Vertical profiles of trace gas and aerosol properties over the Eastern North Atlantic: Variations with season and synoptic condition

Because of their extensive coverage, marine low clouds greatly impact the global climate. Presently, the response of marine low clouds to the changes in atmospheric aerosols remains a major source of uncertainty in climate simulations. One key contribution to this large uncertainty derives from the poor understanding of the properties and processes of marine aerosols under natural conditions, and the perturbation by anthropogenic emissions. The Eastern North Atlantic (ENA) is a region of persistent but diverse subtropical marine boundary layer (MBL) clouds, where cloud albedo and precipitation are highly susceptible to perturbations in aerosol properties. Here we examine the key processes that drive the cloud condensation nuclei (CCN) population in the MBL using comprehensive characterizations of aerosol and trace gas vertical profiles during the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) field campaign. During ACE-ENA, a total of 39 research flights were conducted in the Azores, 20 during summer 2017, and 19 during winter 2018. During summer, long-range transported aerosol layers were periodically observed in the lower free troposphere (FT), leading to elevated FT CCN concentrations (NCCN). Both biomass burning and pollution from North America contribute to submicron aerosol mass in these layers, with pollution likely the dominant contributor. In contrast, long-range transported continental emissions have a much weaker influence on the aerosol properties in the ENA during the winter season. While the entrainment of FT air is a major source of particle number in the MBL for both seasons, on average, it does not serve as a direct source of CCN in the MBL because the average FT NCCN is the same or even lower than that in the MBL. The particle number flux due to FT entrainment is dominated by pre-CCN (particles that are too small to form cloud droplets under typical conditions, i.e., particles with sizes below the Hoppel minimum) due to the elevated Npre-CCN in the lower FT. Once these pre-CCN are entrained into the MBL, they can grow and reach CCN size range through condensational growth, representing an indirect and major source of MBL CCN at ENA. The impact of synoptic condition on the aerosol properties is examined. Under pre-front and post-front conditions, shallow convective activity often leads to a deep and decoupled boundary layer. Coalescence scavenging and evaporation of drizzle below clouds leads to much reduced NCCN and larger accumulation-mode particle sizes in the upper, cloud-containing decoupled layer, indicating that surface measurements overestimate the NCCN relevant to the formation of MBL clouds under decoupled conditions.

Numerical Study of Meteorological Factors for Tropospheric Nocturnal Ozone Increase in the Metropolitan Area of São Paulo

One of the central problems in large cities is air pollution, mainly caused by vehicular emissions. Tropospheric ozone is an atmospheric oxidizing gas that forms in minimal amounts naturally, affecting peoples’ health. This pollutant is formed by the NO2 photolysis, creating a main peak during the day. Nighttime secondary peaks occur in several parts of the world, but their intensity and frequency depend on the local condition. In this sense, this works aims to study the local characteristics for tropospheric nocturnal ozone levels in the Metropolitan Area of São Paulo, in Brazil, using the Simple Photochemical Module coupled to the Brazilian Developments on the Regional Atmospheric Modeling System. For this, three different situations of nocturnal occurrence were studied. The results show that the nocturnal maximum of ozone concentrations is related to the vertical transport of this pollutant from higher levels of the atmosphere to the surface and is not related to the synoptic condition.

On the multi-day haze in the Asian continental outflow: An important role of synoptic condition combined with regional and local sources

Air quality of the megacities in the populated and industrialized regions like East Asia is affected by both local and regional emission sources. A combined effect of regional transport and local emissions on multi-day haze was investigated by synthetic analysis of PM2.5, sampled at both an urban site in Seoul, South Korea and an upwind background site in Deokjeok Island over the Yellow Sea, during a severe multi-day haze episode in late February 2014. Inorganic components as well as carbonaceous species of daily PM2.5 samples were measured, and gaseous pollutants, local meteorological factors and synoptic meteorological conditions were also determined. Dominance of fine-mode particles, a large secondary inorganic fraction (76 %), high OC/EC ratio (7.3), and highly oxidized aerosols under relatively warm, humid, and stagnant conditions characterize the multi-day haze episode; however, the early and late stages of the episode show different chemical compositions of PM2.5. High concentrations of sulfate in both Seoul and the upwind background in the early stage suggest a significant regional influence on the onset of the multi-day haze. At the same time, high concentrations of nitrate and organic compounds in Seoul, which are local and highly correlated with meteorological factors, suggest the contribution of local emissions and secondary formation under the stagnant meteorological condition to the haze. A slow eastward-moving high-pressure system from southern China to the East China Sea induces the regional transport of aerosols and potential gaseous precursors for secondary aerosols from the North China Plain in the early stage but provides stagnant conditions conducive to the accumulation and the local formation of aerosols in the late stage. A blocking ridge over Alaska developed during the episode hinders the zonal propagation of synoptic-scale systems and extends the haze period to several days. This study provides chemical insights of haze development sequentially by regional transport and local sources, and shows that the synoptic condition plays an important role for the dynamical evolution of long-lasting haze in the Asian continental outflow region.

Influence of airmass transport events on the variability of surface ozone at Xianggelila Regional Atmosphere Background Station, Southwest China

In situ measurements of ozone (O3), carbon monoxide (CO) and meteorological parameters were made from December 2007 to November 2009 at the Xianggelila Regional Atmosphere Background Station (28.006° N, 99.726° E, 3580 m a.s.l.), Southwest China. It is found that both O3 and CO peaked in spring while the valleys of O3 and CO occurred in summer and winter, respectively. A normalized indicator (marked as "Y") of transport events on the basis of the monthly normalized O3, CO, and water vapor, is proposed to evaluate the occurrence of O3 transport events from the upper, O3-rich atmosphere. This composite indicator has the advantage of being less influenced by and seasonal or occasional variations of individual factors. It is shown that the most frequent transport events occurred in winter and they can make a significant contribution to surface O3 at Xianggelila. A case of strong O3 transport event under the synoptic condition of a~deep westerly trough is studied by the combination of the Y indicator, potential vorticity, total column ozone, and trajectory analysis. A 9.6 ppb increase (21.0%) of surface ozone is estimated based on the impacts of deep transport events in winter. Asian Monsoon plays an important role in suppressing O3 accumulation in summer and fall.