An inversion of NOx and non-methane volatile organic compound (NMVOC) emissions using satellite observations during the KORUS-AQ campaign and implications for surface ozone over East Asia
Authors: Souri Amir H. , Nowlan Caroline R. , González Abad Gonzalo , Zhu Lei , Blake Donald R. , Fried Alan , Weinheimer Andrew J. , Wisthaler Armin , Woo Jung-Hun , Zhang Qiang , Chan Miller Christopher E. , Liu Xiong , Chance Kelly 
Published: August 2020

Abstract. The absence of up-to-date emissions has been a major impediment to
accurately simulating aspects of atmospheric chemistry and to precisely
quantifying the impact of changes in emissions on air pollution. Hence, a
nonlinear joint analytical inversion (Gauss–Newton method) of both volatile
organic compounds (VOCs) and nitrogen oxide (NOx) emissions is made by
exploiting the Smithsonian Astrophysical Observatory (SAO) Ozone Mapping and
Profiler Suite Nadir Mapper (OMPS-NM) formaldehyde (HCHO) and the National
Aeronautics and Space Administration (NASA) Ozone Monitoring Instrument
(OMI) tropospheric nitrogen dioxide (NO2) columns during the
Korea–United States Air Quality (KORUS-AQ) campaign over East Asia in
May–June 2016. Effects of the chemical feedback of NOx and VOCs on both
NO2 and HCHO are implicitly included by iteratively optimizing the
inversion. Emission uncertainties are greatly narrowed (averaging
kernels > 0.8, which is the mathematical presentation of the
partition of information gained from the satellite observations with respect
to the prior knowledge) over medium- to high-emitting areas such as cities
and dense vegetation. The prior amount of total NOx emissions is mainly
dictated by values reported in the MIX-Asia 2010 inventory. After the
inversion we conclude that there is a decline in emissions (before, after, change) for
China (87.94±44.09 Gg d−1, 68.00±15.94 Gg d−1, −23 %), North
China Plain (NCP) (27.96±13.49 Gg d−1, 19.05±2.50 Gg d−1,
−32 %), Pearl River Delta (PRD) (4.23±1.78 Gg d−1, 2.70±0.32 Gg d−1, −36 %), Yangtze River Delta (YRD) (9.84±4.68 Gg d−1,
5.77±0.51 Gg d−1, −41 %), Taiwan (1.26±0.57 Gg d−1,
0.97±0.33 Gg d−1, −23 %), and Malaysia (2.89±2.77 Gg d−1,
2.25±1.34 Gg d−1, −22 %), all of which have effectively implemented
various stringent regulations. In contrast, South Korea (2.71±1.34 Gg d−1, 2.95±0.58 Gg d−1, +9 %) and Japan (3.53±1.71 Gg d−1, 3.96±1.04 Gg d−1, +12 %) are experiencing an increase in
NOx emissions, potentially due to an increased number of diesel vehicles and
new thermal power plants. We revisit the well-documented positive bias (by a
factor of 2 to 3) of MEGAN v2.1 (Model of Emissions of Gases and Aerosols
from Nature) in terms of biogenic VOC emissions in
the tropics. The inversion, however, suggests a larger growth of VOCs (mainly
anthropogenic) over NCP (25 %) than previously reported (6 %) relative
to 2010. The spatial variation in both the magnitude and sign of NOx and
VOC emissions results in nonlinear responses of ozone production and loss. Due
to a simultaneous decrease and increase in NOx∕VOC over NCP and YRD, we
observe a ∼53 % reduction in the ratio of the chemical loss
of NOx (LNOx) to the chemical loss of ROx
(RO2+HO2) over the surface transitioning toward NOx-sensitive regimes, which
in turn reduces and increases the afternoon chemical loss and production of ozone
through NO2+OH (−0.42 ppbv h−1)∕HO2 (and RO2)+NO
(+0.31 ppbv h−1). Conversely, a combined decrease in NOx and
VOC emissions in Taiwan, Malaysia, and southern China suppresses the
formation of ozone. Simulations using the updated emissions indicate
increases in maximum daily 8 h average (MDA8) surface ozone over China
(0.62 ppbv), NCP (4.56 ppbv), and YRD (5.25 ppbv), suggesting that emission
control strategies on VOCs should be prioritized to curb ozone production
rates in these regions. Taiwan, Malaysia, and PRD stand out as regions
undergoing lower MDA8 ozone levels resulting from the NOx reductions
occurring predominantly in NOx-sensitive regimes.