Understanding and constraining tropospheric oxidants such as the hydroxyl radical (OH) remains a key challenge of direct relevance to understanding the oxidizing power of the atmosphere, radiative forcing, and surface air quality. Despite the critical role in atmospheric chemistry, OH abundances differ widely among chemistry–climate and chemical transport models (Stevenson et al., 2020; Zhao et al., 2019). In addition, global annual mean OH response to historical anthropogenic emission changes (all species) between the preindustrial and the present-day ranged from a 12.7 % decrease to a 14.6 % increase across 17 global models (Naik et al., 2013b), with similar discrepancies across simulations of future composition and climate (Voulgarakis et al., 2013). These differences between model estimates of OH suggest major knowledge gaps in our understanding of the drivers of OH. One potential driver of tropospheric oxidant changes that has not received sufficient study is aerosols, which can uptake radical species (chemical effect) and scatter or absorb incoming solar radiation (radiative effect), thereby impacting OH and other important chemical species (Jacob, 2000; Wild et al., 2000).

Anthropogenic emissions of sulfur dioxide (SO2), a precursor to sulfate aerosol, have significantly decreased in the US and Europe for the last several decades and are projected to continue to decline (Riahi et al., 2011; Vuuren et al., 2011; Westervelt et al., 2015). In China, emissions of anthropogenic aerosols began to decline in about 2013 after increasing for decades (Fontes et al., 2017; Li et al., 2017; Samset et al., 2018). Previous research has indicated that these past and forthcoming emission changes have the potential to influence the tropospheric oxidation capacity on both a regional and global basis (Dentener and Crutzen, 1993; Dickerson et al., 1997; Martin et al., 2003). The aerosol decreases in China were associated with subsequent increases in summertime surface ozone (O3) in China, attributed to a reduction in the sink of radical species such as the hydroperoxyl radical (HO2) that promote O3 production (Li et al., 2019b). Using a model and observations, the authors found that a 40 % decrease in fine particulate matter (PM2.5) in China between 2013 and 2017 led to an increasing ozone trend of up to 3 ppb per year in eastern China and was a more important factor than NOx emissions reductions over the same time period (Li et al., 2019a, b). These findings confirm earlier modeling work and point to an important role for aerosol impacts on tropospheric oxidation with implications for surface O3 concentrations, especially over China (Li et al., 2018; Lou et al., 2014).

On a global scale, the impact of aerosols on tropospheric oxidants has received little attention. Often, aerosol impacts are assumed to be negligible in constraining present and future OH concentrations (Voulgarakis et al., 2013). Primary production of OH depends on the amount of water vapor and O(1D) present (formed via O3 photolysis) and is the dominant pathway of OH formation in most locations except for high latitudes (Spivakovsky et al., 2000). Secondary production involves reactions of HO2 or RO2 (organic peroxy) radicals generated from oxidation of volatile organic compounds (VOCs) or carbon monoxide (CO) with nitric oxide (NO), which regenerates OH. Concentrations of these atmospheric constituents and certain meteorological factors such as absolute humidity, temperature, and ultraviolet radiation are thought to predominantly control OH abundance (Spivakovsky et al., 2000). However, by differencing a Goddard Earth Observing System Chemistry Transport Model (GEOS-Chem) control simulation of late 1990s atmospheric composition with a sensitivity simulation in which the offline global aerosols are excluded, Martin et al. (2003) find that the presence of all aerosols decreases OH by 9 % globally and 5 %–35 % in the Northern Hemisphere boundary layer. The authors also find 15–45 ppbv decreases in boundary layer O3 over India in March associated with the presence of all aerosols compared to all aerosols removed. In a similar global study, Tie et al. (2005) use the Model for Ozone and Related Chemical Tracers version 2 (MOZART-2) to show that the net effect of all aerosols (natural and anthropogenic) reduces HOx (defined as OH + HO2) and O3 by 30 % and 20 %, respectively, improving on past methodology (e.g., Martin et al., 2003) by calculating aerosol abundances interactively. Past studies only considered global distributions of aerosols and often focused on natural aerosols such as dust or sea salt (Bian and Zender, 2003; Liao et al., 2003). The impact of rapidly changing spatially heterogeneous anthropogenic aerosol abundances on tropospheric OH and O3 in response to regional air pollution control programs is thus an open question.

We expand on past studies by considering sulfate aerosol decrease via SO2 emissions reductions within individual regions (China, Europe, and the US), and quantify the local and remote impacts of changing these emissions on atmospheric HOx, NOx, and O3 concentrations on a seasonal basis within a chemistry–climate model nudged to observed meteorology. We focus on anthropogenic SO2 emissions, which have decreased most dramatically in many regions compared to anthropogenically sourced carbonaceous aerosols or natural aerosols such as dust and sea salt. We seek mechanistic understanding of the interactions between aerosols, oxidants, and radical species and photolysis rates over different regions and in different seasons. We consider two main pathways through which aerosols can affect oxidation: modification of photolysis rates via extinction of incoming solar radiation (radiative effect) and heterogeneous uptake of radical species onto aerosol surfaces (chemical effect). Finally, we consider the impact of anthropogenic SO2 emissions reductions on boreal summertime surface O3 concentrations in China, Europe, and the US.

We use the National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory Atmospheric Model version 3 (GFDL-AM3), which is the atmosphere-only component of the GFDL-coupled climate model, CM3 (Donner et al., 2011). The model has been rigorously evaluated against observations in previous work, including against surface observations of O3 over the US, Europe, and China (Donner et al., 2011; Naik et al., 2013a, Westervelt et al., 2019). Paulot et al. (2016) evaluate sulfate concentrations in GFDL-AM3 over the US (Interagency Monitoring of Protected Visual Environments, IMPROVE) and Europe (European Monitoring and Evaluation Programme, EMEP) and find a normalized mean bias of 0.07 in model surface concentrations compared against IMPROVE and a -0.43 mean normalized bias over Europe against EMEP. The model has 48 vertical layers from the surface up to about 0.01 hPa and a six-face cubed-sphere grid with 48 cells along each edge (C48), which is regridded to a 2∘ latitude by 2.5∘ longitude Cartesian grid. Emissions of anthropogenic trace gases and aerosols for year 2015 emissions are from the Representative Concentration Pathway 8.5 (RCP8.5) scenario (Riahi et al., 2011). The tropospheric chemical mechanism for aerosols and gas-phase species follows the work of Horowitz et al. (2003, 2007) with updates to photolysis, radical uptake by aerosols and convective wet scavenging of aerosols. The Fast-JX module (Bian et al., 2003; Wild et al., 2000) calculates the impact of online aerosols and clouds on photolysis rates and actinic fluxes implemented into GFDL-AM3 according to Mao et al. (2013b). Heterogeneous uptake of radical species is simulated according to Mao et al. (2013b) and Mao et al. (2013a) using a first order reactive uptake rate constant k (Eq. 1): 1k=-reDg+4γν-1A, where re is the aerosol effective radius (m), Dg is the gas-phase molecular diffusion coefficient, ν is the mean molecular speed of the gas, and A is the aerosol surface area per unit volume of air. Here we set the heterogeneous reactive uptake coefficient (γ) of HO2 to 0.2 instead of the value of 1.0 in Mao et al. (2013a). Though estimates of γ are uncertain, recent literature suggests such high values of 1.0 are not supported by observations and that the parameter is likely closer to 0.2 (Abbatt et al., 2012; Li et al., 2019a, b; Taketani et al., 2012). Taketani et al. (2012) recommend a middle γ value of 0.24 based on measurements at two high-altitude sites in China. Reactive uptake coefficients for all other reactions including N2O5, NO3, and NO2 are shown in Table 1, taken from Jacob (2000). We allow uptake of HO2, N2O5, NO3, and NO2 onto all aerosol types, including sulfate, black carbon, organic carbon, sea salt, and dust using the same coefficients for each composition. We also include updates to convective wet scavenging of aerosols in the form of finer vertical discretization of convective updraft plumes, resulting in improvements in aerosol budgets (Paulot et al., 2016). Horizontal wind velocities are nudged using a pressure-dependent technique towards reanalysis values from the National Centers for Environmental Prediction Global Forecast System (NCEP GFS; Lin et al., 2012). Further model description and model evaluation against observations can be found in Donner et al. (2011), Naik et al. (2013a), and Rasmussen et al. (2012).

We conduct a two-year (2014–2015) nudged control simulation in which emissions of aerosols and their precursors follow RCP8.5 and contrast it with three perturbations: one in which all anthropogenic SO2 emissions are set to zero over the US (30–50∘ N, 70–125∘ W) , all anthropogenic SO2 emissions are set to zero over Europe (35–70∘ N, 15∘ W–55∘ E), and all anthropogenic SO2 emissions are set to zero over China (15–50∘ N, 95–130∘ E). SO2 is oxidized by the hydroxyl radical in the gas phase and by ozone and hydrogen peroxide in clouds to form sulfate aerosol, which is a dominant component of total aerosol in GFDL-AM3 (Westervelt et al., 2015, 2017). We separately subtract each regional SO2 perturbation simulation from the control simulation, thereby isolating the impact of regional SO2 emissions (and subsequent sulfate formation) on tropospheric oxidants and related species. We test for statistical significance using a Student's t test on seasonal mean responses with the null hypothesis being that the difference between the control and the perturbation simulation is zero. Only the full year of 2015 is used for analysis to allow for a full year of initialization. SO2 perturbations from our simulations are 10.8, 12.4, and 16.2 Tg SO2 y-1 for US, Europe, and China, respectively.

The global annual mean OH for the 2015 control simulation is 7.0×105 molecules cm-3, which is within the range of the 14 Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) for year 2000 and 14 Chemistry Climate Model Initiative (CCMI) models (Voulgarakis et al., 2013; Zhao et al., 2019) for years 2000–2010. The global annual tropospheric burden of O3 in the 2015 control simulation is 356 Tg, which compares well to the year 2000 O3 burden mean across the ACCMIP models of 337±23 Tg (Young et al., 2013).

Figure 1 shows the percent increase in seasonal (March–April–May, MAM) sulfate concentrations at the surface (right column) and at altitude (left column) due to the presence of all anthropogenic US SO2 (first row), all European SO2 (second row), and all Chinese SO2 (third row) based on year 2015 anthropogenic emissions. Additional seasons are shown in the Supplement (Figs. S1–S3). The zeroing of 2015 SO2 emissions in each location results in the largest relative perturbation in China, where emissions are highest, followed by Europe and the US. Sulfate increases are largest closest to the source region, but all three regional simulations show statistically significant remote impacts both horizontally and vertically in the atmosphere, as evidenced by the spatial and zonal plots in Fig. 1. Emissions from US, Europe, and China perturbations all significantly increase sulfate throughout the troposphere up to 200 hPa and higher towards the North Pole, with the largest increases of up to 30 %–40 % resulting from the China SO2 perturbation. Transport to the Arctic is a common feature in all three perturbations and is consistent with previous studies on aerosol transport to the Arctic (Shindell et al., 2008; Stohl, 2006, Yang et al., 2017, 2018; Ren et al., 2020). The US perturbation impacts sulfate concentrations significantly at the surface and at altitude over the North Atlantic Ocean, while emissions from China exert a heavy influence over the Pacific reaching all of the way to the western US. European SO2 emissions have widespread influence on the Northern Hemisphere, but especially in the Arctic, the Mediterranean, and northern Africa. In all cases, sulfate changes are nearly entirely confined to the Northern Hemisphere.

We analyze the impact of sulfate changes on atmospheric oxidation capacity, starting in Fig. 2 with OH (left column) and HO2 (right column) for each of the three regional perturbations (rows of Fig. 2). Sulfate aerosol surfaces directly uptake HO2 radicals as described in Sect. 2, resulting in significant decreases of HO2 and OH (via their rapid cycling). For each perturbation, decreases in both OH and HO2 occur throughout most of the Northern Hemisphere up to about 200 hPa vertically during the boreal spring (MAM). The largest decreases in OH and HO2 occur in spring for each of the perturbations, followed by winter (December–January–February, DJF), autumn (September–October–November, SON), and summer (June–July–August, JJA). These additional seasons are plotted in Figs. S4–S6 in the Supplement. In MAM, SO2 emissions over the US decrease OH and HO2 by about 5 % within the US planetary boundary layer. In the mid-troposphere (400–600 hPa), OH decreases are 5 % or greater and are located spatially above the Arctic. For the Europe SO2 and China SO2 cases during MAM, the Arctic middle troposphere OH decreases are larger in percent change (>10 %) than the local changes near the surface (∼8 %). The presence of 2015 China SO2 emissions also decreases OH and HO2 by about 10 % over the North Pacific Ocean middle troposphere (about 400–600 hPa) in the model. By comparing the first row of Fig. 2 with the second and third rows, we find that the zonal structure of the OH and HO2 response to anthropogenic SO2 emissions is very similar across the three regional perturbations, while the magnitude is largest in response to China SO2 emissions, followed by Europe SO2 and US SO2. We conclude that regional SO2 emissions may have stronger impacts remotely than locally, and OH may be relatively more sensitive to aerosol changes in the Arctic and remote oceans at higher altitudes where its production is more limited.

In Fig. 3 we plot spring (MAM) changes in NOx (defined as NO + NO2) concentrations in response to anthropogenic SO2 emissions in the US, Europe, and China. While HO2 and OH strongly decreased in response to SO2 emissions, NOx significantly increases throughout most of the Northern Hemisphere. In the model, aerosols can take up NO2 directly but with a very low reaction probability (0.0001; Table 1), such that little uptake actually occurs and is easily offset by feedbacks onto other chemical reactions involving NOx. Instead, reduction in the sinks of NOx via OH (nitric acid formation) during the day and uptake of NO3 at night dominates the response to SO2 emission changes, increasing NOx in the model as OH decreases. At night, NOx is removed by reaction with the nitrate radical (NO3), which forms dinitrogen pentoxide (N2O5) (Chang et al., 2011; Jacob, 2000). Sulfate aerosols are effective at removing NO3 via reactive uptake (reaction probability of 0.1), slowing down this nighttime NOx sink and thus increasing NOx abundance. This hindering of day and night NOx sinks is most effective during MAM and DJF in the Northern Hemisphere mid-troposphere (Fig. 3a–c for MAM; additional seasons shown in Figs. S7–S9 in the Supplement). Mid-tropospheric Northern Hemisphere NOx increases reach about 7 %–8 % in response to Chinese SO2 emissions specifically, with smaller effects for both US and Europe SO2 perturbations. NOx at the surface increases slightly less at about 5 %–7 % depending on the regional emissions perturbation, though these changes are still statistically significant. N2O5 is removed by aerosols also with a reaction probability of 0.1, although several previous studies have used smaller reactive uptake coefficients for N2O5 (Evans and Jacob, 2005; Holmes et al., 2019; Macintyre and Evans, 2010; McDuffie et al., 2019) based on more recent laboratory experiments, but only find impacts on mean tropospheric O3 burden of 2 %–4 %. Using a box modeling approach, McDuffie et al. (2019) find a median γ for N2O5 of 0.076, reasonably close to our assumed value of 0.1.

In most of the Northern Hemisphere troposphere, O3 decreases in MAM by up to 4 % in response to US, European, or Chinese SO2 emissions increases in the model (Fig. 4), mostly coinciding with regions of large HOx decreases (Fig. 2), despite the increase in NOx (Fig. 3). O3 decreases are the largest in response to Chinese anthropogenic SO2 emissions, owing to the larger SO2 perturbation compared to the US and Europe emissions perturbations. The O3 increases in the upper troposphere are mostly not significant. We examine model diagnostics of gross ozone production (the sum of HO2+NO and all RO2+NO reaction pathways) and O3 loss (which includes reaction of O3 with HOx and with alkenes, plus O3 photolysis followed by O1D+H2O) to interpret further the O3 decrease. While both O3 production (PO3) and loss (LO3) rates decline (Figs. S10 and S11 in the Supplement), production decreases more strongly than loss, lowering O3 concentrations. We confirm that transport of O3 from other latitudes is unlikely to contribute much to the modeled O3 response as the change in zonal mean advective or convective tendency in O3 (Fig. S12 in the Supplement) is far smaller than the chemical production and loss terms (Fig. S10 in the Supplement). The O3 production and loss rates decrease most strongly in the lower troposphere over the source regions (Figs. S10 and S11 in the Supplement) while the O3 decreases (Fig. 4) propagate more widely through the free troposphere, indicating reduced export from these source regions. Additional seasons for O3 change are shown in Figs. S13–S15 in the Supplement.

We find here that the decline in HOx and its impact on PO3 outweighs the aerosol-induced increases in NOx and decreases in O3–HOx sinks, even during summer in all three source regions. We show the response of summertime surface 8 h maximum daily average (MDA) O3 to increasing anthropogenic SO2 emissions in the US, Europe, and China in Fig. 5. Increasing sulfate aerosol increases the sink of HO2 radicals and thus slows down O3 production (Fig. S11 in the Supplement), resulting in surface O3 concentration decreases, which are largest and mostly confined to the emissions source region. Sulfate aerosol can also reduce NO2 and O3 photolysis rates. The combined effect of sulfate aerosol on changes in photolysis rates and heterogeneous chemistry is a statistically significant decrease of about 5 ppbv over most of eastern China, Korea, and Japan when Chinese SO2 emissions are introduced, a decrease of about 3 ppbv over the eastern US for US SO2 emissions, and a decrease of about 3 ppbv over Eurasia for the Europe SO2 emissions perturbation. Changes in similar magnitude have been reported over China using both a chemistry transport model and observations (Li et al., 2019). Large sulfate decreases have occurred since the 1970s in both Europe and the US. The SO2 perturbation in our study (zero-out 2015 level emissions) is 10.8, 12.4, and 16.2 Tg SO2 y-1 in the US, Europe, and China, respectively. These results imply that the sulfate decreases from clean air regulations and technologies have had the unintended consequence of driving O3 up by a few ppb during the summertime in the US and Europe. While this may be a small amount of the total surface O3 concentration and not entirely outside the range of typical variability, our study only considers the impact of sulfate aerosol and not carbonaceous aerosols, which make up greater than 50 % of the total aerosol mass in many environments (Jiminez et al., 2009). Additionally, even O3 changes on the order of 3–5 ppbv may be important for holistically meeting tightening air quality standards. NOx emissions have also decreased dramatically over roughly the same time period and have likely more than offset any O3 increase from decreasing sulfate. However, the full potential of possible O3 improvement via NOx and anthropogenic volatile organic carbon (VOC) decreases may have been partially masked by sulfate decreases. These findings highlight the importance of a multipollutant strategy for effective clean air regulation.

Finally, in Fig. 6 we plot the relative change in MAM O3→O(1D) and NO2 photolysis rates, denoted jO(1D) and jNO2 in response to SO2 emissions in each region. Photolysis of both species is slightly influenced by changing SO2 emissions, especially over China in response to China's SO2 emissions, where decreases in both photolysis rates are about 7 %. For each of the perturbations, especially the US and Europe cases, changes in photolysis rates rarely rise above the noise, which is likely caused by meteorological factors such as slight changes in cloud cover. We conclude that while radiative effects via photolysis are non-negligible, they are significantly less important than chemical effects for aerosol impacts on oxidation, consistent with previous findings (Li et al., 2019).

Using the updated GFDL-AM3 nudged chemistry–climate model with online aerosol heterogeneous chemistry and interactions with radiation, we estimate the impact of Northern Hemisphere mid-latitude regional anthropogenic SO2 emissions on tropospheric OH, HO2, O3, and NOx. Regional SO2 emissions perturbations lead to significant changes to sulfate aerosol in far-reaching regions of the world, particularly in the Arctic and the mid- and upper troposphere. OH and HO2 decrease throughout the northern hemisphere mid-troposphere by up to 10 %, which in turn increase NOx concentrations by at least 5 %. NOx is not efficiently removed by heterogeneous reactions on aerosols, while species that contribute to NOx sinks such as OH (via HO2 uptake) and NO3 are efficiently removed, slowing down the NOx sink and increasing NOx concentrations. However, any influence of NOx increases on tropospheric O3 are overwhelmed by HO2 decreases, and the resulting decrease in O3 production offsets decreases in O3 sinks, resulting in up to 4 % decrease in O3 in the free troposphere and at the surface. Aerosols impact oxidation primarily through heterogeneous reactive uptake pathways over photolysis pathways.

Surface ozone decreases by 3 to 5 ppbv in response to the introduction of regional SO2 emissions. If SO2 emissions decline in developing regions of the world such as South Asia and sub-Saharan Africa, a goal attained through air quality improvements to protect human health, there could be an unintended increase in surface O3 concentrations. Decreasing surface O3 in these regions will require a multipollutant approach in which NOx and VOCs are simultaneously decreased with aerosols in order to offset the effect of decrease in aerosols and their precursors. While SO2 and NOx emissions decreases coincided to some extent in the US, end-of-pipe technologies at power plants allow for control of SO2 and NOx individually, and other sources of fine particulate matter (PM2.5) such as waste burning and vehicle emissions will have a similar effect on ozone as sulfate aerosols. PM2.5 and SO2 have decreased dramatically in recent years in the US and Europe, such that O3 improvements may have been partially masked by the aerosol impact. SO2 perturbations from our simulations are 10.8, 12.4, and 16.2 Tg yr-1 for US, Europe, and China, respectively, which result in a 3 ppbv surface ozone response over the US and Europe, and a 5 ppbv surface ozone response over China, where SO2 emissions are the largest in 2015. Model overprediction of surface O3 over urban areas in China (Westervelt et al., 2019) likely make this 5 ppbv change an upper estimate of the surface O3 response to China SO2 emissions.

Future work is needed to improve estimates of reactive uptake of HO2 and other radical species by aerosols, as great uncertainty still exists surrounding this parameter as well as the dependence of aerosol composition on reactive uptake parameters (George et al., 2013). We focus here on anthropogenic aerosols as they are changing rapidly and expected to continue to change. Previous work finds a large influence of Saharan dust aerosols on oxidation (Tie et al., 2005). Regions of biomass burning such as Africa and South America are also potential contributors to aerosol-driven oxidation changes. In order to avoid trading one problem for another in areas of the world that are experiencing both rapid emissions changes and high exposures to air pollutants, we must better understand the impact of aerosols on atmospheric photochemistry.