Abstract In September 2022, National Aeronautics and Space Administration (NASA) conducted the Convective Processes Experiment–Cabo Verde (CPEX-CV) campaign over the data-sparse eastern Atlantic Ocean. Over this region, CPEX-CV collected a suite of dense observations to aid in the study of convective systems. Tropical Storm (TS) Hermine formed in late September and had an unusual northward trajectory. Hermine was sampled by two consecutive research flights prior to becoming a TS, which provided an opportunity to improve Hermine’s forecast via data assimilation and verify model forecasts. With an improved forecast after assimilating CPEX-CV observations, model data are used to study the processes controlling Hermine’s evolution more accurately. Two experiments were conducted. One experiment assimilated CPEX-CV observations (WCPEX), while the other did not (WoCPEX). Compared to WoCPEX, the assimilation of CPEX-CV observations in the WCPEX analysis produced a stronger Saharan air layer, more intense dry-air intrusion, more easterly wind bias corrections, and a stronger midlevel circulation within pre-Hermine. Forecasts show that the strengthening of pre-Hermine into a TS in WoCPEX was delayed by 12 h due to the large vertical tilt of the vortex and weak midlevel vorticity; it also had a westward track bias. Compared to WoCPEX, in WCPEX, while convection near pre-Hermine was weaker at early forecast times due to a more intense dry-air intrusion, stronger, more organized midlevel vorticity and better vertical alignment of the vortex improved the intensity and track forecast. Additional sensitivity tests revealed that assimilating only CPEX-CV remote sensing observations improved Hermine’s forecast nearly as much as assimilating all CPEX-CV observations. Significance Statement In September 2022, NASA conducted a field campaign whose goal was to collect meteorological observations over the data-sparse eastern Atlantic Ocean. Field observations of Tropical Storm Hermine, which occurred during the campaign, were used in a weather forecast model to assess the effects of the observations on Hermine’s forecast. The modeling results showed that the additional observations from the field campaign improved the forecast of both the track and the strength of Hermine. The modeling results also showed that with the additional observations, the processes that contributed to Hermine’s formation were better represented.

A technique was developed to provide cloud phase information using data collected by the NASA Langley airborne High Spectral Resolution Lidar systems with a particular emphasis on mixed-phase cloud conditions, where boundaries and gradients in the distribution of ice and liquid water are critically important for microphysical and radiative processes. The method is based on the established use of depolarization to identify ice particles but incorporates a new method to separate the ice depolarization from the depolarization produced by multiple scattering in dense liquid clouds. Clouds known to be liquid-only based on ambient temperature were used to train an empirical model of the multiple-scattering depolarization that results at different ranges from the lidar. The method classifies lidar observations as liquid-dominant, mixed-phase, and ice-dominant and has an additional categorization for oriented ice. For evaluation of the retrieval, a two-aircraft approach was used with the lidar observing the same clouds that were concurrently being sampled with in situ microphysical probes. Aircraft matchups were able to track the individual cloud elements and capture marked changes in the distribution of liquid and ice across flight segments of typically 20-100 km. Qualitative features relating to localized changes in the cloud-top temperature, cloud morphology, and convective circulations were generally replicated between the lidar phase classification and the in situ microphysical data. Quantitative evaluation of the phase classification was carried out using a subset of 15 cloud scenes that satisfied strict aircraft collocation and microphysical requirements. Using the in situ microphysical data, it was found that ice extinction fractions of 14 % and 76 % most closely matched the upper and lower bounds of the lidar mixed-phase classification.

NASA's Vapor In-cloud Profiling Radar (VIPR) is a tunable G-band radar designed for in-cloud and precipitation humidity remote sensing. VIPR estimates humidity using the differential absorption radar (DAR) technique. This technique exploits the difference between atmospheric attenuation at different frequencies (“on” and “off” an absorption line) and combines it with the ranging capabilities of the radar to estimate the absorbing gas concentration along the radar path. We analyze the VIPR humidity measurements during two NASA field campaigns: (1) the Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) campaign, with the objective of studying wintertime snowstorms focusing on east coast cyclones; and (2) the Synergies Of Active optical and Active microwave Remote Sensing Experiment (SOA2RSE) campaign, which studied the synergy between DAR (VIPR) and differential absorption lidar (DIAL, the High altitude Lidar Observatory – HALO) measurements. We discuss a comparison with dropsondes launched during these campaigns as well as an intercomparison against the ERA5 reanalysis fields. Thus, this study serves as an additional evaluation of ERA5 lower tropospheric humidity fields. Overall, in-cloud and in-snowstorm comparisons suggest that ERA5 and VIPR agree within 20 % or better against the dropsondes. The exception is during SOA2RSE (i.e., in fair weather), where ERA5 exhibits up to a 50 % underestimation above 4 km. We also show a smooth transition in water vapor profiles between the in-cloud and clear-sky measurements obtained from VIPR and HALO respectively, which highlights the complementary nature of these two measurement techniques for future airborne and space-based missions.

The objective of this study is to understand rainfall processes over tropical islands by identifying synoptic conditions that influence fl uence the diurnal cycle of rainfall over western Puerto Rico. Summer rainfall over Puerto Rico is dominated by its afternoon peak, yet there is large variability in its behavior that remains challenging to predict. We use radiosonde and airborne data collected through the NASA Convective Processes Experiment Aerosols and Winds (CPEX-AW) fi eld campaign (August-September 2021) to achieve our objective, in addition to the network of surface station data over the island. We fi nd that the background wind speed and humidity have strong influences fl uences on afternoon rainfall through different mechanisms. A stronger background wind inhibits afternoon rainfall likely by reducing land-sea thermal contrast and weakening sea-breeze convergence over the island. At the same time, an inversion layer often forms with a stronger background wind that further inhibits deep convection. When the background wind is weak and sea breezes are prominent, afternoon rainfall increases exclusively over the island, while limited rainfall appears over the surrounding ocean. However, enhanced rainfall still occurs over the island with weak sea breezes if humidity is high, accompanied by enhanced rainfall over the surrounding ocean due to the offshore movement and development of convective storms. The sources of variability in background wind and humidity are mostly independent, resulting in a wide range of synoptic conditions and associated effects on the island rainfall. This expanded understanding of the mechanisms causing variability of diurnal rainfall can lead to improved forecasts over Puerto Rico and other tropical islands.

The 2022 Convective Processes Experiment – Cabo Verde (CPEX-CV) collected observations using the NASA DC-8 to study dynamics and microphysics related to the Saharan Air Layer (SAL), African easterly waves and jets, and deep convection within the Intertropical Convergence Zone (ITCZ) in the tropical East Atlantic. CPEX-CV measurements also collected data to help calibrate and validate ESA’s Aeolus Doppler wind lidar. CPEX-CV is part of combined effort with the European Space Agency (ESA) and their partner laboratories and universities called the Joint Aeolus Tropical Atlantic Campaign (JATAC) to validate ESA’s Aeolus satellite. As part of the CPEX-CV – JATAC collaboration from Cabo Verde in September 2022, the NASA DC-8, outfitted with a comprehensive suite of remote and in-situ sensors, coordinated with the ASKOS ground site at Mindelo and the Slovenian WT-10 aircraft to validate AEOLUS wind and aerosol products as well as to link quantitatve aerosol observations from multiple vantage points to better understand the role of the SAL in toprical dynamics.During CPEX-CV the NASA DC-8 was outfitted with the Advanced Vertical Atmospheric Profiling System (AVAPS) dropsondes, Doppler Aerosol Wind Lidar (DAWN), High-Altitude Lidar Observatory (HALO), third generation Airborne Precipitation Radar (APR-3), High-Altitude Monolithic Microwave Integrated Circuit Sounding Radiometer (HAMSR), and the Cloud Aerosol and Precipitation Spectrometer. This presentation will highlight the HALO water vapor DIAL and aerosol/cloud HSRL observations collected during CPEX-CV and discuss the synergies with the JATAC campaign. HALO HSRL observations are compared and contrasted with those taken from the ground site at Mindelo to better understand the influence of orographic island effects on the transport of aerosols and better constrain aerosol processes such as aerosol electrification being studied by ASKOS. Additionally, HALO aerosol backscatter, depolarization, and extinction products at 532 nm and 1064 nm are used to evaluate Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aerosol retrievals at those same wavelengths and, after adjustment to 355 nm, are compared against Aeolus backscatter and extinction products. Areas for future collaborative efforts will also be discussed including Aeolus and Earthcare validation.

Mixing layer height (MLH) and land-sea breeze in the coastal area play an important role in ozone (O3) dispersion and transport. This study presents the multiple-platform observations of MLH dynamics for a high O3 event during Long Island Sound Tropospheric Ozone Study (LISTOS) in summer 2018. The diurnal evolution and spatial variability of MLH are observed from ground-based aerosol Lidar, Vaisala Ceilometer, Coherent Doppler Wind Lidar, and airborne lidar in New York City (NYC) urban and the coastal area in Long Island on August 28–29, 2018. The results indicate that the MLH growth in the morning shows a time lag of 1–2 h in the coastal area versus that in the NYC urban area. The spatial gradient variation of MLH from the urban to the coastal area is demonstrated from the NASA High Altitude Lidar Observatory (HALO) airborne high spectral resolution lidar (HSRL) observations by showing lower values in the coastal and marine area. Meanwhile, the ground O3 are consistently much higher at noon in the coastal sites than those in the NYC urban sites reaching the maximum value of 100–120 ppb in exceedance of the O3 NAAQS, which is closely relevant to the urban pollutant transport, MLH dynamics, and sea breeze. The observation data are further used to assess the model product of PBLH and O3 at the NYC urban and coastal sites.

The NASA Langley Research Center High Altitude Lidar Observatory (HALO) is a multi-functional and modular lidar developed to address the observational needs of NASA's weather, climate, carbon cycle, and atmospheric composition focus areas. HALO measures atmospheric H2O mixing ratios, CH4 mole fractions, and aerosol/cloud optical properties using the differential absorption lidar (DIAL) and high-spectral-resolution lidar (HSRL) techniques. In 2019 HALO participated in the NASA Atmospheric Carbon and Transport – America campaign on board the NASA C-130 to complement a suite of greenhouse gas in situ sensors and provide, for the first time, simultaneous measurements of column CH4 and aerosol/cloud profiles. HALO operated in 18 of 19 science flights where the DIAL and integrated path differential absorption (IPDA) lidar techniques at 1645 nm were used for column and multi-layer measurements of CH4 mole fractions, and the HSRL and backscatter techniques were used at 532 and 1064 nm, respectively, for retrievals of aerosol backscatter, extinction, depolarization, and mixing layer heights. In this paper we present HALO's measurement theory for the retrievals of column and multi-layer XCH4, retrieval accuracy, and precision including methods for bias correction and a comprehensive total column XCH4 validation comparison to in situ observations. Comparisons of HALO XCH4 to in situ-derived XCH4, collected during spiral ascents and descents, indicate a mean difference of 2.54 ppb and standard deviation (SD) of the differences of 16.66 ppb when employing 15 s along-track averaging (<3 km). A high correlation coefficient of R=0.9058 was observed for the 11 in situ spiral comparisons. Column XCH4 measured by HALO over regional scales covered by the ACT-America campaign is compared against in situ CH4 measurements carried out within the planetary boundary layer (PBL) from both the C-130 and B200 aircraft. Favorable correlation between the in situ point measurements within the PBL and the remote column measurements from HALO elucidates the sensitivity of a column-integrating lidar to CH4 variability within the PBL, where surface fluxes dominate the signal. Novel capabilities for CH4 profiling in regions of clear air using the DIAL technique are presented and validated for the first time. Additionally, profiling of CH4 is used to apportion the PBL absorption from the total column and is compared to previously reported IPDA cloud slicing techniques that estimate PBL columns using strong echoes from fair weather cumulus. The analysis presented here points towards HALO's ability to retrieve accurate and precise CH4 columns with the prospects for future multi-layer profiling in support of future suborbital campaigns.

Airborne differential absorption lidar (DIAL) offers a uniquely capable solution to the problem of measuring water vapor (WV) with high precision, accuracy, and resolution throughout the troposphere and lower stratosphere. The High Altitude Lidar Observatory (HALO) airborne WV DIAL was recently developed at NASA Langley Research Center and was first deployed in 2019. It uses four wavelengths near 935 nm to achieve sensitivity over a wide dynamic range and simultaneously employs 1064 nm backscatter and 532 nm high-spectral-resolution lidar (HSRL) measurements for aerosol and cloud profiling. A key component of the WV retrieval framework is flexibly trading resolution for precision to achieve optimal datasets for scientific objectives across scales. An approach to retrieving WV in the lowest few hundred meters of the atmosphere using the strong surface return signal is also presented. The five maiden flights of the HALO WV DIAL spanned the tropics through midlatitudes with a wide range of atmospheric conditions, but opportunities for validation were sparse. Comparisons to dropsonde WV profiles were qualitatively in good agreement, though statistical analysis was impossible due to systematic error in the dropsonde measurements. Comparison of HALO to in situ WV measurements aboard the aircraft showed no substantial bias across 3 orders of magnitude, despite variance (R2=0.66) that may be largely attributed to spatiotemporal variability. Precipitable water vapor measurements from the spaceborne sounders AIRS and IASI compared very well to HALO with R2>0.96 over ocean.

Air pollution associated with wildfire smoke transport during the summer can significantly affect ozone (03) and particulate matter (PM) concentrations, even in heavily populated areas like New York City (NYC). Here, we use observations from aircraft, ground -based lidar, in-situ analyzers and satellite to study and assess wildfire smoke transport, vertical distribution, optical properties, and potential impact on air quality in the NYC urban and coastal areas during the summer 2018 Long Island Sound Tropospheric Ozone Study (LISTOS). We investigate an episode of dense smoke transported and mixed into the planetary boundary layer (PBL) on August 15-17, 2018. The horizontal advection of the smoke is shown to be characterized with the prevailing northwest winds in the PBL (velocity > 10 in /s) based on Doppler wind lidar measurements.The wildfire sources and smoke transport paths from the northwest US/Canada to northeast US are identified from the NOAA hazard mapping system (HMS) fires and smoke product and NOAA-HYbrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) backward trajectory analysis. The smoke particles are distinguished from the urban aerosols by showing larger lidar-ratio (70-sr at 532-nm) and smaller depolarization ratio (0.02) at 1064-nm using the NASA High Altitude Lidar Observatory (HALO) airborne high -spectral resolution lidar (HSRL) measurements. The extinction related angstrom exponents in the near-infrared (IR at 1020-1640 nm) and Ultraviolet (UV at 340-440 nm) from NASA-Aerosol Robotic Network (AERONET) product show a reverse variation trend along the smoke loadings, and their absolute differences indicate strong correlation with the smoke-Aerosol Optical Depth (AOD) (R > 0.94). We show that the aloft smoke plumes can contribute as much as 60-70% to the column ADD and that concurrent high-loadings of O-3, carbon monoxide (CO), and black carbon (BC) were found in the elevated smoke layers from the University of Maryland (UMD) aircraft in-siLu observations. Meanwhile, the surface PM2.5 (PM with diameter <= 2.5 mu m), organic carbon (OC) and CO measurements show coincident and sharp increase (e.g., PM2.5 from 5 mu g/m(3) before the plume intrusion to similar to 30 mu g/m(3)) with the onset of the plume intrusions into the PBL along with hourly 03 exceedances in the NYC region. We further evaluate the NOAA-National Air Quality forecasting Capability (NAQTC) model PBL-height, PM2.5, and O-3 with the observations and demonstrate good consistency near the ground during the convective PBL period, but significant bias at other times. The aloft smoke layers are sometimes missed by the model. (C) 2021 Elsevier B.V. All rights reserved.

Lidars are uniquely capable of collecting high-precision and high spatiotemporal resolution observations that have been used for atmospheric process studies from the ground, aircraft, and space for many years. The Aeolus mission, the first space-borne Doppler wind lidar, was developed by the European Space Agency (ESA) and launched in August 2018. Its novel Atmospheric LAser Doppler INstrument (ALADIN) observes profiles of the component of the wind vector and aerosol/cloud optical properties along the instrument's line-of-sight (LOS) direction on a global scale. A total of two airborne lidar systems have been developed at NASA Langley Research Center in recent years that collect measurements in support of several NASA Earth Science Division focus areas. The coherent Doppler Aerosol WiNd (DAWN) lidar measures vertical profiles of LOS velocity along selected azimuth angles that are combined to derive profiles of horizontal wind speed and direction. The High Altitude Lidar Observatory (HALO) measures high resolution profiles of atmospheric water vapor (WV) and aerosol and cloud optical properties. Because there are limitations in terms of spatial and vertical detail and measurement precision that can be accomplished from space, airborne remote sensing observations like those from DAWN and HALO are required to fill these observational gaps and to calibrate and validate space-borne measurements. Over a 2-week period in April 2019, during their Aeolus Cal/Val Test Flight campaign, NASA conducted five research flights over the eastern Pacific Ocean with the DC-8 aircraft. The purpose was to demonstrate the following: (1) DAWN and HALO measurement capabilities across a range of atmospheric conditions, (2) Aeolus Cal/Val flight strategies and comparisons of DAWN and HALO measurements with Aeolus, to gain an initial perspective of Aeolus performance, and (3) ways in which atmospheric dynamic processes can be resolved and better understood through simultaneous observations of wind, WV, and aerosol profile observations, coupled with numerical model and other remote sensing observations. This paper provides a brief description of the DAWN and HALO instruments, discusses the synergistic observations collected across a wide range of atmospheric conditions sampled during the DC-8 flights, and gives a brief summary of the validation of DAWN, HALO, and Aeolus observations and comparisons.

First of its kind combined atmospheric and ocean profile data were collected by the recently upgraded NASA Langley Research Center’s (LaRC) High Spectral Resolution Lidar (HSRL-1) during the 17 July – 7 August 2014 Ship-Aircraft Bio-Optical Research Experiment (SABOR). This mission sampled over a region that covered the Gulf of Maine, open-ocean near Bermuda, and coastal waters from Virginia to Rhode Island. The HSRL-1 and the Research Scanning Polarimeter from NASA Goddard Institute for Space Studies collected data onboard the NASA LaRC King Air aircraft and flight operations were closely coordinated with the Research Vessel Endeavor that made in situ ocean optical measurements. The lidar measurements provided profiles of atmospheric backscatter and particulate depolarization at 532nm, 1064nm, and extinction (532nm) from approximately 9km altitude. In addition, for the first time HSRL seawater backscatter, depolarization, and diffuse attenuation data at 532nm were collected and compared to both the ship measurements and the Moderate Resolution Imaging Spectrometer (NASA MODIS-Aqua) satellite ocean retrievals.

Linear particle depolarization ratio is presented for three case studies from the NASA Langley airborne High Spectral Resolution Lidar-2 (HSRL-2). Particle depolarization ratio from lidar is an indicator of non-spherical particles and is sensitive to the fraction of non-spherical particles and their size. The HSRL-2 instrument measures depolarization at three wavelengths: 355, 532, and 1064 nm. The three measurement cases presented here include two cases of dust-dominated aerosol and one case of smoke aerosol. These cases have partial analogs in earlier HSRL-1 depolarization measurements at 532 and 1064 nm and in literature, but the availability of three wavelengths gives additional insight into different scenarios for non-spherical particles in the atmosphere. A case of transported Saharan dust has a spectral dependence with a peak of 0.30 at 532 nm with smaller particle depolarization ratios of 0.27 and 0.25 at 1064 and 355 nm, respectively. A case of aerosol containing locally generated wind-blown North American dust has a maximum of 0.38 at 1064 nm, decreasing to 0.37 and 0.24 at 532 and 355 nm, respectively. The cause of the maximum at 1064 nm is inferred to be very large particles that have not settled out of the dust layer. The smoke layer has the opposite spectral dependence, with the peak of 0.24 at 355 nm, decreasing to 0.09 and 0.02 at 532 and 1064 nm, respectively. The depolarization in the smoke case may be explained by the presence of coated soot aggregates. We note that in these specific case studies, the linear particle depolarization ratio for smoke and dust-dominated aerosol are more similar at 355 nm than at 532 nm, having possible implications for using the particle depolarization ratio at a single wavelength for aerosol typing.

A 2-μm backscatter lidar system has been developed by utilizing tunable pulsed laser and infrared phototransistor for the transmitter and the receiver, respectively. To validate the system, the 2-μm atmospheric backscatter profiles were compared to profiles obtained at 1 and 0.5 μm using avalanche photodiode and photomultiplier tube, respectively. Consequently, a methodology is proposed to compare the performance of different lidar systems operating at different wavelengths through various detection technologies. The methodology is based on extracting the system equivalent detectivity and comparing it to that of the detectors, as well as the ideal background detectivity. Besides, the 2-μm system capability for atmospheric CO2 temporal profiling using the differential absorption lidar (DIAL) technique was demonstrated. This was achieved by tuning the laser at slightly different wavelengths around the CO2 R22 absorption line in the 2.05-μm band. CO2 temporal profiles were also compared to in situ measurements. Preliminary results indicated average mixing ratios close to 390 ppm in the atmospheric boundary layer with 3.0% precision. The development of this system is an initial step for developing a high-resolution, high-precision direct-detection atmospheric CO2 DIAL system. A successful development of this system would be a valuable tool in obtaining and validating global atmospheric CO2 measurements.

A 2-mu m backscatter lidar system has been developed by utilizing tunable pulsed laser and infrared phototransistor for the transmitter and the receiver, respectively. To validate the system, the 2-mu m atmospheric backscatter profiles were compared to profiles obtained at 1 and 0.5 mu m using avalanche photodiode and photomultiplier tube, respectively. Consequently, a methodology is proposed to compare the performance of different lidar systems operating at different wavelengths through various detection technologies. The methodology is based on extracting the system equivalent detectivity and comparing it to that of the detectors, as well as the ideal background detectivity. Besides, the 2-mu m system capability for atmospheric CO2 temporal profiling using the differential absorption lidar (DIAL) technique was demonstrated. This was achieved by tuning the laser at slightly different wavelengths around the CO2 R22 absorption line in the 2.05-mu m band. CO2 temporal profiles were also compared to in situ measurements. Preliminary results indicated average mixing ratios close to 390 ppm in the atmospheric boundary layer with 3.0% precision. The development of this system is an initial step for developing a high-resolution, high-precision direct-detection atmospheric CO2 DIAL system. A successful development of this system would be a valuable tool in obtaining and validating global atmospheric CO2 measurements.

A compact ozone (O3) and aerosol lidar system is being developed for conducting global atmospheric investigations from the NASA Global Hawk Uninhabited Aerial Vehicle (UAV) and for enabling the development and test of a space-based O3 and aerosol lidar. GOLD incorporates advanced technologies and designs to produce a compact, autonomously operating O3 and aerosol Differential Absorption Lidar (DIAL) system for a UAV platform. The GOLD system leverages advanced Nd:YAG and optical parametric oscillator laser technologies and receiver optics, detectors, and electronics. Significant progress has been made toward the development of the GOLD system, and this paper provides an overview of this program with a focus on the American Recovery and Reinvestment Act (ARRA) funded tasks that will enable the GOLD system for flight on the NASA Global Hawk..

This paper presents the design, development, and field testing of a high sensitivity ground-based Differential Absorption Lidar (DIAL) system that was developed under the NASA Instrument Incubator Program. The investigation presents a significant advancement towards the development of future CO2 profiling capability as it incorporates key elements of technologies needed for a future development of global CO2 measuring systems including: (1) 2-μm laser technologies that have been developed under a number of NASA programs including the Laser Risk Reduction Program (LRRP) (2) A novel high quantum efficiency (QE), high gain (without excess noise factor), and low noise phototransistor, and (3) Direct detection DIAL system using a large collection area receiver that is insensitive to speckle and coherence length effects from atmospheric turbulence that influences heterodyne detection systems. The objective of the project was a system TRL of 4, and the goal was TRL 5. Development and testing of the laser, new detector, and receiver systems during the project, integration into a complete lidar system into a trailer, field testing of system at West Branch, IA and comparison of the lidar CO2 measurements with in situ sensors advanced the system to a TRL of 5. The system demonstrated high vertical resolution CO2 measurement capability within the boundary layer and column measurements to long ranges. This is the first demonstration of a direct detection 2-micron CO2 DIAL system with a high vertical resolution CO2 measurement capability. This project advanced from instrument concept to field demonstration.

A three‐dimensional off‐line chemistry transport model, driven by European Centre for Medium‐Range Weather Forecasts winds and temperatures, is used to interpret measurements of NO and NO2 taken from the DC‐8 during the second Airborne Arctic Stratospheric Expedition. The model was run in three configurations: gas phase chemistry alone, inclusion of the N2O5 aerosol reaction, and inclusion of both N2O5 and ClONO2 aerosol reactions. The run including the N2O5 aerosol reaction alone usually agreed best with measured NOx/NOy ratios in midlatitude air masses. The NOx/NOy ratios of the run with both aerosol reactions were always too low, while the gas phase ratios were usually too high, especially during March. All three simulations generated extremely low NO2/NOy ratios in air parcels that had spent several days or more in the polar night. Measured NO2/NOy ratios in these types of air masses were sometimes equally low but could also be considerably higher. Observed NO/NO2 ratios differed strongly from known theory.