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Trends in air quality across the Northern Hemisphere over a 21-year period(1990–2010) were simulated using the Community Multiscale AirQuality (CMAQ) multiscale chemical transportmodel driven by meteorology from Weather Research and Forecasting (WRF) simulations and internally consistenthistorical emission inventories obtained from EDGAR. Thorough comparisonwith several ground observation networks mostly over Europe and NorthAmerica was conducted to evaluate the model performance as well as theability of CMAQ to reproduce the observed trends in air quality over thepast 2 decades in three regions: eastern China, the continental UnitedStates and Europe. The model successfully reproduced the observed decreasing trends inSO2, NO2, 8 h O3 maxima, SO42− and elemental carbon (EC) in the USand Europe. However, the model fails to reproduce the decreasing trends inNO3− in the US, potentially pointing to uncertainties of NH3emissions. The model failed to capture the 6-year trends of SO2 andNO2 in CN-API (China – AirPollution Index) from 2005 to 2010, but reproduced the observed pattern ofO3 trends shown in three World Data Centre for Greenhouse Gases (WDCGG) sites over eastern Asia. Due to thecoarse spatial resolution employed in these calculations, predicted SO2and NO2 concentrations are underestimated relative to all urbannetworks, i.e., US-AQS (US – Air Quality System; normalized mean bias (NMB) = −38% and −48%), EU-AIRBASE(European Air quality data Base; NMB = −18 and −54%) and CN-API (NMB = −36 and −68%).Conversely, at the rural network EU-EMEP (European Monitoringand Evaluation Programme), SO2 is overestimated (NMB from4 to 150%) while NO2 is simulated well (NMB within ±15%)in all seasons. Correlations between simulated and observed O3 wintertime daily 8 h maxima (DM8) are poor compared to other seasons forall networks. Better correlation between simulated and observedSO42− was found compared to that for SO2. Underestimation ofsummer SO42− in the US may be associated with the uncertainty inprecipitation and associated wet scavenging representation in the model. Themodel exhibits worse performance for NO3− predictions,particularly in summer, due to high uncertainties in the gas/particlepartitioning of NO3− as well as seasonal variations of NH3emissions. There are high correlations (R > 0.5) between observedand simulated EC, although the model underestimates the EC concentration by65% due to the coarse grid resolution as well as uncertainties in the PMspeciation profile associated with EC emissions. The almost linear response seen in the trajectory of modeled O3 changesin eastern China over the past 2 decades suggests that controlstrategies that focus on combined control of NOx and volatile organic compound (VOC) emissions witha ratio of 0.46 may provide the most effective means for O3 reductionsfor the region devoid of nonlinear response potentially associated withNOx or VOC limitation resulting from alternate strategies. The responseof O3 is more sensitive to changes in NOx emissions in the easternUS because the relative abundance of biogenic VOC emissions tends to reducethe effectiveness of VOC controls. Increasing NH3 levels offset therelative effectiveness of NOx controls in reducing the relativefraction of aerosol NO3− formed from declining NOx emissionsin the eastern US, while the control effectiveness was assured by thesimultaneous control of NH3 emission in Europe

Observations and modeling of air quality trends over 1990–2010 across the Northern Hemisphere: China, the United States and Europe