ALBERT

Description: Regional and global chemical transport models underpredict NOx (NO+NO2) in the upper troposphere where it is a precursor to the greenhouse gas ozone. The NOx bias been shown in model evaluations using aircraft data (Singh et al., 2007) and total column NO2 (molecules cm−2) from satellite observations (Napelenok et al., 2008). The causes of NOx underpredictions have yet to be fully understood due to the interconnected nature of simulated emission, transport, and chemistry processes. Recent observation-based studies suggest that, in the upper troposphere, simulated chemistry overpredicts hydrogen radicals (OH• and HO2•) and would convert NOx to HNO3 too quickly (Olson et al., 2006; Bertram et al., 2007; Ren et al., 2008). Since typical chemistry evaluation techniques are not available for upper tropospheric conditions, this study develops an evaluation platform from in situ observations, stochastic convection, and deterministic chemistry. We derive a stochastic convection model and optimize it using two simulated datasets of time since convection, one based on meteorology and the other on chemistry. The chemistry surrogate for time since convection is calculated using seven different chemical mechanisms, all of which predict shorter time since convection than our meteorological analysis. We evaluate chemical simulations by inter-comparison and by pairing results with observations based on NOx:HNO3, a photochemical aging indicator. Inter-comparison reveals individual chemical mechanism biases and recommended updates. Evaluation against observations shows that all chemical mechanisms overpredict NOx removal relative to long-lived methanol and carbon monoxide. All chemical mechanisms underpredict observed NOx by at least 30%, and further evaluation is necessary to refine simulation sensitivities to initial conditions and chemical rate uncertainties.

Description: The contribution of marine organic emissions to the air quality in coastal areas of the western United States is studied using the latest version of the US Environmental Protection Agency (EPA) regional-scale Community Multiscale Air Quality (CMAQv4.7) modeling system. Emissions of marine isoprene, monoterpenes, and primary organic matter (POM) from the ocean are implemented into the model to provide a comprehensive view of the connection between ocean biology and atmospheric chemistry and air pollution. Model simulations show that marine organics can increase the concentration of PM2.5 by 0.1–0.3 μg m−3 (up to 5%) in some coastal cities such as San Francisco, CA. This increase in the PM2.5 concentration is primarily attributed to the POM emissions, with small contributions from the marine isoprene and monoterpenes. When marine organic emissions are included, organic carbon (OC) concentrations over the remote ocean are increased by up to 50% (25% in coastal areas), values consistent with recent observational findings. This study is the first to quantify the air quality impacts from marine POM and monoterpenes for the United States, and it highlights the need for inclusion of marine organic emissions in air quality models.

Description: Regional and global chemical transport models underpredict NOx (NO + NO2) in the upper troposphere where it is a precursor to the greenhouse gas ozone. The NOx bias has been shown in model evaluations using aircraft data (Singh et al., 2007) and total column NO2 (molecules cm−2) from satellite observations (Napelenok et al., 2008). The causes of NOx underpredictions have yet to be fully understood due to the interconnected nature of simulated emission, transport, and chemistry processes. Recent observation-based studies, in the upper troposphere, identify chemical rate coefficients as a potential source of error (Olson et al., 2006; Ren et al., 2008). Since typical chemistry evaluation techniques are not available for upper tropospheric conditions, this study develops an evaluation platform from in situ observations, stochastic convection, and deterministic chemistry. We derive a stochastic convection model and optimize it using two simulated datasets of time since convection, one based on meteorology, and the other on chemistry. The chemistry surrogate for time since convection is calculated using seven different chemical mechanisms, all of which predict shorter time since convection than our meteorological analysis. We evaluate chemical simulations by inter-comparison and by pairing results with observations based on NOx:HNO3, a photochemical aging indicator. Inter-comparison reveals individual chemical mechanism biases and recommended updates. Evaluation against observations shows that all chemical mechanisms overpredict NOx removal relative to long-lived methanol and carbon monoxide. All chemical mechanisms underpredict observed NOx by at least 30%, and further evaluation is necessary to refine simulation sensitivities to initial conditions and chemical rate uncertainties.

Description: We present in situ measurements of particle-phase liquid water. Measurements were conducted from 3 June to 15 July 2013 during the Southern Oxidant and Aerosol Study (SOAS) in the southeastern US. The region is photochemically active, humid, dominated by biogenic emissions, impacted by anthropogenic pollution, and known to contain high concentrations of organic aerosol mass. Measurements characterized mobility number size distributions of ambient atmospheric aerosols in three states: unperturbed, dry, and dry-humidified. Unperturbed measurements describe the aerosol distribution at ambient temperature and relative humidity. For the dry state, the sample was routed through a cold trap upstream of the inlet then reheated, while for the dry-humidified state the sample was rehumidified after drying. The total volume of water and semi-volatile compounds lost during drying was quantified by differencing dry and unperturbed volumes from the integrated size spectra, while semi-volatile volumes lost during drying were quantified differencing unperturbed and dry-humidified volumes. Results indicate that particle-phase liquid water was always present. Throughout the SOAS campaign, median water mass concentrations at the relative humidity (RH) encountered in the instrument typically ranged from 1 to 5 μg m−3 but were as high as 73 μg m−3. On non-raining days, morning time (06:00–09:00) median mass concentrations exceeded 15 μg m−3. Hygroscopic growth factors followed a diel cycle and exceed 2 from 07:00 to 09:00 local time. The hygroscopicity parameter kappa ranged from 0.14 to 0.46 and hygroscopicity increased with increasing particle size. An observed diel cycle in kappa is consistent with changes in aerosol inorganic content and a dependency of the hygroscopicity parameter on water content. Unperturbed and dry-humidified aerosol volumes did not result in statistically discernible differences, demonstrating that drying did not lead to large losses in dry particle volume. We anticipate that our results will help improve the representation of aerosol water content and aqueous-phase-mediated partitioning of atmospheric water-soluble gases in photochemical models.

Description: Recent field and laboratory evidence indicates that the oxidation of isoprene, (2-methyl-1,3-butadiene, C5H8) forms secondary organic aerosol (SOA). Global biogenic emissions of isoprene (600 Tg yr−1) are sufficiently large that the formation of SOA in even small yields results in substantial production of atmospheric particulate matter, likely having implications for air quality and climate. Here we present a review of field measurements, experimental work, and modeling studies aimed at understanding the mechanisms, yield, and atmospheric importance of isoprene-derived SOA. SOA yields depend on a number of factors, including organic aerosol loading (Mo), NOx level (RO2 chemistry), and, because of the importance of multigenerational chemistry, the degree of oxidation. These dependences are not always included in SOA modules used in atmospheric transport models, and instead most yield parameterizations rely on a single set of chamber experiments (carried out over a limited range of conditions); this may lead to very different estimates of the atmospheric importance of isoprene SOA. New yield parameterizations, based on all available laboratory data (Mo=0–50 μg m−3), are presented here, so that SOA formation may be computed as a function of Mo, NOx level, and temperature. Current research needs and future research directions are identified.

Description: Tropospheric ozone is the third strongest greenhouse gas, and has the highest uncertainty in radiative forcing of the top five greenhouse gases. Throughout the troposphere, ozone is produced by radical oxidation of nitrogen oxides (NOx = NO + NO2). In the upper troposphere (8–10 km), current chemical transport models under-estimate nitrogen dioxide (NO2) observations. Improvements to simulated NOx production from lightning have increased NO2 predictions, but the predictions in the upper troposphere remain biased low. The upper troposphere has low temperatures (T 〈 250 K) that increase the uncertainty of many important chemical reaction rates. This study constrains uncertain reaction rates by combining model predictions with measurements from the Intercontinental Chemical Transport Experiment-North America observational campaign. The results show that the nitric acid formation rate, which is the dominant sink of NO2 and radicals, is currently over-estimated by 22% in the upper troposphere. The results from this study suggest that the temperature sensitivity of nitric acid formation is lower than currently recommended. Since the formation of nitric acid removes nitrogen dioxide and radicals that drive the production of ozone, the revised reaction rate will affect ozone concentrations in upper troposphere impacting climate and air quality in the lower troposphere.

Description: Secondary organic aerosols (SOA) exert a significant influence on ambient air quality and regional climate. Recent field, laboratorial and modeling studies have confirmed that in-cloud processes contribute to a large fraction of SOA production with large space-time heterogeneity. This study evaluates the key factors that govern the production of cloud-process SOA (SOAcld) on a global scale based on the GFDL coupled chemistry-climate model AM3 in which full cloud chemistry is employed. The association between SOAcld production rate and six factors (i.e., liquid water content (LWC), total carbon chemical loss rate (TCloss), temperature, VOC/NOx, OH, and O3) is examined. We find that LWC alone determines the spatial pattern of SOAcld production, particularly over the tropical, subtropical and temperate forest regions, and is strongly correlated with SOAcld production. TCloss ranks the second and mainly represents the seasonal variability of vegetation growth. Other individual factors are essentially uncorrelated spatiotemporally to SOAcld production. We find that the rate of SOAcld production is simultaneously determined by both LWC and TCloss, but responds linearly to LWC and nonlinearly (or concavely) to TCloss. A parameterization based on LWC and TCloss can capture well the spatial and temporal variability of the process-based SOAcld formation (R2 = 0.5) and can be easily applied to global three dimensional models to represent the SOA production from cloud processes.

Description: Despite critical importance for air quality and climate predictions, accurate representation of secondary organic aerosol (SOA) formation remains elusive. An essential addition to the ongoing discussion of improving model predictions is an acknowledgement of the linkages between experimental conditions, parameter optimization and model output, as well as the linkage between empirically-derived partitioning parameters and the physicochemical properties of SOA they represent in models. In this work, a "best available" set of SOA modeling parameters is selected by comparing predicted SOA yields and mass concentrations with observed yields and mass concentrations from a comprehensive list of published smog chamber studies. Evaluated SOA model parameters include existing parameters for two product (2p) and volatility basis set (VBS) modeling frameworks, and new 2p-VBS parameters; 2p-VBS parameters are developed to exploit advantages of the VBS approach within the computationally-economical and widely-used 2p framework. Fine particulate matter (PM2.5) and SOA mass concentrations are simulated for the continental United States using CMAQv.4.7.1; results are compared for a base case (with default CMAQ parameters) and two best available parameter cases to illustrate the high- and low-NOx limits of biogenic SOA formation from monoterpenes. Results are discussed in terms of implications for current chemical transport model simulations and recommendations are provided for future modeling and measurement efforts. The comparisons of SOA yield predictions with data from 22 published chamber studies illustrate that: (1) SOA yields for naphthalene, and cyclic and 〉 C5 straight-chain/branched alkanes are not well represented using either the newly developed or existing parameters for low-yield aromatics and lumped alkanes, respectively; and (2) for four of seven volatile organic compound+oxidant systems, the 2p-VBS parameters better represent chamber data than do the default CMAQ v.4.7.1 parameters. Using the "best available" parameters (combination of published 2p and newly derived 2p-VBS), predicted SOA mass and PM2.5 concentrations increase by up to 15% and 7%, respectively, for the high-NOx case and up to 215% (~3 μg m−3) and 55%, respectively, for the low-NOx case. Percent bias between model-based and observationally-based secondary organic carbon (SOC) improved from −63% for the base case to −15% for the low-NOx case. The ability to robustly assign "best available" parameters in all volatile organic compound+oxidant systems, however, is critically limited due to insufficient data; particularly for photo-oxidation of diverse monoterpenes, sesquiterpenes, and alkanes under a range of atmospherically relevant conditions.

Description: Gas-phase water-soluble organic matter (WSOMg) is ubiquitous in the troposphere. In the summertime, the potential for these gases to partition to particle-phase liquid water (H2Optcl) where they can form secondary organic aerosol (SOAAQ) is high in the Eastern US and low elsewhere, with the exception of an area near Los Angeles, CA. This spatial pattern is driven by mass concentrations of H2Optcl, not WSOMg. H2Optcl mass concentrations are predicted to be high in the Eastern US, largely due to sulfate. The ability of sulfate to increase H2Optcl is well established and routinely included in atmospheric models; however WSOMg partitioning to this water and subsequent SOA formation is not. The high mass concentrations of H2Optcl in the southeast (SE) US but not the Amazon may help explain why biogenic SOA mass concentrations are high in the SE US but low in the Amazon. Furthermore, during the summertime in the Eastern US, the potential for organic gases to partition into liquid water is greater than their potential to partition into organic matter (OM) because concentrations of WSOMg and H2Optcl are higher than semi-volatile gases and OM. Thus, unless condensed phase yields are substantially different (〉 ~ order of magnitude), we expect that SOA formed through aqueous-phase pathways (SOAAQ) will dominate in the Eastern US. These findings also suggest that H2Optcl is largely anthropogenic and provide a previously unrecognized mechanism by which anthropogenic pollutants impact the amount of SOA mass formed from biogenic organic emissions. The previously reported estimate of the controllable fraction of biogenic SOA in the Eastern US (50%) is likely too low.

Description: Tropospheric ozone is the third strongest greenhouse gas, and has the highest uncertainty in radiative forcing of the top five greenhouse gases. Throughout the troposphere, ozone is produced by radical oxidation of nitrogen oxides (NOx = NO + NO2). In the upper troposphere (8–10 km), current chemical transport models under-estimate nitrogen dioxide (NO2) observations. Improvements to simulated NOx emissions from lightning have increased NO2 predictions, but the predictions in the upper troposphere remain biased low. The upper troposphere has low temperatures (T 〈 250 K) that increase the uncertainty of many important chemical reaction rates. This study constrains uncertain reaction rates by combining model predictions with measurements from the Intercontinental Chemical Transport Experiment-North America observational campaign. The results show that the nitric acid formation rate, which is the dominant sink of NO2 and radicals, is currently over-estimated by 22 % in the upper troposphere. The results from this study suggest that the temperature sensitivity of nitric acid formation is lower than currently recommended. Since the formation of nitric acid removes nitrogen dioxide and radicals that drive the production of ozone, the revised reaction rate will affect ozone concentrations in upper troposphere impacting climate and air quality in the lower troposphere.