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

Abstract. Trends in air quality across the Northern Hemisphere over a 21-year period (1990–2010) were simulated using the Community Multiscale Air Quality (CMAQ) multiscale chemical transport model driven by meteorology from Weather Research and Forecasting (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 2 decades in three regions: eastern China, the continental United States and Europe.

The model successfully reproduced the observed decreasing trends in SO₂, NO₂, 8 h O₃ maxima, SO₄²⁻ and elemental carbon (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 (China – Air Pollution Index) from 2005 to 2010, but reproduced the observed pattern of O₃ trends shown in three World Data Centre for Greenhouse Gases (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 (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 Monitoring and Evaluation Programme), SO₂ is overestimated (NMB from 4 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 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 eastern China over the past 2 decades suggests that control strategies that focus on combined control of NO_x and volatile organic compound (VOC) emissions with a ratio of 0.46 may provide the most effective means for O₃ reductions for the region devoid of nonlinear response potentially associated with NO_x or VOC limitation resulting from alternate strategies. The response of O₃ is more sensitive to changes in NO_x 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 NO_x controls in reducing the relative fraction of aerosol NO₃⁻ formed from declining NO_x emissions in the eastern US, while the control effectiveness was assured by the simultaneous control of NH₃ emission in Europe.