ALBERT

Description: Secondary organic aerosol (SOA) was formed in an environmental reaction chamber from the ozonolysis of β-caryophyllene (β-C) at low concentrations (5 ppb or 20 ppb). Experimental parameters were varied to characterize the effects of hydroxyl radicals, light and the presence of lower molecular weight terpene precursor (isoprene) for β-C SOA formation and cloud condensation nuclei (CCN) characteristics. Changes in β-C SOA chemicophysical properties (e.g., density, volatility, oxidation state) were explored with online techniques to improve our predictive understanding of β-C CCN activity. In the absence of OH scavenger, light intensity had negligible impacts on SOA oxidation state and CCN activity. In contrast, when OH reaction was effectively suppressed (〉 11 ppm scavenger), SOA showed a much lower CCN activity and slightly less oxygenated state consistent with previously reported values. Though there is significant oxidized material present (O / C 〉 0.25), no linear correlation existed between the mass ratio ion fragment 44 in the bulk organic mass (f44) and O / C for the β-C-O3 system. No direct correlations were observed with other aerosol bulk ion fragment fraction (fx) and κ as well. A mixture of β-C and lower molecular weight terpenes (isoprene) consumed more ozone and formed SOA with distinct characteristics dependent on isoprene amounts. The addition of isoprene also improved the CCN predictive capabilities with bulk aerosol chemical information. The β-C SOA CCN activity reported here is much higher than previous studies (κ 〈 0.1) that use higher precursor concentration in smaller environmental chambers; similar results were only achieved with significant use of OH scavenger. Results show that aerosol formed from a mixture of low and high molecular weight terpene ozonolysis can be hygroscopic and can contribute to the global biogenic SOA CCN budget.

Description: In order to improve the surface ozone forecast over Beijing and surrounding regions, data assimilation method integrated into a high-resolution regional air quality model and a regional air quality monitoring network are employed. Several advanced data assimilation strategies based on ensemble Kalman filter are designed to adjust O3 initial conditions, NOx initial conditions and emissions, VOCs initial conditions and emissions separately or jointly through assimilating ozone observations. As a result, adjusting precursor initial conditions demonstrates potential improvement of the 1-h ozone forecast almost as great as shown by adjusting precursor emissions. Nevertheless, either adjusting precursor initial conditions or emissions show deficiency in improving the short-term ozone forecast at suburban areas. Adjusting ozone initial values brings significant improvement to the 1-h ozone forecast, and its limitations lie in the difficulty in improving the 1-h forecast at some urban site. A simultaneous adjustment of the above five variables is found to be able to reduce these limitations and display an overall better performance in improving both the 1-h and 24-h ozone forecast over these areas. The root mean square errors of 1-h ozone forecast at urban sites and suburban sites decrease by 51% and 58% respectively compared with those in free run. Through these experiments, we found that assimilating local ozone observations is determinant for ozone forecast over the observational area, while assimilating remote ozone observations could reduce the uncertainty in regional transport ozone.

Description: Secondary organic aerosol (SOA) was formed in an environmental reaction chamber from the ozonolysis of β-caryophyllene (β-C) at very low concentrations (5 ppb or 20 ppb) near ambient conditions. Experimental parameters were varied to characterize the effects of hydroxyl radicals, light and the presence of lower molecular weight terpene precursor (isoprene) for β-C SOA formation and Cloud Condensation Nuclei (CCN) characteristics. Changes in β-C SOA chemicophysical properties (e.g. density, volatility, oxidation state) were explored with online techniques to improve our predictive understanding of β-C CCN activity. In the absence of OH scavenger, light intensity had negligible impacts on SOA oxidation state and CCN activity. In contrast, when OH reaction was effectively suppressed (〉11ppm scavenger), SOA showed a much lower CCN activity and slightly less oxygenated state consistent with previously reported values. Though there is significant oxidized material present (O/C〉0.25), no linear correlation existed between the mass ratio ion fragment 44 in the bulk organic mass (f44) and O/C for the β-C-O3 system. No direct correlations were observed with other aerosol bulk ion fragment fraction (fx) and κ as well. A mixture of β-C and lower molecular weight terpenes (isoprene) consumed more ozone and formed SOA with distinct characteristics dependent on isoprene amounts. The addition of isoprene also improved the CCN predictive capabilities with bulk aerosol chemical information. The β-C SOA CCN activity reported here is much higher than previous studies (κ〉0.1) that use higher precursor concentration in smaller environmental chambers; similar results were only achieved with significant use of OH scavenger. Results show that aerosol formed from a mixture of low and high molecular weight terpene ozonolysis can be hygroscopic and can contribute to the global biogenic SOA CCN budget.

Description: Chinese scientists will start to drill a deep ice core at Kunlun station near Dome A in the near future. Recent work has predicted that Dome A is a location where ice older than 1 million years can be found. We model flow, temperature and the age of the ice by applying a three-dimensional, thermo-mechanically coupled full-Stokes model to a 70 km × 70 km domain around Kunlun station, using isotropic non-linear rheology and different prescribed anisotropic ice fabrics that vary the evolution from isotropic to single maximum at 1/3 or 2/3 depths. The variation in fabric is about as important as the uncertainties in geothermal heat flux in determining the vertical advection which in consequence controls both the basal temperature and the age profile. We find strongly variable basal ages across the domain since the ice varies greatly in thickness and any basal melting effectively removes very old ice in the deepest parts of the subglacial valleys. Comparison with dated radar isochrones in the upper one third of the ice sheet cannot sufficiently constrain the age of the deeper ice, with uncertainties as large as 500 000 yr in the basal age. We also assess basal age and thermal state sensitivities to geothermal heat flux and surface conditions. Despite expectations of modest changes in surface height over a glacial cycle at Dome A, even small variations in the evolution of surface conditions cause large variation in basal conditions which is consistent with basal accretion features seen in radar surveys.

Description: Chinese scientists will start to drill a deep ice core at Kunlun station near Dome A in the near future. Recent work has predicted that Dome A is a location where ice older than 1 million years can be found. We model flow, temperature and the age of the ice by applying a three-dimensional, thermomechanically coupled full-Stokes model to a 70 × 70 km2 domain around Kunlun station, using isotropic non-linear rheology and different prescribed anisotropic ice fabrics that vary the evolution from isotropic to single maximum at 1/3 or 2/3 depths. The variation in fabric is about as important as the uncertainties in geothermal heat flux in determining the vertical advection which in consequence controls both the basal temperature and the age profile. We find strongly variable basal ages across the domain since the ice varies greatly in thickness, and any basal melting effectively removes very old ice in the deepest parts of the subglacial valleys. Comparison with dated radar isochrones in the upper one third of the ice sheet cannot sufficiently constrain the age of the deeper ice, with uncertainties as large as 500 000 years in the basal age. We also assess basal age and thermal state sensitivities to geothermal heat flux and surface conditions. Despite expectations of modest changes in surface height over a glacial cycle at Dome A, even small variations in the evolution of surface conditions cause large variation in basal conditions, which is consistent with basal accretion features seen in radar surveys.

Description: Using the monthly precipitation data at 14 stations from 1961 to 2012, the precipitation trends at the seasonal and annual scales are analyzed using the Mann–Kendall test in the Southeast Tibetan Plateau (STP). The area-averaged precipitation shows an increasing trend in the STP with strong temporal and spatial variations. The seasonal and annual precipitation increased, except in the summer; the annual precipitation increased by about one millimeter pre year over the last 52 years. The spring precipitation significantly increased at the 99% confidence level, while the mean summer precipitation insignificantly decreased at the 95% confidence level. The extreme precipitation, including the maxima and minima, also experienced overall increases. More than 78% of the stations exhibited increases in the annual precipitation (93% in spring). The precipitation variation with elevation was not obvious, but the variation with complex topography was obvious in the STP. The largest precipitation increases and decreases occurred in high-precipitation areas, while the increasing precipitation was dominant in or near the main area of the Tibetan Plateau (TP). The results of this study reveal the spatio-temporal variability in the precipitation trends in the STP for the first time. The results are beneficial for understanding the local climate characteristics in the STP and in the entire TP.

Description: The objectives of this study were to investigate seasonal variation of greenhouse gas fluxes from soils on sites dominated by plantation (Robinia pseudoacacia, Punica granatum, and Ziziphus jujube) and natural regenerated forests (Vitex negundo var. heterophylla, Leptodermis oblonga, and Bothriochloa ischcemum), and to identify how tree species, litter exclusion, and soil properties (soil temperature, soil moisture, soil organic carbon, total N, soil bulk density, and soil pH) explained the temporal and spatial variance in soil greenhouse gas fluxes. Fluxes of greenhouse gases were measured using static chamber and gas chromatography techniques. Six static chambers were randomly installed in each tree species. Three chambers were randomly designated to measure the impacts of surface litter exclusion, and the remaining three were used as a control. Field measurements were conducted biweekly from May 2010 through April 2012. Soil CO2 emissions from all tree species were significantly affected by soil temperature, soil moisture, and their interaction. Driven by the seasonality of temperature and precipitation, soil CO2 emissions demonstrated a clear seasonal pattern, with fluxes significantly higher during the rainy season than during the dry season. Soil CH4 and N2O fluxes were not significantly correlated with soil temperature, soil moisture, or their interaction, and no significant seasonal differences were detected. Soil CO2 and N2O fluxes were significantly correlated with soil organic carbon, total N, and soil bulk density, while soil pH was not correlated with CO2 and N2O emissions. Soil CH4 fluxes did not display pronounced dependency on soil organic carbon, total N, soil bulk density, and soil pH. Removal of surface litter resulted in significant decreases in CO2 emissions and CH4 uptakes, but had no significant influence on N2O fluxes. Soils in six tree species acted as sinks for atmospheric CH4. With the exception of Ziziphus jujube, Soils in all sites acted as sinks for atmospheric N2O. Tree species had a significant effect on CO2 and N2O fluxes but not on CH4 uptake. The lower net global warming potential in natural regenerated vegetation suggested that natural regenerated vegetation were more desirable plant species in reducing global warming.

Description: This study investigates a cross-variable ozone data assimilation (DA) method based on an ensemble Kalman filter (EnKF) that has been validated as an efficient approach for improving ozone forecasts. The main purpose is to delve into the impacts of the cross-variable adjustment of nitrogen oxides (NOx) emissions on the nitrogen dioxide (NO2) forecasts over Beijing and surrounding regions during the 2008 Beijing Olympic Games. A mixed effect on the NO2 forecasts was observed during the application of the cross-variable assimilation approach in real-data assimilation (RDA) experiments. The method improved the NO2 forecast over almost half of the urban sites with reductions of the root mean square errors (RMSEs) by 15–36 % in contrast to big increases of the RMSEs over other urban stations by 56–239 %. Over the urban stations with negative DA impacts, improvement of the NO2 forecasts with 7 % reduction of the RMSEs was noticed during the night and the morning vs. significant deterioration of the forecasts during daytime with 190 % increase of the RMSEs, suggesting the negative DA impacts mainly occurred during daytime. Ideal data assimilation (IDA) experiments with a box model and the same cross-variable assimilation method, as a further investigation, confirmed the mixed effects found in the RDA experiments. An improvement of the NOx emission estimation was obtained from the cross-variable assimilation under relatively small errors in the prior estimation of NOx emissions during daytime, while deterioration of the NOx emission estimation was found under large biases in the prior estimation of NOx emissions during daytime. However, the cross-variable assimilation improved the NOx emission estimations during the night and the morning even with large biases in the prior estimations. The mixed effects observed in the cross-variable assimilation, i.e., positive DA impacts on NO2 forecast over some urban sites, negative DA impacts over the other urban sites and weak DA impacts over suburban sites, were found to be strongly associated with the linearization of the EnKF at the analysis step and the fast variability of the relationship between ozone concentrations and NOx emissions. When the uncertainties of the daytime ozone concentrations were strongly nonlinearly related to those of the NOx emissions, the linearization analysis of the EnKF brought out an inefficient or a wrong adjustment to the NOx emissions during the daytime. The results of this study provided further explanations for the negative DA impacts observed in previous chemical DA studies and highlighted the limitation of the existing chemical DA methods under high nonlinearity.

Description: Aliphatic amines can form secondary aerosol via oxidation with atmospheric radicals (e.g. hydroxyl radical and nitrate radical). The particle composition can contain both secondary organic aerosol (SOA) and inorganic salts. The fraction of organic to inorganic materials in the particulate phase influences aerosol hygroscopicity and cloud condensation nuclei (CCN) activity. SOA formed from trimethylamine (TMA) and butylamine (BA) reactions with hydroxyl radical (OH) is composed of organic material of low hygroscopicity (single hygroscopicity parameter, κ ≤ 0.25). Secondary aerosol formed from the tertiary aliphatic amine (TMA) with N2O5 (source of nitrate radical, NO3), contains less volatile compounds than the primary aliphatic amine (BA) aerosol. TMA + N2O5 form semi-volatile organics in low RH conditions that have κ ~ 0.20, indicative of slightly soluble organic material. As RH increases, several inorganic amine salts are formed as a result of acid-base reactions. The CCN activity of the humid TMA-N2O5 aerosol obeys Zdanovskii, Stokes, and Robinson (ZSR) ideal mixing rules. Higher CCN activity (κ 〉 0.3) was also observed for humid BA+N2O5 aerosols compared with dry aerosol (κ ~ 0.2), as a result of the formation of inorganic salts such as NH4NO3 and butylamine nitrate (C4H11N · HNO3). Compared with TMA, BA+N2O5 reactions produce more volatile aerosols. The BA+N2O5 aerosol products under humid experiments were found to be very sensitive to the temperature within the stream-wise continuous flow thermal gradient CCN counter. The CCN counter, when set above a 21 °C temperature difference, evaporates BA+N2O5 aerosol formed at RH ≥ 30%; κ ranges from 0.4 to 0.7 and is dependent on the instrument supersaturation (ss) settings. The aerosol behaves non-ideally, hence simple ZSR rules cannot be applied to the CCN results from the primary aliphatic amine system. Overall, aliphatic amine aerosol systems κ ranges from 0.2 〈 κ 〈 0.7. This work indicates that aerosols formed via nighttime reactions with amines are likely to produce hygroscopic and volatile aerosol whereas photochemical reactions with OH produce secondary organic aerosol of lower CCN activity. Thermal gradient CCN counters measurement will impact the observed CCN activity of volatile aerosol formed via a nitric acid pathway. The contributions of semi-volatile secondary organic and inorganic material from aliphatic amines must be considered for accurate hygroscopicity and CCN predictions from aliphatic amine systems.

Description: We performed ozone data assimilation by simultaneously adjusting the ozone initial conditions, precursor initial conditions and emissions based on the Ensemble Kalman Filter (EnKF) and assessed its impacts on ozone modeling and forecasting in Beijing and nearby regions. A high-resolution regional air quality model and a newly established regional monitoring network covering Beijing and its surrounding areas were employed. At each assimilation cycle, the forecast error covariance was sampled from a set of forecast ensembles that were generated by perturbing ozone precursor initial conditions, emissions, photolysis rates and deposition velocity. A model-error module and a local analysis scheme have been introduced to reduce the impact of filter divergence and spurious correlation that accompanied with EnKF. The results showed significant improvement of 1-hour ozone forecast in Beijing and its surrounding areas through separately adjusting ozone initial conditions, precursor initial conditions and emissions with ozone observations. However, adjustment of precursor initial conditions and emissions had minor effect on the 1-hour ozone forecast in suburban area. The best ozone forecast skill was obtained through jointly adjusting ozone initial conditions, NOx and VOC initial conditions and emissions. The root mean square errors of 1-hour ozone forecast at urban sites and suburban sites decreased by 54% and 59% respectively compared with those in free run. Furthermore, the specific impacts of observations from urban and suburban sites on ozone data assimilation were evaluated by implementing sensitivity experiments. Both urban and suburban sites were found to be very important for the improvement of regional ozone forecast. The importance of observational data at urban sites was particularly highlighted through its role in constraining the uncertainty of precursor initial conditions and emission rates. Further improvement of regional ozone forecast might therefore be expected with more routine regional air pollution monitoring stations.