Coarse particulate matter (coarse PM; particulate matter between 2.5 and 10 µm aerodynamic diameter) is a severe air pollution problem in East Asia, contributing a particle mass comparable to fine particulate matter (PM2.5) and thus about half of PM10 (Chen et al., 2019; Lee et al., 2015; Qiu et al., 2014; X. Wang et al., 2018). It is mainly fugitive mineral dust, with contributions from both natural desert dust and anthropogenic sources including on-road traffic, construction, and agriculture (Wu et al., 2016; Zhao et al., 2017; Liu et al., 2021; Katra, 2020). Atmospheric chemistry models used in air quality applications generally do not include anthropogenic fugitive dust, due to the lack of available emission inventories except for a few urban areas (Li et al., 2021a, b, c). Aside from its direct interest as an air pollutant, coarse PM can suppress PM2.5 by heterogeneously taking up acids (HNO3, SO2, and H2SO4) that would otherwise lead to PM2.5 formation. This uptake has been observed for natural dust events (Wang et al., 2017; Heim et al., 2020; Z. Wang et al., 2018; Park et al., 2004; Stone et al., 2011), but the more ubiquitous effect from anthropogenic dust has received little study (Kakavas and Pandis, 2021; Hodzic et al., 2006). With increasingly stringent control measures to decrease fugitive dust air pollution in East Asia (Chinese State Council, 2019; Noh et al., 2018; Wu et al., 2016; Xing et al., 2018), it is important to better understand the impact on PM2.5 air quality.

A specific issue is the effect of anthropogenic dust on PM2.5 nitrate. Nitrate is a major component of PM2.5 in urban regions of East Asia including the North China Plain (NCP) (Li et al., 2019; Zhai et al., 2021a) and the Seoul Metropolitan Area (SMA) (Jeong et al., 2022; Kim et al., 2020), and it can dominate haze pollution events in both regions (Fu et al., 2020; Li et al., 2018; Xu et al., 2019; Kim et al., 2017, 2020). PM2.5 nitrate over North China in winter has not decreased in recent years despite reductions in emissions of the precursor nitrogen oxides (NOx≡NO+NO2) (Zhai et al., 2021a; Fu et al., 2020) from fossil fuel combustion. This has been attributed to limitation by ammonia (NH3) emissions, since PM2.5 nitrate is mainly present as ammonium nitrate (Zhai et al., 2021a). Decreasing coarse PM emissions is another possible explanation, as it would allow more HNO3 to be available for PM2.5 nitrate formation, and it could also shift PM2.5 nitrate formation to be more NH3-limited. Better understanding this sensitivity of PM2.5 nitrate to coarse PM is of crucial importance because of recent efforts by the Chinese government to decrease NH3 emissions (Liao et al., 2022), which are mainly from agriculture with additional urban contributions from vehicle, industrial, and waste disposal sources (Mgelwa et al., 2022).

In this work, we show that coarse PM over the NCP and the SMA is mainly anthropogenic and decreased by 21 % during the 2015–2019 period. We find that accounting for this anthropogenic coarse PM in the GEOS-Chem atmospheric chemistry model greatly improves the ability of the model to simulate PM2.5 nitrate during the Korea–United States Air Quality (KORUS-AQ) aircraft campaign over Korea where previous GEOS-Chem simulations found a large overestimate (Travis et al., 2022; Zhai et al., 2021b). From there we examine the implications for the effects of emission controls on long-term trends of PM2.5 nitrate in China and South Korea.

Figure 1 shows the annual mean concentrations of coarse PM in 2015, 2019, and 2020 measured at air quality networks in China and South Korea as the PM10–PM2.5 difference. Data for China are from the Ministry of Ecology and Environment (MEE) network (http://www.quotsoft.net/air/, last access: 18 December 2022), and data for South Korea are from the AirKorea network (https://www.airkorea.or.kr, last access: 18 December 2022). We remove spurious data when PM2.5 is higher than PM10, which account for 1.7 % and 0.2 % of the dataset, respectively, in China and South Korea.

We see from Fig. 1 that coarse PM concentrations in China and South Korea are highest in the NCP and the SMA, respectively, indicating a dominant urban anthropogenic origin. Coarse PM in year 2015 averaged 52 µg m-3 in the NCP and 23 µg m-3 in the SMA, contributing nearly half of total PM10 (120 µg m-3 in the NCP and 50 µg m-3 in the SMA). National air quality standards for annual mean PM10 are 70 µg m-3 in China (urban) and 50 µg m-3 in South Korea, well above the World Health Organization (WHO) recommended annual standard of 15 µg m-3. Coarse PM decreased by 21 % in both the NCP and the SMA from 2015 to 2019, reflecting emission controls on fugitive dust (Chinese State Council, 2013, 2018; Noh et al., 2018; Wu et al., 2016) and further decreased strongly in 2020 because of COVID-19 restrictions on traffic and construction. The COVID-19 impact is evident in China by comparing concentrations before and after the sharp 24 January 2020 lockdown (Fig. 2).

Figure 3 shows further evidence of the dominant anthropogenic contribution to coarse PM as the daily correlation with carbon monoxide (CO) in 2015. CO is emitted by incomplete combustion and is a tracer of urban influence. We find strong correlations between coarse PM and CO with consistent slopes, except in spring, which features high coarse PM outliers attributable to desert dust events (Heim et al., 2020; Shao and Dong, 2006). Similar correlations to 2015 are found in other years (Fig. S1 in the Supplement). The desert dust events drive the seasonal maximum of coarse PM in Fig. 1h.

We simulated the effect of anthropogenic coarse PM on PM2.5 nitrate using the GEOS-Chem model and evaluated the model with observations from the KORUS-AQ aircraft campaign over South Korea in May–June 2016 (Crawford et al., 2021). KORUS-AQ offers a unique dataset of detailed aerosol and gas-phase composition over East Asia. Previous GEOS-Chem simulations showed a large overestimate of fine particulate nitrate and a large underestimate of coarse PM (Travis et al., 2022; Zhai et al., 2021b). Particulate nitrate concentrations were measured during KORUS-AQ at the Korea Institute of Science and Technology (KIST) surface site and on the aircraft by aerosol mass spectrometers (AMS) with size cut of 1 µm diameter (PM1 nitrate) (Kim et al., 2017, 2018). The AMS only detects non-refractory nitrate, taken here to be ammonium nitrate (Fig. S2). Total particulate nitrate with size cut of 4 µm diameter (PM4 nitrate) was also sampled on the aircraft by the soluble acidic gases and aerosol (SAGA) instrument (Dibb et al., 2003; McNaughton et al., 2007). Additional measurements on the aircraft included HNO3 concentrations with a chemical ionization time of flight mass spectrometer (CIT-ToF-CIMS), and aerosol size distributions including coarse PM with a DMT CPSPD (Droplet Measurement Technologies cloud particle spectrometer with polarization detection) probe. We focus on the observations over the SMA and exclude observations from two process-directed flights (RF7 and RF8) and the Daesan power plant plume following Park et al. (2021).

We use GEOS-Chem version 13.0.2 (https://zenodo.org/record/4681204, last access: 18 December 2022) in a nested-grid simulation over East Asia (100–150∘ E, 20–50∘ N) with a horizontal resolution of 0.5∘×0.625∘. The model simulates detailed oxidant-aerosol chemistry relevant to PM2.5 nitrate formation (Zhai et al., 2021a) and is driven by meteorological data from the NASA Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2). Formation of semi-volatile ammonium nitrate aerosol is governed by ISORROPIA version 2.2 thermodynamics (Fountoukis and Nenes, 2007). Dry deposition of gases and particles follows a standard resistance-in-series scheme (Wesely, 1989). Wet deposition of gases and particles includes contributions from rainout, washout, and scavenging in convective updrafts (Liu et al., 2001; Luo et al., 2019). The model includes reactive uptake of HNO3 on dust limited by dust alkalinity and mass transfer (Fairlie et al., 2010), assuming 7.1 % Ca2+ and 1.1 % Mg2+ as carbonates per mass in emitted dust (Shah et al., 2020a; Tang and Han, 2017; Zhang et al., 2014). The relative humidity (RH)-dependent reactive uptake coefficient (γ) of HNO3 is based on laboratory studies (Liu et al., 2008; Huynh and McNeill, 2020) and observations during natural dust events in Beijing (Tian et al., 2021; Wang et al., 2017), and increases from 0.06 to 0.21 as RH increases from 40 % to 80 %. Monthly anthropogenic emissions for China are from the Multi-resolution Emission Inventory for China (MEIC) (Zheng et al., 2018, Zheng et al., 2021a, b), and emissions for other Asian countries including South Korea are from the KORUSv5 inventory (Woo et al., 2020). Fine anthropogenic mineral dust emissions from combustion and industrial sources (ash) are derived from the MEIC and KORUSv5 inventories as the residual of anthropogenic primary PM2.5 emissions after excluding primary organic aerosol, black carbon, and primary sulfate (Philip et al., 2017).

We compare the results from the standard model as described above to a simulation where we add anthropogenic coarse PM by using 24 h average observed coarse PM concentrations from the air quality networks (Fig. 1) as boundary conditions at the lowest model level. For this purpose, we linearly interpolate the daily mean coarse PM data from the network to the GEOS-Chem model horizontal grid and apply them to the coarse dust GEOS-Chem model component with an effective diameter of 4.8 µm. This concentration boundary condition in the lowest model level serves as an implicit source and defines the vertical concentration profile. The resulting vertical profiles of coarse PM in GEOS-Chem over South Korea are consistent with KORUS-AQ aircraft observations (Fig. S3). Anthropogenic coarse PM is assumed to be mainly fugitive dust with the same alkalinity properties as natural dust (Zhang et al., 2014; Tang and Han, 2017).

Figure 4 compares GEOS-Chem to the KORUS-AQ observations including median diurnal PM1 nitrate at the KIST site and median aircraft vertical profiles over the SMA. The model is sampled along the aircraft flight tracks at the times of the observations, all in daytime. PM1–4 nitrate is derived as the difference between SAGA PM4 nitrate and AMS PM1 nitrate. Here we take ammonium nitrate in the model for comparison to PM1 observations and size-resolved dust nitrate for comparison to PM1–4 observations. In this way, any dust-associated refractory PM1 nitrate is included in the PM1–4 profiles, for both observations and the GEOS-Chem model. Such classification does not allow for supermicron ammonium nitrate, but KORUS-AQ observations found ammonium nitrate to be mainly submicron (Kim et al., 2018). GEOS-Chem results are shown both for the standard model (not including anthropogenic coarse PM) and with the addition of anthropogenic coarse PM. In both simulations, we adjusted the diurnal variation of NH3 emission to match the NH3 observations made at the Olympic Park site, 7 km southeast of KIST (Fig. S4).

The standard GEOS-Chem simulation without anthropogenic fugitive dust overestimates daytime PM1 nitrate (aircraft and surface) by about a factor of 2, while underestimating PM1–4 nitrate by about a factor of 2 (Fig. 4a–c). Coarse PM in the standard simulation (from natural dust and sea salt) is near zero, in contrast to observations (Fig. 4d). Adding anthropogenic coarse PM to the model corrects this bias and further corrects the PM1 and PM1–4 nitrate biases by providing an added sink for HNO3. We find that anthropogenic coarse PM takes up HNO3 3 times faster than dry deposition and that this uptake is limited by mass-transfer rather than alkalinity (only 60 %–70 % of the coarse dust alkalinity in surface air is neutralized on average). The shift from PM1 to PM1–4 nitrate is consistent with the uptake of HNO3 by coarse PM, with some of this uptake in the model taking place on dust coarser than 4 µm and so not observed by PM1–4 nitrate. Half of the model overestimate of HNO3 is corrected (Fig. 4e), with the remainder possibly due to an underestimate of HNO3 deposition velocity (Travis et al., 2022). The model overestimates nighttime nitrate in surface air at the KIST site, even with anthropogenic coarse PM. This nighttime nitrate in the model is driven by heterogeneous NO2 and N2O5 chemistry under stratified conditions, which could be subject to large local errors (Travis et al., 2022).

We also examined the effect of anthropogenic coarse PM on PM2.5 nitrate concentrations in the NCP. PM2.5 nitrate observations in NCP are mostly filter-collected bulk PM2.5 nitrate, which could be biased low in summer due to volatilization (Chow et al., 2005). Previous evaluation of GEOS-Chem with 2013 and 2015 PM2.5 nitrate observations across China in summer and winter found no significant bias in 2015 or winter 2013 but an overestimate in summer 2013 (Zhai et al., 2021a). That simulation did not include HNO3 uptake by dust (natural or anthropogenic). We find here that including HNO3 uptake by fine (PM2.5) dust has little effect on total PM2.5 nitrate but partitions 10 % of ammonium nitrate mass to fine dust nitrate in winter and 23 % in summer (Fig. S5). Adding anthropogenic coarse PM in GEOS-Chem decreases modeled ammonium nitrate in the NCP by 10 %–20 % in winter and by 25 %–30 % in summer, a relatively more modest effect than over the SMA because of larger excess of NH3. The comparison with PM2.5 nitrate observations here indicates that fine dust associated nitrate should be considered when comparing modeled particle nitrate to bulk PM2.5 nitrate data.

There are to our knowledge no readily accessible continuous long-term records of PM2.5 nitrate concentrations in China or South Korea. Figure 5 shows a multi-year compilation of winter and summer mean PM1 and PM2.5 nitrate observations from individual field campaigns in Beijing and Seoul over 2015–2021 (Table S1 in the Supplement). We find no significant trends in winter, consistent with previous studies in the NCP that examined earlier periods (Fu et al., 2020). In summer, observations tend to show a decrease over the period but with large interannual variations driven by meteorology (Li et al., 2018; Zhai et al., 2021a).

Changes in anthropogenic emissions of NOx, SO2, NH3, PM2.5, and coarse PM could all affect PM2.5 nitrate, and we used GEOS-Chem to investigate these effects for the 2015–2019 period. The Multi-resolution Emission Inventory for China (MEIC) reports that NOx emissions in the NCP decreased by 11 % from 2015 to 2019, SO2 emissions decreased by 54 %, and primary PM2.5 from combustion decreased by 35 % (Zheng et al., 2021a). This primary PM2.5 includes a 40 % contribution from mineral ash that we treat as anthropogenic fine dust, and it decreased by 27 % from 2015 to 2019. The MEIC also reports a 15 % decrease of NH3 emissions over China from 2015 to 2019 (19 % for the NCP), while the PKU-NH3 emission inventory reports a 6 % decrease over China from 2015 to 2018 (Liao et al., 2022). Observations of surface NO2 and SO2 over the SMA imply a 22 % decrease of NOx emissions and a 40 % decrease of SO2 emissions from 2015 to 2019 (Bae et al., 2021; Colombi et al., 2023). Coarse PM decreased by 33 % over the NCP and by 31 % over SMA during the same period (considering winter and summer data only).

Figure 6 shows the resulting emission-driven changes of PM2.5 nitrate over the NCP and SMA from 2015 to 2019 as simulated by GEOS-Chem in sensitivity simulations, applying either 2015 or 2019 emissions to the same meteorological year (2019), and with or without anthropogenic coarse PM. The sum of changes driven by individual emission changes amounts to the total emission-driven net change. Sensitivities to NH3 and primary PM2.5 emissions in the SMA are solely driven by emission trends in China, since we assume no emission trends for these species in South Korea.

The model reproduces the lack of trend in winter and the decreasing trend in summer seen in the observations for both the NCP and SMA. The lack of trend in winter reflects offsetting influences from decreasing NOx, NH3, and primary PM2.5 emissions on the one hand, and decreasing SO2 and coarse PM emissions on the other hand. Decreasing SO2 increases the availability of NH3 for nitrate formation (Fu et al., 2020; Zhai et al., 2021a). Decreasing primary PM2.5 reduces the aerosol volume available for heterogeneous conversion of NOx to HNO3 (Shah et al., 2020b). Decreasing coarse PM has relatively little direct effect on PM2.5 nitrate in winter in the NCP because abundant atmospheric NH3 combined with low temperatures drives HNO3 near-quantitively to ammonium-nitrate particles, and subsequent mass transfer of HNO3 from ammonium nitrate to coarse PM is very slow because of the weak HNO3 partial pressure (Wexler and Seinfeld, 1992). The decrease of coarse PM still quantitatively offsets the benefit from NOx emission controls, which has been the main vehicle for controlling PM2.5 nitrate. Consideration of coarse PM in the model further increases the sensitivity of PM2.5 nitrate to NH3 and SO2 emissions, respectively, by 30 % and 46 %. This is because coarse PM provides an additional sink for the small fraction of HNO3 that remains in the gas phase, which increases the sensitivity of the atmospheric lifetime of total nitrate (ammonium nitrate + HNO3) to changes in NH3 or SO2 emissions (Zhai et al., 2021a).

In summer, we find that the decrease in coarse PM over the 2015–2019 period directly cancels half of the benefit from decreasing NOx, SO2, NH3, and primary PM2.5 emissions in the NCP, with less effect in the SMA. Over the NCP, the decrease of coarse PM offsets 80 % of the benefits from NOx and NH3 emission controls. Unlike in winter, decreasing SO2 suppresses nitrate formation by decreasing the aerosol liquid water content (Stelson and Seinfeld, 1982). The effect of decreasing coarse PM emissions in summer is larger than in winter because warmer temperatures allow more HNO3 to remain in the gas phase under NH3–HNO3–H2SO4 thermodynamics and can thus be scavenged by coarse PM.

Coarse PM (PM10–PM2.5) in urban areas of China and South Korea is very high year-round and is mainly of anthropogenic origin as fugitive dust except for natural desert dust events in spring. Annual mean coarse PM concentrations decreased by 21 % from 2015 to 2019 in both the North China Plain (NCP) and the Seoul Metropolitan Area (SMA), with steeper decreases in 2020 because of COVID-19 restrictions on traffic and construction. Considering only winter and summer when the influence of natural dust is small, we find that anthropogenic fugitive dust emissions decreased by about 30 % from 2015 to 2019 in both the NCP and the SMA.

Anthropogenic coarse PM is of direct air quality concern because it accounts for about half of total PM10 in the NCP and the SMA, but it also takes up HNO3 effectively and can thus suppress formation of fine particulate nitrate which is a major component of PM2.5 pollution. Comparison of GEOS-Chem model simulations to surface and aircraft observations from the KORUS-AQ campaign over the SMA in May–June 2016 shows that accounting for anthropogenic coarse PM largely corrects previous model overestimates of fine particulate nitrate.

Decrease in anthropogenic coarse PM emissions to improve PM10 air quality could have the unintended consequence of increasing PM2.5 nitrate, offsetting the gains from decreases in NOx and NH3 emissions. Compilation of 2015–2021 observations of fine particulate nitrate in Beijing and Seoul suggests little trend in winter and a decrease in summer, consistent with GEOS-Chem. Decreasing coarse PM in the model in winter offsets the benefit of decreasing NOx emissions, and coarse PM further increases the sensitivity of PM2.5 nitrate to changes in NH3 and SO2 emissions by affecting the lifetime of total inorganic nitrate (ammonium nitrate + HNO3). In summer, decreasing coarse PM in the NCP offsets 80 % of the PM2.5 nitrate benefit of decreasing NOx and NH3 emissions. As coarse PM continues to decrease in response to fugitive dust pollution control, there is a greater need to reduce NH3 and NOx emissions in order to decrease fine particulate nitrate air pollution in East Asia.