Soil Emissions of Reactive Nitrogen Accelerate Summertime Surface Ozone Increases in the North China Plain

Abstract

Summertime surface ozone in China has been increasing since 2013 despite the policy-driven reduction in fuel combustion emissions of nitrogen oxides (NO x ). Here we examine the role of soil reactive nitrogen (N r , including NO x and nitrous acid (HONO)) emissions in the 2013−2019 ozone increase over the North China Plain (NCP), using GEOS-Chem chemical transport model simulations. We update soil NO x emissions and add soil HONO emissions in GEOS-Chem based on observation-constrained parametrization schemes. The model estimates significant daily maximum 8 h average (MDA8) ozone enhancement from soil N r emissions of 8.0 ppbv over the NCP and 5.5 ppbv over China in June−July 2019. We identify a strong competing effect between combustion and soil N r sources on ozone production in the NCP region. We find that soil N r emissions accelerate the 2013−2019 June-July ozone increase over the NCP by 3.0 ppbv. The increase in soil N r ozone contribution, however, is not primarily driven by weather-induced increases in soil N r emissions, but by the concurrent decreases in fuel combustion NO x emissions, which enhance ozone production efficiency from soil by pushing ozone production toward a more NO x -sensitive regime. Our results reveal an important indirect effect from fuel combustion NO x emission reduction on ozone trends by increasing ozone production from soil N r emissions, highlighting the necessity to consider the interaction between anthropogenic and biogenic sources in ozone mitigation in the North China Plain.

Atmospheric Reactive Nitrogen Species Weaken the Air Quality Response to Emission Reductions in China

“…HONO plays a pivotal role in atmospheric chemistry as an important precursor of the hydroxyl radical (OH), which fuels atmospheric chemical cycling and drives the formation of secondary pollutants. The dense agricultural activities result in high primary HONO emissions from various sources, including fertilized soils, livestock farming, and biomass burning in China . Furthermore, the increasing frequency of wildfire events under a warming climate intensifies HONO emissions, accelerating oxidative wildfire chemistry and, consequently, O 3 production.…”

“…Collectively, these factors contribute to increased HONO levels and hence increase the atmospheric oxidizing capacity, resulting in severe environmental consequences. For instance, soil-emitted HONO alone was found to enhance MDA8 O 3 by 2.6 ppb in the North China Plain and up to 7 ppb in the Sichuan Basin in June and July 2019 . Accordingly, HONO mitigation may provide a new O 3 control strategy beyond traditional VOCs or NO x control in China.…”

“…Nitrate, a significant component of PM 2.5 , continues to persist at increased levels, displaying a limited decline despite substantial anthropogenic reductions in NO x . Furthermore, in contrast to PM 2.5 trends, ozone (O 3 ) exhibits an increasing trajectory, with a rate of 2.4 ppb year –1 in major Chinese cities . Consequently, the frequency of O 3 and PM 2.5 co-pollution is increasing, indicating that air quality improvement is limited with respect to current mitigation strategies.…”

Large contribution of soil emissions to the atmospheric nitrogen budget and their impacts on air quality and temperature rise in North China

“…Only a few studies simultaneously considered the impact of soil HONO emissions (denoted as SHONO) along with SNOx on O3 and other secondary pollutants (Tan et al, 2023;Wang et al, 2023b). Wang et al (2023b) found that the NOx and HONO emissions from natural soils (i.e., soil background emissions) increased daily average O3 concentrations by 2.0% in Northeast Plain during August 2016 without considering the contribution from fertilized croplands.…”

“…Wang et al (2023b) found that the NOx and HONO emissions from natural soils (i.e., soil background emissions) increased daily average O3 concentrations by 2.0% in Northeast Plain during August 2016 without considering the contribution from fertilized croplands. Tan et al (2023) believed that the contribution of soil NOx and HONO to O3 pollution has been in an increasing trend from 2013 (5.0 pptv) to 2019 (8.0 pptv) in the summer season over the North China Plain by using the GEOS-Chem model; however the coarse resolution of GEOS-Chem simulation may not insufficient to resolve the spatial heterogeneity in soil emission distribution (Lu et al, 2021). Associated with the decreasing anthropogenic emissions is the increasing contribution of soil emissions to the atmospheric nitrogen budget in China.…”