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The ozonesonde observations in Hanoi, Vietnam, over 14 years since 2004, have confirmed an enhancement in lower tropospheric ozone concentration at about 3 km altitude in the spring season. We investigated the evolution of the ozone enhancement from analysis of meteorological data, backward trajectories, and model sensitivity experiments. In spring, air masses over Hanoi exhibit strong height dependence. At 3 km, the high‐ozone air masses originate from the land area to the west of Hanoi, while low‐ozone air masses below about 1.5 km are from the oceanic area to the east. Above 4 km, the air masses are mostly traced back to the farther west area. The chemical transport model simulations revealed that precursor emissions from biomass burning in the inland Indochina Peninsula have the largest contribution to the lower tropospheric ozone enhancement, which is transported upward and eastward and overhangs the clean air intrusion from the ocean to the east of Hanoi. At this height level, the polluted air has the horizontal extent of about 20° in longitude and latitude. The polluted air observed in Hanoi is transported further east and widely spread over the northern Pacific Ocean. Plain Language Summary: Ozone in the lower atmosphere acts as an air pollutant and is one of the strong greenhouse gases. Understanding chemical and transport processes that control ozone variations provide important implications for the origin of air pollution and for climate change. In the past 20 years, the global tropospheric ozone amount has increased in the tropics and subtropics, with particularly large increases in Southeast Asia. We analyze a seasonal enhancement in ozone over Hanoi, Vietnam, using the 14‐year record of upper air ozone measurements that began in 2004. The measurements reveal high levels of ozone particularly during the spring that peak at around 3 km. Numerical model simulations show that agricultural burning in the inland Indochina Peninsula is the major contributor to these high ozone amounts that are transported upward and eastward and overhang the clean air from the ocean to the east of Hanoi. Over time, the high‐ozone air observed in Hanoi is transported further east and widely spread over the northern Pacific Ocean. The identified source of the ozone enhancements and its three‐dimensional distribution provide new insights into the impact of agricultural burning over Southeast Asia on large‐scale ozone distributions and are relevant to global air quality and climate studies. Key Points: Ozonesonde observations in Hanoi, Vietnam, for about 14 years since 2004 display an ozone enhancement at ∼3 km altitude in March–AprilEnhanced ozone air at ∼3 km originates from burning areas west of Hanoi whereas low‐ozone near‐surface air is marine from the eastModel experiments show biomass fires in the Indochina Peninsula are the cause of Hanoi's enhanced ozone [ABSTRACT FROM AUTHOR]

A network of balloon-borne radiosonde observations employing chilled-mirror hygrometers for water and electrochemical concentration cells for ozone has been operated since the late 1990s in the Tropical Pacific to capture the evolution of dehydration of air parcels advected quasihorizontally in the Tropical Tropopause Layer (TTL). The analysis of this dataset is made on isentropes taking advantage of the conservative properties of tracers moving adiabatically. The existence of ice particles is diagnosed by lidars simultaneously operated with sonde flights. Characteristics of the TTL dehydration are presented on the basis of individual soundings and statistical features. Supersaturations close to 80% in relative humidity with respect to ice (RHice) have been observed in subvisible cirrus clouds located near the cold point tropopause at extremely low temperatures around 180 K. Although further observational evidence is needed to confirm the credibility of such high values of RHice, the evolution of TTL dehydration is evident from the data in isentropic scatter plots between the sonde-observed mixing ratio (OMR) and the minimum saturation mixing ratio (SMRmin) along the back trajectories associated with the observed air mass. Supersaturation exceeding the critical value of homogeneous ice nucleation (OMR > 1.6×SMRmin) is frequently observed on the 360 and 365K surfaces indicating that cold trap dehydration is in progress in the TTL. The near correspondence between the two (OMR ~ SMRmin) at 380K on the other hand implies that this surface is not sufficiently cold for the advected air parcels to be dehydrated. Above 380 K, cold trap dehydration would scarcely function while some moistening occurs before the air parcels reach the lowermost stratosphere at around 400K where OMR is generally smaller than SMRmin. [ABSTRACT FROM AUTHOR]

Variation in stratospheric water vapor is controlled mainly by the dehydration process in the tropical tropopause layer (TTL) over the western Pacific; however, this process is poorly understood. To address this shortcoming, in this study the match method is applied to quantify the dehydration process in the TTL over the western Pacific. The match pairs are sought from the Soundings of Ozone and Water in the Equatorial Region (SOWER) campaign network observations using isentropic trajectories. For the pairs identified, extensive screening procedures are performed to verify the representativeness of the air parcel and the validity of the isentropic treatment, and to check for possible water injection by deep convection, consistency between the sonde data and analysis field, and conservation of the ozone content. Among the pairs that passed the screening test, we found some cases corresponding to the first quantitative value of dehydration associated with horizontal advection in the TTL. The statistical features of dehydration for the air parcels advected in the lower TTL are derived from the match pairs. Match analysis indicates that ice nucleation starts before the relative humidity with respect to ice (RHice) reaches 207±81% (1 σ) and that the air mass is dehydrated until RHice reaches 83±30% (1σ). The efficiency of dehydration is estimated as the relaxation time required for the relative humidity of the supersaturated air parcel to approach the saturation state. This is empirically estimated from the match pairs as the quantity that reproduces the second water vapor observation, given the first observed water vapor amount and the history of the saturation mixing ratio of the match air mass exposed during the advection. The relaxation time is found to range from 2 to 3 h, which is consistent with previous studies. [ABSTRACT FROM AUTHOR]

A network of balloon-born radiosonde observations employing chilled-mirror hygrometers for water and electrochemical concentration cells for ozone has been operated since late 1990s in the Tropical Pacific trying to capture the progress of dehydration for the air parcels advected horizontally in the Tropical Tropopause Layer (TTL). The analyses of this dataset are made on isentropes taking advantage of the conservative properties of tracers in adiabatic motion. The existence of ice particles is diagnosed by lidars simultaneously operated with sonde flights. Characteristics of the TTL dehydration are presented on the basis of individual soundings and statistical features. Supersaturations close to 80% in the relative humidity with respect to ice (RHice) have been observed in subvisible cirrus clouds located near the cold point tropopause at extremely low temperatures around 180 K. Further observational evidence is needed to confirm the credibility of such high values of RHice. The progress of TTL dehydration is reflected in isentropic scatter plots between the sonde-observed mixing ratio (OMR) and the minimum saturation mixing ratio (SMRmin) along the back trajectories associated with the observed air mass. The supersaturation exceeding the critical value of the homogeneous ice nucleation (OMR > 1.6xSMRmin) is frequently observed on 360 and 365 K surfaces indicating that the cold trap dehydration is under progress in the TTL. The near correspondence between the two (OMR~SMRmin) on 380 K on the other hand implies that this surface is not significantly cold for the advected air parcels to be dehydrated. Above 380 K, the cold trap dehydration would scarcely function while some moistening in turn occurs before the air parcels reach the lowermost stratosphere at around 400 K where OMR is generally smaller than SMRmin. [ABSTRACT FROM AUTHOR]

An optical particle counter (OPC) is used in conjunction with lidar measurements to examine the characteristics of the particle size distribution in cirrus cloud in the tropical tropopause layer (TTL) over Thailand where the TTL is defined as the height at which temperature is lower than -75°C in this paper. Of 11 OPC launches, cirrus cloud was detected at 10-15 km high on 7 occasions, cirrus was detected in the TTL in 6 cases, and simultaneous OPC and lidar measurements were made on two occasions. Comparison of lidar and OPC measurements reveal that the cloud heights of cirrus in the TTL varies by several hundred meters over distances of tens kilometers; hence the height is not always horizontally uniform. The mode radii of particles constituting the clouds are estimated by lidar and OPC measurements to be less than approximately 10 µm. The regression lines of the particle size distribution with and without cirrus cloud exhibit similar features at equivalent radii of <0.8 µm. Enhancement in the integrated number concentration at radii greater than 0.8 µm indicates that liquid particles tend to be frozen at a radius of 0.8 µm, with cirrus clouds above 10 km exhibiting similar features. On the other hand, enhancement in the particle size distribution at radii greater than 0.9 µm and a peak at around 0.8 µm in the ratio of the standard deviation of count values to that of the Poisson distribution of the averaged count values are common features of cirrus clouds in the TTL, where the ratio shows the vertical homogeneity of the particle number. These typical features suggest that the transition from liquid, sulfuric acid aerosol, to ice is more observable in the TTL and the timing of freezing may vary with height in the TTL. [ABSTRACT FROM AUTHOR]

An optical particle counter (OPC) is used in conjunction with lidar measurements to examine the characteristics of the particle size distribution in cirrus cloud at the tropical tropopause (TT) over Thailand. Of 11 OPC launches, cirrus cloud was detected at 10-15 km high on 7 occasions, cirrus was detected at the TT in 6 cases, and simultaneous OPC and lidar measurements were made on two occasions. Comparison of lidar and OPC measurements reveal that the cloud height of cirrus in the TT varies by several hundred meters over distances of tens kilometers; hence the height is not horizontally uniform. The mode radii of particles constituting the clouds are estimated by lidar and OPC measurements to be less than approximately 10 µm. The regression lines of the particle size distribution with and without cirrus cloud exhibit similar features at equivalent radii of <0.7 µm. Enhancement in the integrated number concentration at radii greater than 0.7 µm indicates that liquid particles tend to be frozen at a radius of 0.7 µm, with cirrus clouds above 10 km exhibiting similar features. In addition, common features of cirrus clouds at the TT include a local maximum in the particle size distribution at 2.0 µm and a peak between 0.5 µm and 1.7 µm in the ratio of the standard deviation of count values to that of the Poisson distribution of the averaged count values. Each feature implies that all ice particles in the clouds may be nucleated by the same mechanism and particles in this size range are actively frozen at the TT. These parameters are thus good indicators for checking the results of cirrus cloud models in the TT. [ABSTRACT FROM AUTHOR]