ALBERT

Description: Trends in air quality across the Northern Hemisphere over a 21 year period (1990–2010) were simulated using the CMAQ multiscale chemical transport model driven by meteorology from WRF simulations and internally consistent historical emission inventories obtained from EDGAR. Thorough comparison with several ground observation networks mostly over Europe and North America was conducted to evaluate the model performance as well as the ability of CMAQ to reproduce the observed trends in air quality over the past two decades in three regions: eastern China, the continental United States and Europe. The model successfully reproduced the observed decreasing trends in SO2, NO2, maxima 8 h O3, SO42− and EC in the US and Europe. However, the model fails to reproduce the decreasing trends in NO3− in the US, potentially pointing to uncertainties of NH3 emissions. The model failed to capture the 6 year trends of SO2 and NO2 in CN-API from 2005–2010, but reproduced the observed pattern of O3 trends shown in three WDCGG sites over eastern Asia. Due to the coarse spatial resolution employed in these calculations, predicted SO2 and NO2 concentrations are underestimated relative to all urban networks, i.e., US-AQS (NMB = −46 and −54%), EU-AIRBASE (NMB = −12 and −57%) and CN-API (NMB = −36 and −68%). Conversely, at the rural network EU-EMEP SO2 is overestimated (NMB from 4 to 150%) while NO2 is simulated well (NMB within ±15%) in all seasons. Correlations between simulated and observed winter time daily maxima 8 h (DM8) O3 are poor compared to other seasons for all networks. Better correlation between simulated and observed SO42− was found compared to that for SO2. Underestimation of summer SO42− in the US may be associated with the uncertainty in precipitation and associated wet scavenging representation in the model. The model exhibits worse performance for NO3− predictions, particularly in summer, due to high uncertainties in the gas/particle partitioning of NO3− as well as seasonal variations of NH3 emissions. There are high correlations (R 〉 0.5) between observed and simulated EC, although the model underestimates the EC concentration by 65% due to the coarse grid resolution as well as uncertainties in the PM speciation profile associated with EC emissions. The almost linear response seen in the trajectory of modeled O3 changes in the eastern China over the past two decades, suggests that control strategies that focus on combined control of NOx and VOC emissions with a ratio of 0.46 may provide the most effective means for O3 reductions for the region devoid of non-linear response potentially associated with NOx or VOC limitation resulting from alternate strategies. The response of O3 is more sensitive to changes in NOx emissions in the eastern US because the relative abundance of biogenic VOC emissions tends to reduce the effectiveness of VOC controls. Increasing NH3 levels offset the relative effectiveness of NOx controls in reducing the relative fraction of aerosol NO3− formed from declining NOx emissions in the eastern US, while the control effectiveness was assured by the simultaneous control of NH3 emission in Europe.

Description: The ability of a coupled meteorology-chemistry model, i.e., WRF-CMAQ, in reproducing the historical trend in AOD and clear-sky short-wave radiation (SWR) over the Northern Hemisphere has been evaluated through a comparison of 21 year simulated results with observation-derived records from 1990–2010. Six satellite retrieved AOD products including AVHRR, TOMS, SeaWiFS, MISR, MODIS-terra and -aqua as well as long-term historical records from 11 AERONET sites were used for the comparison of AOD trends. Clear-sky SWR products derived by CERES at both TOA and surface as well as surface SWR data derived from seven SURFRAD sites were used for the comparison of trends in SWR. The model successfully captured increasing AOD trends along with the corresponding increased TOA SWR (upwelling) and decreased surface SWR (downwelling) in both eastern China and the northern Pacific. The model also captured declining AOD trends along with the corresponding decreased TOA SWR (upwelling) and increased surface SWR (downwelling) in eastern US, Europe and northern Atlantic for the period of 2000–2010. However, the model underestimated the AOD over regions with substantial natural dust aerosol contributions, such as the Sahara Desert, Arabian Desert, central Atlantic and north Indian Ocean. Estimates of aerosol direct radiative effect (DRE) at TOA are comparable with those derived by measurements. Compared to GCMs, the model exhibits better estimates of surface- aerosol direct radiative efficiency (Eτ). However, surface-DRE tends to be underestimated due to the underestimated AOD in land and dust regions. Further investigation of TOA-Eτ estimations as well as the dust module used for estimates of windblown-dust emissions is needed.

Description: An innovative Extended Response Surface Modeling technique (ERSM v1.0) is developed to characterize the nonlinear response of fine particles (PM2.5) to large and simultaneous changes of multiple precursor emissions from multiple regions and sectors. The ERSM technique is developed starting from the conventional Response Surface Modeling (RSM) technique; it first quantifies the relationship between PM2.5 concentrations and precursor emissions in a single region with the conventional RSM technique, and then assesses the effects of inter-regional transport of PM2.5 and its precursors on PM2.5 concentrations in the target region. We apply this novel technique with a widely used regional air quality model over the Yangtze River Delta (YRD) region of China, and evaluate the response of PM2.5 and its inorganic components to the emissions of 36 pollutant-region-sector combinations. The predicted PM2.5 concentrations agree well with independent air quality model simulations; the correlation coefficients are larger than 0.98 and 0.99, and the mean normalized errors are less than 1 and 2% for January and August, respectively. It is also demonstrated that the ERSM technique could reproduce fairly well the response of PM2.5 to continuous changes of precursor emission levels between zero and 150%. Employing this new technique, we identify the major sources contributing to PM2.5 and its inorganic components in the YRD region. The nonlinearity in the response of PM2.5 to emission changes is characterized and the underlying chemical processes are illustrated.

Description: An innovative extended response surface modeling technique (ERSM v1.0) is developed to characterize the nonlinear response of fine particles (PM2.5) to large and simultaneous changes of multiple precursor emissions from multiple regions and sectors. The ERSM technique is developed based on the conventional response surface modeling (RSM) technique; it first quantifies the relationship between PM2.5 concentrations and the emissions of gaseous precursors from each single region using the conventional RSM technique, and then assesses the effects of inter-regional transport of PM2.5 and its gaseous precursors on PM2.5 concentrations in the target region. We apply this novel technique with a widely used regional chemical transport model (CTM) over the Yangtze River delta (YRD) region of China, and evaluate the response of PM2.5 and its inorganic components to the emissions of 36 pollutant–region–sector combinations. The predicted PM2.5 concentrations agree well with independent CTM simulations; the correlation coefficients are larger than 0.98 and 0.99, and the mean normalized errors (MNEs) are less than 1 and 2% for January and August, respectively. It is also demonstrated that the ERSM technique could reproduce fairly well the response of PM2.5 to continuous changes of precursor emission levels between zero and 150%. Employing this new technique, we identify the major sources contributing to PM2.5 and its inorganic components in the YRD region. The nonlinearity in the response of PM2.5 to emission changes is characterized and the underlying chemical processes are illustrated.

Description: Long-term simulations with the coupled WRF–CMAQ (Weather Research and Forecasting–Community Multi-scale Air Quality) model have been conducted to systematically investigate the changes in anthropogenic emissions of SO2 and NOx over the past 16 years (1995–2010) across the United States (US), their impacts on anthropogenic aerosol loading over North America, and subsequent impacts on regional radiation budgets. In particular, this study attempts to determine the consequences of the changes in tropospheric aerosol burden arising from substantial reductions in emissions of SO2 and NOx associated with control measures under the Clean Air Act (CAA) especially on trends in solar radiation. Extensive analyses conducted by Gan et al. (2014a) utilizing observations (e.g., SURFRAD, CASTNET, IMPROVE, and ARM) over the past 16 years (1995–2010) indicate a shortwave (SW) radiation (both all-sky and clear-sky) "brightening" in the US. The relationship of the radiation brightening trend with decreases in the aerosol burden is less apparent in the western US. One of the main reasons for this is that the emission controls under the CAA were aimed primarily at reducing pollutants in areas violating national air quality standards, most of which were located in the eastern US, while the relatively less populated areas in the western US were less polluted at the beginning of this study period. Comparisons of model results with observations of aerosol optical depth (AOD), aerosol concentration, and radiation demonstrate that the coupled WRF–CMAQ model is capable of replicating the trends well even though it tends to underestimate the AOD. In particular, the sulfate concentration predictions were well matched with the observations. The discrepancies found in the clear-sky diffuse SW radiation are likely due to several factors such as the potential increase of ice particles associated with increasing air traffic, the definition of "clear-sky" in the radiation retrieval methodology, and aerosol semi-direct and/or indirect effects which cannot be readily isolated from the observed data.

Description: Satellite remote sensing of tropospheric nitrogen dioxide (NO2) can provide valuable information for estimating surface nitrogen oxides (NOx) emissions. Using an exponentially modified Gaussian (EMG) method and taking into account the effect of wind on observed NO2 distributions, we estimate 3-year moving-average emissions of summertime NOx from 35 US (United States) urban areas directly from NO2 retrievals of the Ozone Monitoring Instrument (OMI) during 2005–2014. Following conclusions of previous studies that the EMG method provides robust and accurate emission estimates under strong-wind conditions, we derive top-down NOx emissions from each urban area by applying the EMG method to OMI data with wind speeds greater than 3–5 m s−1. Meanwhile, we find that OMI NO2 observations under weak-wind conditions (i.e., 〈 3 m s−1) are qualitatively better correlated to the surface NOx source strength in comparison to all-wind OMI maps; therefore, we use them to calculate the satellite-observed NO2 burdens of urban areas and compare with NOx emission estimates. The EMG results show that OMI-derived NOx emissions are highly correlated (R 〉 0.93) with weak-wind OMI NO2 burdens as well as with bottom-up NOx emission estimates over 35 urban areas, implying a linear response of the OMI observations to surface emissions under weak-wind conditions. The simultaneous EMG-obtained effective NO2 lifetimes (~ 3.5 ± 1.3 h), however, are biased low in comparison to the summertime NO2 chemical lifetimes. In general, isolated urban areas with NOx emission intensities greater than ~ 2 Mg h−1 produce statistically significant weak-wind signals in 3-year average OMI data. From 2005 to 2014, we estimate that total OMI-derived NOx emissions over all selected US urban areas decreased by 49 %, consistent with reductions of 43, 47, 49, and 44 % in the total bottom-up NOx emissions, the sum of weak-wind OMI NO2 columns, the total weak-wind OMI NO2 burdens, and the averaged NO2 concentrations, respectively, reflecting the success of NOx control programs for both mobile sources and power plants. The decrease rates of these NOx-related quantities are found to be faster (i.e., −6.8 to −9.3 % yr−1) before 2010 and slower (i.e., −3.4 to −4.9 % yr−1) after 2010. For individual urban areas, we calculate the R values of pair-wise trends among the OMI-derived and bottom-up NOx emissions, the weak-wind OMI NO2 burdens, and ground-based NO2 measurements, and high correlations are found for all urban areas (median R= 0.8), particularly large ones (R up to 0.97). The results of the current work indicate that using the EMG method and considering the wind effect, the OMI data allow for the estimation of NOx emissions from urban areas and the direct constraint of emission trends with reasonable accuracy.

Description: The ability of a coupled meteorology–chemistry model, i.e., Weather Research and Forecast and Community Multiscale Air Quality (WRF-CMAQ), to reproduce the historical trend in aerosol optical depth (AOD) and clear-sky shortwave radiation (SWR) over the Northern Hemisphere has been evaluated through a comparison of 21-year simulated results with observation-derived records from 1990 to 2010. Six satellite-retrieved AOD products including AVHRR, TOMS, SeaWiFS, MISR, MODIS-Terra and MODIS-Aqua as well as long-term historical records from 11 AERONET sites were used for the comparison of AOD trends. Clear-sky SWR products derived by CERES at both the top of atmosphere (TOA) and surface as well as surface SWR data derived from seven SURFRAD sites were used for the comparison of trends in SWR. The model successfully captured increasing AOD trends along with the corresponding increased TOA SWR (upwelling) and decreased surface SWR (downwelling) in both eastern China and the northern Pacific. The model also captured declining AOD trends along with the corresponding decreased TOA SWR (upwelling) and increased surface SWR (downwelling) in the eastern US, Europe and the northern Atlantic for the period of 2000–2010. However, the model underestimated the AOD over regions with substantial natural dust aerosol contributions, such as the Sahara Desert, Arabian Desert, central Atlantic and northern Indian Ocean. Estimates of the aerosol direct radiative effect (DRE) at TOA are comparable with those derived by measurements. Compared to global climate models (GCMs), the model exhibits better estimates of surface-aerosol direct radiative efficiency (Eτ). However, surface-DRE tends to be underestimated due to the underestimated AOD in land and dust regions. Further investigation of TOA-Eτ estimations as well as the dust module used for estimates of windblown-dust emissions is needed.

Description: Photodegradation of secondary organic aerosol (SOA) prepared by ozone-initiated oxidation of D-limonene is studied with an action spectroscopy approach, which relies on detection of volatile photoproducts with chemical ionization mass-spectrometry as a function of the UV irradiation wavelength. Efficient photodegradation is observed for a broad range of ozone and D-limonene concentrations (0.1–300 ppm) used in the preparation of SOA. The observed photoproducts are dominated by oxygenated C1-C3 compounds such as methanol, formic acid, acetaldehyde, acetic acid, and acetone. The irradiation wavelength dependence of the combined yield of the photoproducts closely tracks the absorption spectrum of the SOA material suggesting that photodegradation is not limited to the UV wavelengths. Kinetic simulations suggest that RO2+HO2/RO2 reactions represent the dominant route to photochemically active carbonyl and peroxide species in the limonene SOA material. Similar photodegradation processes are likely to occur in realistic SOA produced by OH- or O3-initiated oxidation of biogenic volatile organic compounds in clean air.

Description: Statistical response surface method (RSM) is successfully applied in Community Multi-scale Air Quality model (CMAQ) analysis on ozone sensitivity studies. Prediction performance has been validated through cross validation, out of sample validation and isopleths validation. Sample methods and key parameters including the maximum numbers for variables involving in statistic interpolation as well as training sample number have been tested and selected through computational experiments. Overall impacts from individual sources including local/regional NOx and VOC emission sources and NOx emissions from power plants for three megacities as Beijing, Shanghai and Guangzhou have been evaluated through RSM analysis under a July 2005 modeling study. NOx control appears to be beneficial for ozone reduction in the downwind areas where usually have higher ozone levels, and it's likely to be more effective than anthropogenic VOC control during heavy photochemical pollution period. Regional NOx sources are strong contributors to surface ozone mixing. Local NOx emission control without regional involvement may bring the risk of increasing urban ozone levels due to the VOC-limited conditions, but it gives considerable control benefit for ozone in upper layers (up to 1 km, where the ozone chemistry is changed to NOx-limited condition) and helps to improve regional air quality in the downwind areas. Effectiveness of NOx emission control is growing along with stricter control efforts, therefore an integrated regional and multi-pollutant control policy is necessary to mitigate ozone problem in China.

Description: Dust invasion is an important type of particle pollution in China. During 1 to 6 May in 2011, a dust event was observed in the Yangtze River Delta region (YRD). The highest PM10 (particles up to 10 μ in diameter) concentration reached over 1000 μg m−3 and the visibility was below 3 km. In this study, the Community Multi-scale Air Quality modeling system (CMAQ5.0) coupled with an in-line windblown dust model was used to simulate the formation, spatial and temporal characteristics of this dust event, and analyze its impacts. The threshold friction velocity for loose, fine-grained soil with low surface roughness in the dust model was revised based on Chinese data to improve the model performance. This dust storm broke out in Xinjiang and Mongolia during 28 to 30 April and arrived in the YRD region on 1 May. The transported dust particles contributed to the mean surface layer concentrations of PM10 in the YRD region 78.9% during 1 to 6 May with their impact weakening from north to south due to the removal of dust particles along the path. The dry deposition, wet deposition and total deposition of PM10 in the YRD reached 184.7 kt, 172.6 kt and 357.32 kt, respectively. The dust particles also had significant impacts on optical/radiative characteristics by absorption and scattering. In Shanghai, the largest perturbations of aerosol optical depth (AOD) and irradiance were about 0.8 DU and −130 W m−2, which could obviously influence the radiation balance in this region. The decrease of actinic fluxes impacts future photochemistry. In Shanghai, the negative effects on the NO2 and O3 photolysis could be −35% when dust particles arrived. The concentrations of O3 and OH were reduced by 1.5% and 3.1% in the whole of China, and by 9.4% and 12.1% in the YRD region, respectively. Such changes in O3 and OH levels can affect the future formation of secondary aerosols in the atmosphere by directly determining the oxidation rate of their precursors. The work of this manuscript is meaningful for understanding the dust emissions in China as well as for the application of CMAQ in Asia. It is also helpful for understanding the formation mechanism and impacts of dust pollution in the YRD.