provides the simplest conceptual framework for tropical cyclogenesis resulting from tropical waves, their interaction with the mean flow, and with diabatic processes associated with deep moist convection. The critical layer represents a sweet spot for tropical cyclogenesis in which a proto-vortex may form and grow within its parent wave. A common location for storm development within the critical layer is given by the intersection of the wave's critical latitude and trough axis, with analyzed vorticity centroid nearby. The wave and vortex live together for a time, and initially propagate at approximately the same speed. In most cases this coupled propagation continues for a few days after a tropical depression is identified. For easterly waves, as the name suggests, the propagation is westward. It is shown that in order to visualize optimally this "marsupial paradigm" one should view the flow streamlines, or stream function, in a frame of reference translating horizontally with the phase propagation of the parent wave. This translation requires an appropriate "gauge" that renders translating streamlines and isopleths of translating stream function approximately equivalent to flow trajectories. In the translating frame, the closed circulation is stationary, and a dividing streamline effectively separates air within the critical layer from air outside. The critical layer equatorward of the easterly jet axis is important to tropical cyclogenesis because it provides (i) a region of cyclonic vorticity and weak deformation by the resolved flow, (ii) containment of moisture entrained by the gyre and/or lofted by deep convection therein, (iii) confinement of mesoscale vortex aggregation, (iv) a predominantly convective type of heating profile, and (v) maintenance or enhancement of the parent wave until the vortex becomes a self-sustaining entity and emerges from the wave as a tropical depression. These ideas are formulated in three new hypotheses describing the flow kinematics and dynamics, moist thermodynamics and wave/vortex interactions comprising the marsupial paradigm. A survey of 55 named tropical storms in 1998–2001 reveals that actual critical layers sometimes resemble the ideal east-west train of cat's eyes, but are usually less regular, with one or more recirculation regions in the translating frame. It is shown that a "wave gauge" given by the translation speed of the parent wave is the appropriate choice, as well, for isolated proto-vortices carried by the wave. Some implications for entrainment/containment of vorticity and moisture in the cat's eye are discussed from this perspective, based on the observational survey.