Modeling regional aerosol and aerosol precursor variability over California and its sensitivity to emissions and long-range transport during the 2010 CalNex and CARES campaigns (original) (raw)
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2014
The performance of the Weather Research and Forecasting regional model with chemistry (WRF-73 Chem) in simulating the spatial and temporal variations in aerosol mass, composition, and size over 74 California is quantified using the extensive meteorological, trace gas, and aerosol measurements collected 75 during the California Nexus of Air Quality and Climate Experiment (CalNex) and the Carbonaceous 76 Aerosol and Radiative Effects Study (CARES) conducted during May and June of 2010. The overall 77 objective of the field campaigns was to obtain data needed to better understand processes that affect both 78 climate and air quality, including emission assessments, transport and chemical aging of aerosols, aerosol 79 radiative effects. Simulations were performed that examined the sensitivity of aerosol concentrations to 80 anthropogenic emissions and to long-range transport of aerosols into the domain obtained from a global 81 model. The configuration of WRF-Chem used in this study is shown to reproduce the overall synoptic 82 conditions, thermally-driven circulations, and boundary layer structure observed in region that controls 83 the transport and mixing of trace gases and aerosols. Reducing the default emissions inventory by 50% 84 led to an overall improvement in many simulated trace gases and black carbon aerosol at most sites and 85 along most aircraft flight paths; however, simulated organic aerosol was closer to observed when there 86 were no adjustments to the primary organic aerosol emissions. We found that sulfate was better 87 simulated over northern California whereas nitrate was better simulated over southern California. While 88 the overall spatial and temporal variability of aerosols and their precursors were simulated reasonably 89 well, we show cases where the local transport of some aerosol plumes were either too slow or too fast, 90 which adversely affects the statistics quantifying the differences between observed and simulated 91 quantities. Comparisons with lidar and in-situ measurements indicate that long-range transport of 92 aerosols from the global model was likely too high in the free troposphere even though their 93 concentrations were relatively low. This bias led to an over-prediction in aerosol optical depth by as 94 much as a factor of two that offset the under-predictions of boundary-layer extinction resulting primarily 95 from local emissions. Lowering the boundary conditions of aerosol concentrations by 50% greatly 96 reduced the bias in simulated aerosol optical depth for all regions of California. This study shows that 97 quantifying regional-scale variations in aerosol radiative forcing and determining the relative role of 98 emissions from local and distant sources is challenging during 'clean' conditions and that a wide array of 99 measurements are needed to ensure model predictions are correct for the right reasons. In this regard, the 100 combined CalNex and CARES datasets are an ideal testbed that can be used to evaluate aerosol models in 101 great detail and develop improved treatments for aerosol processes.
Atmospheric Chemistry and Physics, 2012
We describe the synoptic and regional-scale meteorological conditions that affected the transport and mixing of trace gases and aerosols in the vicinity of Sacramento, California during June 2010 when the Carbonaceous Aerosol and Radiative Effects Study (CARES) was conducted. The meteorological measurements collected by various instruments deployed during the campaign and the performance of the chemistry version of the Weather Research and Forecasting model (WRF-Chem) are both discussed. WRF-Chem was run daily during the campaign to forecast the spatial and temporal variation of carbon monoxide emitted from 20 anthropogenic source regions in California to guide aircraft sampling. The model is shown to reproduce the overall circulations and boundary-layer characteristics in the region, although errors in the upslope wind speed and boundary-layer depth contribute to differences in the observed and simulated carbon monoxide. Thermally-driven upslope flows that transported pollutants from Sacramento over the foothills of the Sierra Nevada occurred every afternoon, except during three periods when the passage of mid-tropospheric troughs disrupted the regional-scale flow patterns. The meteorological conditions after the passage of the third trough were the most favorable for photochemistry and likely formation of secondary organic aerosols. Meteorological measurements and model forecasts indicate that the Sacramento pollutant plume was likely transported over a downwind site that col-lected trace gas and aerosol measurements during 23 time periods; however, direct transport occurred during only eight of these periods. The model also showed that emissions from the San Francisco Bay area transported by intrusions of marine air contributed a large fraction of the carbon monoxide in the vicinity of Sacramento, suggesting that this source likely affects local chemistry. Contributions from other sources of pollutants, such as those in the Sacramento Valley and San Joaquin Valley, were relatively low. Aerosol layering in the free troposphere was observed during the morning by an airborne Lidar. WRF-Chem forecasts showed that mountain venting processes contributed to aged pollutants aloft in the valley atmosphere that are then entrained into the growing boundary layer the subsequent day.
Carbonaceous aerosols impact climate directly by scattering and absorbing radiation, and hence play a major, although highly uncertain, role in global radiative forcing. Commonly, ambient carbonaceous aerosols are internally mixed with secondary species such as nitrate, sulfate, and ammonium, which influences their optical properties, hygroscopicity, and atmospheric lifetime, thus impacting climate forcing. Aircraft-aerosol time-of-flight mass spectrometry (A-ATOFMS), which measures single-particle mixing state, was used to determine the fraction of organic and soot aerosols that are internally mixed and the variability of their mixing state in California during the Carbonaceous Aerosols and Radiative Effects Study (CARES) and the Research at the Nexus of Air Quality and Climate Change (Cal-Nex) field campaigns in the late spring and early summer of 2010. Nearly 88 % of all A-ATOFMS measured particles (100-1000 nm in diameter) were internally mixed with secondary species, with 96 % and 75 % of particles internally mixed with nitrate and/or sulfate in southern and northern California, respectively. Even though atmospheric particle composition in both regions was primarily influenced by urban sources, the mixing state was found to vary greatly, with nitrate and soot being the dominant species in southern California, and sulfate and organic carbon in northern California. Furthermore, mixing state varied temporally in northern California, with soot becoming the prevalent particle type towards the end of the study as regional pollution levels increased. The results from these studies demonstrate that the majority of ambient carbonaceous particles in California are internally mixed and are heavily influenced by secondary species that are most prevalent in the particular region. Based on these findings, considerations of regionally dominant sources and secondary species, as well as temporal variations of aerosol physical and optical properties, will be required to obtain more accurate predictions of the climate impacts of aerosol in California.
Fine scale modeling of wintertime aerosol mass, number, and size distributions in central California
1] In light of nonattainment of PM 2.5 in central California, the CMAQ-MADRID 1 model is applied to simulate PM 2.5 mass, number, and size distributions observed during the California Regional PM 10 /PM 2.5 Air Quality Study (CRPAQS) winter episode of 25-31 December 2000. The simulations with 12 and 24 size sections at a horizontal grid resolution of 4 km reproduce well the 24 h average mass concentrations of PM 2.5 (with normalized mean biases (NMBs) of −6.2% to 0.5%), but with larger biases for organic matter, nitrate, and elemental carbon (with NMBs of −67% to 40.2%) and a weaker capability of replicating temporal variation of PM 2.5 and its components. The coagulation process leads to a 40%-91% reduction in simulated PM 2.5 number concentrations. The 24 section simulation with coagulation shows the best agreement with the observed PM number and size distributions (with an NMB of −13.9%), indicating the importance of coagulation for predicting particle number and the merits of using a fine particle size resolution. Accurately simulating PM 2.5 number and size distributions continue to be a major challenge, due to inaccuracies in model inputs (e.g., meteorological fields, precursor emissions, and the initial size distribution of PM emissions and concentrations), uncertainties in model formulations (e.g., heterogeneous chemistry and aerosol formation, growth, and removal processes), as well as inconsistencies and uncertainties in observations obtained with different methods.
Atmospheric Chemistry and Physics, 2012
The new global anthropogenic emission inventory (EDGAR-CIRCE) of gas and aerosol pollutants has been incorporated in the chemistry general circulation model EMAC (ECHAM5/MESSy Atmospheric Chemistry). A relatively high horizontal resolution simulation is performed for the years 2005-2008 to evaluate the capability of the model and the emissions to reproduce observed aerosol concentrations and aerosol optical depth (AOD) values. Model output is compared with observations from different measurement networks (CASTNET, EMEP and EANET) and AODs from remote sensing instruments (MODIS and MISR). A good spatial agreement of the distribution of sulfate and ammonium aerosol is found when compared to observations, while calculated nitrate aerosol concentrations show some discrepancies. The simulated temporal development of the inorganic aerosols is in line with measurements of sulfate and nitrate aerosol, while for ammonium aerosol some deviations from observations occur over the USA, due to the wrong temporal distribution of ammonia gas emissions. The calculated AODs agree well with the satellite observations in most regions, while negative biases are found for the equatorial area and in the dust outflow regions (i.e. Central Atlantic and Northern Indian Ocean), due to an underestimation of biomass burning and aeolian dust emissions, respectively. Aerosols and precursors budgets for five different regions (North America, Europe, East Asia, Central Africa and South America) are calculated. Over East-Asia most of the emitted aerosols (precursors) are also deposited within the region, while in North America and Europe transport plays a larger role. Further, it is shown that a simulation with monthly varying anthropogenic emissions typically improves the temporal correlation by 5-10 % compared to one with constant annual emissions.
Journal of Geophysical Research: Atmospheres, 2013
Carbonaceous aerosols have the potential to impact climate directly through absorption of incoming solar radiation and indirectly by affecting cloud and precipitation. Recent modeling studies have made great efforts to simulate both the spatial and temporal distributions of carbonaceous aerosol's optical properties and radiative forcing. This study makes the first observationally constrained assessment of the direct radiative forcing of carbonaceous aerosols over California. By exploiting multiple observations (including ground sites and satellites), we constructed the distribution of aerosol optical depths and aerosol absorption optical depths (AAOD) over California for a 10 year period (2000-2010). We partitioned the total solar absorption into individual contributions from elemental carbon (EC), organic carbon (OC), and dust aerosols, using a newly developed scheme. Our results show that AAOD due to carbonaceous aerosols (EC and OC) at 440 nm was 50%-200% larger than natural dust, with EC contributing the bulk (70%-90%). Observationally constrained EC absorption agrees reasonably well with estimates from global and regional chemical transport models, but the models underestimate the OC AAOD by at least 50%. We estimated that the top of the atmosphere (TOA) forcing from carbonaceous aerosols was 0.7 W/m 2 and the TOA forcing due to OC was close to zero. The atmospheric heating of carbonaceous aerosol was 2.2-2.9 W/m 2 , of which EC contributed about 80-90%. We estimated the atmospheric heating of OC at 0.1-0.4 W/m 2 , larger than model simulations. EC reduction over the last two decades may have caused a surface brightening of 1.5-3.5 W/m 2 .
The California Climate Change Center Report Series details ongoing Center-sponsored research. As interim project results, these reports receive minimal editing, and the information contained in these reports may change; authors should be contacted for the most recent project results. By providing ready access to this timely research, the Center seeks to inform the public and expand dissemination of climate change information; thereby leveraging collaborative efforts and increasing the benefits of this research to California's citizens, environment, and economy.
Atmospheric Chemistry and Physics Discussions, 2015
Co-located measurements of fine particulate matter (PM 2.5) organic carbon (OC), elemental carbon, radiocarbon (14 C), speciated volatile organic compounds (VOCs), and OH radicals during the CalNex field campaign provide a unique opportunity to evaluate the Community Multiscale Air Quality (CMAQ) model's representation of organic species from VOCs to particles. Episode average daily 23 h average 14 C analysis indicates PM 2.5 carbon at Pasadena and Bakersfield during the CalNex field campaign was evenly split between contemporary and fossil origins. CMAQ predicts a higher contemporary carbon fraction than indicated by the 14 C analysis at both locations. The model underestimates measured PM 2.5 organic carbon at both sites with very little (7 % in Pasadena) of the modeled mass represented by secondary production, which contrasts with the ambient-based SOC / OC fraction of 63 % at Pasadena. Measurements and predictions of gas-phase anthropogenic species, such as toluene and xylenes, are generally within a factor of 2, but the corresponding SOC tracer (2,3dihydroxy-4-oxo-pentanoic acid) is systematically underpredicted by more than a factor of 2. Monoterpene VOCs and SOCs are underestimated at both sites. Isoprene is underestimated at Pasadena and overpredicted at Bakersfield and isoprene SOC mass is underestimated at both sites. Systematic model underestimates in SOC mass coupled with reasonable skill (typically within a factor of 2) in predicting hydroxyl radical and VOC gas-phase precursors suggest error(s) in the parameterization of semivolatile gases to form SOC. Yield values (α) applied to semivolatile partitioning species were increased by a factor of 4 in CMAQ for a sensitivity simulation, taking into account recent findings of underestimated yields in chamber experiments due to gas wall losses. This sensitivity resulted in improved model perfor-Published by Copernicus Publications on behalf of the European Geosciences Union. 5244 K. R. Baker et al.: Gas and aerosol carbon in California mance for PM 2.5 organic carbon at both field study locations and at routine monitor network sites in California. Modeled percent secondary contribution (22 % at Pasadena) becomes closer to ambient-based estimates but still contains a higher primary fraction than observed.