ALBERT

Description: The study uses a Monte Carlo radiative transfer model to examine the sensitivity of the UV Erythemal radiation (UVER) enhancement to broken liquid water clouds of the cumulus and stratocumulus type. The model uses monochromatic radiation at 310 nm corresponding approximately to the peak of the product between irradiance and the erythemal curve. All scattering, absorption, extinction coefficients and spectral albedos are tuned to this wavelength. In order of importance, fractional cloud cover, the area of individual cloud patches and cloud thickness exert a strong influence on the enhancement, with smaller contributions from cloud optical depth, cloud base height and solar zenith angle. In order to produce realistic enhancements for our study area located in the Valencia region of Spain (39° 30’ N; 0° 25’W), measurements were obtained from a Landsat image of the region in combination with a spectral Fourier transform model. The Monte Carlo model, as applied to the Fourier transform cloud distribution, produced satisfactory results compared to one year of measured UVER enhancement for the study region provided fractional cloud cover was equal to or greater than 3/10. At smaller cloud fractions, the neglect of cloud patches less than 50 m x 50 m in area by the model created significant discrepancies.

Description: The urban population of South America has grown at 1.05% per year, greater urbanization increasing problems related to air pollution. In most large cities in South America, there has been no continuous long-term measurement of regulated pollutants. One exception is São Paulo, Brazil, where an air quality-monitoring network has been in place since the 1970s. In this paper, we used an air quality-based approach to determine pollutant trends for emissions of carbon monoxide (CO), nitrogen oxides (NO x ), ozone (O 3 ) and coarse particulate matter (PM 10 ), mostly from mobile sources, in the Metropolitan Region of São Paulo (MRSP) for the 2000–2013 period. Mobile sources included light-duty vehicles (LDVs, comprising gasoline- or ethanol-powered cars and motorcycles) and heavy-duty vehicles (HDVs, comprising diesel-powered trucks and buses). Pollutant concentrations for mobile source emissions were measured and correlated with fuel sales by the emission factors. Over the 2000–2013 period, concentrations of NO x , CO, and PM 10 decreased by 0.65, 0.37, and 0.71% month −1 , respectively, whereas sales of gasoline, ethanol, and diesel increased by 0.26, 1.96, and 0.38% month −1 , respectively. LDVs were the major mobile source of CO, whereas LDVs were the major source of NO x and PM 10 . Increases in fuel sales and in the corresponding traffic volume were partially offset by decreases in pollutant concentrations. Between 2000 and 2013, there was a sharp (−5 ppb month −1 ) decrease in the concentrations of LDV-emitted CO, together with (less dramatic) decreases in the concentrations of HDV-emitted NO x and PM 10 (−0.25 and −0.09 ppb month −1 , respectively). Variability was greater for HDV-emitted NO x and PM 10 (R = −0.47 and −0.41, respectively) than for LDV-emitted CO (R = −0.72). We draw the following conclusions: the observed concentrations of LDV-emitted CO decreased at a sharper rate than did those of HDV-emitted NO x and PM 10 ; mobile source contributions to O 3 formation varied significantly, LDVs making a greater contribution during the 2000–2008 period, whereas HDVs made a greater contribution during the 2009–2013 period; and decreases in NO x emissions resulted in increases in O 3 observations.

Description: We examine in detail a one-year global reanalysis of carbon monoxide (CO) that is based on joint assimilation of conventional meteorological observations and Measurement of Pollution in The Troposphere (MOPITT) multispectral CO retrievals in the Community Earth System Model (CESM). Our focus is to assess the impact to the chemical system when CO distribution is constrained in a coupled full chemistry-climate model like CESM. To do this, we first evaluate the joint reanalysis (MOPITT Reanalysis) against four sets of independent observations and compare its performance against a reanalysis with no MOPITT assimilation (Control Run). We then investigate the CO burden and chemical response with the aid of tagged sectoral CO tracers. We estimate the total tropospheric CO burden in 2002 (from ensemble mean and spread) to be 371 ±12% Tg for MOPITT Reanalysis and 291 ± 9 % Tg for Control Run. Our multi-species analysis of this difference suggests that: a) direct emissions of CO and hydrocarbons are too low in the inventory used in this study; and b) chemical oxidation, transport, and deposition processes are not accurately and consistently represented in the model. Increases in CO led to net reduction of OH and subsequent longer lifetime of CH 4 (Control Run: 8.7 years versus MOPITT Reanalysis: 9.3 years). Yet, at the same time, this increase led to 5-10% enhancement of northern hemisphere O 3 and overall photochemical activity via HO X recycling. Such nonlinear effects further complicate the attribution to uncertainties in direct emissions alone. This has implications to chemistry-climate modeling and inversion studies of longer-lived species.

Description: Abstract The values of the atmospheric variables follow from mass, momentum and energy conservation equations. These equations involve gradients which, in heterogeneous terrain, lead to a connection of the values at one point to the values in their nearby surroundings, due to turbulence mixing or advective local transports. In this work the observed variability of the air and soil variables is analysed for a 1 km2 surface in a semi‐rural area for a number of weather stations separated typically 150 m. Data show a large variability at the hectometre scale, with several degrees of difference in temperature sustained in time between neighbouring points. While in the daytime turbulence contributes to keep the differences moderate, in weak wind nights the spatial variability increases significantly. The variability of soil variables can be large depending on the soil moisture and the vegetation cover and is less sensitive to the diurnal cycle. The inspection of the vertical gradients of temperature and humidity reveals that their sign and intensity, and consequently the corresponding sensible and latent heat fluxes, differ depending on the measuring point. The thermal advection is obtained from the hectometre‐scale network, which is comparable in magnitude to the turbulent fluxes and to the imbalance of the surface energy budget for clear and calm nights. During the day, the advection term may explain part of the energy budget imbalance, particularly when it is computed on a 30‐minute time scale. A similar method is applied to the water vapour, finding that in the very dry conditions of the experiment, moisture advection is significant and with values comparable to the evapotranspiration.

Description: Abstract The Amazon forests and climatological precipitation patterns in South America are interrelated. A fundamental question is how these patterns depend on the presence of forests. Here we investigate this relationship by studying how precipitation varies with distance from the ocean along wind streamlines linking the Atlantic ocean to northwestern and southern South America through the Amazon forests. Through a robust observation‐based analysis we found that precipitation exponentially increases with distance from the ocean along wind streamlines flowing over forests, while it exponentially decreases downwind of the forests. These patterns are consistent with multiple mechanisms through which forests influence the transport of atmospheric moisture and precipitation production over the continent. We propose a conceptual explanation of this forest influence based on the atmospheric water balance. Our results imply that a major consequence of the degradation or loss of forests may be a disruption of these mechanisms, with widespread impacts on continental precipitation.

Description: Abstract We present airborne observations of the vertical gradient of atmospheric oxygen (δ(O2/N2)) and carbon dioxide (CO2) through the atmospheric boundary layer (BL) over the Drake Passage region of the Southern Ocean, during the O2/N2 Ratio and CO2 Airborne Southern Ocean Study (ORCAS), from January 15 to February 29, 2016. Gradients were predominately anti‐correlated, with excesses of δ(O2/N2) and depletions of CO2 found within the BL, relative to a mean reference height of 1.7 km. Through analysis of the molar ratio of the gradients (GR), the behavior of other trace gases measured in situ, and modeling experiments with the Community Earth System Model (CESM), we found that the main driver of gradients was air‐sea exchange of O2 and CO2 driven by biological processes, more so than solubility effects. An exception to this was in the eastern Drake Passage, where positive GRs were occasionally observed, likely due to the dominance of thermal forcing on the air‐sea flux of both species. GRs were more spatially consistent than the magnitudes of the gradients, suggesting that GRs can provide integrated process constraints over broad spatial scales. Based on the model simulation within a domain bounded by 45 °S, 75 °S, 100 °W, and 45 °W, we show that the sampling density of the campaign was such that the observed mean GR (± standard error), –4:0±0.8 mol O2 per mol CO2, was a reasonable proxy for both the mean GR and the mean molar ratio of air‐sea uxes of O2 and CO2 during ORCAS.