Emission of nitrous acid from soil and biological soil crusts 1 represents a dominant source of HONO in the remote 2 atmosphere in Cyprus 3

Soil and biological soil crusts can emit nitrous acid (HONO) and nitric oxide (NO). The terrestrial ground 14 surface in arid and semi-arid regions is anticipated to play an important role in the local atmospheric HONO budget, 15 deemed to represent one of the unaccounted HONO sources frequently observed in field studies. In this study HONO 16 and NO emissions from a representative variety of soil and biological soil crust samples from the Mediterranean 17 island Cyprus were investigated under controlled laboratory conditions. A wide range of fluxes was observed, 18 ranging from 0.6 to 264 ng m -2 s -1 HONO-N at optimal soil water content (20-30% of water holding capacity, WHC). 19 Maximum NO-N at this WHC fluxes were lower (0.8-121 ng m -2 s -1 ). Highest emissions of both reactive nitrogen 20 species were found from bare soil, followed by light and dark cyanobacteria-dominated biological soil crusts 21 (biocrusts), correlating well with the sample nutrient levels (nitrite and nitrate). Extrapolations of lab-based HONO 22 emission studies agree well with the unaccounted HONO source derived previously for the extensive CYPHEX field 23 campaign, i.e., emissions from soil and biocrusts may essentially close the Cyprus HONO budget. 24

HONO and NO can be formed during biological processes (nitrification and denitrification; Pilegaard, 2013), in which NH 3 or NH 4 + is oxidized stepwise or NO 3 -is reduced (Fig. 1).Depending on soil-pH and according to Henry´s law soil nitrite (NO 2 -) can be converted into gaseous HONO.
In Cyprus, an island in the semi-arid eastern Mediterranean area, biocrusts are ubiquitously covering ground surfaces and hence can be anticipated to play an important role in the local HONO budget.In the CYPHEX campaign 2014 (CYprus PHotochemical EXperiment) the observed diel cycles of HONO ambient air concentrations revealed strong unaccounted sources of HONO and NO, being well correlated with each other (Meusel et al., 2016).With low NO 2 concentrations and high HONO/NO x ratios, respectively, direct emissions from combustion and heterogeneous reactions of NO 2 could be excluded as significant HONO sources, leaving emissions from soil and the respective surface cover to be the most plausible common source for both nitrogen species (Meusel et al., 2016).
In the present study we measured HONO and NO fluxes from soil and biocrust samples from Cyprus by means of a dynamic chamber system.The aim of this study was to characterize and quantify direct trace gas emissions and demonstrate their impact on the atmospheric chemistry in the remote coastal environment of Cyprus.

Sampling
Bare soil and biocrust samples were collected on 27th April 2016 on the South/South-East side of the military station in Ineia, Cyprus (34.9638°N, 32.3778°E), where the CYPHEX campaign took place in 2014.It is a rural site about 600 m above sea level, approximately 5-8 km from the coast and is surrounded by typical Mediterranean vegetation (olive and pine trees, small shrubs like Pistacia lentiscus, Sacopoterium spinosum and Inula viscosa).More details about the site can be found in Meusel et al. (2016).
In an area of about 8580 m² (South/South-East direction of the station) 50 grids (25x25 cm) were placed at randomly selected spots for systematic ground cover assessment.At each grid point occurrence of nine types of surface cover (i.e., light and dark cyanobacteria-, chlorolichen-, cyanolichen-, and moss-dominated biocrust, bare soil, stone, litter, vascular vegetation/shrub) were assigned and quantified.Spatially independent replicate samples were collected of light cyanobacteria-dominated biocrusts (light BSC), dark cyanobacteria-dominated biocrusts with cyanolichens (dark BSC), chlorolichen-dominated biocrusts (chlorolichen BSC I, chlorolichen BSC II), moss-dominated biocrusts (moss BSC) and of bare soil (Fig. S1 of the supplement).Each sample was collected in a plastic petri dish, sealed and stored in the dark at room temperature until further analysis (storage time less than 15 weeks).
In total 43 samples were collected (Table 1) of which 18 samples, i.e., 3 replicates of each HONO emitting surface cover type were used direct (upfront) for nutrient analysis, while all others were first used for trace gas exchange measurements, prior to nutrient and chlorophyll content analysis.

Meteorological data
During CYPHEX the meteorological parameters were even measured at about 5 m above ground, considered not representative for the micro-habitat of the soil ground surface.Hence we placed three humidity Dec. 2016).

Soil characteristics: nutrient, chlorophyll and pH
Soil characteristics (nutrient, pH) have an effect on soil emission, e.g., higher nutrient level and lower pH would enhance emission according to Henry law (Su et al., 2011).Nutrient analysis was conducted on samples without gas exchange measurements (n = 3) and on replicate samples after gas exchange measurements in order to analyze potential effects of the applied 'wetting-drying' cycle.Nitrate (NO 3 -), nitrite (NO 2 -) and ammonium (NH 4 + ) were analyzed via flow injection analysis with photometric detection (FIAstar 5000, Foss, Denmark).Prior to that, the samples comprised of soil and its biocrust-cover were gently ground and an aliquot of 7 g was solved in 28 mL of 0.0125 M CaCl 2 .After shaking for 1 hour the mixture was filtered on a N-free filter.
Chlorophyll analysis, as an indicator of biomass of photo-autotrophic organisms, was done according to the dimethyl sulfoxide (DMSO) method (Ronen and Galun, 1984).Ground samples were extracted twice with CaCO 3 saturated DMSO (20 mL, 10 mL) at 65°C for 90 min.Both extracts were combined and centrifuged (3000 G) at 15°C for 10 min.The light absorption at 648, 665 and 700 nm was detected with a spectral photometer (Lambda 25 UV/VIS Spectrometer, Perkin Elmer, Rodgau).The amount of chlorophyll a (Chl a ) was calculated according to Arnon et al.
( 1974).Chlorophyll a+b (Chl a+b ) content was calculated according to Lange, Bilger and Pfanz (pers. comm. in Weber et al., 2013): where Chl a+b [μg], Chl a [μg] is the chlorophyll content of the sample, E 648 , E 665 , E 700 are light absorption at the given wavelength, and a is the amount of DMSO used in mL.
The pH was determined for each surface cover type (n = 3-4) according to Weber et al. (2015, Suppl.).Here, 1.5 g of the ground sample was mixed with 3.75 mL of pure water and shaken for 15 min.Then the slurry was centrifuged (3000 G, 5 min) to separate the solid phase from the liquid solution.The latter was used for pH determination by means of a pH electrode (Inlab Export Pro-ISM, Mettler Toledo).

Trace gas exchange measurements
The dynamic chamber method for analyzing NO and HONO emissions from soil samples was already introduced before (Oswald et al., 2013;Weber et al., 2015;Wu et al., 2014).and water vapor were quantified.HONO was analyzed with a commercial long path absorption photometer (LOPAP, QUMA GmbH; Wuppertal, Germany), with a detection limit of ~4 ppt and 10% uncertainty (based on the uncertainties of liquid and gas flow, concentration of calibration standard and regression of calibration).To avoid any transformation of HONO in the tubing, the sampling unit including the stripping coil from LOPAP was directly connected to the chamber.NO x (NO + NO 2 ) was detected with a commercial chemiluminescence detector (42i TL, Thermo Scientific; Watham, USA) modified with a photolytic converter with a detection limit of ~50 ppt (NO) and ~200 ppt (NO 2 ).An infrared CO 2 and H 2 O analyzer (Li-840A, LICOR; Lincoln, USA) was used to log the drying and to calculate the soil water content (SWC) of the samples as follows: with t=0 denoting the measurement start (wetted sample inserted into chamber), t=n: any time between 0 and N, t=N: time when sample had dried out and measurement was stopped, S Licor : absolute H 2 O signal at a given time, m H2O,0 : mass of water added to sample (water holding capacity, WHC), SWC: soil water content in % WHC.

Meteorological conditions
One month before sampling, three sensors measuring temperature and relative humidity directly above the soil surface were installed in the field to represent the micro-climate of the ground surface.Reference air temperature, humidity and precipitation measurements at Paphos airport and Prodromos showed one rain event on 11-12 April which is reflected by higher soil humidity (80-100%) and lower temperatures on these days (see Fig. 2).As a consequence, the biological soil crusts were activated and went through one full wetting and drying cycle before sample collection.Temperature above the soil ranged from 10°C in the night to 50°C during the day when solar radiation was most intense.Air temperature was similar during the night but not as hot during the day ranging between 20° and 30°C.Humidity above the ground was low during daytime (<30% rH) and increased during the night up to 80%, while the atmospheric relative humidity (at Paphos airport) ranged between 47 and 73% (without rain event).Thus there were only little variations of humidity with height above the soil surface.Above the ground surface the relative humidity was somewhat lower during the day (mainly caused by higher temperatures) but with humidity between 20-60% rH.Hence we can assume that soil surface temperatures were higher and ground rH in the same range during CYPHEX compared to sampling period.

Cyprus soil and biocrust characteristics
Systematic mapping of surface covers revealed that moss-dominated biocrusts are the most frequent in the investigated Cyprus field site area (21.3%), followed by light (10.4%) and dark BSC (6.5%), whereas chlorolichen-(3.2%)and cyanolichen-dominated BSC (1.8%)only played a minor role (Fig. 3, Fig. S1).The soil surface was partially covered by litter (26.3%), stones (19.5%) and vascular vegetation (8.5%), whereas open soil was rarely found (2.5%).It was previously established that soil and biocrusts emit HONO and NO (Weber et al., 2015;Oswald et al., 2013), jointly accounting for 45.6% of surface area in our studied region.To the best of our knowledge, no data on reactive nitrogen emissions from vascular vegetation and plant litter have been published yet.
Nutrient analysis revealed large variations in concentrations of nitrogen species ranging from 0 to 6.48, 0 to 0.57 and 0 to 22.2 mg (N) kg -1 of dry soil/crust mass for NO 3 -, NO 2 -, and NH 4 + , respectively (Fig. 4a, Tab.S1 of the supplement).In general, no significant change in reactive nitrogen contents was found before and after the trace gas exchange experiments (Fig. 4a), indicating no significant impact of one wetting-drying cycle on the nutrient content.
Bare soil samples had significantly higher levels of NO 3 -and NO 2 -content compared to dark, chlorolichen and moss BSC.Among the latter three, no significant differences in nutrient levels were observed.Light BSC had NO 2 contents similar to bare soil.The NH 4 + content was very similar in all samples, except for one outlier in the group of light BSC with strongly elevated NH 4 + .Higher nitrate and ammonium levels in bare soil compared to crust-covered samples were also reported recently for a warm desert site in South Africa (Weber et al., 2015), indicative of nutrient consumption/integration by the biocrusts.Nitrite, on the other hand, was lower for bare soil samples compared to biocrust samples.While NO 3 -was slightly higher, NH 4 + and NO 2 -contents (especially of bare soil samples) were lower in the South African arid ecosystem compared to Cyprus.
Chlorophyll was only determined in the samples used for flux measurements.Chl a ranged from 4.1 (bare soil) to 144.2 mg m -2 (moss BSC) and Chl a+b from 9.3 (bare soil) to 211.3 mg m -2 (moss BSC), respectively (Fig. 4b, Tab.S1).From bare soil, via light BSC and chlorolichen BSC II, to dark BSC the chlorophyll content increased, but not significantly (p > 0.2).Nevertheless, Chl a and Chl a+b contents of chlorolichen BSC I and moss BSC were significantly higher than these of bare soil, light BSC and chlorolichen BSC II (p<0.05,Fig. 4b)..The range of chlorophyll contents is comparable to previous arid ecosystem studies (Weber et al., 2015).
The pH of soil and biocrusts ranged between slightly acidic (6.2) and slightly alkaline (7.6; Fig. 4c).The mean pH of 17 samples was 7.0, i.e., neutral.Only the pH of moss BSC samples was significantly lower than that of bare soil, light BSC and chlorolichen BSC samples (p=0.05).Soil and biocrust samples from South Africa were slightly more alkaline (7.1-8.2) with no significant difference among biocrust types (Weber et al., 2015).

NO and HONO flux measurements
All samples showed HONO and NO emissions during full wetting and drying cycles.Maximum emission rates of HONO were observed at about 17-33% WHC, and of NO at 20-36% with no significant differences between all soil cover types (Fig 5).Emissions declined to zero at 0% WHC and to very small rates >70%.Emission maxima strongly varied between soil cover types, but also between samples of the same cover type (see Fig. 5 and 6, and Table S1).Highest emissions of both HONO-N and NO-N were detected for bare soil (175 ± 87.3 and 92.2 ± 34.7 ng m -2 s -1 ), followed by light (48.6 ± 48.5 and 34.5 ± 42.1 ng m -2 s -1 ) and dark BSC (27.1 ± 35.9 and 16.7 ± 18.3 ng m -2 s -1 ).Both types of chlorolichen-and moss-dominated biocrusts showed very low emission rates of reactive nitrogen (on average < 10 ng m -2 s -1 ).Maximum HONO emissions were somewhat higher than maximum NO emissions, especially for bare soil.Integrating full wetting and drying cycles (6-8 hours), 0.04-1.9mg m -2 HONO-N and 0.06-1.6mg m -2 NO-N were released (Fig. 6, lower panel).While the maximum fluxes of reactive nitrogen emission were higher for HONO than NO, especially from bare soil, the integrated emissions were similar or even larger for NO, which is released over a wider range of SWC.
In general, it is difficult to compare chamber flux measurements of different studies due to different experimental configurations, such as chamber dimension, flow rate, resident time and drying rate etc.Here, we compare our results to studies which applied the same method (with the same or very similar conditions).The emission rates are consistent with these studies where HONO-N or NO-N emissions from soil between 1-3000 ng m -2 s -1 were found (Su et al., 2011;Oswald et al., 2013;Mamtimin et al., 2016;Wu et al., 2014;Weber et al., 2015).Mamtimin et al. (2016) observed NO-N fluxes at 25°C of 57.5 ng m -2 s -1 , 18.9 ng m -2 s -1 and 4.1 ng m -2 s -1 for soil of grape and cotton fields and desert soil from an oasis in China, respectively.Oswald et al. (2013) found HONO-N and NO-N emissions between 2 and 280 ng m -2 s -1 (each) from different soil from all over the world covering a wide range of pH, nutrient content and organic matter.Biogenic NO emissions of 44 soil samples from arid and semi-arid regions were reviewed by Meixner and Yang (2006) with N-fluxes ranging from 0 to 142 ng m -2 s -1 .
In contrast to the results of the present study, where bare soil showed highest emissions, Weber et al. (2015) found lowest emission from bare soil in samples from South Africa.In that study, dark cyanobacteria-dominated biocrusts revealed highest emission rates (each HONO-N and NO-N up to 200 ng m -2 s -1 ), followed by light cyanobacteriadominated biocrusts (up to 120 ng m -2 s -1 ), whereas in the present study, emissions of dark cyanobacteria-dominated biocrusts tended to be lower.No significant difference of HONO-N and NO-N emissions from light BSC between both sample origins were found.HONO-N and NO-N emissions of moss-and chlorolichen-dominated biocrusts were low in both studies (each <60 ng m -2 s -1 ) but still significantly higher for samples from South Africa than from Cyprus.In the present study HONO maximum emissions were higher than for NO (while integrated emissions being comparable) while in the study of Weber et al. (2015) HONO maximum fluxes were somewhat lower than those of NO.The present results of nitrogen emissions correlate well with the nutrient contents (especially NO 2 -and NO 3 -, Fig. 7).Bare soil, in which highest NO 3 -and NO 2 -levels were found, also showed highest HONO and NO emissions.
A very good correlation was found between NO 2 -contents and emission of both nitrogen gas phase species for all samples (R² = 0.84 for HONO and 0.85 for NO; p<0.001).The level of correlation between NO 3 -and HONO and NO was lower, but still significant (R² = 0.68 and 0.67, respectively, p<0.seems that reactive nitrogen emissions predominantly depend on NO 2 -and NO 3 -contents and not on surface cover types, although biocrusts (especially with cyanobacteria and cyanolichens) are able to fix atmospheric nitrogen (Belnap, 2002;Elbert et al., 2012;Barger et al., 2013;Patova et al., 2016).The results of a two-factorial ANOVA showed that HONO or NO emissions are not significantly related to soil cover type but rather with nitrite content, i.e., its direct aqueous precursor.For nitrate, the two-factorial ANOVA indicated dependencies of both cover type and nutrient content.These results differ from those obtained by Weber et al. (2015) on South African samples, as there HONO and NO emissions were not correlated with bulk concentrations of ammonium, nitrite and nitrate.In their study nitrite content was lowest for bare soil compared to other biocrust types.Ammonium and nitrites levels were also lower than in the present study.Therefore Weber et al. (2015) indicated that biocrusts can enhance N-cycle and emission of reactive nitrogen.

Comparison of soil emission and observed missing source
To quantify the flux rate of HONO emissions from soil to the local atmosphere and to compare it to the unaccounted source found in Cyprus in 2014 (Meusel et al., 2016), we applied a standard formalism describing the atmospheresoil exchange of trace gases as a function of the difference between the atmospheric concentration and the equilibrium concentration at the soil solution surface [HONO]* (Su et al., 2011): where [HONO] is the ambient HONO concentration measured on Cyprus (mean daytime average 60 ppt) and [HONO]* is the equilibrium concentration at soil surface.
[HONO]* can be determined from measurements in a static chamber.In a dynamic chamber system, there is a concentration gradient of HONO between the headspace (where HONO was measured) and the soil surface.Here we use the measurements of water vapor to correct for the soil surface concentration and equilibrium concentration of HONO by assuming a similar gradient for the two species.A correction coefficient of 3.8 was determined, which is the ratio of the equilibrium rH of 100% over wet soil surface to the initial headspace rH of 25-30% after inserting the wet sample into the chamber.The transfer velocity, v t , depends primarily on meteorological and soil conditions, and is typically on the order of ~1 cm s -1 .The flux rate of NO was calculated accordingly with mean daytime NO concentrations of 38 ppt.The calculated flux F* is about (67±3) % of the flux measured in the chamber.
The distribution of nine different surface cover types was mapped (Fig. 2), including stones, vascular vegetation and litter not being attributed to emit significant amounts of HONO and NO to the atmosphere.The residual HONO emitting surface covers comprised 45.6% of total surface in the investigated area.Combining the information on soil/biocrust population and the calculated flux F*, a site-specific community emission F comm of HONO and NO can be estimated via following equation (eq.6).contribution from bare soil dominated with up to 69% (HONO) and 55% (NO), respectively, followed by moss BSC (HONO: 23%; NO: 32%).At high levels of total emission, the contribution from light BSC dominated (HONO: 43%, NO: 49%), decreasing the contribution of bare soil down to about 25% (HONO) and 13% (NO).Emissions from dark BSC contribute about 20% or 24% to the total HONO or NO flux while the contribution from moss BSC decreased to 10% or 12%, respectively.Emissions from chlorolichen BSC didn´t play a significant role (< 2.4%) in general (see Fig. 8).
After heavy rainfalls moistening the soil to full water-holding capacity, 11-113 µg m -2 of HONO-N and 10-131 µg m -2 of NO-N can be calculated for one complete wetting-and-drying period.Assuming 30 rain events per year (based on the statistic of 4 years precipitation data), a wetting-drying cycle time of 7 days, and constant emissions in between them (at 10% WHC) up to 160 mg m -2 yr -1 of nitrogen can be emitted directly by the sum of HONO-N and NO-N from Cyprus natural ground surfaces, i.e., excluding heterogeneous conversion of NO 2 on ground surface.
The release of HONO from the ground surface to the atmosphere can be related to the atmospheric HONO production rate via eq.7 (adapted from Su et al., 2011) and then compared to the missing source.
Based on the studies by Likos (2008) and Leelamanie (2010) and the meteorological conditions during CYPHEX (no rain event, but high rH, usually > 75%) a soil water content, slightly lower than the optimal water content for HONO and NO emissions, of 10% WHC was estimated, at which emissions of about 35% of the maximum was found.
In Cyprus during the summer of 2014 a mean boundary layer height of 300 m was observed by means of a ceilometer..The mean air temperature during the campaign was comparable to the lab based chamber studies (25°C) but soil temperatures at the Cyprus field site could largely vary during daytime and reach maximum temperatures of up to 50°C (Fig. 4).At these high temperatures 6-10 fold higher emissions can be expected in general (Mamtimin et al., 2016), but also a quicker drying of the soil and biocrusts.At 25°C HONO emissions from the ground would equal a source strength of 1.1x10 5 -9.8x10 5 cm -3 s -1 and would cover up to 75% of the missing mean source of 1.3x10 6 cm -3 s -1 (Meusel et al., 2016).In some mornings of the campaign dew formation was expected causing an increase in soil humidity.Combined with rising temperatures after sun-rise these optimized meteorological conditions may have led to enhanced soil emissions and would confer a reasonable explanation for the strong HONO morning peaks observed during the campaign.Similarly, the NO source strength from ground emission at 25°C is in the range from 8.3x10 4 to 8.0x10 5 cm -3 s -1 .As the observed unaccounted source of NO in Cyprus was of the order of 10 7 cm -3 s -1 soil emissions can only contribute up to 8% indicating other NO sources.Note that during CYPHEX there were two periods with lower rH, in which even a NO sink was detected.

Conclusions
HONO and NO emission rates from soil and biological soil crusts were derived by means of lab-based enclosure trace gas exchange measurements, and revealed quite similar ranges of reactive nitrogen source strengths.Emissions of both compounds strongly correlated with NO 2 -and NO 3 -content of the samples.Emissions from bare soil were . Phys.Discuss., doi:10.5194/acp-2017-356,2017 Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 2 May 2017 c Author(s) 2017.CC-BY 3.0 License.HONO + OH → NO 2 + H 2 O (R3) (and temperature) sensors (HOBO Pro v2) just on top of the soil surface about 4 weeks prior to sample collection.Reference meteorological data (air temperature, humidity and precipitation) from Paphos airport (about 20 km south of the Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2017-356,2017 Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 2 May 2017 c Author(s) 2017.CC-BY 3.0 License.sample area, 12 m asl) and Prodromos (about 40 km east of the sampling area, 1380 m asl) during the sampling period as well as the precipitation data from the last 4 years (2013-2016) were provided by the Department of Meteorology, Cyprus (http://www.moa.gov.cy/moa/ms/ms.nsf/DMLmeteo_reports_en/MLmeteo_reports_en?opendocument; last access: Soil and biocrust samples (25-35 g) were wetted with 8-13 g of pure water (18.2MΩ) up to water holding capacity and placed into a dynamic Teflon film chamber (≈47 L) flushed with 8 L min -1 dry pure air (PAG 03, Ecophysics, Switzerland).Typical drying cycles lasted between 6 and 8 hours.A Teflon coated internal fan ensured complete mixing of the chamber headspace volume.During the experiments the chamber was kept at constant temperature (25°C, the mean daytime air temperature during CYPHEX) and in darkness to avoid photochemical reactions.At the chamber outlet the emitted gases HONO, NOAtmos.Chem.Phys.Discuss., doi:10.5194/acp-2017-356,2017   Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 2 May 2017 c Author(s) 2017.CC-BY 3.0 License.
a+b , Chl a , NO and HONO optimum flux and NO and HONO integrated flux did not follow a normal distribution.Rather, log-transformed data were normally distributed (Shapiro-Wilk) and therefore used for statistical analysis (Pearson correlation, ANOVA including Tukey Test with significance level of p = 0.05) executed with OriginPro (version 9.0; OriginLab coporation, Northampton, Massachusetts, USA).Precipitation data from the last 4 years (2013-2016) provided by the Department of Meteorology of Cyprus indicate about 30 rain events per year (precipitation > 1 mm with following one or more dry days) were used to estimate annual emissions of total nitrogen by way of HONO and NO.
Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2017-356,2017   Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 2 May 2017 c Author(s) 2017.CC-BY 3.0 License.somewhat higher during the night, compared to respective weather station data.During and shortly after the main rain event humidity at ground level was higher (80 and 100% rH) compared to ambient air humidity (70-85% rH).Ambient air temperatures were somewhat lower during sample collection of this study as compared to the CYPHEX field campaign in 2014.During CYPHEX, nighttime temperatures (3 m above ground level) did not drop below 18°C.Relative humidity (3 m above ground level) was mostly between 70 and 100% with only two short periods Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2017-356,2017   Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 2 May 2017 c Author(s) 2017.CC-BY 3.0 License.
Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2017-356,2017   Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 2 May 2017 c Author(s) 2017.CC-BY 3.0 License.highest, but bare soil surface spots were rarely found at the investigated CYPHEX field study site.The estimated total ground surface HONO flux in the natural habitat is consistent with the previously unaccounted source estimated for Cyprus, i.e., the unaccounted HONO source can essentially be explained by emissions from soil/biocrusts.For NO, the measured and simulated fluxes cannot account for the unaccounted NO source (during the humid periods of the CYPHEX campaign 2014), indicating that emission from soil was not the only missing source of NO. nighttime radical reservoirs; in Los Angeles and their contribution to the urban uadical budget, EnvironmentalScience & Technology, 46, 10965-10973, 10.1021/es302206a, 2012.