Abstract. We developed a top-down methodology combining the inversed chemistry
transport modeling and satellite-derived tropospheric vertical column of
NO2 and estimated the NOx emissions of the Yangtze River Delta (YRD)
region at a horizontal resolution of 9 km for January, April, July, and
October 2016. The effect of the top-down emission estimation on air quality
modeling and the response of ambient ozone (O3) and inorganic aerosols
(SO42-, NO3-, and NH4+, SNA) to the changed
precursor emissions were evaluated with the Community Multi-scale Air
Quality (CMAQ) system. The top-down estimates of NOx emissions were
smaller than those (i.e., the
bottom-up estimates) in a national emission inventory, Multi-resolution Emission Inventory for China (MEIC), for all the 4 months, and the monthly mean was
calculated to be 260.0 Gg/month, 24 % less than the bottom-up one. The
NO2 concentrations simulated with the bottom-up estimate of NOx
emissions were clearly higher than the ground observations, indicating the
possible overestimation in the current emission inventory, attributed to its
insufficient consideration of recent emission control in the region. The
model performance based on top-down estimate was much better, and the
biggest change was found for July, with the normalized mean bias (NMB) and
normalized mean error (NME) reduced from 111 % to −0.4 % and from
111 % to 33 %, respectively. The results demonstrate the improvement of
NOx emission estimation with the nonlinear inversed modeling and
satellite observation constraint. With the smaller NOx emissions in the
top-down estimate than the bottom-up one, the elevated concentrations of
ambient O3 were simulated for most of the YRD, and they were closer to
observations except for July, implying the VOC (volatile organic
compound)-limited regime of O3 formation. With available ground
observations of SNA in the YRD, moreover, better model performance of
NO3- and NH4+ was achieved for most seasons, implying
the effectiveness of precursor emission estimation on the simulation of
secondary inorganic aerosols. Through the sensitivity analysis of O3
formation for April 2016, the decreased O3 concentrations were found
for most of the YRD region when only VOC emissions were reduced or the reduced
rate of VOC emissions was 2 times of that of NOx, implying the
crucial role of VOC control in O3 pollution abatement. The SNA level
for January 2016 was simulated to decline 12 % when 30 % of NH3
emissions were reduced, while the change was much smaller with the same
reduced rate for SO2 or NOx. The result suggests that reducing
NH3 emissions was the most effective way to alleviate SNA pollution of the
YRD in winter.
