<p><strong>Abstract.</strong> Increasing wildfire activities in the mountainous western US may present a challenge for the region to attain a recently revised ozone air quality standard in summer. Using current Eulerian chemical transport models to examine the wildfire ozone influences is difficult due to uncertainties in fire emissions, inadequate model chemistry and resolution. Here we quantify the wildfire influence on the ozone variability, trends, and number of high MDA8 (daily maximum 8-h average) ozone days over this region in summers (June, July and August) 1989&#8211;2010 using a new approach. We define a Fire Index using retroplumes (plumes of back-trajectory particles) computed by a Lagrangian dispersion model (FLEXPART), and develop statistical models based on the Fire Index and meteorological parameters to interpret MDA8 ozone concentrations measured at 13 Intermountain West surface sites. We show that the statistical models are able to capture the ozone enhancements by wildfires and give results with some features different from the GEOS-Chem Eulerian chemical transport model. Wildfires enhance the Intermountain West regional summer mean MDA8 ozone by 0.3&#8211;1.5&#8201;ppbv (daily episodic enhancements reach 10&#8211;20&#8201;ppbv at individual sites) with large interannual variability, which are strongly correlated with the total MDA8 ozone. Wildfires also contribute 15&#8201;% of the measured increasing but statistically insignificant trends of 0.14&#8211;0.19&#8201;ppbv year<sup>&#8722;1</sup> in 1989&#8211;2010. We find large fire impacts on the number of exceedance days; for the 13 CASTNet sites, 31&#8201;% of the summer days with MDA8 ozone exceeding 70&#8201;ppbv would not occur in the absence of wildfires.</p>
Abstract. Improving the ability of global models to predict concentrations of black carbon (BC) over the Pacific Ocean is essential to evaluate the impact of BC on marine climate. In this study, we tag BC tracers from 13 source regions around the globe in a global chemical transport model MOZART-4. Numerous sensitivity simulations are carried out varying the aging timescale of BC emitted from each source region. The aging timescale for each source region is optimized by minimizing errors in vertical profiles of BC mass mixing ratios between simulations and HIAPER Pole-to-Pole Observations (HIPPO). For most HIPPO deployments, in the Northern Hemisphere, optimized aging timescales are less than half a day for BC emitted from tropical and mid-latitude source regions, and about 1 week for BC emitted from high latitude regions in all seasons except summer. We find that East Asian emissions contribute most to the BC loading over the North Pacific, while South American, African and Australian emissions dominate BC loadings over the South Pacific. Dominant source regions contributing to BC loadings in other parts of the globe are also assessed. The lifetime of BC originating from East Asia (i.e., the world's largest BC emitter) is found to be only 2.2 days, much shorter than the global average lifetime of 4.9 days, making East Asia's contribution to global burden only 36 % of BC from the second largest emitter, Africa. Thus, evaluating only relative emission rates without accounting for differences in aging timescales and deposition rates is not predictive of the contribution of a given source region to climate impacts. Our simulations indicate that lifetime of BC increases nearly linearly with aging timescale for all source regions. When aging rate is fast, the lifetime of BC is largely determined by factors that control local deposition rates (e.g. precipitation). The sensitivity of lifetime to aging timescale depends strongly on the initial hygroscopicity of freshly emitted BC. Our findings suggest that the aging timescale of BC varies significantly by region and season, and can strongly influence the contribution of source regions to BC burdens around the globe. Improving parameterizations of the aging process for BC is important for enhancing the predictive skill of air quality and climate models. Future observations that investigate the evolution of hygroscopicity of BC as it ages from different source regions to the remote atmosphere are urgently needed.
<p><strong>Abstract.</strong> Parameterizations that impact wet removal of black carbon remain uncertain in global climate models. In this study, we enhance the default wet deposition scheme for BC in the Community Earth System Model (CESM) to (a) add relevant physical processes that were not resolved in the default model, and (b) facilitate understanding of the relative importance of various cloud processes on BC distributions. We find that the enhanced scheme greatly improves model performance against HIPPO observations relative to the default scheme. We find that convection scavenging, aerosol activation, ice nucleation, evaporation of rain/snow, and below cloud scavenging dominate wet deposition of BC. BC conversion rates for processes related to in-cloud water/ice conversion (i.e., riming, the Bergeron processes, and evaporation of cloud water sedimentation) are relatively smaller, but have large seasonal variations. We also conduct sensitivity simulations that turn off each cloud process one at a time to quantify the influence of cloud processes on BC distributions and radiative forcing. Convective scavenging is found to most significantly influence BC concentrations at mid-altitudes over the tropics and even globally. In addition, BC is sensitive to all cloud processes over the Northern Hemisphere at high latitudes. As for BC vertical distributions, convective scavenging has a dominant influence. Aerosol activation mainly increases the fraction of column BC below 5&#8201;km whereas ice nucleation decreases that above 10&#8201;km. During wintertime, the Bergeron process also significantly increases BC concentrations at lower altitudes over the Arctic. Our simulation yields a global BC burden of 85&#8201;Gg; corresponding direct radiative forcing (DRF) of BC estimated using the Parallel Offline Radiative Transfer (PORT) is 0.13&#8201;W&#8201;m<sup>&#8722;2</sup>, much lower than previous studies. The range of DRF derived from sensitivity simulations is large, 0.09&#8211;0.33&#8201;W&#8201;m<sup>&#8722;2</sup>, corresponding to BC burdens varying from 73&#8201;Gg to 151&#8201;Gg. Due to differences in BC vertical distributions among each sensitivity simulation, fractional changes in DRF (relative to the baseline simulation) are always higher than fractional changes in BC burdens; this occurs because relocating BC in the vertical influences the radiative forcing per BC mass. Our results highlight the influences of cloud microphysical processes on BC concentrations and radiative forcing.</p>
<p><strong>Abstract.</strong> The response of surface O<sub>3</sub> concentrations to basin-scale warming and cooling of Northern Hemispheric oceans is investigated using the Community Earth System Model (CESM). Idealized spatially uniform sea surface temperature (SST) anomalies of &#177;1&#8201;&#186;C are superimposed onto the North Pacific, North Atlantic, and North Indian oceans, individually. Our simulations suggest seasonal and regional variability of surface O<sub>3</sub> in response to SST anomalies, especially in boreal summer. Increasing (decreasing) SST by 1&#8201;&#186;C in one of the regions of focus induces decreases (increases) in surface O<sub>3</sub> concentrations, ranging from 1 to 5&#8201;ppbv. With fixed emissions, SST increases of a specific ocean in the Northern Hemisphere tend to increase summertime surface O<sub>3</sub> concentrations over upwind continents, accompanied with a widespread reduction over downwind regions. We implement the integrated process analysis (IPR) in CESM and find that meteorological O<sub>3</sub> transport in response to SST changes is the key process causing surface O<sub>3</sub> perturbations in most cases. During boreal summer, basin-scale SST warming facilitates vertical transport of O<sub>3</sub> to the surface over upwind regions while significantly reducing vertical transport over continents that are downwind. This process, as confirmed by tagged CO tracers, implicates a considerable suppression of O<sub>3</sub> intercontinental transport due to increased stagnation at mid-latitudes induced by SST increases. Changes in O<sub>3</sub> chemical production associated with regional SST increases, on the other hand, can increase surface O<sub>3</sub> over highly polluted continents except for South Asia. In South Asia, intensified cloud loading in response to North Indian SST warming depresses both surface air temperature and solar radiation, and thus photochemical production of O<sub>3</sub>. Our findings indicate a robust linkage between basin-scale SST variability and continental surface O<sub>3</sub> pollution, which should be taken into account for regional air quality management.</p>
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