W. Wieprecht

Strong daytime production of OH from HNO2 at a rural mountain site

Nitrous acid and OH were measured concurrently with a number of other atmospheric components and relevant photolysis frequencies during two campaigns at the Meteorological Observatory Hohenpeissenberg (980 m a.s.l.) in summer 2002 and 2004. On most of the 26 measurement days the HNO 2 concentration surprisingly showed a broad maximum around noon (on average 100 pptv) and much lower concentrations during the night (∼30 pptv). The results indicate a strong unknown daytime source of HNO 2 with a production rate on the order of 2-4 x 10 6 cm -3 s -1 . The data demonstrate an important contribution of HNO 2 to local HO x levels over the entire day, comparable with the photolysis of O 3 and HCHO. On average during the 2004 campaign, 42% of integrated photolytic HO x formation is attributable to HNO 2 photolysis.

Occurrence and formation of nitrated phenols in and out of cloud

In this study, the concentrations of phenol, four nitrated phenols, their precursors and reactants in air and cloud water, are presented. The concentrations in air and cloud water were measured simultaneously at the summit of Great Dun Fell (GDF). The measured concentrations were compared with emission data, leading to the conclusion, that in particular dinitrophenols are formed by atmospheric reactions, while car exhaust accounts to a significant extent for the mononitrophenols observed. The experimental results point to a formation of dinitrophenols in the liquid phase (cloud droplets). This is corroborated by flow tube experiments which show that phenol in aqueous solution reacts with N2O5 and ClNO2 to form nitrophenols.

Night-time formation and occurrence of new particles associated with orographic clouds

Abstract The formation and occurrence of new ultrafine aerosol particles were studied in association with an orographic cloud during a field experiment at Great Dun Fell (GDF), Northern England. Three size spectrometers to measure submicrometer aerosol particles were located upwind, on top, and downwind of GDF Summit to investigate changes in the aerosol size distribution. During two night-time cloud periods, ultrafine particles were observed downwind of the hill while no particles were detected upwind of the hill. During one cloud event, there was some evidence of entrainment. In this case, the occurrence of ultrafine particles may have been due to entrainment from aloft or by homogenous nucleation downwind of the hill. During the other cloud event, the formation of an ultrafine particle mode (nucleation mode) occurred probably after the cloud passage. There was no evidence of entrainment during this time period. Multicomponent homogeneous nucleation models were used to simulate the formation of new particle downwind of an orographic cloud. Possible homogeneous nucleation processes for this could be the formation of sulphuric acid or ammonium chloride due to outgassing of hydrochloric acid. It was not possible, however, to simulate formation rates of new particles as observed downwind the hill using a model for the binary or ternary homogeneous nucleation process of ammonia and hydrochloric acid. During the first event with high sulphur dioxide concentrations, the formation of new particle via binary homogeneous nucleation of sulphuric acid and water could be only predicted using a high nighttime hydroxyl radical concentration. No formation of sulphuric acid particle could be simulated during the second event with low sulphur dioxide concentrations.

Cloud processing of soluble gases

Abstract Experimental data from the Great Dun Fell Cloud Experiment 1993 were used to investigate interactions between soluble gases and cloud droplets. Concentrations of H 2 O 2 , SO 2 , CH 3 COOOH, HCOOH, and HCHO were monitored at different sites within and downwind of a hill cap cloud and their temporal and spatial evolution during several cloud events was investigated. Significant differences were found between in-cloud and out-of-cloud concentrations, most of which could not be explained by simple dissolution into cloud droplets. Concentration patterns were analysed in relation to the chemistry of cloud droplets and the gas/liquid equilibrium. Soluble gases do not undergo similar behaviour: CH 3 COOH simply dissolves in the aqueous phase and is outgassed upon cloud dissipation; instead, SO 2 is consumed by its reaction with H 2 O 2 . The behaviour of HCOOH is more complex because there is evidence for in-cloud chemical production. The formation of HCOOH interferes with the odd hydrogen cycle by enhancing the liquid-phase production of H 2 O 2 . The H 2 O 2 concentration in cloud therefore results from the balance of consumption by oxidation of SO 2 in-cloud production, and the rate by which it is supplied to the system by entrainment of new air into the clouds.

Observations and modelling of the processing of aerosol by a hill cap cloud

Abstract Observations are presented of the aerosol size distribution both upwind and downwind of the Great Dun Fell cap cloud. Simultaneous measurements of the cloud microphysics and cloud chemistry, and of the chemical composition of the aerosol both upwind and downwind of the hill were made along with measurements of sulphur dioxide, hydrogen peroxide and ozone. These observations are used for initialisation of, and for comparison with the predictions of a model of the air flow, cloud microphysics and cloud chemistry of the system. A broad droplet size distribution is often observed near to the hill summit, seemingly produced as a result of a complex supersaturation profile and by mixing between parcels with different ascent trajectories. The model generates several supersaturation peaks as the airstream ascends over the complex terrain, activating increasing numbers of droplets. In conditions where sulphate production in-cloud (due to the oxidation of S(IV) by hydrogen peroxide and ozone) is observed, there is a marked effect on the chemical evolution of the aerosol particles on which the droplets form. When sulphate production occurs, a significant modification of the aerosol size distribution and hygroscopic properties is both predicted and observed. The addition of sulphate mass to those aerosol particles nucleation scavenged by the cloud generally increases the ease with which they are subsequently able to act as cloud condensation nuclei (CCN). Often, this will lead to an increase in the number of CCN available for subsequent cloud formation, although this latter effect is shown to be strongly dependent upon the activation history of the droplets and the concentration of pollutant gases present in the interstitial air. Situations are also identified where cloud processing could lead to a reduction in the capacity of smaller aerosol to act as CCN.

Case study of cloud physical and chemical processes in low clouds at Mt. Brocken

Frequency distributions of cloud base height and cloud type of low clouds observed between May and October 1998 at Mt. Brocken (Germany) have been derived from ceilometer measurements and synoptic observations. The summit at 1142 m a.s.l. was about 50% of that time in cloud. During daytime, Stratus clouds were the dominant cloud type (65%), whereas Cumulus clouds amounted to 27% and Stratocumulus clouds to 8%. Evidence was found that the increase of the cloud base height observed at Mt. Brocken continues since the end of the 1980s. An example for a clear anticorrelation between the liquid water content (LWC) of the cloud and the height above cloud base is shown. Other results of this detailed case study of a cloud event on October 8, 1998 concerning phase partitioning of water-soluble inorganic compounds, black carbon (BC) and organic carbon (OC) between the liquid and the interstitial phase will also be presented. The observed ion-specific increase in the solute mass per cubic meter of air with decrease of the distance between sampling position and cloud base was caused mainly by entrainment of air from the below-cloud layer. As expected, for sulfate, ammonia and nitrate, high scavenging coefficients (>0.8) were found. OC exhibits a high scavenging fraction of between 0.4 and 0.7; the value for black carbon (0.2–0.4) implies that soot was possibly to some extent internally mixed in the cloud condensation nuclei (CCN). Simultaneous measurements during a cloud event of HNO2 and HNO3 in the gas phase and N(III) and N(V) in the liquid phase were made for the first time. D 2002 Elsevier Science B.V. All rights reserved.

METEOROLOGY OF THE GREAT DUN FELL CLOUD EXPERIMENT 1993

Synoptic and local meteorological conditions during the Spring 1993 Ground-based Cloud Experiment on Great Dun Fell are described, including cloud microphysics, general pollution levels and sources of air, especially for five case studies selected for detailed analysis. Periods when air was flowing across the hill are identified and the extent to which air mixed into the cloud from above reached the ground is estimated. To aid the interpretation of cloud chemistry and microphysics measurements, the horizontal and vertical extent of the cloud are used to estimate droplet lifetimes and to comment on the influence of complex terrain on peak supersaturation.

PHENOLS AND NITRATED PHENOLS IN CLOUDS AT MOUNT BROCKEN

Abstract Two phenols and four nitrophenols have been determined in the gas and liquid phase of clouds during a field campaign in May 1994 at Mount Brocken (Germany). If the total amount in both phases is considered, 2-nitrophenol is the dominant compound which is mainly present in the gas phase while 4-nitrophenol is much more abundant in the liquid phase, which reflects the differences in the distribution constants. The experimental data suggest that mononitrophenols may partially originate from automobile exhaust while dinitrophenols are almost completely formed by atmospheric reactions. Furthermore, comparison of the concentration of dinitrophenols and the extent to which air masses are transported in clouds indicates that these compounds are mainly formed in cloud droplets.

The reduced nitrogen budget of an orographic cloud

Field data collected during the GDF 93 project indicated that during polluted conditions (SO2(g) > 2 ppbv, NH3(g) > 0.5 ppbv), sulphate and ammonium concentrations in air increased through cloud chemistry by as much as 25%. Similarly, ammonia was seen to be consumed by cloud processing and decreased by up to 20%. In comparatively clean conditions (SO2(g) < 0.5 ppbv, NH3(g) < 0.5 ppbv), the sulphate loading of the aerosol was seen to remain constant, and ammonium was lost from the aerosol and outgassed as ammonia, increasing ambient ammonia concentrations by as much as 0.5 ppbv. An ideal cloud chemistry model predicted up to 20% more sulphate production than is implied by the bulk aerosol data set. A non-ideal cloud chemistry model was used to estimate the final ammonium loading of the aerosol, which is determined by the transformation from wet cloud droplet to dry aerosol particles below their deliquescence point. The non-ideal model showed that in three of the four cases ammonia outgassed from evaporating cloud droplets, consistent with field observations, but at variance with the ideal chemistry model. The results indicate that in low pollution conditions clouds act to re-equilibrate reduced nitrogen in the aerosol phase with gaseous ammonia. The outgassed ammonia will then be rapidly deposited to semi-natural ecosystems downwind of such clouds.

Atmospheric research program for studying changing emission patterns after German unification

Abstract A dramatic decrease in emission of sulphur dioxide (SO 2 , 60%), dust (82%), nitrogen oxide (NO, 30%) and other pollutants (hydrochloric acid (HCl); ammonia (NH 3 )) has been observed in East Germany after German unification in 1989. The smaller decrease in total NO is due to significant increase of NO from traffic. Air-pollutant concentrations in both parts of Germany before and after 1989 will be compared and their development will be discussed, based on precipitation and cloud chemistry data from long-term continuous record. Sectoral rain data, based on backward trajectory calculations, clearly show that the significant decline in annual volume weighted precipitation ions (sulphate, calcium, ammonium, chloride) in rain water from sector “East” since 1989 and particularly since about 1992 is roughly correlated with a similar percentage decline in SO 2 , dust and NH 3 emissions for the region East Germany. The acidity of the precipitation from sector “East” has increased very strikingly from 1993 up to 1995 threefold more than from sector “West”. Despite the strong decrease in SO 2 emission, this decrease of the atmospheric neutralising capacity was caused by the much steeper decline in atmospheric base cations in that region. Precipitation data from all transport directions show high variation, but no significant trend in the annual average H + ion concentration. At Mt. Brocken the annual average volume weighted cloud-water acidity during frost-free periods increased by a factor of three between 1992 and 1995. This result, confirmed also by the annual frequency distributions of the pH values, reflects the atmospheric pollutant loading also from all entry sectors. After 1995 this trend is reversed. Detailed classification of the cloud-water data by entry sector, and by meteorological and especially microphysical factors is currently being carried out and preliminary results are presented.