2600 G. McFiggans et al. aerosol effects on warm cloud activation Background Near-city Urban marylebone(GB) 0oo10010.1 1E·5 E+5 E+5 Mept (Dj 010101 Jungtaujoch (CH pra(n 0o010011 Fig. 6. Median particle number size distributions during summer, during morning hours(black dashed line), afternoon(grey full line) and night(black full line). From Van Dingenen et al. (2004) Table 1. Table of s(Xi)=aIn Na/aln Xi where Xi is one of Na mately the same(although opposite in sign) whereas S(w is small. Under polluted conditions, the relative influence indicates Na>1000 cm-3. The ranges of x are w: 20 cms-I to of rg, Og and w on Na increases significantly while S(Na) 300, Na: 20 cm-3 to 3000 cm-3, rg: 0.03 to 0. 1 um, a: 1.3 decreases in importance. S(e) is relatively small compared to2.2,E:0.10to1.00 to the other terms. although we caution that this term only reflects composition changes associated with the fraction all Clean polluted of soluble material. The signs of S(Xi) are as expected specific mention is made of S(og) which is negative because Na0.880.92 0.7 rg0.320.280.39 the tail of the distribution at large sizes results in activation a-0.39-0.310.53 of larger drops, and suppression of supersaturation which tends to suppress Nd. This combination of effects makes E0.110.09 0.13 S(og) quite large, particularly under polluted conditions when the larger particles are abundant (e.g. O Dowd et al 1999, Sect. 3.1.2). Rissman et al.(2004)performed a more detailed analysis of the effect of various composition ange of parameter space. Aerosol composition was repre factors such as solubility and surface tension, as well as size sented in a simplified fashion by considering an ammonium distribution parameters. Their results were derived from sulphate and insoluble mix, and varying the mass fraction analytical solutions, and presented in terms of a sensitivity of ammonium sulphate, over the range 0. 1 to 1.0. The out- relative to the sensitivity of drop number concentration to put was then used to examine the relative sensitivity of cloud updraft velocity (x)=(x/w)(aNa/a x)/(aNa/aw),where drop size to the various input parameters using the model of x is a composition factor such as organic mass fraction Feingold(2003). Here we repeat this analysis for sensitiv- Eo. The authors concluded that when defined this way, ity of drop number concentration Na. The sensitivities defined as S(Xi)=aIn Nd/aIn Xi where Xi is one of Na,r sensitivity to composition factors (x) is highest for aerosol e). In this form, values of S(Xi) can be compared updraught velocity. However, these are conditions under with one-another to assess their relative importance. Values which supersaturation and activated fractions of S(Xi) for conditions similar to Feingold (2003)are given an increase in w does not add many new drops(aNa/dw in Table 1 is small). The appea of aNd/aw in the denominator Under clean conditions, arbitrarily defined as tends to increase (x). Thus at high S, even though Na<1000 cm -', S(Na) is close to its theoretical upper o(x) is large, composition effects may not be important limit of 1, indicating a high level of in-cloud activation in an absolute sense. Because the individual sensitivities Sensitivity to rg and og under clean conditions is approxi tmos.Chem.Phys,6,2593-2649,2006 www.atmos-chem-phys.net/6/2593/2006/2600 G. McFiggans et al.: Aerosol effects on warm cloud activation Fig. 6. Median particle number size distributions during summer, during morning hours (black dashed line), afternoon (grey full line) and night (black full line). From Van Dingenen et al. (2004); van Din￾genen et al. (2004). 125 Fig. 6. Median particle number size distributions during summer, during morning hours (black dashed line), afternoon (grey full line) and night (black full line). From Van Dingenen et al. (2004). Table 1. Table of S(Xi )=∂lnNd /∂lnXi where Xi is one of Na, rg, σg, w, ε. “Clean” indicates Na<1000 cm−3 and “Polluted” indicates Na>1000 cm−3 . The ranges of Xi are w: 20 cm s−1 to 300 cm s−1 , Na: 20 cm−3 to 3000 cm−3 , rg: 0.03 to 0.1µm, σ: 1.3 to 2.2, ε: 0.10 to 1.00. All Clean Polluted Na 0.88 0.92 0.73 rg 0.32 0.28 0.39 σ −0.39 −0.31 −0.53 w 0.29 0.18 0.47 ε 0.11 0.09 0.13 range of parameter space. Aerosol composition was repre￾sented in a simplified fashion by considering an ammonium sulphate and insoluble mix, and varying the mass fraction of ammonium sulphate, over the range 0.1 to 1.0. The out￾put was then used to examine the relative sensitivity of cloud drop size to the various input parameters using the model of Feingold (2003). Here we repeat this analysis for sensitiv￾ity of drop number concentration Nd . The sensitivities are defined as S(Xi)=∂lnN d/∂lnXi where Xi is one of Na, rg, σg, w or ε). In this form, values of S(Xi) can be compared with one-another to assess their relative importance. Values of S(Xi) for conditions similar to Feingold (2003) are given in Table 1. Under clean conditions, arbitrarily defined as Na<1000 cm−3 , S(Na) is close to its theoretical upper limit of 1, indicating a high level of in-cloud activation. Sensitivity to rg and σg under clean conditions is approxi￾mately the same (although opposite in sign) whereas S(w) is small. Under polluted conditions, the relative influence of rg, σg and w on Nd increases significantly while S(Na) decreases in importance. S(ε) is relatively small compared to the other terms, although we caution that this term only reflects composition changes associated with the fraction of soluble material. The signs of S(Xi) are as expected; specific mention is made of S(σg) which is negative because the tail of the distribution at large sizes results in activation of larger drops, and suppression of supersaturation which tends to suppress Nd . This combination of effects makes S(σg) quite large, particularly under polluted conditions when the larger particles are abundant (e.g. O’Dowd et al., 1999, Sect. 3.1.2). Rissman et al. (2004) performed a more detailed analysis of the effect of various composition factors such as solubility and surface tension, as well as size distribution parameters. Their results were derived from analytical solutions, and presented in terms of a sensitivity relative to the sensitivity of drop number concentration to updraft velocity φ(χ )=(χ/w)(∂Nd /∂χ )/(∂Nd /∂w), where χ is a composition factor such as organic mass fraction o. The authors concluded that when defined this way, sensitivity to composition factors φ(χ ) is highest for aerosol typical of marine condition, and increases with increasing updraught velocity. However, these are conditions under which supersaturation and activated fractions are high, and an increase in w does not add many new drops (∂Nd /∂w is small). The appearance of ∂Nd /∂w in the denominator tends to increase φ(χ ). Thus at high S, even though φ(χ ) is large, composition effects may not be important in an absolute sense,. Because the individual sensitivities Atmos. Chem. Phys., 6, 2593–2649, 2006 www.atmos-chem-phys.net/6/2593/2006/