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A cloud‐resolved storm and chemistry simulation of a severe convective system in Oklahoma constrained by anvil aircraft observations of NOxwas used to estimate the mean production of NOxper flash in this storm. An upward ice flux scheme was used to parameterize flash rates in the model. Model lightning was also constrained by observed lightning flash types and the altitude distribution of flash channel segments. The best estimate of mean NOxproduction by lightning in this storm was 80–110 mol per flash, which is smaller than many other literature estimates. This result is likely due to the storm having been a high flash rate event in which flash extents were relatively small. Over the evolution of this storm a moderate negative correlation was found between the total flash rate and flash extent and energy per flash. A longer‐term simulation at 36‐km horizontal resolution with parameterized convection was used to simulate the downwind transport and chemistry of the anvil outflow from the same storm. Convective transport of low‐ozone air from the boundary layer decreased ozone in the anvil outflow by up to 20–40 ppbv compared with the initial conditions, which contained stratospheric influence. Photochemical ozone production in the lightning‐NOxenhanced convective plume proceeded at a rate of 10–11 ppbv per day in the 9–11 km outflow layer over the 24‐hr period of downwind transport to the Southern Appalachians. Photochemical production plays a large role in the restoration of upper tropospheric ozone following deep convection. Nitrogen oxides are important precursors for tropospheric ozone, an important greenhouse gas. The global amount of nitrogen oxides produced by lightning remains highly uncertain, primarily because of uncertainty in the amount produced per flash. In this paper we use an approach that involves cloud‐resolved modeling with chemistry, constrained by observed flash rates and aircraft measurements of nitrogen oxides, to make an estimate of the mean production per flash for an observed severe storm over Oklahoma. We estimate that the mean production rate was 80–110 mol per flash for this high flash rate storm, which is at the lower end of the range reported in the literature. We use a regional model to follow the outflow of the Oklahoma storm downwind to estimate the amount of ozone produced in the upper tropospheric outflow plume by the lightning‐generated nitrogen oxides. Our estimate is in the range found in previous studies, and we note that the ozone production by photochemistry is an important process in restoring upper tropospheric ozone following storms that lofted low values of ozone from the layer of air near the surface. Mean lightning NOxproduction was 80–110 mol flash−1for a high flash rate storm, based on a model constrained by flash and NOxobservationsNet O3production in the upper outflow of a high flash rate storm was at a rate of 10–11 ppbv day−1during 24 hr of downwind transportProduction of O3due to lightning NOxcontributed to the recovery of upper tropospheric O3following reductions due to convective transport Mean lightning NOxproduction was 80–110 mol flash−1for a high flash rate storm, based on a model constrained by flash and NOxobservations Net O3production in the upper outflow of a high flash rate storm was at a rate of 10–11 ppbv day−1during 24 hr of downwind transport Production of O3due to lightning NOxcontributed to the recovery of upper tropospheric O3following reductions due to convective transport more...

This study examines the transport of chemical species to the upper troposphere/lower stratosphere (UTLS) by Typhoon Mireille (1991). We follow an integrated research approach, using in situ flight data when available and results from a high‐resolution chemical transport model. The Weather Research and Forecasting model coupled with chemistry (WRF‐Chem) was used with an innermost grid spacing of 3 km to explicitly resolve the convection being studied. Mireille was well simulated based on comparisons between the simulated fields and in situ chemical measurements from NASA's Pacific Exploratory Mission‐West A field project. Results from the simulated fields show that pollution from distant sources is ingested by Mireille and subsequently lofted by eyewall convection to the upper troposphere, enhancing concentrations in this region. Flux calculations suggest that a strong tropical cyclone (TC), such as Mireille, can impact UTLS chemistry as much as a continental middle‐latitude cyclone. Furthermore, overshooting convective cells in Mireille produced values of chemical flux density at tropopause level that are as much as 15–30 times greater than that of theTC as a whole. Although overshooting tops comprise only a small area of the simulated total TC, they transport large quantities of gaseous species to the upper troposphere because of their strong updrafts. Mireille was well simulated by WRF‐Chem based on tropical cyclone structure, its ability to capture large‐scale chemical trends, and its ability to produce realistic chemical concentrations compared to in situ aircraft measurementsTrajectory analyses showed pollutants from distant source regions being wrapped cyclonically into the center of Mireille at low altitudes and subsequently lofted by eyewall convection to the UTLS regionA strong TC such as Mireille can impact UTLS chemistry as much as a middle‐latitude cyclone more...

Deep convective transport of gaseous precursors to ozone (O3) and aerosols to the upper troposphere is affected by liquid phase and mixed‐phase scavenging, entrainment of free tropospheric air and aqueous chemistry. The contributions of these processes are examined using aircraft measurements obtained in storm inflow and outflow during the 2012 Deep Convective Clouds and Chemistry (DC3) experiment combined with high‐resolution (dx≤3 km) WRF‐Chem simulations of a severe storm, an air mass storm, and a mesoscale convective system (MCS). The simulation results for the MCS suggest that formaldehyde (CH2O) is not retained in ice when cloud water freezes, in agreement with previous studies of the severe storm. By analyzing WRF‐Chem trajectories, the effects of scavenging, entrainment, and aqueous chemistry on outflow mixing ratios of CH2O, methyl hydroperoxide (CH3OOH), and hydrogen peroxide (H2O2) are quantified. Liquid phase microphysical scavenging was the dominant process reducing CH2O and H2O2outflow mixing ratios in all three storms. Aqueous chemistry did not significantly affect outflow mixing ratios of all three species. In the severe storm and MCS, the higher than expected reductions in CH3OOH mixing ratios in the storm cores were primarily due to entrainment of low‐background CH3OOH. In the air mass storm, lower CH3OOH and H2O2scavenging efficiencies (SEs) than in the MCS were partly due to entrainment of higher background CH3OOH and H2O2. Overestimated rain and hail production in WRF‐Chem reduces the confidence in ice retention fraction values determined for the peroxides and CH2O. Methyl hydroperoxide mixing ratios are decreased mainly by entrainment and liquid phase and mixed‐phase scavengingHydrogen peroxide and formaldehyde mixing ratios affected more by liquid phase scavenging than by entrainment or aqueous chemistryOverestimated rain/hail production in WRF‐Chem reduces confidence in ice retention fraction values determined for peroxides and formaldehyde more...

Exposure to unhealthy air causes millions of premature deaths and damages crops sufficient to feed a large portion of the South Asian population every year. However, little is known about how future air quality in South Asia will respond to changing human activities. Here we examine the combined effect of changes in climate and air pollutant emissions projected by the Representative Concentration Pathways (RCP) 8.5 and RCP6.0 on air quality of South Asia in 2050 using a state‐of‐the‐science Nested Regional Climate model with Chemistry (NRCM‐Chem). RCP8.5 and RCP6.0 are selected to represent scenarios of highest and lowest air pollution in South Asia by 2050. NRCM‐Chem shows the ability to capture observed key features of variability in meteorological parameters, ozone and related gases, and aerosols. NRCM‐Chem results show that surface ozone and particulate matter of less than 2.5 μm in diameter will increase significantly by midcentury in South Asia under the RCP8.5 but remain similar to present day under RCP6.0. No RCP suggest an improvement in air pollution in South Asia by midcentury. Under RCP8.5, the frequency of air pollution events is predicted to increase by 20–120 days per year in 2050 compared to the present‐day conditions, with particulate matter of less than 2.5 μm in diameter predicted to breach the World Health Organization ambient air quality guidelines on an almost daily basis in many parts of South Asia. These results indicate that while the RCP scenarios project a global improvement in air quality, they generally result in degrading air quality in South Asia. Future air quality in South Asia is projected under RCP8.5 and RCP6.0None of the RCP projects an improvement in South Asian air quality by 2050PM2.5is projected to breach the WHO ambient air quality guidelines on an almost daily basis in many parts of South Asia by 2050 more...

Deep convective transport of surface moisture and pollution from the planetary boundary layer to the upper troposphere and lower stratosphere affects the radiation budget and climate. This study analyzes the deep convective transport in three different convective regimes from the 2012 Deep Convective Clouds and Chemistry field campaign: 21 May Alabama air mass thunderstorms, 29 May Oklahoma supercell severe storm, and 11 June mesoscale convective system (MCS). Lightning data assimilation within the Weather Research and Forecasting (WRF) model coupled with chemistry (WRF‐Chem) is utilized to improve the simulations of storm location, vertical structure, and chemical fields. Analysis of vertical flux divergence shows that deep convective transport in the 29 May supercell case is the strongest per unit area, while transport of boundary layer insoluble trace gases is relatively weak in the MCS and air mass cases. The weak deep convective transport in the strong MCS is unexpected and is caused by the injection into low levels of midlevel clean air by a strong rear inflow jet. In each system, the magnitude of tracer vertical transport is more closely related to the vertical distribution of mass flux density than the vertical distribution of trace gas mixing ratio. Finally, the net vertical transport is strongest in high composite reflectivity regions and dominated by upward transport. Lightning data assimilation improves model simulations of storm location and vertical structureRear inflow jets within mesoscale convective systems may weaken boundary layer trace gas vertical transportThe tracer vertical transport is more controlled by the vertical gradient of mass flux than the vertical gradient of trace gas more...

We examine wet scavenging of soluble trace gases in storms observed during the Deep Convective Clouds and Chemistry (DC3) field campaign. We conduct high‐resolution simulations with the Weather Research and Forecasting model with Chemistry (WRF‐Chem) of a severe storm in Oklahoma. The model represents well the storm location, size, and structure as compared with Next Generation Weather Radar reflectivity, and simulated CO transport is consistent with aircraft observations. Scavenging efficiencies (SEs) between inflow and outflow of soluble species are calculated from aircraft measurements and model simulations. Using a simple wet scavenging scheme, we simulate the SE of each soluble species within the error bars of the observations. The simulated SEs of all species except nitric acid (HNO3) are highly sensitive to the values specified for the fractions retained in ice when cloud water freezes. To reproduce the observations, we must assume zero ice retention for formaldehyde (CH2O) and hydrogen peroxide (H2O2) and complete retention for methyl hydrogen peroxide (CH3OOH) and sulfur dioxide (SO2), likely to compensate for the lack of aqueous chemistry in the model. We then compare scavenging efficiencies among storms that formed in Alabama and northeast Colorado and the Oklahoma storm. Significant differences in SEs are seen among storms and species. More scavenging of HNO3and less removal of CH3OOH are seen in storms with higher maximum flash rates, an indication of more graupel mass. Graupel is associated with mixed‐phase scavenging and lightning production of nitrogen oxides (NOx), processes that may explain the observed differences in HNO3and CH3OOH scavenging. Observed deep convective scavenging efficiencies simulated by simple schemeSimulated scavenging efficiencies highly sensitive to ice retention fractionScavenging efficiencies vary among storms with different microphysics more...

Chemical processes in clouds and fogs can substantially alter atmospheric oxidant budgets and lead to aerosol mass formation. However, many regional and global models do not include detailed aqueous‐phase chemical mechanisms due to the (a) lack of complete understanding of the chemical processes and (b) computational burden of adding constituents. Current gas‐aqueous chemistry 0‐dimensional models were evaluated in a cloud‐chemistry box model intercomparison based on a mid‐September 2016 cloud chemistry event at Whiteface Mountain, New York. Multiphase mechanisms in the five participating models ranged from those appropriate for 3‐d models to highly complex with thousands of reactions. This study focused on oxidant levels in both phases and aqueous‐phase sulfate and organic acid formation. Comparison of gas‐phase‐only chemistry gives very similar oxidant predictions at night but shows significant differences during daytime with the hydroxyl radical (OH) variability of about an order of magnitude. The variability in the model results increases substantially with aqueous chemistry due to different Henry's Law constants, aqueous‐phase reaction rate constants, and chemical mechanisms. Using a prescribed liquid water content and pH value of 4.5, modeled aqueous OH, aldehyde, and organic acid concentrations differ by over an order of magnitude in daytime. Simulations were also conducted at a pH = 5.1, predicted variable pH, and with added transition metal ion chemistry. While we compare predicted and measured inorganic anions and water‐soluble organic carbon, we cannot do so for aqueous‐phase oxidant concentrations due to the lack of measurements. We highlight a need for recommended equilibrium and aqueous‐phase rate constants. Accurate description of cloud chemistry is needed in models used to predict air quality and climate. A comprehensive international cloud box model intercomparison with physical and chemical observational constraints from mountain top sampling highlights the need for a concerted effort to develop robust aqueous‐phase chemical mechanisms. Prediction of some key species differ among the models by orders of magnitude. We investigate the underlying, fundamental chemical explanations for the discrepancy as a diagnosis to understand the chemical parameters most in need of further investigation efforts. Five box models participated in a cloud chemistry intercomparison representing a case at Whiteface Mountain ObservatoryModel results were often quite different indicating a need for recommended equilibria and aqueous‐phase reaction rate constantsComparisons of simple to detailed chemistry schemes show need for development of effective simplified organic aqueous chemistry schemes Five box models participated in a cloud chemistry intercomparison representing a case at Whiteface Mountain Observatory Model results were often quite different indicating a need for recommended equilibria and aqueous‐phase reaction rate constants Comparisons of simple to detailed chemistry schemes show need for development of effective simplified organic aqueous chemistry schemes more...

Recent observational studies have shown that stratospheric air rich in ozone (O3) is capable of being transported into the upper troposphere in association with tropopause‐penetrating convection (anvil wrapping). This finding challenges the current understanding of upper tropospheric sources of O3, which is traditionally thought to come from thunderstorm outflows where lightning‐generated nitrogen oxides facilitate O3formation. Since tropospheric O3is an important greenhouse gas and the frequency and strength of tropopause‐penetrating storms may change in a changing climate, it is important to understand the mechanisms driving this transport process so that it can be better represented in chemistry‐climate models. Simulations of a mesoscale convective system (MCS) around which this transport process was observed are performed using the Weather Research and Forecasting model coupled with Chemistry. The Weather Research and Forecasting model coupled with Chemistry model adequately simulates anvil wrapping of ozone‐rich air. Possible mechanisms that influence the transport, including small‐scale static and dynamic instabilities and MCS‐induced mesoscale circulations, are evaluated. Model results suggest that anvil wrapping is a two‐step transport process (1) compensating subsidence surrounding the MCS, which is driven by mass conservation as the MCS transports tropospheric air into the upper troposphere and lower stratosphere, followed by (2) differential advection beneath the core of the MCS upper‐tropospheric outflow jet which wraps high O3air around and under the MCS cloud anvil. Static and dynamic instabilities are not a leading contributor to this transport process. Continued fine‐scale modeling of these events is needed to fully represent the stratosphere‐to‐troposphere transport process. WRF‐Chem is capable of reproducing downward transport around the MCS anvilDownward transport results from compensating vertical motions driven by mass continuity and differential advection between the anvil outflow and environmental flowAir transported downward primarily originated in the upper parts of the troposphere instead of the stratosphere more...

We investigate the impacts of cloud aqueous processes on the chemistry and transport of biogenic volatile organic compounds (BVOC) using the National Center for Atmospheric Research's large‐eddy simulation code with an updated chemical mechanism that includes both gas‐ and aqueous‐phase reactions. We simulate transport and chemistry for a meteorological case with a diurnal pattern of nonprecipitating cumulus clouds from the Baltimore‐Washington area DISCOVER‐AQ campaign. We evaluate two scenarios with and without aqueous‐phase chemical reactions. In the cloud layer (2–3 km), the addition of aqueous phase reactions decreases HCHO by 18% over the domain due to its solubility and the fast depletion from aqueous reactions, resulting in a corresponding decrease in radical oxidants (e.g., 18% decrease in OH). The decrease of OH increases the mixing ratios of isoprene and methacrolein (MACR) (100% and 15%, respectively) in the cloud layer because the reaction rate is lower. Aqueous‐phase reactions can modify the segregation between OH and BVOC by changing the sign of the segregation intensity, causing up to 55% reduction in the isoprene‐OH reaction rate and 40% reduction for the MACR‐OH reaction when clouds are present. Analysis of the isoprene‐OH covariance budget shows the chemistry term is the primary driver of the strong segregation in clouds, triggered by the decrease in OH. All organic acids except acetic acid are formed only through aqueous‐phase reactions. For acids formed in the aqueous phase, turbulence mixes these compounds on short time scales, with the near‐surface mixing ratios of these acids reaching 20% of the mixing ratios in the cloud layer within 1 h of cloud formation. Adding aqueous chemistry to a LES model reduces OH by up to 25% and increases isoprene, promoting their segregation in cloudsCovariance of isoprene oxidation products with OH changes sign (from positive to negative) when aqueous‐phase reactions are includedIn‐cloud formation of organic acids increases their mixing ratios throughout the boundary layer more...

To study aerosol-cloud interactions over the Gangetic Basin of India, the Weather Research and Forecasting model coupled with chemistry (WRF-Chem) has been applied to a typical monsoon depression event prevalent between the 23 and 29 August 2009. This event was sampled during the Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX) aircraft campaign, providing measurements of aerosol and cloud microphysical properties from two sorties. Comparison of the simulated meteorological, thermodynamical, and aerosol fields against satellite and in situ aircraft measurements illustrated that the westward propagation of the monsoon depression and the cloud, aerosol, and rainfall spatial distribution was simulated reasonably well using anthropogenic emission rates from Monitoring Atmospheric Composition and Climate project along with cityZEN projects (MACCity)+Intercontinental Chemical Transport Experiment Phase B anthropogenic emission rates. However,the magnitude of aerosol optical depth was underestimated by up to 50%. A simulation with aerosol emissions increased by a factor of 6 over the CAIPEEX campaign domain increased the simulated aerosol concentrations to values close to the observations, mainly within boundary layer. Comparison of the low-aerosol simulation and high-aerosol simulation for the two sorties illustrated that more anthropogenic aerosols increased the cloud condensing nuclei (CCN) and cloud droplet mass concentrations. The number of simulated cloud droplets increased while the cloud droplet effective radii decreased, highlighting the importance of CCN-cloud feedbacks over this region. The increase in simulated anthropogenic aerosols (including absorbing aerosols) also increased the temperature of air parcels below clouds and thus the convective available potential energy (CAPE). The increase in CAPE intensified the updraft and invigorated the cloud, inducing formation of deeper clouds with more ice-phase hydrometeors for both cases. These case studies provide evidence of aerosol-induced cloud invigoration over the Gangetic Basin. Aircraft sampled microphysical and aerosol fields are used to evaluate WRF-ChemWestward propagation of the monsoon depression reduced AOD over central IndiaAerosol-induced perturbations invigorated convection via low-level heating and cloud physics more...