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Aerosols from Fires: An Examination of the Effects on Ozone Photochemistry in the Western United States Xiaoyan Jiang,*,† Christine Wiedinmyer,† and Annmarie G. Carlton‡ †

National Center for Atmospheric Research, Boulder, Colorado, United States Department of Environmental Science, Rutgers University, New Brunswick, New Jersey

‡

S Supporting Information *

ABSTRACT: This study presents a first attempt to investigate the roles of fire aerosols in ozone (O3) photochemistry using an online coupled meteorology-chemistry model, the Weather Research and Foresting model with Chemistry (WRF-Chem). Four 1-month WRFChem simulations for August 2007, with and without fire emissions, were carried out to assess the sensitivity of O3 predictions to the emissions and subsequent radiative feedbacks associated with largescale fires in the Western United States (U.S.). Results show that decreases in planetary boundary layer height (PBLH) resulting from the radiative effects of fire aerosols and increases in emissions of nitrogen oxides (NOx) and volatile organic compounds (VOCs) from the fires tend to increase modeled O3 concentrations near the source. Reductions in downward shortwave radiation reaching the surface and surface temperature due to fire aerosols cause decreases in biogenic isoprene emissions and J(NO2) photolysis rates, resulting in reductions in O3 concentrations by as much as 15%. Thus, the results presented in this study imply that considering the radiative effects of fire aerosols may reduce O3 overestimation by traditional photochemical models that do not consider fireinduced changes in meteorology; implementation of coupled meteorology-chemistry models are required to simulate the atmospheric chemistry impacted by large-scale fires. study fire impacts on air quality, weather, and climate.9−11 By including the direct and semidirect effects of aerosols on radiation in a coupled global climate model, Tosca et al.10 assessed the radiative and climate effects of biomass burning and determined that fire aerosols reduced net shortwave radiation at the surface during August−October by 10% (19.1 ± 12.9 Wm−2). Using a coupled regional meteorologychemistry model, Wong et al.12 found that regional fires could lead to more than 250 Wm−2 decrease in downward shortwave radiation in the fire source regions. Reduced downward solar radiation lessens isoprene emissions,13 a known contributor to O3 production (e.g., ref 14), and can lower photolysis rates (e.g., decrease J(NO2) values) (e.g., ref 15). Liao et al.16 studied the effects of aerosols on tropospheric photolysis rates under clear and cloudy sky conditions using a one-dimensional radiative transfer model. They found that soot aerosol reduced photolysis rates from 9 to 19% under both clear and cloudy sky conditions. Photolytic reactions play a crucial role in the abundance of O3, as NO2 photodissociation is the most significant source of atomic and molecular oxygen7 in the troposphere. Accounting for fire

1. INTRODUCTION Large-scale fires are recognized to impact air quality through direct emissions of particulate black carbon (BC),1,2 carbon monoxide (CO),3 and other pollutants.3 Air quality can also be impacted by increased fire-related emissions of nitrogen oxides (NOx) and volatile organic compounds (VOC), and the subsequent production of ozone (O3) downwind of fire locations.4 Yet, three-dimensional photochemical models often cannot adequately describe these fire-related air quality impacts and often misrepresent O3 formation in and downwind of fire plumes.3,5 Inaccuracies in modeled O3 can arise for a variety of reasons: for example, the vertical mixing of O3 is not well described,3 fire NOx emission factors are highly uncertain,3,5 emissions of oxygenated VOCs are not properly considered,6 or conversion of NOx to PAN is incorrect in chemical mechanisms.5 Another possibility, explored in this work, is that air quality models that employ “offline” meteorology are insufficient in “big fire” applications. Specifically, the use of radiation values that do not include the fire aerosols induced changes to drive photochemistry and biogenic emissions in the shadow of fire plumes is inadequate. Fire-induced reduction in downward solar radiation, a consequence of BC absorption and scattering aloft by other aerosol species, has been observed directly in field experiments7 and through analysis of long-term surface and satellite observational data.8 Numerical models have been used to © 2012 American Chemical Society

Received: Revised: Accepted: Published: 11878

April 18, 2012 September 18, 2012 September 26, 2012 September 26, 2012 dx.doi.org/10.1021/es301541k | Environ. Sci. Technol. 2012, 46, 11878−11886

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Aerosol-radiation interaction processes, which include particles emitted from fires, are implemented in WRF-Chem. The modeled radiative impacts of aerosol include direct scattering and absorption, as well as the semidirect effect (radiative changes through differences in cloud cover). In this study, we did not consider aerosol indirect radiative effects. Each MOSAIC aerosol chemical constituent was linked with aerosol optical properties through a complex refractive index. The refractive index was calculated by volume averaging for each size bin based on a refractive index mixing rule described in Bond et al.29 For BC, Bond and Bergstrom30 defined a range of plausible complex refractive indices from 1.75 to 1.95 for the real part, and 0.62 to 0.79 for the imaginary part. In this study, the real and imaginary parts of the refractive index were set to 1.85 and 0.71, the midpoints of those values reported in Bond and Bergstrom.30 For more details about the refractive indices for other aerosols and how the aerosol optical properties are calculated in WRF-Chem, readers are referred to Barnard et al.31 Mie theory was used to calculate space- and timedependent aerosol optical properties including extinction efficiency, single-scattering albedo, and asymmetry factor for scattering at different wavelengths (300, 400, 600, and 1000 nm). Also, internal to the WRF-Chem modeling system, aerosol distributions and the associated radiative properties were transferred to the Goddard shortwave radiative transfer model32 to calculate the direct aerosol radiative forcing at the surface. The inclusion of online chemistry and meteorology in WRFChem does not only include the feedbacks directly to the radiation; chemical processes are also impacted. The interaction of the smoke particles with the solar radiation was considered by the calculation of the photolysis rates. The aerosol optical properties calculated from Mie theory were used to simulate the aerosol radiative effect on photolysis rates such as NO2 photolysis rate (jNO2). Photolysis rates (j-values) in the presence of aerosols were computed using the Fast-J radiation scheme.33,34 2.2. Emissions. Fire emissions were calculated for the modeled time period using the Fire INventory from NCAR (FINN) version 1,35 downloaded directly from http://bai.acd. ucar.edu/Data/fire/. This method used a combination of observations derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments aboard the NASA Terra and Aqua satellites, including rapid response fire detections, land cover type, and vegetation continuous fields. Emission factors were based on the type of vegetation burned (e.g., ref 36). The VOC emissions were speciated to the SAPRC99 mechanism using published speciation profiles35 and were further speciated to the CBMZ mechanism following the details in Carter37 and other studies.38,39 Fine smoke particles (PM2.5) were divided into several MOSAIC aerosol species (sulfate, nitrate, organic carbon, elemental carbon, and other unspecified primary PM2.5), which were externally mixed, following Fast et al.40 Coarse particulate emissions were not assigned chemical composition. Anthropogenic emissions were obtained from the 2005 U.S. Environmental Protection Agency (EPA) national emissions inventory (NEI-2005). U.S. EPA-provided temporal allocation factors, representative of a typical summer day and specific to each source classification code, were used to temporally allocate emissions. In this study, same day emissions were recycled for multiday simulations.13,41 Biogenic emissions were calculated online in WRF-Chem using the Model of Emissions of Gases

aerosol-induced reductions in radiation and the subsequent modulation of photolysis rates (e.g., J(NO2)) in Lagrangian and 3-dimensional photochemical models applied to East Asia results in predicted O3 reductions up to 6 ppbv.15 More recently, using a radiative transfer model and photochemical box model, Flynn et al.17 showed that the combined cloud and aerosol effects reduced J(NO2) by 17%, resulting in 8 ppbv reduction in O3 concentrations. All of these studies used offline radiation models with photochemical models; the feedback between meteorology and chemistry is not considered. Online meteorological-chemical modeling of the atmosphere enables the assessment of the interactions between aerosols, weather, and chemistry.18 The goal of this study is to quantify the sensitivity of O3 chemistry to the emissions from large-scale fires and to the aerosol radiative forcing from fire aerosols. Here, an online chemistry and weather model is used to explore the effects on fire-related air quality in the Western United States (U.S.) and compare results to control simulations employing “offline” meteorology or without feedbacks between fire aerosols and meteorology. Sensitivity simulations of a wildfire event that occurred in Idaho and Montana in August 2007 were carried out to understand these impacts.

2. MATERIALS AND METHODS 2.1. WRF-Chem Model Description. A fully coupled meteorology-chemistry model, the Weather Research and Forecasting with Chemistry (WRF-Chem) model, version 3.218 was used for this study. The components of this modeling system, including chemistry and meteorology, employ the same transport, grid, boundary layer, land surface model, and time step. Thus, the continuity equations for all chemical species are performed “online.” In this study, the mass coordinate version of the model, Advanced Research WRF (ARW), was applied. Gas-phase chemistry was calculated with the CBM-Z chemical mechanism,19 which includes 67 prognostic species and 164 reactions. This version of WRF-Chem implements a regimedependent approach to partition kinetics into background, anthropogenic, and biogenic submechanisms to reduce the computational time. Dry deposition for trace gases used a surface resistance parametrization developed by Wesely,20 and for aerosols it was based on Binkowski and Shankar.21 In this work, aerosols were simulated using a sectional aerosol model, the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC).22 Four size bins were used in this study, and each size bin was assumed to be internally mixed so that all particles within a size bin are assumed to have the same chemical composition. The aerosol species used in MOSAIC include sulfate, nitrate, ammonium, organic matters, BC, and water. Brown carbon,23 one type of strong light-absorbing carbon from fires, was not explicitly considered in the current aerosol model. Thus, the results presented in this study may underestimate the impacts of fires on O3 formation. Simplified wet deposition using the employed convective parametrization was used for aerosols. Other selected parametrizations used in all simulations include Lin et al.’s microphysics scheme,24 the Kain-Fritsch Cumulus Parameterization scheme,25 the Yonsei University Planetary Boundary Layer (PBL) scheme,26 the Rapid Radiative Transfer Model Longwave Radiation scheme,27 and the Noah Land Surface Model.28 Observation-based nudging method (i.e., four-dimensional data assimilation) is not used in this study in order to allow changes in meteorology in response to aerosol changes. 11879

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and Aerosols from Nature (MEGAN) biogenic emissions module, version 2.04,42 which is released with the WRF-Chem version 3.2. MEGAN v2.1 maps of isoprene emission factors, leaf area index, and plant functional types were applied.43 MEGAN includes light, temperature, leaf age, and leaf area index controls on the emissions of isoprene and other biogenic emissions. Biogenic emissions were emitted at the surface below any BC-induced effects and were driven by the simulated temperature and radiation conditions within WRF-chem. Emissions from sea-salt and dust were not considered. 2.3. Experimental Design. To investigate the impact of smoke plumes on downwind chemistry, an episode of high fire activity and emissions was selected as a case study. Moderate to severe drought occurred in late July and contributed to an active fire season in the northern Rocky Mountains of Idaho and Montana. Wildfires burned more than 1.1 million ha in Montana and Idaho in 2007 (http://www.nifc.gov/nicc/ predictive/intelligence/2007_statssumm/fires_acres.pdf); most of that fire activity took place during July and August. Westerly winds blew smoke plumes from the Rocky Mountain region to the eastern plains. Figure 1a displays a satellite photo from NASA's Earth Observatory on 13 August 2007 showing the extent of the fires and smoke plumes during the peak of the fire episode (http://earthobservatory.nasa.gov/ NaturalHazards/view.php?id=18829). Four 1 month simulations for August 2007, with and without fire emissions, were performed using WRF-Chem to assess the

sensitivity of air quality predictions to the emissions and subsequent radiative feedbacks associated with large-scale fires in the Western U.S. To reduce the effects of boundary conditions, all simulations were performed over the entire contiguous U.S. at 30 km spatial resolution. It should be noted that the model resolution could have important effects on model results.44 The first nine days of each simulation were not included in the analysis to allow for model “spin up”. The simulation time step for meteorology was two minutes, meaning fire aerosols impact meteorology every two minutes. Our analysis is focused on 10−16 August when the fires were most active. In all simulations, North American Regional Reanalysis (NARR) data from the National Center for Environmental Prediction (NCEP)45 were used to provide the initial and lateral boundary meteorological conditions in all model simulations. Lateral boundary conditions were updated every 3 h. The chemistry was initialized with idealized profiles. The first simulation (CTRL), or base case, contains no fire emissions. Simulation “EXP1” included the addition of only fire-related gas phase emissions, and “EXP2” included only fireemitted aerosol species. “EXP3” contained both gas- and aerosol-phase fire emissions. In the experiments with fire emissions, only emissions were used and the transport of fire emissions was treated the same as other emissions through the vertical transport scheme used in WRF-Chem. That means both meteorological component and chemical species used the same transport scheme.18 Future studies should apply the fire plume rise algorithm for wildfires in WRF-Chem.11

Figure 1. (a) Fires captured on Monday, 14:55 LST, 13 August 2007 by the Moderate Resolution Imaging Spectroradiometer (MODIS) on the NASA Aqua satellite (downloaded from http://earthobservatory. nasa.gov/NaturalHazards/view.php?id=18829). (b) PM2.5 emissions from fires during 10−16 August 2007. Size of the dots represents the rate of emissions ranging from 0.2 to 3.0 ug m−2s−1. Star (*) marker indicates Location 1 (latitude: 46.3°N and longitude: −114°W). Plus (+) marker indicates Location 2 (latitude: 46.3°N and longitude: −109°W). Green dots, triangles and squares represent the 10 IMPROVE sites, five CASTNet sites, and five AERONET sites, respectively.

3. RESULTS 3.1. Model Performance. Our analysis of the model simulations covers the period of the strongest fire activity (10− 16 August 2007) during which time significant emissions, including CO, NOx, VOC, and PM, were released from the wildfires. We compared the averaged daily emissions of BC, OC, CO, and NO during 10−16 August 2007 over the study domain (shown in Figure 1b) under the conditions of without and with fires (Supporting Information (SI) Table S1). It is clear that fires emit a large amount of BC, OC, CO, and NO as compared to anthropogenic emissions. Figure 1b shows the PM2.5 estimated from the fires over the period of 10−16 August 2007. Without the inclusion of fire aerosols, the model produces much lower PM2.5 concentrations when compared to measurements of PM2.5 mass concentrations from the IMPROVE Network (http://vista.cira.colostate.edu/improve, SI Figure S1a). The PM2.5 concentrations from the sensitivity run (EXP3) are much closer to the observations, although there are still biases. The biases between model results (EXP3) and observations could arise for several reasons: for example, the anthropogenic emissions used in this study were for 2005 and might not be representative of 2007. The magnitude and spatial (horizontal and vertical) allocation of the fire emissions are also highly uncertain (e.g., ref 35). Changes in the aerosol concentrations due to fire aerosols lead to large increases in the total observed aerosol optical depth (AOD) (SI Figure S1b). Elevated AODs are seen near the fire source and downwind area (AERONET Sites: Missoula and Bratts-Lake) over the periods when the fire intensity is strong. Other AERONET sites including BSRN-BAO-Boulder, Kelowna, and Rimrock, which are either at upwind areas or further away from the fire source, show much lower AODs. The maximum AODs (up to 0.8) simulated by WRF-Chem occurred at the same time when the AERONET observations 11880

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Figure 2. Changes in clear-sky and cloudy-sky downward shortwave radiation, surface air temperature, and PBLH between EXP2 and CTRL. The left panels show the daytime (06:00−18:00 LST) means over 10−16 August 2007. The right panels show the means over 09:00−16:00 LST on 15 August 2007.

inclusion of fire aerosol emissions and resulting direct aerosol radiative feedbacks. Figure 2 also presents changes in shortwave radiation during the daytime on 15 August 2007. Carbonaceous aerosols (BC and organic particulate carbon (OC)) from fires absorb and scatter solar radiation.46 In particular, the presence of BC can result in a large atmospheric solar absorption,47 which substantially reduces the solar radiation reaching the surface. Thus, the reduction in downward shortwave radiation at the surface results from the combined effect of aerosol absorption and reflection (Figures 2). The large increase in AOD results in about 40 Wm−2 decrease in clear-sky radiation during the daytime averaged over 10−16 August. The decrease on 15 August exceeds 50 Wm−2. The region of strong cooling coincides with the region of high fire activity and emissions, but also extends downwind (Figure 2). In response to decreases in downward shortwave radiation at the surface, surface temperature decreases as much as 2 K in the region of high fire activity (Figures 2). There is also strong cooling outside of the fire region (Figure 2) related to increases in total cloud cover (not shown). The slight increase in surface temperature outside of the fire region could be due to the fast and nonuniform responses in regional clouds. Because of these

show large values of AOD, although the modeled magnitudes are smaller. The above-mentioned causes for the model biases could be the reason for the simulated lower AODs. The result from CTRL shows smaller AODs at these AERONET sites. The significant increases in AODs in the areas influenced by the fires can impact on meteorology and photochemistry, as discussed below (Sections 3.2 and 3.3). Simulated daytime O3 concentrations are compared with CASTNet O3 data (http://epa.gov/castnet/javaweb/index. html) over the period of 10−16 August. SI Figure S2 shows that without the consideration of gas and aerosol emissions from fires (CTRL), the model underestimates O3 concentrations. The model when including both gas and aerosol emissions from the fires (EXP3) simulated O3 concentrations in better agreement with observations. The details in regard to how fires are impacting O3 formation are discussed below. 3.2. Impacts of Fires on Meteorology/Radiation. The direct radiative impact of fire aerosols is estimated by calculating the differences in clear-sky and cloudy-sky downward shortwave radiation between EXP2 and CTRL during the daytime periods (06:00−18:00 LST) for 10−16 August 2007 (Figure 2). The difference between these two simulations is the 11881

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Figure 3. Changes in NOx (ppbv, left panels) and O3 (% change, right panels) for the three sensitivity experiments (EXP1, EXP2, and EXP3) compared to the CTRL simulation for 09:00−16:00 LST, 15 August 2007.

chemical mechanism, and episode differ between this study and that of Wong et al.;12 thus, it is not surprising to see varying magnitudes in the aerosol direct effects. 3.3. Impacts of Fires on Chemistry. Daytime changes in NOx and O3 concentrations resulting from the inclusion of fire emissions in each scenario are shown in Figure 3. Increases in O3 concentrations when only fire-emitted gaseous emissions are considered (EXP1) are associated with increases in NOx and VOCs emitted from the fires (not shown). The strongest O3 production occurs in the fire source region and regions directly downwind, with increases up to 60% greater than those areas not impacted by fires. The impact of fires on O3 production in this study is higher than some other studies. For example, Pfister et al.4 found that O3 increases resulting from fires were about 10 ppbv in September to October, which resulted in a 7−10% enhancement over that region. On larger scale studies, Hudman et al.50 and Pfister et al.9 found that the O3 enhancements were lower, about 1−3%, over the northern hemisphere. Those studies applied global models with relatively coarse spatial resolutions. The model spatial resolution used in this work, which is much higher than those studies, is expected to impact the simulated O3 enhancement. When only the direct aerosol radiative forcing of fire aerosols is considered (EXP2), decreases of O3 concentrations up to 15% (10 ppbv) over the fire source region and the downwind regions are observed. The regions showing decreased O3 concentrations are consistent with the areas of strong reductions in downward solar radiation. When both gaseous and particulate fire emissions are included in the simulation (EXP3), a slight decrease in O3 concentrations outside the fire

interactions, clouds decrease in some areas, leading to a slight increase in downward radiation and surface temperature (not shown). Hourly surface temperatures decrease during the daytime, while they increase slightly at night (SI Figure S3). This is attributed to the longwave effects of the fire aerosols, a phenomenon that has been observed in field experiments.48 Overall, our findings are consistent with previous studies (e.g., refs 48,49) that observe significant daytime surface cooling from wildfire smoke and negligible impacts at night. BC and OC from fires also influence the PBLH, which controls the vertical dilution of atmospheric constituents, including O3. The aerosol effects tend to heat the atmosphere strongest at the top of the boundary layer through aerosol absorption within the atmosphere (Figure S4), leading to a more stable and shallower PBLH (Figure 2 and SI Figure S3). Decreases in PBLH over the fire source region and the downwind area could reduce the vertical dilution of O3 precursors and O3, leading to enhanced O3 concentrations. In some regions, slight increases in PBLH are simulated (Figure 2). The impacts of fire aerosols on meteorology are comparable with recent findings.11,12 Using the online WRF-CMAQ model, Wong et al.12 show that fire-related reductions in surface temperature and PBLH are associated with a strong reduction in shortwave radiation reaching the surface. In their study, the reduction in shortwave radiation is a factor of 6 greater, 250 Wm−2, than what is reported here. They also found, in the areas influenced by fires, AOD increases from 0.5 to more than 3, and are associated with 75−300 Wm−2 decreases in downward shortwave radiation. The modeling domain, emissions, 11882

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Figure 4. Top panels (a, b) show differences in CO (ppbv) and O3 (%) between EXP1 and CTRL at Locations 1 and 2 (shown in Figure 1b). Bottom panels (c, d) show differences in PM2.5 (ug kg−1) and O3 (%) between EXP2 and CTRL at Locations 1 and 2 (shown in Figure 1b).

downward solar radiation and temperatures, and associated changes in chemistry) that contribute to O3 reductions. 3.4. Changes in Biogenic Emissions. Changes in nearsurface meteorology, particularly temperature and downward solar radiation, control biogenic isoprene emissions, an important O3 precursor (e.g., ref 51). Decreases in simulated biogenic isoprene are observed in areas with reductions in surface air temperature and downward solar radiation (SI Figure S6a). The largest decrease in biogenic isoprene emissions can be as much as 30% (Figure S6c). In addition to the impacts on O3 formation from changes in NOx and PAN, changes in biogenic isoprene emissions contribute to decreases in O3 concentrations over the fire source region. The spatial extent of decreased O3 (Figure 3) is larger than that of decreased biogenic isoprene emissions (SI Figure S6a). This indicates that while reduced isoprene emissions contribute to lower O3 concentrations, there are a variety of nontrivial contributors including meteorology, NOx, PAN, and VOCs. 3.5. Changes in Radiation-Photochemistry. In response to the direct radiative forcing of fire aerosols, the simulated J(NO2) photolysis rate decreased over the regions of strong reductions in the downward solar radiation at the surface (SI Figure S6b). This is consistent with previous studies that found absorbing aerosols reduce UV actinic flux throughout the troposphere, leading to a reduction in near-surface O 3 production.17,52,53 The spatial distribution of the changes in J(NO2) tends to spatially match changes in the downward solar radiation reaching the surface. In the fire source region, there is up to 75% decrease in J(NO2), while the decrease is smaller in

regions is observed, including some downwind areas, but large increases in O3 concentrations in the fire source region remain. This suggests that the gaseous fire emissions (e.g., CO, NOx, VOCs) have a strong effect on O3 production near the fire source regions. If only considering gaseous emissions from the fires, the increases in O3 production correspond with the increases in CO from fires in the fire source region (Figure 4a). During the peak fire activity on 12−13 August, O3 production near the fire source location increased as much as 200%. Over the fire source region, the ratio of O3 and CO differences is smaller as compared to that in the downwind area (SI Figure S5). This is mainly caused by the high concentrations of CO from fires in the source region. The impact of fire aerosols on reducing O3 formation is manifest in the downwind region as indicated by the high ratio of O3 and CO differences between EXP3 and CTRL. Gaseous emissions, including NOx and VOCs, lead to a strong increase in NOx and PAN formation over the fire region (SI Figure S5), resulting in increases in O3 concentrations.5 This effect is reduced in downwind areas (Figure 4b). The differences between EXP2 and CTRL (Figure 4c,d) show that in the fire source region, the high peaks of reduction in O3 concentrations during the daytime correspond well with the times when there are strong PM2.5 emissions from fires, which reduce downward solar radiation reaching the surface and surface air temperatures. During the times when there are high peaks of O3 reductions, the PBLH decreases (SI Figure S3c,d). This suggests that the role the PBLH plays in enhancing O3 formation is outcompeted by other factors (i.e., reduced 11883

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Notes

the downwind area. The decrease in J(NO2) due to the inclusion of fire aerosols is much larger than that caused by anthropogenic aerosols and clouds (e.g., 17% in Flynn et al.17 and 20% in Tang et al.15). J(NO2) controls the photolysis of NO2 in the atmosphere; reductions in NO2 photolysis reduces the production of O(1D), required for O3 production. The decrease in J(NO2) along with changes in other variables leads to the decrease in O3 concentrations over the modeling domain when only considering aerosols from fires in our simulations.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The National Center for Atmospheric Research (NCAR) is sponsored by the U.S. National Science Foundation. We are grateful for the help provided by Serena Chung, Mary Barth, and Gabriele Pfister. We thank Jerome Fast for his helpful comments.

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4. DISCUSSION The simulations in this study show that the aerosols from fires lead up to 50 Wm−2 decrease in downward solar radiation reaching the surface, a 2 K cooling at the surface, and a 75% decrease in photolysis rates. These effects, in addition to reductions in biogenic isoprene emissions, cause up to 15% reductions in hourly O3 concentrations over the source region and immediately downwind. Other competing interactions, such as reductions in PBLH and emissions of NOx and VOC from the fires, lead to increased ambient O3 mixing ratios. Overall, the models show an increase in O3 production resulting from the large-scale emissions from wild fires; however, this effect is dampened significantly by the interactions of the fire-emitted aerosol and the atmospheric radiation budget. Thus, while previous modeling studies (e.g., refs 3 and 5), which investigated one effect at a time or did not consider the meteorology−chemistry interactions, have pointed out the overestimation of O3 formation due to the overestimation of NOx in regions of fire, the results presented in this study imply that considering the effects of radiative forcing of fire aerosols may be important when assessing the role of fires in air quality and could reduce the overestimation of O3 formation. Improving model simulations of the effect of fires on air quality is contingent on improving the interactions between meteorology and chemistry. In this work, we only examined the direct radiative forcing of fire aerosols; the indirect effects are not considered. In future work, changes in meteorology as well as chemistry caused by the radiative forcing (both direct and indirect) of aerosols should, therefore, be taken into account when assessing the effects of fires on photochemistry. This study demonstrates the importance of including the online meteorology−chemistry feedbacks in air quality studies and quantified the impact of the emissions and the aerosol direct radiative forcing on O3 chemistry. It should be noted that in this study we only studied one fire case, and more work needs to be done to assess the statistical significance for different fire cases and for different regions. Fire aerosols not only impact meteorology, but also have strong effects on biogenic emissions and actinic flux, which controls photolysis rates in the troposphere and at the surface.

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ASSOCIATED CONTENT

* Supporting Information S

One table and six figures are available. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: 303-497-1435; e-mail: [email protected]. 11884

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