Development and application of the WRFDA-Chem three-dimensional variational (3DVAR) system: aiming to improve air quality forecasting and diagnose model deficiencies

To improve the operational air quality forecasting over China, a new aerosol or gas-phase pollutants assimilation capability is developed within the WRFDA system using the three-dimensional variational (3DVAR) algorithm. In this first application, the interface for the MOSAIC (Model for Simulating Aerosol Interactions and Chemistry) aerosol scheme is built with the potential for flexible extension. Based on the new WRFDA-Chem system, five experiments assimilating different surface observations, including PM2.5, PM10, SO2, NO2, O3, and CO, are conducted for January 2017 along with a control experiment without data assimilation (DA). Results show that the WRFDA-Chem system evidently improves the air quality forecasting. From the analysis aspect, the assimilation of surface observations reduces the bias and RMSE in the initial condition (IC) remarkably; from the forecast aspect, better forecast performances are acquired up to 24 h, in which the experiment assimilating the six pollutants simultaneously displays the best forecast skill overall. With respect to the impact of the DA cycling frequency, the responses toward IC updating are found to be different among the pollutants. For PM2.5, PM10, SO2, and CO, the forecast skills increase with the DA frequency. For O3, although improvements are acquired at the 6 h cycling frequency, the advantage of more frequent DA could be consumed by the disadvantages of the unbalanced photochemistry (due to inaccurate precursor NOx ∕ VOC (volatile organic compound) ratios) or the changed titration process (due to changed NO2 concentrations but not NO) from assimilating the existing observations (only O3 and NO2, but no VOC and NO). As yet the finding is based on the 00:00 UTC forecast for this winter season only, and O3 has strong diurnal and seasonal variations. More experiments should be conducted to draw further conclusions. In addition, considering one aspect (IC) in the model is corrected by DA, the deficiencies of other aspects (e.g., chemical reactions) could be more evident. This study explores the model deficiencies by investigating the effects of assimilating gaseous precursors on the forecast of related aerosols. Results show that the parameterization (uptake coefficients) in the newly added sulfate–nitrate–ammonium (SNA)-relevant heterogeneous reactions in the model is not fully appropriate although it best simulates observed SNA aerosols without DA; since the uptake coefficients were originally tuned under the inaccurate gaseous precursor scenarios without DA, the biases from the two aspects (SNA reactions and IC DA) were just compensated. In future chemistry development, parameterizations (such as uptake coefficients) for different gaseous precursor scenarios should be adjusted and verified with the help of the DA technique. According to these results, DA ameliorates certain aspects by using observations as constraints and thus provides an opportunity to identify and diagnose the model deficiencies; it is useful especially when the uncertainties of various aspects are mixed up and the reaction paths are not clearly revealed. In the future, besides being used to improve the forecast through updating IC, DA could be treated as another approach to explore necessary developments in the model.

Harsh Sliding Wear of a Zirconia Ball against a-C:H Coated CoCrMo Disc in Hyaluronic Gel

The a-C:H (amorphous carbon-hydrogen) films belong to the family of DLC (diamond-like carbon) coatings. The a-C:H coating was deposited on medical grade CoCrMo substrates by plasma-assisted chemical vapor deposition (PA-CVD) using benzene as gaseous precursor. Benzene offers an aromatic structure, which affects the a-C:H properties after plasma decomposition. A zirconia ball was sliding at two different frequencies, 50 Hz or 1Hz, against the uncoated and a-C:H coated CoCrMo. The frequency of 1 Hz is typical for human movement during fast walking. The harsh sliding conditions with a normal load of 100 N and 50 Hz frequency simulate extreme overloading of the biomedical sliding partners. It gives insight into the failure mechanisms. The wear tests were carried out in laboratory air (dry, RH: 15.6%) or using hyaluronic gel as lubricant. The hyaluronic gel acts as an effective intermediate medium. It adheres very well to both, a-C:H coating and zirconia. No wear was evident on the ZrO2 ball at 1 Hz and 100 N. Minor wear traces were observed on the a-C:H coating only. A wear coefficient of 0.16 × 10−6 mm3/N·m were calculated for a-C:H coated CoCrMo after ZrO2 ball sliding with 1 Hz and 100 N in hyaluronic gel. This is two orders of magnitude lower in comparison to dry sliding of ZrO2 ball against DLC coated CoCrMo with 1 Hz. The coefficient of friction (COF) remained below 0.09 until the hyaluronic gel starts to lose viscosity. This finding pronounces the importance of a proper homogeneous lubrication during operation of the biomedical joints. For extreme harsh tribological loading like sudden jumps of a patient with artificial joints, the application of an intermediate layer before a-C:H coating needs further evaluation.

Development and application of the WRFDA-Chem 3DVAR system: aiming to improve air quality forecast and diagnose model deficiencies

To improve the operational air quality forecasting over China, a new aerosol/gas phase pollutants assimilation capability is developed within the WRFDA system using 3DVAR algorithm. In this first application, the interface for MOSAIC aerosol scheme is built with flexible extending potentials. Based on the new WRFDA-Chem system, five experiments assimilating different surface observations, including PM2.5, PM10, SO2, NO2, O3, and CO are conducted for January 2017 along with a control experiment without DA. Results exhibit that the WRFDA-Chem system evidently improves the air quality forecasting. On the analysis aspect, the assimilation of surface observations reduces the bias and RMSE in the initial condition (IC) remarkably; on the forecast aspect, better forecast performances are acquired up to 24-h, in which the experiment assimilating the six pollutants simultaneously displays the best forecast skill overall. With respect to the impact of DA cycling frequency, the responses toward IC updating are found out to be different among the pollutants. For PM2.5, PM10, SO2 and CO, the forecast skills increase with the DA frequency; for O3, although improvements are acquired at the 6-h cycling frequency, the advantage of more frequent DA could be consumed by the disadvantage of unbalanced photochemistry (due to inaccurate precursor NOx/VOC ratios) from assimilating the existing observations (only O3 and NO2, but no VOC). Considering after one aspect (IC) in the model is corrected by DA, the deficiencies from other aspects (e.g., chemical reactions) could be more evident, this study further explores the model deficiencies by investigating the effects of assimilating gaseous precursors on the forecast of related aerosols. Results exhibit that the parameterization (uptake coefficients) in the newly added Sulfate-Nitrate-Ammonium (SNA) relevant heterogeneous reactions in the model are not fully appropriate although it best simulates observed SNA aerosols without DA; since the uptake coefficients were originally tuned under the inaccurate gaseous precursor scenarios without DA, the biases from the two aspects (SNA reactions and IC DA) were just compensated. In the future chemistry development, parameterizations (such as uptake coefficients) for different gaseous precursor scenarios should be adjusted and verified with the help of DA technique. According to these results, DA ameliorates certain aspects by using observation as constraints, and thus provides an opportunity to identify and diagnose the model deficiencies; it is useful especially when the uncertainties of various aspects are mixed up and the reaction paths are not clearly revealed. In the future, besides being used to improve the forecast through updating IC, DA could be treated as another approach to explore necessary developments in the model.

Possible heterogeneous chemistry of hydroxymethanesulfonate (HMS) in northern China winter haze

The chemical mechanisms responsible for rapid sulfate production, an important driver of winter haze formation in northern China, remain unclear. Here, we propose a potentially important heterogeneous hydroxymethanesulfonate (HMS) chemical mechanism. Through analyzing field measurements with aerosol mass spectrometry, we show evidence for a possible significant existence in haze aerosols of organosulfur primarily as HMS, misidentified as sulfate in previous observations. We estimate that HMS can account for up to about one-third of the sulfate concentrations unexplained by current air quality models. Heterogeneous production of HMS by SO2 and formaldehyde is favored under northern China winter haze conditions due to high aerosol water content, moderately acidic pH values, high gaseous precursor levels, and low temperature. These analyses identify an unappreciated importance of formaldehyde in secondary aerosol formation and call for more research on sources and on the chemistry of formaldehyde in northern China winter.

Dominant contribution of oxygenated organic aerosol to haze particles from real-time observation in Singapore during an Indonesian wildfire event in 2015

Recurring transboundary haze from Indonesian wildfires in previous decades significantly elevated particulate matter (PM) concentrations in Southeast Asia. During that event on 10 to 31 October 2015, we conducted a real-time observation of non-refractory submicron PM (NR-PM1) in Singapore using an Aerodyne aerosol mass spectrometer. Simultaneously, we characterized carbonaceous components and organic aerosol (OA) tracers from fine PM (PM2.5) samples to support source apportionment of the online measurements. The real-time analysis demonstrated that OA accounted for approximately 80 % of NR-PM1 mass during the wildfire haze period. Source apportionment analysis applied to the OA mass spectra using the multilinear-engine (ME-2) approach resulted in four factors: hydrocarbon-like OA (HOA), biomass burning OA (BBOA), peat burning OA (PBOA), and oxygenated OA (OOA). The OOA can be considered as a surrogate of both secondary organic aerosol (SOA) and oxidized primary organic aerosol (OPOA), while the other factors are considered as surrogates of primary organic aerosol (POA). The OOA accounted for approximately 50 % of the total OA mass in NR-PM1, while POA subtypes from wildfires (BBOA and PBOA) contributed to approximately 30 % of the total OA mass. Our findings highlight the importance of atmospheric chemical processes, which likely include POA oxidation and SOA formation from oxidation of gaseous precursors, to the OOA concentration. As this research could not separately quantify the POA oxidation and SOA formation processes, further studies should attempt to investigate the contribution of gaseous precursor oxidation and POA aging to the OOA formation in wildfire plumes.

Possible heterogeneous hydroxymethanesulfonate (HMS) chemistry in northern China winter haze and implications for rapid sulfate formation

Chemical mechanisms responsible for rapid sulfate production, an important driver of winter haze formation in northern China, remain unclear. Here, we propose a potentially important heterogeneous hydroxymethanesulfonate (HMS) chemical mechanism. Through analyzing field measurements with aerosol mass spectrometry, we show evidence for a possible significant existence in haze aerosols of organosulfur primarily as HMS, misidentified as sulfate in previous observations. We estimate that HMS can account for up to about one-third of the sulfate concentrations unexplained by current air quality models. In addition, HMS in the presence of hydroxyl radicals can trigger rapid sulfate production in aerosol water. Heterogeneous production of HMS by SO2 and formaldehyde is favored under northern China winter haze conditions due to high aerosol water content, moderately acidic pH values, high gaseous precursor levels, and low temperature. These analyses identify an unappreciated importance of formaldehyde in secondary aerosol formation and calls for more research on sources and on the chemistry of formaldehyde in northern China winter.